# Why can we detect gravitational waves?

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Now that LIGO has finally measured gravitational waves using a huge laser interferometer, to me, the question remains, why was it possible? As it is explained in many news articles, gravitational waves are similar to water waves or electromagnetic waves, they just do not exist in a medium like water or space, but space-time itself is the transport medium. If space-time itself gets contracted and expanded by the gravitational waves, so does any means of measurement. The ruler you use for measurement (the laser beam) gets deformed while the wave travels through the measuring device. Otherwise the "ruler" had to live outside of space-time, but there is no outside. If space-time was a cup filled with pudding, on which we had painted a straight line with 10 marks, pushing into the pudding slightly with our thumb does bend the line, but for us, there remain 10 marks on the line, because to measure the extension, we had to use a ruler, outside of our space-time (pudding) to measure, let's say, 11 marks. But, well, there is no outside. I assume the same happens not only to the 3 spacial dimensions but also to the time dimension. Because they "did it", what am I missing?

The short answer is that waves that are "in the apparatus" are indeed stretched. However the "fresh waves" being produced by the laser are not. So long as the "new" waves spend much less time in the interferometer than it takes to expand them (which takes roughly 1/gravitational wave frequency), then the effect you are talking about can be neglected.

Details:

There is an apparent paradox: you can think about the detection in two ways. On the one hand you can imagine that the lengths of the detector arms change and that the round-trip travel time of a light beam is subsequently changed and so the difference in the time-of-arrival of wavecrests translates into a phase difference that is detected in the interferometer. On the other hand you have the analogy to the expansion of the universe - if the arm length is changed, then isn't the wavelength of the light changed by exactly the same factor and so there can be no change in the phase difference? I guess this latter is your question.

Well clearly, the detector works so there must be a problem with the second interpretation. There is an excellent discussion of this by Saulson 1997, from which I give a summary.

Interpretation 1:

If the two arms are in the $$x$$ and $$y$$ directions and the incoming wave the $$z$$ direction, then the metric due to the wave can be written $$ds^2 = -c^2 dt^2 + (1+ h(t))dx^2 + (1-h(t))dy^2,$$ where $$h(t)$$ is the strain of the gravitational wave.

For light travelling on geodesic paths the metric interval $$ds^2=0$$, this means that (considering only the arm aligned along the x-axis for a moment) $$c dt = sqrt{(1 + h(t))}dx simeq (1 + frac{1}{2}h(t))dx$$ The time taken to travel the path is therefore increased to $$au_+ = int dt = frac{1}{c}int (1 + frac{1}{2}h(t))dx$$

If the original arm is of length $$L$$ and the perturbed arm length is $$L(1+h/2)$$, then the time difference for a photon to make the round trip along each arm is $$Delta au = au_+ - au_- simeq frac{2L}{c}h$$ leading to a phase difference in the signals of $$Delta phi = frac{4pi L}{lambda} h$$ This assumes that $$h(t)$$ is treated as a constant for the time that the laser light is in the apparatus.

Interpretation 2:

In analogy with the expansion of the universe, the gravitational wave does change the wavelength of light in each arm of the experiment. However, only the waves that are in the apparatus as the gravitational wave passes through can be affected.

Suppose that $$h(t)$$ is a step function so that the arm changes length from $$L$$ to $$L+h(0)/2$$ instantaneously. The waves that are just arriving back at the detector will be unaffected by this change, but subsequent wavecrests will have had successively further to travel and so there is a phase lag that builds up gradually to the value defined above in interpretation 1. The time taken for the phase lag to build up will be $$2L/c$$.

But then what about the waves that enter the apparatus later? For those, the laser frequency is unchanged and as the speed of light is constant, then the wavelength is unchanged. These waves travel in a lengthened arm and therefore experience a phase lag exactly equivalent to interpretation 1.

In practice, the "buildup time" for the phase lag is short compared with the reciprocal of the frequency of the gravitational waves. For example the LIGO path length is about 1,000 km, so the "build up time" would be 0.003 s compared with the reciprocal of the $$sim 100$$ Hz signal of 0.01 s and so is relatively unimportant when interpreting the signal (the detection sensitivity of the interferometer is indeed compromised at higher frequencies because of this effect).

