# Gravitational Waves and the Big bang

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The Ligo website says "Detecting the relic gravitational waves from the Big Bang will allow us to see farther back into the history of the Universe than ever before."

I find this puzzling. How can a Gravitational Wave originating at about the same time and place as the particles that make up their detector (ie in the Big bang) be detected? To use a simple analogy - an explosion in an electronics warehouse causes a flash of light and pieces of electronics gear to be scattered. These pieces are then assembled to make a camera which is able to photograph the explosion. I am confused!

Answers suitable for a high school physics class greatly appreciated.

Firstly, Big Bang didn't happen at a point in space, away from which we are traveling. Big Bang was the creation of space. This space has been expanding ever since, so that the distances between everything increases, but Big Bang happened right where you are, where the Andromeda galaxy is, where GN-z11 is, and so on.

The Universe has evolved ever since. If we want to know how galaxies look 13.8 billion years (Gyr) after Big Bang, we can just look around in our neighborhood. If we look too far away, we don't see 13.8 Gyr old galaxies, because the light has taken some time to reach us. Thus, if we want to see galaxies that are 12.8 Gyr old, we simply look 1 billion lightyears away; if we want to se 10 Gyr old galaxies, we look (roughly) 3.8 billion lightyears away, and so on.$^dagger$ In this way, we look back in time.

If we look away far enough, we would in principle be able to look 13.8 Gyr back in time. However, we face a problem in that until the Universe was 380,000 years old, it was opaque to light (why this is so is another story). It wasn't opaque to gravitational waves, however. And since lots of GWs are thought to have originated during the epoch called inflation, which were responsible for the expansion of the Universe, and which took place a fraction of a second after the creation of space, we say that GWs offer the possibility of looking all the way back to Big Bang.

$^dagger$This is somewhat imprecise, since galaxies weren't created instantly after Big Bang, and since all galaxies weren't created at the same time. But for the sake of the arument, let's pretend they were.

## What will gravitational waves tell us about the universe?

“THERE’S going to be a revolution.” So says Erik Katsavounidis of MIT, one of the team behind the long-awaited discovery of gravitational waves.

On 11 February, the Laser Interferometer Gravitational-Wave observatory, or LIGO, announced it had spotted gravitational waves, the stretching and squeezing of space-time caused by the movement of massive objects.

### LIGO Detection: Behind the scenes of the discovery of the decade

#### Watch the exclusive world premiere on 7 February 2017

The announcement caused a sensation among physicists and astronomers across the world, and they are now gearing up to exploit this new window on the universe. Gravitational waves will allow us to explore fundamental physics, examine the weirdest objects in the universe and possibly even peer back to the universe’s earliest moments. “We can potentially see almost all the way to the big bang,” says Dejan Stojkovic of the State University of New York in Buffalo.

The signal was picked up by LIGO’s two observatories in Hanford, Washington, and Livingston, Louisiana, on 14 September 2015. It was created by two black holes colliding, each about 30 times the mass of the sun. The details of the signal suggest they circled each other closer and closer until they finally merged into one.

This immediately resolved one open question for astronomers. Before the signal came in, the very existence of such black hole binaries was contested. Because they are dark, black holes of these masses are almost impossible to spot unless something bright – like a star – orbits them.

“Signals from black hole mergers could help us understand the nature of dark energy“

The next target is to observe gravitational waves from the death spiral of two neutron stars. Unlike black holes, which hide their mass behind an event horizon even as they crash, colliding neutron stars spew hot, bright matter across space, which could help us explore other mysteries. For example, studying these explosions may explain short gamma-ray bursts – mysterious and incredibly bright electromagnetic phenomena. They might also help explain where much of the universe’s heavy elements, like uranium, thorium and gold, are forged.

Within the next two years, LIGO should be sensitive enough to detect gravitational waves from any neutron star mergers that happen within the nearest 300,000 galaxies. That means we should see about one signal per month.

