# Influences from outside the observable universe explaining dark energy and expansion?

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Could some influence from outside the observable universe be causing the expansion?

EDIT - The original question was edited to enable it to be reopened, but my answer below responded to the way the question was initially framed. I've left my answer intact as it may be useful in addressing confusion about the difference between "the universe" and "the observable universe".

Could some pressure from outside of our universe

According to Wikipedia, the Universe is "all of space and time and their contents". So it makes no sense to talk of something "outside" all of space and time.

You might, of course, be referring just to the observable universe, "a spherical region of the Universe comprising all matter that can be observed from Earth at the present time."

However, if the conditions beyond the observable universe are what is causing expansion, then the Universe is neither homogenous nor isotropic. While we have no way of disproving this idea, all the evidence points to a homogenous and isotropic Universe and the vast majority of physicists subscribe to this model.

causing the expansion

The metric expansion of the Universe is a generic property of the Universe under the FLRW metric, an exact solution of Einstein's field equations of general relativity. There is no need to propose an external cause for the expansion.

Yes, it's possible, but there's absolutely no evidence for it.

String theory -- which is, I emphasize, a theory for which there is no evidence -- has many incompatible variations, and some of them (e.g., the Ekpyrotic theories) suggest that our 4D universe may be one of many that exist on "branes" in a higher-dimensional space.

Collisions between the branes may be possible and would have an impact (pun unavoidable) on our universe -- and may even have been the cause of the Big Bang. Such collisions ought to have left an imprint on spacetime, but searches for the expected fingerprints have come up negative.

None-the-less, String theory and all its many children are flexible enough and unconstrained by observation enough that a solution where the cosmological constant is affected by other brane universes is plausible.

But I wouldn't believe any of that until there's some observational evidence for String theory. At the moment it's basically a modern version of Kepler's Mysterium Cosmographicum -- theory running wild.

Influences from outside the observable universe explaining dark energy and expansion?

Could some influence from outside the observable universe be causing the expansion?

Outside of the "observable universe" is the portion of the universe which is unobservable.

The whole universe (observable or not) is all that there exists within our dimensions.

Outside of our universe is nothing, our universe is the full extent of existence in our dimensions.

There is no proof that something outside of our universe can influence something within our universe, it's not as though we are contained in something.

Even the rate of expansion is disagreed upon.

In the paper: "Milky Way Cepheid Standards for Measuring Cosmic Distances and Application to Gaia DR2: Implications for the Hubble Constant" published in the journal of The American Astronomical Society (July 12 2018), by Adam G. Riess, Stefano Casertano, Wenlong Yuan, Lucas Macri, Beatrice Bucciarelli, Mario G. Lattanzi, John W. MacKenty, J. Bradley Bowers, WeiKang Zheng, Alexei V. Filippenko, Caroline Huang, and Richard I. Anderson they wrote:

"… the value of the Hubble constant has been determined to be $$H_0 = 73.24 ±1.7,$$km s$$^{−1},$$Mpc$$^{−1}$$, from R16.". [Note: R16 is Riess' paper: "A 2.4% Determination of the Local Value of the Hubble Constant".]

… is inconsistent with the scale needed to match the Planck 2016 cosmic microwave background data combined with ΛCDM at the 2.9σ confidence level (99.6%). At 96.5% confidence we find that the formal DR2 errors may be underestimated as indicated. We identify additional errors associated with the use of augmented Cepheid samples utilizing ground-based photometry and discuss their likely origins. Including the DR2 parallaxes with all prior distance-ladder data raises the current tension between the late and early universe route to the Hubble constant to 3.8σ (99.99%).".

In the "Planck 2018 Results" published as: "Planck 2018 results. VI. Cosmological parameters" (Jul 17 2018), by the Planck Collaboration: N. Aghanim, Y. Akrami, M. Ashdown, J. Aumont, C. Baccigalupi, M. Ballardini, A. J. Banday, R. B. Barreiro, N. Bartolo, S. Basak, R. Battye, K. Benabed, J.-P. Bernard, M. Bersanelli, P. Bielewicz, J. J. Bock, J. R. Bond, J. Borrill, F. R. Bouchet, F. Boulanger, M. Bucher, C. Burigana, R. C. Butler, E. Calabrese, J.-F. Cardoso, J. Carron, A. Challinor, H. C. Chiang, J. Chluba, L. P. L. Colombo, C. Combet, D. Contreras, B. P. Crill, F. Cuttaia, P. de Bernardis, G. de Zotti, J. Delabrouille, J.-M. Delouis, E. Di Valentino, J. M. Diego, O. Doré, M. Douspis, A. Ducout, X. Dupac, S. Dusini, G. Efstathiou, F. Elsner, T. A. Enßlin, H. K. Eriksen, Y. Fantaye, M. Farhang, J. Fergusson, R. Fernandez-Cobos, F. Finelli, F. Forastieri, M. Frailis, E. Franceschi, A. Frolov, et al. (120 additional authors not shown) they wrote:

"The Planck base-ΛCDM cosmology requires a Hubble constant $$H_0 = 67.4±0.5,$$km s$$^{−1},$$Mpc$$^{−1}$$, in substantial 3.6σ tension with the latest local determination by Riess et al. (2018b). The Planck measurement is in excellent agreement with independent inverse-distance ladder measurements using BAO, supernovae, and element abundance results. None of the extended models that we have studied in this paper convincingly resolves the tension with the Riess et al. (2018b) value of $$H_0$$.".

Wikipedia's webpage "Expansion of the universe" explains:

"The expansion of the universe is the increase of the distance between two distant parts of the universe with time. It is an intrinsic expansion whereby the scale of space itself changes. The universe does not expand "into" anything and does not require space to exist "outside" it.

Technically neither space, nor objects in space, move. Instead it is the metric governing the size and geometry of spacetime itself that changes in scale. Although light and objects within spacetime cannot travel faster than the speed of light, this limitation does not restrict the metric itself. To an observer it appears that space is expanding and all but the nearest galaxies are receding into the distance.

I think the original question is valid and I have been pondering that question myself for years

First I am having difficulty understanding the logic in stating there is nothing outside the observable universe. The term 'observable universe' by definition limits knowledge to whatever is observable and does not exclude any thing outside of it that is not observable. This has been mentioned in earlier posts.

However there could be observable effects within our universe that may give a clue to the existence of an outside influence upon the known universe.

One of these is the fact that distant galaxies are accelerating, apparently, away from us. One phenomenon that has this ability to cause acceleration is gravity. One hypothesis could be that the distant galaxies are caught in a gravitational field outside our known universe. This would place our known universe within, for want of a better word, a 'bubble' within this gravitational influence causing it to expand in all directions.

Of course this would change the idea of a big bang theory, not in its effect but its origins. One has difficulty in imagining the whole of our universe being crammed into a single point as unstable as that may be. However one could imagine it coming into existence by, for want of a better word, 'squirted' into a void within an outside existence of a meta-universe. This also does away with the need for dark energy as we once thought we needed the ether to carry radio waves.

Karl Popper says that any hypothesis/theory/etc must be falsifiable. The above is certainly one of those things. It only needs to be proven non-falsifiable, possibly, with physics not yet known.

## Dark Energy

I was wondering if anyone knows much about dark matter and dark energy. Is there anyway that we can detect it? And is it possible for either to travel faster than the speed of light? Thanks for any help.

Iɽ recommend Isaac Arthur's Cosmology series on YouTube, he's done episodes on dark matter and dark energy. It's a couple of years old but as far as I know there hasn't been any major developments in that time.

Such a great channel he has. One in a million

That man is ahead of his time

Under our current understanding of special and general relativity (which have been tested time and again and found to be consistent with observations), nothing with mass can break the light barrier. So dark matter, since it has mass, cannot travel faster than light.

Dark matter can currently be detected by its effect on visible objects. For example, we can see its effect on the velocity of stars orbiting inside galaxies. In the case of spiral galaxies, the presence of dark matter keeps the orbital velocity of stars roughly constant and independent of distance from the center, whereas classical Keplerian orbits predict that orbital velocities should fall off with the square root of the distance. In elliptical galaxies, the velocity dispersion of stars (the distribution of orbital velocities) does not match the predicted velocity dispersion unless we include some invisible mass component.

In addition to that, gravitational lensing studies of galaxy clusters demonstrate that the mass present is much greater than the visible mass from stars and galaxies.

Finally, the anisotropies in the cosmic microwave background (specifically, their angular power spectrum) cannot be explained unless there is some large amount of matter that doesn't interact electromagnetically (such as ordinary baryonic matter) but does interact gravitationally.

There are a number of dark matter direct detection experiments currently happening--the details vary, but they all involve extremely low-background, highly-sensitive, large-volume apparatuses.

As for dark energy, it's currently far more of a mystery than dark matter. It is causing the expansion of the Universe to accelerate rather than slow down, as one would expect given the effects of gravity. There are a few competing hypotheses of what it could be, but more data is needed.