## Why the gravitational wave discovery matters

A Laser Interferometer Gravitational-wave Observatory (LIGO) technician preparing an optical mode cleaner in this July 10, 2012 photo released by Caltech/MIT/LIGO Laboratory on February 8, 2016. REUTERS/Caltech/MIT/LIGO Laboratory/Handout via Reuters

Scientists working at the LIGO experiment in the US have for the first time detected elusive ripples in the fabric of space and time known as gravitational waves. There is no doubt that the finding is one of the most groundbreaking physics discoveries of the past 100 years. But what are they?

To best understand the phenomenon, let’s go back in time a few hundred years. In 1687 when Isaac Newton published his Philosophiæ Naturalis Principia Mathematica, he thought of the gravitational force as an attractive force between two masses – be it the Earth and the Moon or two peas on a table top. However the nature of how this force was transmitted was less well understood at the time. Indeed the law of gravitation itself was not tested until British scientist Henry Cavendish did so in 1798, while measuring the density of the Earth.

Fast forward to 1916, when Einstein presented physicists with a new way of thinking about space, time and gravity. Building on work published in 1905, the theory of general relativity tied together that what we commonly consider to be separate entities – space and time – into what is now called “space-time”.

Space-time can be considered to be the fabric of the universe. That means everything that moves, moves through it. In this model, anything with mass distorts the space-time fabric. The larger the mass, the larger the distortion. And since every moving object moves through space-time, it will also follow the distortions caused by objects with big mass.

One way of thinking about this is to consider two children, one heavier than the other, playing on a trampoline. If we treat the surface of the trampoline as the fabric then the more massive child distorts the fabric more than the other. If one child places a ball near the feet of the other then the ball will roll towards, or follow the distortion, towards their feet. Similarly, when the Earth goes around the sun, the huge mass of the sun distorts the space around it, leaving our comparatively tiny planet following as “straight” a path as it can, but in a curved space. This is why it ends up orbiting the sun.

If we accept this simple analogy, then we have the basics of gravity. Moving on to gravitational waves is a small, but very important, step. Let one of the children on the trampoline pull a heavy object across the surface. This creates a ripple on the surface that can be observed. Another way to visualise it is to consider moving your hand through water. The ripples or waves spread out from their origin but quickly decay.

Any object moving through the space-time fabric causes waves or ripples in that fabric. Unfortunately, these ripples also disappear fairly quickly and only the most violent events produce distortions big enough to be detected on Earth. To put this into perspective, two colliding black holes each with a mass of ten times that of our sun would result in a wave causing a distortion of 1% of the diameter of an atom when it reaches the Earth. On this scale, the distortion is of the order of a 0.0000000000001m change in the diameter of the Earth compared to the 1m change due to a tidal bulge.

Laser Interferometer Gravitational-wave Observatory (LIGO) technicians working at LIGO Livingston Observatory near Livington, Louisiana in this undated photo released by Caltech/MIT/LIGO Laboratory on February 8, 2016. REUTERS/Caltech/MIT/LIGO Laboratory/Handout via Reuters

September 3, 2016 at 8:39 am

Thank you, Sky and Telescope. This is the best article that I have read of the development and astrophysical importance of the most significant physical discovery yet of the twenty-first century. I am thrilled that I have lived long enough to see this happen,

January 11, 2017 at 9:59 pm

Please pardon my limited understanding in this field of science. While reading this Q&A, my mind kept poking me with a question. Can you help me with an answer, "Can LIGO be affected by earthquakes?" Something so sensitive-maybe, I have to look deeper for more light on the subject. Thank you, and yes, thank you for this very fine article. keep up the good work.

## Astronomers find possible hints of gravitational waves

An international team of astronomers – including 17 Cornellians – report they have found the first faint, low-frequency whispers that may be gravitational waves from gigantic, colliding black holes in distant galaxies.

The findings were obtained from more than 12.5 years of data collected from the national radio telescopes at Green Bank, West Virginia, and the recently collapsed dish at the Arecibo Observatory, in Arecibo, Puerto Rico.