These single event detections are just the start, however. Put several together and we should be able to get new insights into the history and composition of the universe as a whole, says Avi Loeb of Harvard University. The signals from a number of black hole mergers, for example, can be combined to help understand the nature of dark energy, which is causing the universe’s expansion to accelerate.

From the “shape” of the signal – how the waves’ frequency and volume rise and fall – we can discern the sizes of the black holes involved, and determine how loud the event was at its source. Comparing how powerful it really was to the faint vibrations LIGO detected tells us how far away it occurred. Combined with observations from standard telescopes, this can tell us how space has expanded during the time the waves took to reach us, providing a measure of dark energy’s effect on space.

This measure should be stronger and more reliable than anything we have used so far. Spotting just a few black hole mergers would change everything, Loeb says. “If you have tens of them, it will be a new branch in cosmology.”

Other researchers are hoping to use gravitational wave signals to put Einstein’s general theory of relativity to even more stringent tests. One way is through the equivalence principle, an assumption that gravity affects all masses in the same way. “In the age of GPS and space travel, where even minute deviations from the assumed theory of gravity would have major consequences, it is of enormous importance,” says Xue-Feng Wu of Purple Mountain Observatory in Nanjing, China.

Erminia Calabrese, an astronomer at the University of Oxford, sees gravitational waves as a way to check whether gravity behaves as relativity predicts it should over large distances. “If their strength fell off with distance in a surprising way we could detect this with the upcoming LIGO data,” she says.

## Contents

Ordinary gravitational waves' frequencies are very low and much harder to detect, while higher frequencies occur in more dramatic events and thus have become the first to be observed.

In addition to a merger of black holes, a binary neutron star merger has been directly detected: a gamma-ray burst (GRB) was detected by the orbiting Fermi gamma-ray burst monitor on 2017 August 17 12:41:06 UTC, triggering an automated notice worldwide. Six minutes later a single detector at Hanford LIGO, a gravitational-wave observatory, registered a gravitational-wave candidate occurring 2 seconds before the gamma-ray burst. This set of observations is consistent with a binary neutron star merger, [7] as evidenced by a multi-messenger transient event which was signalled by gravitational-wave, and electromagnetic (gamma-ray burst, optical, and infrared)-spectrum sightings.

### High frequency Edit

In 2015, the LIGO project was the first to directly observe gravitational waves using laser interferometers. [8] [9] The LIGO detectors observed gravitational waves from the merger of two stellar-mass black holes, matching predictions of general relativity. [10] [11] [12] These observations demonstrated the existence of binary stellar-mass black hole systems, and were the first direct detection of gravitational waves and the first observation of a binary black hole merger. [13] This finding has been characterized as revolutionary to science, because of the verification of our ability to use gravitational-wave astronomy to progress in our search and exploration of dark matter and the big bang.

There are several current scientific collaborations for observing gravitational waves. There is a worldwide network of ground-based detectors, these are kilometre-scale laser interferometers including: the Laser Interferometer Gravitational-Wave Observatory (LIGO), a joint project between MIT, Caltech and the scientists of the LIGO Scientific Collaboration with detectors in Livingston, Louisiana and Hanford, Washington Virgo, at the European Gravitational Observatory, Cascina, Italy GEO600 in Sarstedt, Germany, and the Kamioka Gravitational Wave Detector (KAGRA), operated by the University of Tokyo in the Kamioka Observatory, Japan. LIGO and Virgo are currently being upgraded to their advanced configurations. Advanced LIGO began observations in 2015, detecting gravitational waves even though not having reached its design sensitivity yet. The more advanced KAGRA started observation on February 25, 2020. GEO600 is currently operational, but its sensitivity makes it unlikely to make an observation its primary purpose is to trial technology.