It’s possible that we can’t directly observe dark matter or energy as it may exist beyond the dimensions of our observable universe.

There are theories that speculate dark matter and energy could be the interlap of the multiverse with ours intersecting with various other universes. We may be able to indirectly observe the forces and effects of dark matter/energy, but it may remain an unknown as it is basically a pseudonym for mass we cannot account for.

I was wondering if anyone know much about dark matter and dark energy.

Lol nobody knows much about either of those thing. They don't interact with pretty much anything including light (hence the dark part) and we think dark energy could have something to do with the acceleration or slowing of the expansion of the universe. I'm certainly not an expert by any means but these really are some of the biggest scientific mysteries right now. I'm sure someone else has much more insight than I do though

We don't really know what is dark matter or dark energy or if they even exist. We know that dark energy (if it's real) explains expansion of Universe.

Concept of dark matter was created when calculations of galaxy rotation didn't mach observations (graph). We explain that difference by dark matter

Is this trend for the observed rotational velocity the same for each galaxy or slightly different for each. If it is different, how much of the deviation is explainable by considering that the mass distribution isn't linear with radius, or seemingly correlated at all for that matter. I ask because it seems on a glance like computational fluid dynamics could be used to explain at least part of the deviation, if not all.

Edit: Like you could assign any galaxy a "characteristic viscosity" which would explain the deviation for the Keplar orbit, which could be like a case of typical viscosity. EG Stirring water produces very predictable drop off of stir radial velocity as a function of distance from stirrer. However for a more viscous solution these prediction become increasingly incorrect especially for non-newtonian fluids.

If I formalise another way : we had a theory that made predictions on how galaxy rotations should be. Observations did not match these predictions. So, instead of saying that our theory is wrong we explain this with a hypothetic dark matter and dark energy ?

If it is just this, it seems to me like observing that behaviour of high energy particles do not match Newtonian Mechanics and saying : "Hmm . a dark energy is pushing them fron going faster" .

This would have prevented coming up with relativity . Am I wrong ?

From what I know (minimal) the "matter" that we all know and love composes

5% of everything, about 20% is dark matter, the rest is dark energy. We can't directly measure dark matter it because it interacts with normal-matter only very very slightly. Our current instruments arn't sensitive enough to pick it up. However, we know it's there because it is allowed for and it explains the rotation and movement of galaxies (lots and lots of mass interacting with even more dark matter, even if only slightly, to produce a noticeable change we can measure indirectly).

Brian Cox does a good job of summarizing it in his JRE podcast (linked). As for your question about faster than light, nothing with mass can ever travel faster than it. Unless we find out something in the future that breaks everything we know, nothing can go faster than light. But maybe dark energy will be the thing that breaks that rule. We need a unifying theory of physics which people are working on but we don't have yet to know for sure.

To the people who were arguing over whether DM is actually matter or a fault in our theory of gravity, I wanted to respond to your comments before they got deleted. Here's my response and hopefully it helps answer part of OP's question as well.

As proper scientists we should ask ourselves "what are the credences for certain propositions being true?" For this we use Bayesian statistics. On one hand we have the possibility that (1) DM is some type of undetected particle. On the other, (2) DM is the result of trying to apply GR outside of its domain of applicability. I.e. we need an improved theory of gravity to explain our measurements. Now we ask what are our prior credences for (1) and (2)? I'm inclined to put more weight toward (1) because the history of discovering new particles has been very successful and less to (2) because I've heard attempts at new theories of gravity are very complicated and have a hard time explaining other measurements. The exact values for the priors we give to (1) and (2) are not as important as how we update our credences with new evidence. Every successful test of GR then decreases my credence in (2) and increases my credence in (1). Likewise the longer we go without detecting new DM candidate particles the more my credence in (1) will decrease, and my credence in (2) will increase. Since we haven't been searching for DM candidates particles for very long and since there have been so many accurate predictions by GR, my credence for (1) is still higher than for (2). As such I think it's more likely that DM is actual matter (particles), not a fault of gravitational theory.

That's how we should look at the world.

To take the opposite tact, Einstein formulated General Relativity to explain the numerous flaws at the fringes that Newtonian mechanics could not. At that time Newton's laws were just as much in stone having been around 200 or so years. GR is now 100 and we've learned more in the 100 than we learned in the 200 before, so it shouldn't be surprising that we are beginning to see the same sorts of fraying.

I think the "it's just another particle" theory, after 40 years, is doomed to join super-luminous ether. This is especially true of a particle which, for unexplained reasons, only shows up on galactic scales. Where it truly the source of 90% of the matter in the universe that should hold true within the Solar system. There's no evidence for additional dark matter in the movement of the planets in the solar system, or the behavior of nearby binary stars, and that should be a huge red flag.

Quantised Inertia seems to do the trick, but it has some fantastical claims. Then again, so does relativity to the eyes of a 19th century scientist. Blow up a city with a 5 lb. lump of metal? Really? QI disposes of the equivalence principle, but if you follow the graph of what it replaces it with you'll note that almost all the time inertial mass and gravitational mass are effectively the same. They only diverge at very low accelerations, like at the fringes of galaxy. The model isn't in perfect sync with observations, but the margin of error seems to map directly to the margin of error for the Hubble constant - and getting that constant right is crucial to McCullogh's theory just as getting the speed of light correct is crucial to relativity's formulae.

Reading the overview it feels right - but we need to test those predictions before giving it any credence.

Whatever theory comes out, it must agree with Relativity completely, and then explain the anomalies. The same was required of Relativity against Newton's Laws, and the reason why we still use Newton's laws today - it is rare for us to be working with one of the esoteric corner cases where Relativity is more accurate than Newton (though it does happen - GPS satellites spring to mind). As I mentioned earlier, that is a very tall order. So tall in fact that it's understandable most have shrunk away from it and went out looking for a new particle.

The problem with that approach is you can look for a particle for hundreds of years and still not find it if it doesn't exist.

## Why Does Dark Energy Make the Universe Accelerate?

Peter Coles has issued a challenge: explain why dark energy makes the universe accelerate in terms that are understandable to non-scientists. This is a pet peeve of mine — any number of fellow cosmologists will recall me haranguing them about it over coffee at conferences — but I’m not sure I’ve ever blogged about it directly, so here goes. In three parts: the wrong way, the right way, and the math.

The Wrong Way

Ordinary matter acts to slow down the expansion of the universe. That makes intuitive sense, because the matter is exerting a gravitational force, acting to pull things together. So why does dark energy seem to push things apart?

The usual (wrong) way to explain this is to point out that dark energy has “negative pressure.” The kind of pressure we are most familiar with, in a balloon or an inflated tire, pushing out on the membrane enclosing it. But negative pressure — tension — is more like a stretched string or rubber band, pulling in rather than pushing out. And dark energy has negative pressure, so that makes the universe accelerate.

If the kindly cosmologist is both lazy and fortunate, that little bit of word salad will suffice. But it makes no sense at all, as Peter points out. Why do we go through all the conceptual effort of explaining that negative pressure corresponds to a pull, and then quickly mumble that this accounts for why galaxies are pushed apart?

So the slightly more careful cosmologist has to explain that the direct action of this negative pressure is completely impotent, because it’s equal in all directions and cancels out. (That’s a bit of a lie as well, of course it’s really because you don’t interact directly with the dark energy, so you don’t feel pressure of any sort, but admitting that runs the risk of making it all seem even more confusing.) What matters, according to this line of fast talk, is the gravitational effect of the negative pressure. And in Einstein’s general relativity, unlike Newtonian gravity, both the pressure and the energy contribute to the force of gravity. The negative pressure associated with dark energy is so large that it overcomes the positive (attractive) impulse of the energy itself, so the net effect is a push rather than a pull.

This explanation isn’t wrong it does track the actual equations. But it’s not the slightest bit of help in bringing people to any real understanding. It simply replaces one question (why does dark energy cause acceleration?) with two facts that need to be taken on faith (dark energy has negative pressure, and gravity is sourced by a sum of energy and pressure). The listener goes away with, at best, the impression that something profound has just happened rather than any actual understanding.

The Right Way

The right way is to not mention pressure at all, positive or negative. For cosmological dynamics, the relevant fact about dark energy isn’t its pressure, it’s that it’s persistent. It doesn’t dilute away as the universe expands. And this is even a fact that can be explained, by saying that dark energy isn’t a collection of particles growing less dense as space expands, but instead is (according to our simplest and best models) a feature of space itself. The amount of dark energy is constant throughout both space and time: about one hundred-millionth of an erg per cubic centimeter. It doesn’t dilute away, even as space expands.

Given that, all you need to accept is that Einstein’s formulation of gravity says “the curvature of spacetime is proportional to the amount of stuff within it.” (The technical version of “curvature of spacetime” is the Einstein tensor, and the technical version of “stuff” is the energy-momentum tensor.) In the case of an expanding universe, the manifestation of spacetime curvature is simply the fact that space is expanding. (There can also be spatial curvature, but that seems negligible in the real world, so why complicate things.)