The research was announced Jan. 11 at a press conference at the American Astronomical Society’s national meeting, held online due to the COVID-19 pandemic. The press conference highlighted the research, “The NANOGrav 12.5-year Data Set: Search for an Isotropic Stochastic Gravitational-wave Background,” published Dec. 24 in The Astrophysical Journal Letters.

The astronomers are all participants in the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) project, which uses pulsars – rapidly spinning dense stars – that act as wave detectors and cosmic timekeepers.

Merging gargantuan black holes create gravitational waves that can send ripples through space-time and affect a pulsar’s timekeeping regularity – ultimately indicating that Earth’s position in the universe may have slightly shifted.

“We must be clear: We are not yet claiming to have detected gravitational waves,” said Shami Chatterjee, Ph.D. ’03, a Cornell principal research scientist in the College of Arts and Sciences’ (A&S) Department of Astronomy. “We have detected a signal that is consistent with the existence of gravitational waves, but we can’t prove that quite yet. We think this is the tip of the iceberg, but we have to actually demonstrate it to our own satisfaction.”

To get a sense of the size of these gravitational waves, recall the wave detection by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in early 2016, when scientists caught two black holes merging.

The merger set off kilohertz waves that were hundreds of kilometers in length, small enough to allow Earth-based detectors to capture them from kilometer-wide, land-based sensors. That finding confirmed a major prediction of Albert Einstein’s 1915 general theory of relativity.

In the NANOGrav case, gigantic black holes are in the process of merging.

“The masses we’re talking about are the giant black holes that are in the centers of galaxies,” said James Cordes, the George Feldstein Professor of Astronomy (A&S). “They are a billion times the mass of the sun. They’re monsters.”

And these monsters are generating nanohertz-scale gravitational quavers that are light-years in length, said Cordes. Thus, astronomers enlist pulsars to help detect these waves.

The paper notes that 47 pulsars were studied to gather this data currently the astronomers are using 80 pulsars. Cordes said the plan is for the project’s astronomers to use about 200 pulsars, once they secure telescope time on other radio telescopes – to replace the time lost at the Arecibo Observatory, which recently collapsed.

In addition to Cordes and Chatterjee, the other Cornellians who work on this project include:

• Ross Jennings, doctoral candidate
• H. Thankful Cromartie, NASA Einstein Postdoctoral Fellow
• Maura A. McLaughlin, Ph.D. ‘01, professor, West Virginia University, a member of NANOgrav and co-director of the Physics Frontier Center
• Michael T. Lam, Ph.D. ’16, assistant professor, Rochester Institute of Technology
• T. Joseph W. Lazio, Ph.D. ’97, chief scientist of the Interplanetary Network Directorate, Jet Propulsion Laboratory, California Institute of Technology
• Dustin R. Madison, Ph.D. ’15, postdoctoral fellow, University of West Virginia
• David L. Kaplan ’98, visiting associate professor, University of Wisconsin, Madison
• Dan Stinebring M.S. ’78, Ph.D. ’82, emeritus professor of physics, Oberlin College
• Caitlin A. Witt ’16, Brent J. Shapiro-Albert (former summer student), and Jacob E. Turner (former summer student), doctoral students at the University of West Virginia
• Duncan Lorimer, professor and associate dean for research, University of West Virginia, former astronomer at the Arecibo Observatory
• Zaven Arzoumanian, deputy principal investigator and science lead, NASA Goddard Spaceflight Center, former postdoctoral researcher at Cornell and
• Timothy Dolch, assistant professor, Hillsdale College, former postdoctoral researcher at Cornell.

Cordes and Chatterjee are members of Cornell’s Carl Sagan Institute.

NANOGrav – of which Cornell is a founding member – is joint venture between the National Science Foundation and the Natural Sciences and Engineering Research Council of Canada. Both organizations provided funding.

## What’s the point of finding gravitational waves?

Well, gravitational waves give us another way to observe space. For example, waves from the Big Bang would tell us a little more about how the universe formed. Waves also form when black holes collide, supernovae explode, and massive neutron stars wobble. So detecting these waves would give us a new new insight into the cosmic events that produced them.