### Low frequency Edit

An alternative means of observation is using pulsar timing arrays (PTAs). There are three consortia, the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array (PPTA), which co-operate as the International Pulsar Timing Array. These use existing radio telescopes, but since they are sensitive to frequencies in the nanohertz range, many years of observation are needed to detect a signal and detector sensitivity improves gradually. Current bounds are approaching those expected for astrophysical sources. [14]

### Intermediate frequencies Edit

Further in the future, there is the possibility of space-borne detectors. The European Space Agency has selected a gravitational-wave mission for its L3 mission, due to launch 2034, the current concept is the evolved Laser Interferometer Space Antenna (eLISA). [15] Also in development is the Japanese Deci-hertz Interferometer Gravitational wave Observatory (DECIGO).

Astronomy has traditionally relied on electromagnetic radiation. Originating with the visible band, as technology advanced, it became possible to observe other parts of the electromagnetic spectrum, from radio to gamma rays. Each new frequency band gave a new perspective on the Universe and heralded new discoveries. [16] During the 20th century, indirect and later direct measurements of high-energy, massive, particles provided an additional window into the cosmos. Late in the 20th century, the detection of solar neutrinos founded the field of neutrino astronomy, giving an insight into previously inaccessible phenomena, such as the inner workings of the Sun. [17] [18] The observation of gravitational waves provides a further means of making astrophysical observations.

Russell Hulse and Joseph Taylor were awarded the 1993 Nobel Prize in Physics for showing that the orbital decay of a pair of neutron stars, one of them a pulsar, fits general relativity's predictions of gravitational radiation. [19] Subsequently, many other binary pulsars (including one double pulsar system) have been observed, all fitting gravitational-wave predictions. [20] In 2017, the Nobel Prize in Physics was awarded to Rainer Weiss, Kip Thorne and Barry Barish for their role in the first detection of gravitational waves. [21] [22] [23]

Gravitational waves provide complementary information to that provided by other means. By combining observations of a single event made using different means, it is possible to gain a more complete understanding of the source's properties. This is known as multi-messenger astronomy. Gravitational waves can also be used to observe systems that are invisible (or almost impossible to detect) to measure by any other means. For example, they provide a unique method of measuring the properties of black holes.

Gravitational waves can be emitted by many systems, but, to produce detectable signals, the source must consist of extremely massive objects moving at a significant fraction of the speed of light. The main source is a binary of two compact objects. Example systems include:

• Compact binaries made up of two closely orbiting stellar-mass objects, such as white dwarfs, neutron stars or black holes. Wider binaries, which have lower orbital frequencies, are a source for detectors like LISA. [24][25] Closer binaries produce a signal for ground-based detectors like LIGO. [26] Ground-based detectors could potentially detect binaries containing an intermediate mass black hole of several hundred solar masses. [27][28] binaries, consisting of two black holes with masses of 10 5 –10 9 solar masses. Supermassive black holes are found at the centre of galaxies. When galaxies merge, it is expected that their central supermassive black holes merge too. [29] These are potentially the loudest gravitational-wave signals. The most massive binaries are a source for PTAs. [30] Less massive binaries (about a million solar masses) are a source for space-borne detectors like LISA. [31] systems of a stellar-mass compact object orbiting a supermassive black hole. [32] These are sources for detectors like LISA. [31] Systems with highly eccentric orbits produce a burst of gravitational radiation as they pass through the point of closest approach [33] systems with near-circular orbits, which are expected towards the end of the inspiral, emit continuously within LISA's frequency band. [34] Extreme-mass-ratio inspirals can be observed over many orbits. This makes them excellent probes of the background spacetime geometry, allowing for precision tests of general relativity. [35]

In addition to binaries, there are other potential sources:

generate high-frequency bursts of gravitational waves that could be detected with LIGO or Virgo. [36]
• Rotating neutron stars are a source of continuous high-frequency waves if they possess axial asymmetry. [37][38]
• Early universe processes, such as inflation or a phase transition. [39] could also emit gravitational radiation if they do exist. [40] Discovery of these gravitational waves would confirm the existence of cosmic strings.