So: the density of dark energy is constant, which means the curvature of spacetime is constant, which means that the universe expands at a fixed rate.

The tricky part is explaining why “expanding at a fixed rate” means “accelerating.” But this is a subtlety worth clarifying, as it helps distinguish between the expansion of the universe and the speed of a physical object like a moving car, and perhaps will help someone down the road not get confused about the universe “expanding faster than light.” (A confusion which many trained cosmologists who really should know better continue to fall into.)

The point is that the expansion rate of the universe is not a speed. It’s a timescale — the time it takes the universe to double in size (or expand by one percent, or whatever, depending on your conventions). It couldn’t possibly be a speed, because the apparent velocity of distant galaxies is not a constant number, it’s proportional to their distance. When we say “the expansion rate of the universe is a constant,” we mean it takes a fixed amount of time for the universe to double in size. So if we look at any one particular galaxy, in roughly ten billion years it will be twice as far away in twenty billion years (twice that time) it will be four times as far away in thirty billion years it will be eight times that far away, and so on. It’s accelerating away from us, exponentially. “Constant expansion rate” implies “accelerated motion away from us” for individual objects.

There’s absolutely no reason why a non-scientist shouldn’t be able to follow why dark energy makes the universe accelerate, given just a bit of willingness to think about it. Dark energy is persistent, which imparts a constant impulse to the expansion of the universe, which makes galaxies accelerate away. No negative pressures, no double-talk.

So why are people tempted to talk about negative pressure? As Peter says, there is an equation for the second derivative (roughly, the acceleration) of the universe, which looks like this:

(I use a for the scale factor rather than R, and sensibly set c=1.) Here, ρ is the energy density and p is the pressure. To get acceleration, you want the second derivative to be positive, and there’s a minus sign outside the right-hand side, so we want (ρ + 3p) to be negative. The data say the dark energy density is positive, so a negative pressure is just the trick.

But, while that’s a perfectly good equation — the “second Friedmann equation” — it’s not the one anyone actually uses to solve for the evolution of the universe. It’s much nicer to use the first Friedmann equation, which involves the first derivative of the scale factor rather than its second derivative (spatial curvature set to zero for convenience):

Here H is the Hubble parameter, which is what we mean when we say “the expansion rate.” You notice a couple of nice things about this equation. First, the pressure doesn’t appear. The expansion rate is simply driven by the energy density ρ. It’s completely consistent with the first equation, as they are related to each other by an equation that encodes energy-momentum conservation, and the pressure does make an appearance there. Second, a constant energy density straightforwardly implies a constant expansion rate H. So no problem at all: a persistent source of energy causes the universe to accelerate.

Banning “negative pressure” from popular expositions of cosmology would be a great step forward. It’s a legitimate scientific concept, but is more often employed to give the illusion of understanding rather than any actual insight.

## Dark Energy Over Time

This post continues the subject of dark energy, which, as discussed in my previous post is a mysterious form of energy which makes up about 68 per cent of the mass of the Universe and is the reason why the Universe is expanding at an ever-faster speed. This post will discuss how the percentage of dark energy changes over time and how this has influenced and will influence the evolution of the entire Universe.

This post is the fourth in my series on cosmology, the study of the origin and evolution of the Universe as a whole. To view the others please click on the category “Cosmology” at the end of this post.

The influence of dark energy in the early Universe

As mentioned in my previous post, cosmologists have estimated that the amount of dark energy in the Universe works out at about 0.0069 trillionths of a gramme per cubic kilometer of space, about 150 billion times smaller than a small grain of sand. Although this amount is incredibly small, it is still considerably greater than the average density of matter. The most widely accepted explanation of dark energy, the cosmological constant, means that the amount of dark energy in a given volume of space remains constant over time.

The expansion of the Universe is shown in the diagram above, in which the x-axis shows time. It illustrates that, in the distant past, galaxies were much closer together than they are now.

In fact, if we go back in time roughly 7 billion years then, on average, the distance between galaxies was roughly half what it is today.

As shown in the diagram above, when galaxies were two times closer together, the same number of galaxies took up a volume eight times smaller. the density of matter (which is equal to mass divided by the volume) was therefore eight times higher. However, the density of dark energy was exactly the same as it is now. This is shown in the first and third columns of the table and in the pie-chart below.

The percentages of dark matter, dark energy and ordinary matter in the Universe when it was seven billion years old.

If we travel back further and further in time, the density of ordinary matter and dark matter gets progressively higher and higher, because the same amount of matter is in a smaller and smaller space. However, because the density of dark energy is constant, its percentagecontribution to the total mass of the Universe gets smaller and smaller. When the Universe was one tenth its current size then dark energy made up only 0.2% of the total mass of the Universe and at times earlier than this its contribution was insignificant. It is therefore clear that, although it dominates the Universe now, dark energy was completely unimportant in the early Universe and played no part in its initial expansion. The current most widely accepted explanation of the initial expansion of the Universe is that, in the first microscopic fraction of a second after the Universe came into existence, it underwent an incredibly rapid expansion called inflation. After this first minute fraction of a second had passed, inflation switched off and the Universe has been expanding ever since. The exact details of this process are still not fully understood.

The diagram above shows how the Universe expands over time. The Universe expands rapidly after the big bang, but all the time its expansion is slowed by gravity (A). About 7 billion years ago, it is still expanding but its rate of expansion is at a minimum (B). After this time dark energy dominates the Universe and its expansion speeds up (C).

Dark energy in the future

As the Universe expands, and the galaxies get further and further away from each other, the density of ordinary matter will continue to fall. In roughly 10 billion years the Universe will have expanded so that the distance between galaxies will be roughly twice what it is today. This means that because we will have the same amount of matter in a space eight times bigger than it is today, the density of matter will be roughly one eighth.

Because the amount of dark energy in a given volume of space remains the same as the Universe expands, it will make up an even greater proportion of the total mass of the Universe than it does now. This is shown in the pie-chart below.

As the Universe continues to evolve and expand, the contribution of dark energy which accelerates its expansion will continue to get greater and and greater. In fact, in around 30 billion years time when the average distance between galaxies is roughly 10 times its current value, it will consist of 99.95% dark energy.

I will say more about the ultimate fate of the Universe in a future post. As we go further forward in time to around 100 billion years from now, about 7 times longer that the age of the Universe, the distance between our galaxy and other galaxies will be so great that the light from them won’t be able to reach us (see notes). If there are any astronomers in our galaxy at this time then when they look out with their telescopes then rather than seeing hundreds of billions of galaxies in the observable universe that they do today they will only see a single large galaxy – our own.

My next post will be about dark matter, the mysterious substance which makes up about a quarter of the Universe, but about which we know very little indeed.

Related Posts

This post is the fourth my series about cosmology. The other posts are:

(1) The Universe Past, Present and Future. This describes what is meant by the Universe and gives an overview of its origins, evidence for its expansion and discusses its ultimate fate. To view this post click here.

(2) A brief history of the Universe. This gives a history of the Universe from just after the big bang until the current date. To view this post click here.

3) Dark Energy. This post gives the reasons why cosmologist believe dark energy exists and why it makes up nearly 70% of the mass of the Universe. To view this post click here.

Strictly speaking not all galaxies are getting away from us. Our own galaxy the Milky Way and its neighbour, the Andromeda galaxy, together with a number of small satellite galaxies form a group of galaxies called the Local Group. The galaxies in the local group are bound together by the force of gravity so they won’t get further away from each other as the Universe expands.

However, the Milky Way and the Andromeda galaxy are on a collision course. The Andromeda galaxy is approaching the Milky way at 400,000 km/h and they will collide in roughly 5 billion years time to form a giant galaxy. So in in around one hundred billion years time our observable Universe will consist of a single galaxy.

August 25, 2015 at 11:12 am

Interesting article. Dark energy is the repulsive force behind the continuing expansion of the universe. Is it energy in empty space? No. Is it "quintessence" (whatever that is supposed to mean) that does not have to be constant? What is true is this - dark energy does not have to be constant. The rate of acceration of the universe can conceivably change (speed up or slow down). What is wrong with both hypotheses is a great deal - dark energy is not a force that operates FROM WITHIN the cosmos at all. Rather, dark energy is a force that is external to the physical universe, but which acts upon it. There are no chameleon particles. Keep at it, guys, but don't be afraid to think outside the box. Does the phrase "outside of the physical universe" have empirical sense? Yes.

Consider four bodies, each of mass M, at the corners of a square, with sides of length R. Double the distance between all, so the sides are now 2R. How much energy does this take? About 3.41 GM^2/R joules. Now do the calculation with two bodies, each of mass 2M, a distance sqrt(2)R apart, so that the total mass and density is same. Double the distance, as before. How much energy does this take? About 2.83 GM^2/R j.