Finally, gravitational waves could also help physicists understand the fundamental laws of the universe. They are, in fact, a crucial part of Einstein’s general theory of relativity. Finding them would prove that theory—and could also help us figure out where it goes astray. Which could lead to a more accurate, more all-encompassing model, and perhaps point the way toward a theory of everything.

## Kurzweil Tracking the acceleration of intelligence.

Numerical simulations of the gravitational waves emitted by the inspiral and merger of two black holes. The colored contours around each black hole represent the amplitude of the gravitational radiation the blue lines represent the orbits of the black holes and the green arrows represent their spins. (credit: C. Henze/NASA Ames Research Center)

On Sept. 14, 2015 at 5:51 a.m. EDT (09:51 UTC) for the first time, scientists observed ripples in the fabric of spacetime called gravitational waves, arriving at Earth from a cataclysmic event in the distant universe, the National Science Foundation and scientists at the LIGO Scientific Collaboration announced today. This confirms a major prediction of Albert Einstein’s 1915 general theory of relativity and opens an unprecedented new window to the cosmos.

Gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot be obtained from elsewhere. Physicists have concluded that the detected gravitational waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive spinning black hole. This collision of two black holes had been predicted but never observed.

The gravitational-wave event on Sept. 14, 2015 at 09:50:45 UTC was observed by the two LIGO detectors in Livingston, Loiusiana (blue) and Hanford, Washington (orange). The matching waveforms represent gravitational-wave strain inferred to be generated by the merger of two inspiraling black holes. (credit: B. P. Abbott et al./PhysRevLett)

The gravitational waves were detected by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington. The LIGO observatories are funded by the National Science Foundation (NSF), and were conceived, built and are operated by the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT). The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors.

The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10 −21 .

Illustration of the collision of two black holes — an event detected for the first time ever by the Laser Interferometer Gravitational-Wave Observatory, or LIGO — is seen in this still from a computer simulation. LIGO detected gravitational waves, or ripples in space and time, generated as the black holes merged. (credit: SXS)

Based on the observed signals, LIGO scientists estimate that the black holes for this event were about 29 and 36 times the mass of the Sun, and the event took place 1.3 billion years ago. About three times the mass of the Sun was converted into gravitational waves in a fraction of a second — with a peak power output about 50 times that of the whole visible universe. By looking at the time of arrival of the signals — the detector in Livingston recorded the event 7 milliseconds before the detector in Hanford — scientists can say that the source was located in the Southern Hemisphere.

According to general relativity, a pair of black holes orbiting around each other lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes. During the final fraction of a second, the two black holes collide at nearly half the speed of light and form a single more massive black hole, converting a portion of the combined black holes’ mass to energy, according to Einstein’s formula E=mc 2 . This energy is emitted as a final strong burst of gravitational waves. These are the gravitational waves that LIGO observed.

How our sun and Earth warp spacetime is represented here with a green grid. As Albert Einstein demonstrated in his theory of general relativity, the gravity of massive bodies warps the fabric of space and time — and those bodies move along paths determined by this geometry. His theory also predicted the existence of gravitational waves, which are ripples in space and time. These waves, which move at the speed of light, are created when massive bodies accelerate through space and time. (credit: T. Pyle/LIGO)

The existence of gravitational waves was first demonstrated in the 1970s and 1980s by Joseph Taylor, Jr., and colleagues. In 1974, Taylor and Russell Hulse discovered a binary system composed of a pulsar in orbit around a neutron star. Taylor and Joel M. Weisberg in 1982 found that the orbit of the pulsar was slowly shrinking over time because of the release of energy in the form of gravitational waves. For discovering the pulsar and showing that it would make possible this particular gravitational wave measurement, Hulse and Taylor were awarded the 1993 Nobel Prize in Physics.

The new LIGO discovery is the first observation of gravitational waves themselves, made by measuring the tiny disturbances the waves make to space and time as they pass through the earth.

“Our observation of gravitational waves accomplishes an ambitious goal set out over five decades ago to directly detect this elusive phenomenon and better understand the universe, and, fittingly, fulfills Einstein’s legacy on the 100th anniversary of his general theory of relativity,” says Caltech’s David H. Reitze, executive director of the LIGO Laboratory.