While the mass, density and expansion are the same in both cases, the latter takes less energy. It represents a “clumpier” state, like the aging universe. As stars form and galaxies collide, the same expansion requires less energy. Think riding a bicycle, approaching the top of a hill: as the grade flattens, you go faster. The universe is growing clumpier expansion requires less energy the grade is "flattening." Ergo, acceleration may be expected.

Currently the standard model does not include this “clumpiness” effect. It should. As the above calculation shows, R does matter density alone does not define the system.

@Peter Wilson - so then that energy differential between the clumpyness and non-clumpyness states would be the theoretical Dark Energy?

Imagine if you could harness that.

Actually I think the wrong question is being asked, accelerated as compared to what? We can only really see our little part of the universe, so it's like defining the entire world from the perspective of having only lived in Brooklyn.

I was born and raised in Arizona where it's VERY dry. The first time I saw the Mississippi River the person I was with was explaining how people hitch hike up and down the river all year long. It took me several years to reconcile and understand what I heard since it made no sense to me at the time, I mean, what did the hitchhikers do in the summer when the river dried up?

Eventually I realized I was making assumptions based on a limited worldview. I think that's where this science is at as well.

The anti-gravity effect of Dark Energy is indeed strange. But it is true for all four forces of nature that at lower energy levels, time slows down. In the case of gravity, your feet age slower than your head. In the case of the electromagnetic force, an electron in a lower orbital travels slower through time than its higher energy, high orbital brothers. Protons and neutrons in lower orbitals also travel slower through time. It is also true that as objects approach the speed of light, they too travel slower through time (The Twin Paradox). So maybe the principle that unites all 4 forces of nature and the Dark Energy force is simply that time wants to slow down. That would be consistent with the idea that as time slows down less happens and less happening is the definition of stability. So a Universe that is expanding at an ever increasing speed may simply be trying to travel through time more slowly and is, therefore, moving towards a more stable energy state. If no particles or forces can be found to otherwise explain Dark Energy, then we might just have to attribute the strange behavior of Dark Energy to an effect of Relativity which was initially called by Einstein a Theory of Gravity.

August 28, 2015 at 11:56 pm

Peter Wilson. It seems to this poor chemist that you have given us a simple and brilliant proposal. If I understand what you said, we do not need 'dark energy' to explain the accelerated expansion of our Universe. Hopefully, the physics experts will work on your idea to see if it fits all the data. But we still need dark matter (right?) to explain the way galaxies rotate (conservation of angular momentum, etc.)

August 31, 2015 at 10:53 am

Dear Lindsay,
Thank you for your comment. Yes, Dark Matter is needed but if we consider "matter" to be made of protons and neutrons and electron, Dark Matter may not be "matter" at all. This would explain why it is not detectable. Since there is more Dark Matter than matter in the universe and matter is made of 3 quarks, then we would have to assume that if Dark Matter was made in the Big Bang, it was easier for Dark Matter to be make in the Big Bang than was "matter". So I believe that if Dark Matter is made of only one type of particle must be made up of only one or two quarks. If made of one quark, it would have to be a yet undiscovered stable quark if two quarks, it would be a force carrier and since heavy particles decay into lighter particle, it would have to be the lightest of the force carriers. Finally it could be a menagerie of many types of undiscovered particles. Particle physicists would not be surprised if super families of particles exist, in fact, many are expecting this and particles with more than 3 quarks have now been detected.

August 31, 2015 at 12:50 pm

We may still need DE, but the clumping has to be included, if only to rule it out. My fear is that some math-whiz in the physics dept will understand me, as you have, insert R into the Ricci curvature tensor equations, solve the matrix calculus--way over my head--publish the results…and I‘ll read about it here!

## Cosmic expansion in dispute

The results also provide a better insight into some recent controversies about the expansion rate of the universe today and about the geometry of space.

Combining our observations with studies of the universe in its infancy reveals cracks in our description of its evolution. In particular, our measurement of the current rate of expansion of the universe is about 10% lower than the value found using direct methods of measuring distances to nearby galaxies. Both these methods claim their result is correct and very precise, so their difference cannot simply be a statistical fluke.

The universe’s expansion over time. Credit: NASA/WMAP Science Team/ Art by Dana Berry

The precision of eBOSS enhances this crisis. There is no broadly accepted explanation for this discrepancy. It may be that someone made a subtle mistake in one of these studies. Or it may be a sign that we need new physics. One exciting possibility is that a previously unknown form of matter from the early universe might have left a trace on our history. This is known as “early dark energy”, thought to be present when the universe was young, which could have modified the cosmic expansion rate.

Recent studies of the cosmic microwave background suggested that the geometry of space may be curved instead of being simply flat – which is consistent with the most accepted theory of the big bang. But our study concluded that space is indeed flat.

Even after these important advances, cosmologists over the world will remain puzzled by the apparent simplicity of dark energy, the flatness of space and the controversial values of the expansion rate today. There is only one way forward in the quest for answers – making larger and more detailed maps of the universe. Several projects are aiming to measure at least ten times more galaxies than we did.

If the maps from eBOSS were the first to explore a previously missing gap of 11 billion years of our history, the new generation of telescopes will make a high-resolution version of the same period of time. It is exciting to think about the fact that future surveys may be able to resolve the remaining mysteries about the universe’s expansion in the next decade or so. But it would be equally exciting if they revealed more surprises.

Written by Julian Bautista, Research Fellow at University of Portsmouth.

### NASA’s WFIRST Will Study Dark Energy and Help Uncover the Universe’s Fate [Video]

#### 15 Comments on "Dark Energy: Map Gives Clue About What It Is – But Deepens Cosmic Expansion Rate Dispute"

Another way to explain Dark Energy is suggested by String Theory. All matter and energy, including photons (light), have vibrating strings as their basis.

String and anti-string pairs are speculated to be created in the quantum foam, a roiling energy field suggested by quantum mechanics, and they immediately annihilate each other. If light passes near these string/anti-string annihilations, perhaps some of that annihilation energy is absorbed by the string in the light. Then the Fraunhofer lines in that light will move a bit towards the blue and away from the red shift. As this continues in an expanding universe we get the same curve displayed by Perlmutter and colleagues at their Nobel Prize lecture, without the need for Dark Energy.

This speculation has the universe behaving in a much more direct way. Specifics can be found in my YouTube https://www.youtube.com/watch?v=0b6t0jO7IgQ

Picture a stream of fast moving dark energy headed for the sun. When the velocity of the stream and the gravity reach a zero G equivalent (think vomit comet) the dark matter flashes to dark energy creating nano flares. The flares and the expansion of the dark energy would create bubbles with the interface outlined in ionized plasma.

“ The interactions of these ultra-hot electrons and the vacuum created as the tube implodes leads to the flow of electric current. The flow of electric charges is what creates a magnetic field. In this case, the current flow can amplify a pre-existing magnetic field by two to three orders of magnitude, the researchers found.”
https://www.space.com/megatesla-magnetic-fields-earth.html

The dark energy being out of the sweet spot absorbs heat and reverts to dark matter. This would cause coronal rain and in longer streams of dark energy would cool areas all the way to the surface of the sun before regaining sufficient heat to revert to dark matter.

When dark matter leaves the galactic gravity well it flashes to dark energy giving off heat/ nanoflares. This leaves the outside of tha galactic halo hotter than expected and with more hot electrons/ enhanced magnetic fields.

The existence of vacuum energy explains where the quantum fields ground state energies are. We know that they exist from such effects as the Casimir effect and Lamb shift, so observing it at cosmological scales is exciting. What I like most with the eBOSS 20 year survey is that the BOSS collaboration in their cosmology summary paper shortlist some possible explanations for the low vacuum energy density observed, with Weinberg’s anthropic multiverse as the simplest. That is consistent with the type of inflation field that Planck collaboration saw 2016, which naturally results in an infinite number of local universes.

An interesting explanation for the difference between mostly local measurements of the Hubble rate and mostly integrated ones like eBOSS is that the tension can equally well be explained by a tension in the cosmic background temperature calibration. “In conclusion, the authors showed that the Hubble tension can also be expressed as tension in CMB temperature T0. It could, therefore, be solved when a higher T0 is assumed as prior for estimating H0 from Planck data. However, this explanation seems unlikely, as BAO measurements support the lower T0 value. Nevertheless, the analysis showed that the solution to the Hubble tension might not be a change in the cosmological standard model, but rather a careful examination of the assumptions and priors that influence the measurement of H0.” [“Is the Hubble Tension actually a Temperature Tension?”, Linke, astrobites astro-ph reader’s digest.]

The existence of vacuum energy explains where the quantum fields ground state energies are. We know that they exist from such effects as the Casimir effect and Lamb shift, so observing it at cosmological scales is exciting. What I like most with the eBOSS 20 year survey is that the BOSS collaboration in their cosmology summary paper shortlist some possible explanations for the low vacuum energy density observed, with Weinberg’s anthropic multiverse as the simplest. That is consistent with the type of inflation field that Planck collaboration saw 2016, which naturally results in an infinite number of local universes.

Sorry for the duplicate comment – posting difficulties.

@ Howard Jeffrey Bender: “Another way to explain Dark Energy is suggested by String Theory…. quantum foam … my Youtube”.

There is no evidence of a “quantum foam” and string theory has mostly been rejected by LHC, ACME and Fermi-LAT – no natural thermal WIMPs from string theory.

@Priyanka Garai: That links to a pseudoscience site.

It is also self promotion.

@Ed Stauffer: “Picture a stream of fast moving dark energy”. If dark energy is constant density in every volume it cannot “stream”.

I would also point out that it is easy to see in the cosmic background spectra that dark energy, dark matter and normal matter are entirely different phenomena and by comparing with other observations of them at later times that they cannot interconvert.

This tenacious quest for finding the explanation the Einstein cosmological constant (constantly verified experimentally to be a constant) is becoming psychotic. Is there such a pressure concerning Newton constant of gravitational attraction? Or the speed of light? Jumping into the craziesr idéa in the hope to finally find “THE EXPLANATION” and get world recognition does not seam a proper way to proceed. Understanding first Why this is definitely a constant and nothing else seams more productive, and is directly in the line of Einstein first idea (exactly the opposite of Archimedes, Newton and Maxwell). There will be plenty of room after for discussion.

“This tenacious quest for finding the explanation the Einstein cosmological constant (constantly verified experimentally to be a constant) is becoming psychotic. Is there such a pressure concerning Newton constant of gravitational attraction? Or the speed of light?”

The interest from cosmologists is that until recently we didn’t know about vacuum energy – it was discovered 20 years ago – and it is just a few years of constraining it to be constant. It is likely that until the tension between the tendency for low-z (“local”) and high-z (“integrative”) observations of the Hubble rate is resolved there will still be room for debate. And it is understandable as such though likely the explanation is found, same as it seems to be for dark matter (which is an older topic of study).

The interest is rather, as I already commented on, the explanation for the low value of the vacuum energy, which famously is 10^120 times lower than naive expectations. This is analogous to how the homogeneity and isotropy of space is 10^5 times lower than naive expectations and it may be that both apparent finetunings derive from the same source – the inflation field.

From the eBOSS collaboration in their latest cosmology summary paper:

“At the high precision found here, cosmic acceleration remains most consistent with predictions from a cosmological constant. A deviation from consistency with a pure cosmological constant perhaps would have pointed toward specific dark energy and modified gravity models. However, since many of these models have parameter choices that make them indistinguishable from ΛCDM, those models all can be made consistent with our observations. Nevertheless, the observed consistency with flat ΛCDM at the higher precision of this work points increasingly towards a pure cosmological constant solution, for example, as would be produced by a vacuum energy finetuned to have a small value. This fine-tuning represents a theoretical difficulty without any agreed-upon resolution and one that may not be resolvable through fundamental physics considerations alone (Weinberg 1989 Brax & Valageas 2019). This difficulty has been substantially sharpened by the observations presented here.”
[“THE COMPLETED SDSS-IV EXTENDED BARYON OSCILLATION SPECTROSCOPIC SURVEY: COSMOLOGICAL IMPLICATIONS FROM TWO DECADES OF SPECTROSCOPIC SURVEYS AT THE APACHE POINT OBSERVATORY”, eBOSS Collaboration, arxiv 2007.08991]

“It is this “weak anthropic principle” that will be applied here. Its relevance arises from the fact that, in some modern cosmological models, the universe does have parts or eras in which the effective cosmological constant takes a wide variety of values.”

“In models of these types, it is perfectly sensible to apply anthropic considerations to decide which era or part of the universe we could inhabit, and hence which values of the cosmological constant we could observe.

A large cosmological constant would interfere with the appearance of life in different ways, depending on the sign of lambda_eff . For a large positiUe lambda_eff, the universe very early enters an exponentially expanding de Sitter phase, which then lasts forever. The exponential expansion interferes with the formation of gravitational condensations, but once a clump of matter becomes gravitationally bound, its subsequent evolution is unaffected by the cosmological constant. Now, we do not know what weird forms life may take, but it is hard to imagine that it could develop at all without gravitational condensations out of an initially smooth universe. Therefore the anthropic principle makes a rather crisp prediction: lambda_eff must be small enough to allow the formation of sufficiently large gravitational condensations (Weinberg, 1987).

This has been worked out quantitatively, but we can easily understand the main result without detailed calculations. … This result suggests strongly that if it is the anthropic principle that accounts for the smallness of the cosmological constant, then we would expect a vacuum energy density p_V

(10—100)p_M_0, because there is no anthropic reason for it to be any smaller.”
[“The cosmological constant problem”, Weinberg, Reviews of Modern Physics, 1989]

From the Planck collaboration in their last cosmology summary paper:

“Combining with BICEP-Keck 2015 data on B-mode polarization we find a 95% upper limit on the tensor-to-scalar ratio r_0.002 < 0.06. Together with our measurement of n_s , these results favour concave over convex inflation potentials, suggesting a hierarchy between the slow-roll parameters measuring the slope and curvature of the potential"
["Planck 2018 results, VI. Cosmological parameters", Planck Collaboration, Astronomy & Astrophysics]

A description why a scalar ("slowroll") inflation field naturally gives a multiverse where anthropic considerations (selection bias for survivability) may apply:

"If we look at how this physically works, we can visualize inflation as a field: a ball that sits at the top of a hill. The hill has to be particularly flat on top, so that the ball can spend a lot of time up there. The ball rolls, inevitably, down towards the valley below, but it has to roll slowly: only when the ball sits atop the flat part of the hill can inflation occur. When the ball rolls down into the valley, inflation comes to an end, giving rise to a Universe filled with particles, antiparticles, and radiation: it starts the hot Big Bang.

Again, so far, so good. We have one Universe, it inflates, inflation ends, we get the hot Big Bang, and everyone’s happy.

Until you remember one important caveat that we’ve ignored thus far: everything that physically exists, including all particles and fields, must be inherently quantum in nature."

"Imagine you’re up atop this hill, and you’re rolling slowly towards the valley. At the same time, your position has a probability of spreading out, and while there’s a finite chance that you’ll wind up closer to the valley than you would have otherwise, there’s also a chance that you’ll wind up further up the hill than you started."

"What we wind up with, therefore, is a spacetime where, at any time, inflation ends in a few regions, and we get a hot Big Bang where it does. Each of those regions will be surrounded by a broader spacetime that continues to inflate, where in each of the inflating regions, a few small patches will see inflation end and a hot Big Bang ensue. These various hot Big Bangs will each give rise to their own observable Universe, just like ours, with a different starting point and different specific initial conditions for each region. They will be separated by more inflating space, and no two Universes will ever collide or interact with one another.
This is where the concepts of eternal inflation, the multiverse, and the existence of many disconnected Universes come from. If you accept that inflation is a stage that occurred in the Universe’s past prior to the hot Big Bang, and that the Universe itself is inherently quantum in nature, the existence of a multiverse is unavoidable."

["One Universe Is Not Enough", Siegel, Medium Siegel is an astrophysicist who often writes in Forbes.]

“This is analogous to how the homogeneity and isotropy of space is 10^5 times lower than naive expectations” – This is analogous to how the deviations from homogeneity and isotropy of space is 10^5 times lower than naive expectations.

## The Institute for Creation Research

Astronomers have guessed that there are possibly as many as ten thousand billion trillion stars out there in the observable universe, of which perhaps five thousand can be seen without a telescope. If the universe is expanding, as many astronomers believe, there is an outer limit to this vast population of stars, as defined by the distance thus far attained by the expansion. This limit might be considered the boundary of the universe.

This boundary is also, practically by definition, the limit of the so-called background radiation, presumably a cosmos-filling remnant of the postulated primeval Big Bang which its proponents believe must have started the expansion.

Since there is no radiation beyond this limit, there are no stars and no light either&mdashonly outer darkness, so far as we can tell. But even within the known cosmos, evolutionary astronomers seem increasingly obsessed with the idea of darkness. Even with the tremendous mass of those possibly 10 25 stars, these theorists believe that there are still vast amounts of certain mysterious kinds of matter which cannot be seen or measured. They call this dark matter, the gravitational influence of which somehow holds galaxies and galactic clusters together.

The unobservable matter in the universe is greater than all the observable matter&mdash7.5 times greater. Amazing!

However, that still leaves about 66% of the universe's "stuff" to be accounted for beyond that! In particular, there is strong evidence now that the "expansion" of the universe is accelerating near its boundaries. The gravitational influences out there seem to be driving it apart, and this would require some kind of tremendous energy whose source and character are unknown. So cosmologists just call it "dark energy"!

This strange universal anti-gravity field has been given the name "quintessence" by some astronomers. No instruments have been able to measure it, or even record it, but they believe it must be there on the basis of the intensity of light from very distant supernovas, which to them suggests dark energy.