An aerial view of the Laser Interferometer Gravitational-wave Observatory (LIGO) detector in Livingston, Louisiana. LIGO has two detectors: one in Livingston and the other in Hanford, Washington. (credit: LIGO Laboratory)

LIGO research

The discovery was made possible by the enhanced capabilities of Advanced LIGO, a major upgrade that increases the sensitivity of the instruments compared to the first generation LIGO detectors, enabling a large increase in the volume of the universe probed — and the discovery of gravitational waves during its first observation run. NSF is the lead financial supporter of Advanced LIGO. Funding organizations in Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council, STFC) and Australia (Australian Research Council) also have made significant commitments to the project.

LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of more than 1,000 scientists from universities around the United States and in 14 other countries. More than 90 universities and research institutes in the LSC develop detector technology and analyze data approximately 250 students are strong contributing members of the collaboration. The LSC detector network includes the LIGO interferometers and the GEO600 detector. The GEO team includes scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI), Leibniz Universität Hannover, along with partners at the University of Glasgow, Cardiff University, the University of Birmingham, other universities in the United Kingdom and the University of the Balearic Islands in Spain.

“This detection is the beginning of a new era: The field of gravitational wave astronomy is now a reality,” says Gabriela González, LSC spokesperson and professor of physics and astronomy at Louisiana State University.

LIGO was originally proposed as a means of detecting gravitational waves in the 1980s by Rainer Weiss, professor of physics, emeritus, from MIT Kip Thorne, Caltech’s Richard P. Feynman Professor of Theoretical Physics, emeritus and Ronald Drever, professor of physics, emeritus, also from Caltech.

“The description of this observation is beautifully described in the Einstein theory of general relativity formulated 100 years ago and comprises the first test of the theory in strong gravitation. It would have been wonderful to watch Einstein’s face had we been able to tell him,” says Weiss.

“With this discovery, we humans are embarking on a marvelous new quest: the quest to explore the warped side of the universe — objects and phenomena that are made from warped spacetime. Colliding black holes and gravitational waves are our first beautiful examples,” says Thorne.

Virgo research is carried out by the Virgo Collaboration, consisting of more than 250 physicists and engineers belonging to 19 different European research groups: six from Centre National de la Recherche Scientifique (CNRS) in France eight from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy two in the Netherlands with Nikhef the Wigner RCP in Hungary the POLGRAW group in Poland and the European Gravitational Observatory (EGO), the laboratory hosting the Virgo detector near Pisa in Italy.

At each observatory, the 2 1/2-mile (4-km) long, L-shaped LIGO interferometer uses laser light split into two beams that travel back and forth down the arms (four-foot diameter tubes kept under a near-perfect vacuum). The beams are used to monitor the distance between mirrors precisely positioned at the ends of the arms. According to Einstein’s theory, the distance between the mirrors will change by an infinitesimal amount when a gravitational wave passes by the detector. A change in the lengths of the arms smaller than one-ten-thousandth the diameter of a proton (10 -19 meter) can be detected.

Independent and widely separated observatories are necessary to determine the direction of the event causing the gravitational waves, and also to verify that the signals come from space and are not from some other local phenomenon.

Toward this end, the LIGO Laboratory is working closely with scientists in India at the Inter-University Centre for Astronomy and Astrophysics, the Raja Ramanna Centre for Advanced Technology, and the Institute for Plasma to establish a third Advanced LIGO detector on the Indian subcontinent. Awaiting approval by the government of India, it could be operational early in the next decade. The additional detector will greatly improve the ability of the global detector network to localize gravitational-wave sources.

“Hopefully this first observation will accelerate the construction of a global network of detectors to enable accurate source location in the era of multi-messenger astronomy,” says David McClelland, professor of physics and director of the Centre for Gravitational Physics at the Australian National University.

The finding is described in an open-access paper in Physical Review Letters today (Feb. 11).