Some six extra dimensions (in addition to length, width, depth, and time) have been mathematically predicted in string theory, but this also is quite an illusory concept to many.

In any case, the picture now occupying the minds of many evolutionary cosmologists is that of an immense universe with multitudes of stars and other objects in great variety, but which actually comprise only a minute part of the matter and energy of a very "dark" universe.

And, since the law of entropy apparently is of universal extent, even the visible stars will eventually disintegrate into darkness, or worse.

Dark matter, dark energy, and ultimate black holes everywhere certainly sound foreboding&mdashif true. And that's just the known universe! What about the darkness outside of that?

It is well to remember that a great deal of modern cosmology, especially when its practitioners are trying to predict its destiny and retrodict its origin, is sheer mathematical manipulation and evolutionary philosophical imagination. "It ain't necessarily so!"

But it is significant that these speculations lead so often to words like "darkness" and "blackness." These are Biblical terms too.

Speaking of those teachers and leaders who would promote anti-Biblical doctrines of God and His great salvation through the Lord Jesus Christ, the Apostle Peter warned that "these . . . speak evil of the things that they understand not . . . to whom the mist of darkness is reserved for ever" (II Peter 2:12,17).

Similarly Jude compares them to "wandering stars, to whom is reserved the blackness of darkness for ever" (Jude 13).

The Lord Jesus Himself referred to "outer darkness" as the ultimate home of the lost. In His familiar parable of the talents, the judgment on the servant whose unfruitfulness had proved he was not a true servant was to "cast ye the unprofitable servant into outer darkness: there shall be weeping and gnashing of teeth" (Matthew 25:30 see also Matthew 8:12 22:13).

This ultimate home of the lost is also called "hell," of course, and the Lord called it "the lake which burneth with fire and brimstone" (Revelation 21:8), the sea of "everlasting fire, prepared for the devil and his angels" (Matthew 25:41).

As to the location of this fiery lake, we know only that it is not on the earth. The "beast" and the "false prophet" will be dispatched to the lake of fire before the disintegration of the present earth (Revelation 19:20 20:11), and all the unsaved not until after that event (Revelation 20:15). The new earth will be one "wherein dwelleth righteousness" (II Peter 3:13). The obvious inference of all this is that hell will be a vast lake of fire somewhere far out in the "outer darkness" of the created universe, possibly in a dark nebula, or even a black hole.

The fire will be real and easily able to destroy bodies, but it cannot annihilate spirits. All men and women have been created in the image of God and God is eternal. He is the Creator, not the de-Creator, and "whatsoever God doeth, it shall be for ever" (Ecclesiastes 3:14). The outer darkness will be "a world of iniquity . . . set on fire of hell" (James 3:6), where all vestiges of goodness and love and light are gone forever. No wonder Jesus urged men to "Fear Him, which . . . hath power to cast into hell" (Luke 12:5).

God's created universe is also eternal with all its multitude of stars, and the lake of fire will occupy only a small, probably remote, region in comparison.

And all who are saved&mdashthose who believe His word and have received His great salvation through faith in Christ&mdashshall live and praise Him for ever.

Although many have chosen the way that leads to outer darkness, there will also be many living forever in the Holy City on the new earth. "And there shall be no night there . . . for the Lord God giveth them light" (Revelation 22:5).

## The Mystery Of Dark Energy

“Dark energy is not only terribly important for astronomy, it’s the central problem for physics. It’s been the bone in our throat for a long time.” Steven Weinberg, Nobel Laureate, University of Texas at Austin.

More than three years into its quest to solve the nature of dark energy and illuminate the origin, evolution, and fate of our universe, the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) project remains on track to complete the largest map of the cosmos ever.

HEDTEX, a project by Penn State University scientists, aims to create a three-dimensional map of 2.5 million galaxies that will yield valuable insights into the byzantine puzzle of why the expansion of the universe is speeding up over time, a property attributed to the so-called dark energy.

#### But first things first, what exactly is dark energy?

Dark energy in an expanding universe

The observable universe consists of three known components: normal matter, dark matter, and dark energy. Dark energy is the most abundant at 68%, with dark energy making up another 27% of the universe while ordinary matter constitutes just 5%.

Today, there is consensus among astronomers that the universe we inhabit is expanding despite the presence of gravity, and that its expansion is accelerating, giving rise to the notion of a repulsive force that astronomers have dubbed ‘dark energy,’ though the concept has only been around for a little more than 20 years. Generally, astronomers and astrophysicists assign the prefix ‘dark’ to concepts they have little or no clue about.

Dark energy is the name given to the mysterious force that’s causing the rate of expansion of our universe to accelerate, rather than to slow down and go out in a Big Crunch as it ages. That’s contrary to what one might expect from a universe that was birthed by an event like the Big Bang.

Back in 1917 when Albert Einstein came up with the general theory of relativity that laid the foundations of the Big Bang and the universe as a whole, he and most leading scientists were convinced that the cosmos was static and non-expanding. Einstein introduced the Cosmological Constant to help explain why the universe was not collapsing under the attractive force of gravity.

It wasn’t until 12 years later when Edwin Hubble discovered that the universe is in fact expanding, with galaxies farther away from our planet moving away faster than those that are closer. The model of a static universe was finally abandoned, forcing Einstein to quickly modify his theories and come up with two new distinct models of the expanding universe, both of them without the cosmological constant, just a year later.

However, it would be decades later� to be precise–before astronomers discovered that the universe was dominated by dark energy and not normal matter as earlier thought.

#### Solving dark energy

More than two decades after the discovery of dark energy, astronomers remain in the dark regarding what it’s all about.

However, several theories have been advanced to attempt to explain dark energy.

Ironically, Einstein’s previously abandoned cosmological constant is one of the frontrunners, which modern-day physicists describe as vacuum energy.

The vacuum in physics is not a state of nothing. It’s a place where particles and antiparticles are continuously created and destroyed. The energy produced in this perpetual cycle could exert an outward-pushing force on space itself, causing its expansion, initiated in the big bang, to accelerate,” says Penn State University Associate Professor of Astronomy and Astrophysics, Donghui Jeong.

But here’s the rub with the concept of vacuum energy: The theoretical calculations of vacuum energy diverge from actual observations by a factor of as much as ten thousand.

Clearly this is a massive discrepancy that could necessitate a reworking of the current theory.

Another possibility: Einstein’s theory of gravity is wrong from the get-go hence leading to erroneous conclusions.

Nonetheless, the cosmological constant in the form of vacuum energy remains the leading candidate that explains dark energy.

#### HETDEX ambition

Obviously, mapping 2.5 million galaxies is no mean undertaking and requires quite a bit of elbow grease. This is not made any easier by the fact that whereas other comparable studies measure the universe’s expansion using distant supernovae or a phenomenon known as gravitational lensing, HETDEX is focused on sound waves from the big bang, called baryonic acoustic oscillations.

Luckily, HETDEX has secured more than \$40 million in funding and a set of more than 150 spectrographs called VIRUS (Visible Integral-Field Replicable Unit Spectrographs), that gathers light from far-away galaxies into an array of some 35,000 optical fibers where it is split into its component wavelengths.

Another perk: HETDEX is the first probe to try to do a whole lot of spectroscopy and then figure out what they will see by observing broad swaths of sky instead of specific, predetermined objects, meaning they will end up collecting an insane amount of data. Who knows, that treasure trove might yield unexpected insights that might help mankind in its quest to eventually colonize the universe.

## Ask Ethan: Where Does The 'Energy' For Dark Energy Come From?

The farther away we look, the closer in time we're seeing towards the Big Bang. The latest . [+] record-holder for quasars comes from a time when the Universe was just 690 million years old. These ultra-distant cosmological probes also show us a Universe that contains dark matter and dark energy, but doesn't explain where it came from.

Robin Dienel/Carnegie Institution for Science

If you have a Universe full of stuff — whether that's atoms, dark matter, radiation, neutrinos, or anything else — it's virtually impossible to keep it static. The fabric of your Universe, at least in General Relativity, must either expand or contract on the largest scales. But if you have a Universe filled with dark energy, as we appear to have, something even more troubling happens: the total amount of energy contained within our observable Universe increases over time, with no end in sight. Doesn't this violate the conservation of energy? That's what David Ventura wants to know, as he asks:

[T]he total energy of the universe is increasing such that the energy inherent of space-time is kept constant as the universe expands. It is like, in order to build an extra cubic kilometer of space-time you need this quanta of energy. No more and no less. This energy has to come from somewhere. In everything else I know of, energy (including matter via E = mc 2 ), cannot just appear from nowhere. So something must be giving energy into our universe to cause it to expand. [. ] Will it ever stop?

The actual, scientific truth of what's going on is much more troubling than you might imagine.