National Science Foundation | LIGO detects gravitational waves **Begin viewing at 27:14**

## Astronomy Using Gravity

Gravitational waves, also known as gravitational radiation, were predicted by Albert Einstein as a consequence of his theory of general relativity. This theory describes gravity as distortions in the structure of spacetime created by matter and energy. Einstein realized those distortions would travel at the speed of light in the form of waves, much like light itself can be described as a wave.

However, gravity is so weak that even a high-energy gravitational wave barely nudges objects in its path. For that reason, our best hope is to detect gravitational waves from objects that produce very intense gravity because they pack a lot of mass into a very small space. That category includes the sources we’ve detected so far: colliding black holes and neutron stars.

In 2015, a century after Einstein published general relativity, researchers used the Laser Interferometer Gravitational Observatory (LIGO) to detect the collision between two black holes. Two years later, LIGO scientists identified a collision between two neutron stars, an event also observed using light.

Using both gravitational wave and light-based astronomy is known as “multimessenger astronomy”. This is an exciting development for researchers studying the structure of neutron stars, and understanding the creation of many chemical elements such as gold, which are produced in neutron star collisions.

## What Kind of Noise Annoys an Interferometer?

But it gets more complicated. Everything from passing trucks to distant ocean waves can shake the mirrors, causing “noise” that muddies the measurements. That’s why LIGO isn’t just one machine, but a pair of facilities with identical-twin designs , one located in Livingston, Louisiana and the other Hanford, Washington. By putting almost 2,000 miles between the two instruments, physicists can discriminate between local jiggling, which will only be felt by one detector, and authentic cosmic gravitational waves, which should be felt by both.

LIGO's Hanford, Washington, facility

Still, picking out the tiny signal expected from a gravitational wave from mundane background vibrations is like trying to hear the crickets chirping at an AC/DC concert. The trick is to isolate LIGO’s mirrors from external shaking as perfectly as possible. To do that, Advanced LIGO has a completely revamped isolation system that exploits seven different layers of technology to effectively “float” the optics. For LIGO’s first run, the mirrors were hung from simple pendulums. This time around, the mirrors are heavier, and each one is suspended from welded glass fibers that hang from a quadruple pendulum—that is, a pendulum that hangs from a pendulum that hangs from a pendulum that hangs from a pendulum. This quadruple pendulum system is great at cushioning the mirrors from very high frequency vibrations.

“Though LIGO’s suspension system is much more complex than a car’s, it operates on a similar principle,” says MIT research scientist Fabrice Matichard, who leads design and testing of Advanced LIGO’s seismic isolation system. “When you go very fast, the wheels follow the bumps, but the wheels are decoupled from the frame,” so you feel a smooth ride. With LIGO, it’s like that, but times four.

To handle lower-frequency tremors, Advanced LIGO is using a new, “active” isolation system that senses and corrects for vibrations in real-time. Outside the LIGO vacuum chamber, a hydraulic system neutralizes the slow swaying coming from distant effects like tides. Inside the vacuum chamber, another set of seismometers triggers two stages of magnetic actuators that counteract mid-frequency vibrations. This active isolation system also keeps in check the Achilles’ heel of the pendulum system: its inconvenient tendency to resonate at certain frequencies, making the internal vibrations stronger instead of weaker.

A member of the pendulum suspension system

Even with all this noise-canceling technology, today, much of the day-to-day business of operating LIGO is trying to understand all the background noise that it picks up. And now, after eight years spent acclimating to the standard noise of the first LIGO run, physicists find themselves back at the beginning.

“The instrument is so different” after the upgrade, says LIGO spokesperson Gabriela González, a professor at Louisiana State University, that the “noise looks very different—in a good way.” In fact, the very same active seismic isolation system designed to keep the noise out can introduce new noise as it pushes against the mirrors, noise that in the past would have been imperceptible. Like hunters skilled at finding their prey against one landscape, LIGO scientists must now relearn how to pick out their quarry in a totally new environment.

## A bizarre new form of astronomy could unlock these 4 cosmic secrets

Ripples in the fabric of space, called gravitational waves, are careening across the universe, right through everything and everyone.

And apparently there are a lot of them.

On Wednesday, scientists from the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment announced the second-ever detection of gravitational waves — a feat Einstein thought impossible 100 years ago — emanating from two colliding black holes.