The expected fates of the Universe (top three illustrations) all correspond to a Universe where the . [+] matter and energy fights against the initial expansion rate. In our observed Universe, a cosmic acceleration is caused by some type of dark energy, which is hitherto unexplained. All of these Universes are governed by the Friedmann equations, which relate the expansion of the Universe to the various types of matter and energy present within it.

E. Siegel / Beyond the Galaxy

In our physical Universe, there are two things that are inextricably linked together: the expansion rate of the Universe and the breakdown of all the different types of energy present within it. The cardinal rule of General Relativity is that matter tells space how to curve, while curved space tells matter how to move. This is true, but it's not complete. It isn't just matter but also energy that affects the curvature of space, and it isn't simply curvature but also the expansion (or contraction) rate of space that gets affected. In particular, it's the energy density that determines the expansion rate.

But there are different forms of energy in the Universe, and they each play slightly different roles in how the expansion rate changes over time.

While matter and radiation become less dense as the Universe expands owing to its increasing volume, . [+] dark energy is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains constant.

E. Siegel / Beyond The Galaxy

For something like normal matter, its energy contributions are actually intuitive. Matter is made of particles that contain mass, and even as the Universe changes, the individual particles themselves remain the same. Over time, the volume of the Universe increases, and as it does, the total matter density drops. Density is mass over volume: mass remains the same, volume increases, and so the density goes down. If all we had in the Universe was matter, the expansion rate would drop as the matter density dropped.

As the fabric of the Universe expands, the wavelengths of any radiation present gets stretched as . [+] well. This causes the Universe to become less energetic, and makes many high-energy processes that occur spontaneously at early times impossible at later, cooler epochs.

E. Siegel / Beyond The Galaxy

For radiation, there's an extra component to it. Sure, radiation is also made of particles, and as the volume expands, the number density of those particles decreases just as it does for matter. But radiation has a wavelength, and that wavelength gets stretched by the expanding Universe. Longer wavelengths mean lower energies, and so the expansion rate drops faster in a radiation-filled Universe than in a matter-filled one.

But for a Universe filled with dark energy, the story is very different. Dark energy is caused by energy inherent to the fabric of space itself, and as the Universe expands, it's the energy density — the energy-per-unit-volume — that remains constant. As a result, a Universe filled with dark energy will see its expansion rate remain constant, rather than drop at all.

Various components of and contributors to the Universe's energy density, and when they might . [+] dominate. If cosmic strings or domain walls existed in any appreciable amount, they'd contribute significantly to the expansion of the Universe. There could even be additional components that we no longer see, or that haven't appeared yet! Note that by time we reach today, dark energy dominates, matter is still somewhat important, but radiation is negligible. In the very distant past, only radiation was important.

E. Siegel / Beyond The Galaxy

"Hang on," you might object, thinking, "I thought you said the Universe's expansion was accelerating?"

There's a very important point here that doesn't get emphasized enough: there are two different things scientists talk about when it comes to the expansion of the Universe. One is the expansion rate — or the Hubble rate — of the Universe. This behaves exactly as we described above: it drops for matter, it drops faster for radiation, and it asymptotes to a positive constant for dark energy. But the second thing is how quickly an individual galaxy appears to recede from us over time.

An illustration of how redshifts work in the expanding Universe. As a galaxy gets more and more . [+] distant, it must travel a greater distance and for a greater time through the expanding Universe. In a dark-energy dominated Universe, this means that individual galaxies will appear to speed up in their recession from us.

Larry McNish of RASC Calgary Center, via http://calgary.rasc.ca/redshift.htm

As time goes on, a galaxy gets farther and farther away from us. Since the expansion rate is a speed-per-unit-distance (e.g., 70 km/s/Mpc), a galaxy that's farther away (say, 100 Mpc vs. 10 Mpc) will appear to recede at a faster speed (7,000 km/s vs. 700 km/s). If your Universe is filled with matter or radiation, the expansion rate drops faster than your galaxy's distance increases, so the net recession speed will drop over time: your Universe will be decelerating. If your Universe is dominated by dark energy, however, the net recession speed will increase over time: your Universe is accelerating.

Our Universe, today, is made of approximately 68% dark energy. Starting at around 6 billion years ago, our Universe made the switch to accelerating from decelerating, based on the balance of all the different things within it.

The relative importance of different energy components in the Universe at various times in the past. . [+] Note that when dark energy reaches a number near 100% in the future, the energy density of the Universe (and, therefore, the expansion rate) will remain constant arbitrarily far ahead in time.

But how is this okay? It seems like a Universe filled with dark energy doesn't conserve energy. If the energy density — energy-per-unit-volume — remains constant, but the volume of the Universe is increasing, doesn't that mean the total amount of energy in the Universe is increasing? And doesn't that violate the conservation of energy?

This should bother you! After all, we think that energy should be conserved in any and all physical processes that take place in the Universe. Does General Relativity offer a possible violation of energy conservation?

If you had a static spacetime that weren't changing, energy conservation would be guaranteed. But if . [+] the fabric of space changes as the objects you're interested in move through them, there is no longer an energy conservation law under the laws of General Relativity.

David Champion, Max Planck Institute for Radio Astronomy

The scary answer is maybe, actually. There are a lot of quantities that General Relativity does an excellent and precise job of defining, and energy is not one of them. In other words, there is no mandate that energy must be conserved from Einstein's equations global "energy" is not defined by General Relativity at all! In fact, we can make a very general statement about when energy is and isn't conserved. When you have particles interacting in a static background of spacetime, energy is truly conserved. But when the space through which particles move is changing, the total energy of those particles is not conserved. This is true for photons redshifting in an expanding Universe, and it's true for a Universe dominated by dark energy.

But that answer, though technically correct, isn't the end of the story. We can come up with a new definition for energy when the space is changing but we have to be careful when we do.

There is a very smart way of looking at “energy” that allows us to show, in fact, that energy is conserved even in this seemingly paradoxical situation. I want you to remember that, in addition to chemical, electrical, thermal, kinetic, and potential energies, among others, there’s also work. Work, in physics, is when you apply a force to an object in the same direction as the distance it moves this adds energy to the system. If the direction is opposite, you do negative work this subtracts energy from the system.

As individual molecules or atoms move inside a closed container, they exert an outward pressure on . [+] the container walls. As you heat the gas, the molecules move faster, and the pressure increases. (Wikimedia commons user Greg L (A. Greg))

A good analogy is to think of gas. What happens if you heat up (add energy to) that gas? The molecules inside move faster as they gain energy, meaning they increase their speed, and they spread out to take up more space more quickly.

But what happens, instead, if you heat up gas that's enclosed in a container?

Yes, the molecules heat up, they move faster, and they try to spread out, but in this case, they often run into the walls of the container, creating an extra positive pressure on the walls. The container's walls are pushed outward, which costs energy: the molecules are doing work on it!

The effects of increasing the temperature of a gas inside a container. The outward pressure can . [+] result in an increase in volume, where the interior molecules do work on the container walls.

Ben Borland's (Benny B's) science blog

This is very, very analogous to what happens in the expanding Universe. If your Universe were filled with radiation (photons), each quantum would have an energy, given by a wavelength, and as the Universe expands, that photon wavelength gets stretched. Sure, the photons are losing energy, but there is work being done on the Universe itself by everything with a pressure inside of it!

Conversely, if your Universe were filled with dark energy, it also has not only an energy density, but a pressure, too. The big difference, though, is that the pressure from dark energy is negative, which means we have the opposite situation we had for radiation. As the container's walls expand, they're doing work on the fabric of space itself!

Conventionally, we're used to things expanding because there's a positive (outward) pressure coming . [+] from inside of them. The counterintuitive thing about dark energy is that it has a pressure of the opposite sign, but still causes the fabric of space to expand.

'Fun with Astronomy' by Mae and Ira Freeman

So where does the energy for dark energy come from? It comes from the negative work done on the expansion of the Universe itself. There was a paper written in 1992 by Carroll, Press, and Turner, which dealt with this exact issue. In it, they state:

…the patch does negative work on its surroundings, because it has negative pressure. Assuming the patch expands adiabatically, one may equate this negative work to the increase of mass/energy of the patch. One thereby recovers the correct equation of state for dark energy: P = – ρc 2 . So the mathematics is consistent.

Which, again, still doesn't mean that energy is conserved. It simply gives us an intelligent way to look at this problem.

There is a large suite of scientific evidence that supports the picture of the expanding Universe . [+] and the Big Bang, complete with dark energy. The late-time accelerated expansion doesn't strictly conserve energy, but the reasoning behind that is fascinating as well.

This is one of the deepest cosmology questions I've ever fielded for Ask Ethan. The two major takeaways are as follows:

1. When particles interact in an unchanging spacetime, energy must be conserved. When the spacetime they're in changes, that conservation law no longer holds.
2. If you redefine energy to include the work done, both positive and negative, by a patch of space on its surroundings, you can save the conservation of energy in an expanding Universe. This is true for both positive-pressure quantities (like photons) and negative pressure ones (like dark energy).