"It confirms — it super-confirms — that these events are not flukes," astrophysicist Vicky Kalogera, who has been working with LIGO to analyze the signals, told Tech Insider. "They're happening in nature and we can detect them every few months." This summer, Kalogera thinks LIGO may find 10 or more new gravitational waves, and possibly up to 100 a year later on.

Tech Insider spoke with Imre Bartos, also a physicist working with LIGO, and other researchers earlier this year about the "revolutionary" new era of astronomy they say has begun.

Here are just a handful of formerly impossible things astronomers could do with gravitational waves.

## Cosmic News: Astronomers Find the Twisted Fingerprints of Inflation in the Background Glow of the Universe

Update, Jan. 31, 2015: Disappointingly, the big discovery announced here was premature. In their analysis, the scientists used preliminary data from Planck about the amount of dust in our galaxy (which looks very much like the signal from the early Universe for which they were looking), and newer analysis shows they underestimated it. This means the signal of inflation they found was actually far weaker than they first thought, and is not statistically significant that is, they can’t say if it’s real or not.

This is big news: Astronomers have announced that they have seen, for the first time, direct evidence of “inflation” in the extremely early Universe, unlocking an entire chapter in the history of the cosmos. It also ties together relativity and quantum mechanics in a deep and profound way, which has never been done before.

This news is very important and very interesting. However, it’s also very esoteric—probably the most layered and complex announcement I’ve ever written about. It’s not like the Higgs boson, which could at least be summed up in a sentence or two. But this new work unveils a critical point in the history of the Universe and has profound implications for physics.

I’ll note that in preparation for this announcement, Sky and Telescope put up a nice overview, as did the Guardian. My friend, the cosmologist Sean Carroll, has a fantastic writeup about it. It starts off semi-technical and becomes very technical in the second half, so be ye fairly warned, says I. He has a follow-up post with more details as well.

And Leon’s Getting LARGER

We know the Universe is expanding everywhere we look, it appears that galaxies are rushing away from us. If we run the clock backwards, this means the Universe was smaller in the past and at some point must have had (nearly) zero volume. This point in time is commonly referred to as the Big Bang, when the expansion of the Universe started. Here we are, 13.82 billion years later.

But a lot happened in the intervening time, and a lot of it happened at a teeny tiny fraction of a second after the First Moment. One of these things was inflation.

Inflation is a bit of a mind-bender, I’ll admit. It started just about 10 -35 or so seconds after the bang. To give you a better idea of how short a time interval that is, we’re talking 0.00000000000000000000000000000000001 seconds! And it only lasted until about 10 -32 or so seconds later, so it was incredibly brief by human standards. But in such fleeting moments are Universes forged.

During that period, for reasons that are still not clear, the Universe underwent a kind of hyperexpansion. Instead of simply cruising along, getting bigger with time as it does now, the expansion accelerated. Hugely. Hugely hugely. Some models show it increased in size by a factor of 10 50 (some say even more)—that’s 10 trillion trillion trillion trillion times bigger, all in a time frame so small that analogies fail me.

Like I said, inflation is a mind-bender.

Why Inflation?

The reason we think this happened is that the Universe appears very smooth. You’d expect it to be very lumpy, with some parts packed tightly with matter and energy, while other parts would be empty. But when we look at the distant and ancient Universe on really big scales, we see it’s incredibly smooth. Telescopes looking back into the deep Universe can examine the leftover heat from the birth of the Universe, and measure how bumpy it is. Amazingly, it’s smooth to one part in about 100,000 (I explain this in more detail in an earlier post). Inflation solves this problem: The Universe started out lumpy, but during the period of hyperexpansion all the lumpiness got smoothed out. It’s like having a wrinkly sheet, then pulling on it from all sides. The wrinkles vanish.

Not only that, but inflation solves a problem about the geometry, the shape of the Universe. I won’t go into detail here you can read more about it if you like. The point is, astronomers dreamed up this idea of inflation to explain some weird stuff we saw about the Universe, and it did a pretty good job. It’s held up over the years.

Photo by NSF/Harvard CMB group

But all that was indirect evidence for it. Scientists prefer direct evidence, and we don’t have any for inflation.