But this redefinition is not robust it's simply a mathematical redefinition we can use to force energy to be conserved. The truth of the matter is that energy is not conserved in an expanding Universe. Perhaps in a quantum theory of gravity, it will be. But in General Relativity, we have no good way of defining it at all.

Assuming that the standard model of cosmology is correct, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable universe.

The mass-energy of dark matter and ordinary (baryonic) matter contribute 27% and 5%, respectively, and other components such as neutrinos and photons contribute a very small amount.

The density of dark energy is very low (

7 × 10−30 g/cm3) much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the mass-energy of the universe because it is uniform across space.

Two proposed forms for dark energy are the cosmological constant, representing a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space.

Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to the zero-point radiation of space i.e. the vacuum energy.

Scalar fields that change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.

### Einstein’s cosmological constant

The “cosmological constant” is a constant term that can be added to Einstein’s field equation of general relativity.

If considered as a “source term” in the field equation, it can be viewed as equivalent to the mass of empty space (which conceptually could be either positive or negative), or “vacuum energy“.

The cosmological constant was first proposed by Einstein as a mechanism to obtain a solution of the gravitational field equation that would lead to a static universe, effectively using dark energy to balance gravity.

Einstein gave the cosmological constant the symbol Λ (capital lambda). Einstein stated that the cosmological constant required that ’empty space takes the role of gravitating negative masses which are distributed all over the interstellar space’.

The mechanism was an example of fine-tuning, and it was later realized that Einstein’s static universe would not be stable: local inhomogeneities would ultimately lead to either the runaway expansion or contraction of the universe.

The equilibrium is unstable: if the universe expands slightly, then the expansion releases vacuum energy, which causes yet more expansion.

Likewise, a universe which contracts slightly will continue contracting. These sorts of disturbances are inevitable, due to the uneven distribution of matter throughout the universe.

Further, observations made by Edwin Hubble in 1929 showed that the universe appears to be expanding and not static at all. Einstein reportedly referred to his failure to predict the idea of a dynamic universe, in contrast to a static universe, as his greatest blunder.

### Inflationary dark energy

Alan Guth and Alexei Starobinsky proposed in 1980 that a negative pressure field, similar in concept to dark energy, could drive cosmic inflation in the very early universe.

Inflation postulates that some repulsive force, qualitatively similar to dark energy, resulted in an enormous and exponential expansion of the universe slightly after the Big Bang. Such expansion is an essential feature of most current models of the Big Bang.

However, inflation must have occurred at a much higher energy density than the dark energy we observe today and is thought to have completely ended when the universe was just a fraction of a second old.

It is unclear what relation, if any, exists between dark energy and inflation. Even after inflationary models became accepted, the cosmological constant was thought to be irrelevant to the current universe.

Nearly all inflation models predict that the total (matter+energy) density of the universe should be very close to the critical density.

During the 1980s, most cosmological research focused on models with critical density in the matter only, usually 95% cold dark matter and 5% ordinary matter.

These models were found to be successful at forming realistic galaxies and clusters, but some problems appeared in the late 1980s: in particular, the model required value for the Hubble constant lower than preferred by observations, and the model under-predicted observations of large-scale galaxy clustering.

These difficulties became stronger after the discovery of anisotropy in the cosmic microwave background by the COBE spacecraft in 1992, and several modified CDM models came under active study through the mid-1990s: these included the Lambda-CDM model and a mixed cold/hot dark matter model.

The first direct evidence for dark energy came from supernova observations in 1998 of accelerated expansion in Riess et al. and in Perlmutter et al., and the Lambda-CDM model then became the leading model.

Soon after, dark energy was supported by independent observations: in 2000, the BOOMERanG and Maxima cosmic microwave background experiments observed the first acoustic peak in the CMB, showing that the total (matter+energy) density is close to 100% of the critical density.

Then in 2001, the 2dF Galaxy Redshift Survey gave strong evidence that the matter density is around 30% of critical. The large difference between these two supports a smooth component of dark energy making up the difference.

Much more precise measurements from WMAP in 2003–2010 have continued to support the standard model and give more accurate measurements of the key parameters.

The term “dark energy“, echoing Fritz Zwicky’s “dark matter” from the 1930s, was coined by Michael Turner in 1998.

### Nature

The nature of dark energy is more hypothetical than that of dark matter, and many things about it remain in the realm of speculation.

Dark energy is thought to be very homogeneous and not very dense and is not known to interact through any of the fundamental forces other than gravity. Since it is quite rarefied and un-massive — roughly 10−27 kg/m3 — it is unlikely to be detectable in laboratory experiments.

The reason dark energy can have such a profound effect on the universe, making up 68% of universal density in spite of being so dilute, is that it uniformly fills otherwise empty space.

Independently of its actual nature, dark energy would need to have a strong negative pressure (repulsive action), like radiation pressure in a metamaterial, to explain the observed acceleration of the expansion of the universe.

According to general relativity, the pressure within a substance contributes to its gravitational attraction for other objects just as its mass density does.

This happens because the physical quantity that causes matter to generate gravitational effects is the stress-energy tensor, which contains both the energy (or matter) density of a substance and its pressure and viscosity.

In the Friedmann–Lemaître–Robertson–Walker metric, it can be shown that a strong constant negative pressure in all the universe causes an acceleration in the expansion if the universe is already expanding, or deceleration in contraction if the universe is already contracting. This accelerating expansion effect is sometimes labeled “gravitational repulsion“.

### Evidence of existence

The evidence for dark energy is indirect but comes from three independent sources:

• Distance measurements and their relation to redshift, which suggest the universe has expanded more in the last half of its life.
• The theoretical need for a type of additional energy that does not matter or dark matter to form the observationally flat universe (absence of any detectable global curvature).
• Measures of large-scale wave-patterns of mass density in the universe.

### Implications for the fate of the universe

Cosmologists estimate that the acceleration began roughly 5 billion years ago. Before that, it is thought that the expansion was decelerating, due to the attractive influence of matter.

The density of dark matter in an expanding universe decreases more quickly than dark energy, and eventually, the dark energy dominates.

Specifically, when the volume of the universe doubles, the density of dark matter is halved, but the density of dark energy is nearly unchanged (it is exactly constant in the case of a cosmological constant).

Projections into the future can differ radically for different models of dark energy. For a cosmological constant, or any other model that predicts that the acceleration will continue indefinitely, the ultimate result will be that galaxies outside the Local Group will have a line-of-sight velocity that continually increases with time, eventually far exceeding the speed of light.

This is not a violation of special relativity because the notion of “velocity” used here is different from that of velocity in a local inertial frame of reference, which is still constrained to be less than the speed of light for any massive object.

Because the Hubble parameter is decreasing with time, there can actually be cases where a galaxy that is receding from us faster than light does manage to emit a signal which reaches us eventually.

However, because of the accelerating expansion, it is projected that most galaxies will eventually cross a type of cosmological event horizon where any light they emit past that point will never be able to reach us at any time in the infinite future because the light never reaches a point where its “peculiar velocity” towards us exceeds the expansion velocity away from us (these two notions of velocity are also discussed in Uses of the proper distance).

Assuming the dark energy is constant (a cosmological constant), the current distance to this cosmological event horizon is about 16 billion light-years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event were less than 16 billion light years away, but the signal would never reach us if the event were more than 16 billion light-years away.

As galaxies approach the point of crossing this cosmological event horizon, the light from them will become more and more redshifted, to the point where the wavelength becomes too large to detect in practice and the galaxies appear to vanish completely.

Planet Earth, the Milky Way, and the Local Group of which the Milky Way is a part would all remain virtually undisturbed as the rest of the universe recedes and disappears from view.

In this scenario, the Local Group would ultimately suffer heat death, just as was hypothesized for the flat, matter-dominated universe before measurements of cosmic acceleration.

There are other, more speculative ideas about the future of the universe. The phantom energy model of dark energy results in divergent expansion, which would imply that the effective force of dark energy continues growing until it dominates all other forces in the universe.

Under this scenario, dark energy would ultimately tear apart all gravitationally bound structures, including galaxies and solar systems, and eventually overcome the electrical and nuclear forces to tear apart atoms themselves, ending the universe in a “Big Rip“.

On the other hand, dark energy might dissipate with time or even become attractive. Such uncertainties leave open the possibility that gravity might yet rule the day and lead to a universe that contracts in on itself in a “Big Crunch“, or that there may even be a dark energy cycle, which implies a cyclic model of the universe in which every iteration (Big Bang then eventually a Big Crunch) takes about a trillion (1012) years. While none of these are supported by observations, they are not ruled out.

### In the philosophy of science

In the philosophy of science, dark energy is an example of an “auxiliary hypothesis“, an ad hoc postulate that is added to a theory in response to observations that falsify it.

It has been argued that the dark energy hypothesis is a conventionalist hypothesis, that is, a hypothesis that adds no empirical content and hence is unfalsifiable in the sense defined by Karl Popper.