Ripples in the Space-Time Continuum

Until now. That’s what these new results show. Inflationary models predict that other marks were left on the Universe, and one of these is that as the Universe underwent rapid expansion, it would create ripples in the fabric of space-time called gravitational waves. These are literally small expansions and contractions of space itself, like a wave traveling down a Slinky. We know these exist—we see their effects in astronomy, and two astronomers won a Nobel Prize in 1993 for finding an example of gravitational waves—but seeing them coming from the inflationary period of the Universe is incredibly difficult.

We don’t see the waves themselves, but we can detect the effect they had on light coming from the early Universe. The waves would polarize the light, in a sense aligning the waves of light in certain ways. There are many different ways light can be polarized, but gravitational waves left over from inflation would do so in a very specific way (called B mode polarization, which twists and curls the direction of the polarization see the image at the top of this post). Finding this kind of polarization in the light leftover from the fires of the Big Bang would be clear evidence of gravitational waves… and it was precisely this type of polarization that was finally detected by a telescope called BICEP2 (Background Imaging of Cosmic Extragalactic Polarization), located in Antarctica.

Still with me? I know, this seems all very distant and removed from our daily lives, but in fact this is a very big deal indeed. Until now, inflation was a great idea—a critical one to understand the evolution of the Universe from the very first moment after its birth to the huge structures and details we see today—with no direct evidence. Now we have direct evidence.

Filling in the Blanks

Up until now this was all like trying to write a history book about the United States and talking about the Civil War without ever knowing exactly what happened at the time … and then finding photos and diaries and battlefields. This inflation-spawned gravitational-wave-induced B-mode polarized light is like having the words appear on what were before blank pages in a chapter about the Universe itself.

This light is showing us what happened in the tiniest fraction of a second after the birth of our cosmos. This is crucial. There are many different physical models of how inflation might have worked, and observations like this will be able to help us figure out which ones work, which ones don’t, and which ones might need tweaking. The strength of the gravitational waves was stronger than predicted by models, for example, so you know a lot of cosmologists are right now standing in front of blackboards, hunched over papers, or sitting back in their chairs with their hands interlocked behind their heads, puzzling over what variables, what parameters, what equations must be poked at to reproduce these new observations.

Inflation was a time of a huge phase change in the Universe. Finding direct evidence for it will trigger a similar phase change in the way we understand it.

I mentioned the Higgs boson earlier that beast is the linchpin in modern particle physics, and finding evidence of it was a very big deal. This discovery of evidence for gravitational waves from inflation is a similarly important event in the field of cosmology. If the findings stand up, I imagine there might be a Nobel Prize in store for someone (or someones).

But what does this meant to you? Well, that’s up to you, of course. Most of us can live our daily lives without worrying overly much about gravitational waves, subatomic particles, or what the Universe was like in the tiniest sliver of the first moment of its existence.

But think about that: We can understand what the Universe was like in the tiniest sliver of the first moment of its existence! These aren’t wild guesses, or just-so stories, or fanciful myths. This work is the result of an intense amount of research, the application of math, science, physics, and technology over hundreds of years, the painstaking acquisition of knowledge that must withstand the fires of scientific scrutiny and skepticism to survive. And so far, they have.

There are practical concerns here as well. Inflation is based on principles of quantum mechanics, while gravitational waves are the purview of relativity. QM has brought us computers, solar power, atomic energy—a huge amount of modern tech. Relativity is critical in many aspects of our lives as well, including GPS and also nuclear power. In the past these two concepts haven’t played well together, but now we have a direct and profound connection between them. This result is new, and we have a long, long way to go to understand it better. There’s no way to know what will result from this. Yet. But whenever we open up new fields of science, all sorts of interesting things follow. Bet on it.

And a final note. I am not a cosmologist I am an astronomer. But I’m also a human, and when I look out into the dark sky at night or gaze at a gorgeous image from a telescope, I wonder how this all came to be, why things are the way they are, and how they happened to shape themselves into the Universe we see today. I bet you’ve wondered about them too.

These questions have been asked since we’ve been able to ask questions. Science is now answering them.