Gravitational waves interact only weakly with matter. This is what makes them difficult to detect. It also means that they can travel freely through the Universe, and are not absorbed or scattered like electromagnetic radiation. It is therefore possible to see to the center of dense systems, like the cores of supernovae or the Galactic Centre. It is also possible to see further back in time than with electromagnetic radiation, as the early universe was opaque to light prior to recombination, but transparent to gravitational waves. [41]

The ability of gravitational waves to move freely through matter also means that gravitational-wave detectors, unlike telescopes, are not pointed to observe a single field of view but observe the entire sky. Detectors are more sensitive in some directions than others, which is one reason why it is beneficial to have a network of detectors. [42] Directionalization is also poor, due to the small number of detectors.

### In cosmic inflation Edit

Cosmic inflation, a hypothesized period when the universe rapidly expanded during the first 10 −36 seconds after the Big Bang, would have given rise to gravitational waves that would have left a characteristic imprint in the polarization of the CMB radiation. [43] [44]

It is possible to calculate the properties of the primordial gravitational waves from measurements of the patterns in the microwave radiation, and use those calculations to learn about the early universe. [ how? ]

As a young area of research, gravitational-wave astronomy is still in development however, there is consensus within the astrophysics community that this field will evolve to become an established component of 21st century multi-messenger astronomy. [45]

Gravitational-wave observations complement observations in the electromagnetic spectrum. [46] [45] These waves also promise to yield information in ways not possible via detection and analysis of electromagnetic waves. Electromagnetic waves can be absorbed and re-radiated in ways that make extracting information about the source difficult. Gravitational waves, however, only interact weakly with matter, meaning that they are not scattered or absorbed. This should allow astronomers to view the center of a supernova, stellar nebulae, and even colliding galactic cores in new ways.

Ground-based detectors have yielded new information about the inspiral phase and mergers of binary systems of two stellar mass black holes, and merger of two neutron stars. They could also detect signals from core-collapse supernovae, and from periodic sources such as pulsars with small deformations. If there is truth to speculation about certain kinds of phase transitions or kink bursts from long cosmic strings in the very early universe (at cosmic times around 10 −25 seconds), these could also be detectable. [47] Space-based detectors like LISA should detect objects such as binaries consisting of two white dwarfs, and AM CVn stars (a white dwarf accreting matter from its binary partner, a low-mass helium star), and also observe the mergers of supermassive black holes and the inspiral of smaller objects (between one and a thousand solar masses) into such black holes. LISA should also be able to listen to the same kind of sources from the early universe as ground-based detectors, but at even lower frequencies and with greatly increased sensitivity. [48]

Detecting emitted gravitational waves is a difficult endeavor. It involves ultra-stable high-quality lasers and detectors calibrated with a sensitivity of at least 2·10 −22 Hz −1/2 as shown at the ground-based detector, GEO600. [49] It has also been proposed that even from large astronomical events, such as supernova explosions, these waves are likely to degrade to vibrations as small as an atomic diameter. [50]

## A technique to sift out the universe’s first gravitational waves

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In the moments immediately following the Big Bang, the very first gravitational waves rang out. The product of quantum fluctuations in the new soup of primordial matter, these earliest ripples through the fabric of space-time were quickly amplified by inflationary processes that drove the universe to explosively expand.

Primordial gravitational waves, produced nearly 13.8 billion years ago, still echo through the universe today. But they are drowned out by the crackle of gravitational waves produced by more recent events, such as colliding black holes and neutron stars.

Now a team led by an MIT graduate student has developed a method to tease out the very faint signals of primordial ripples from gravitational-wave data. Their results are published this week in Physical Review Letters.

Gravitational waves are being detected on an almost daily basis by LIGO and other gravitational-wave detectors, but primordial gravitational signals are several orders of magnitude fainter than what these detectors can register. It’s expected that the next generation of detectors will be sensitive enough to pick up these earliest ripples.

In the next decade, as more sensitive instruments come online, the new method could be applied to dig up hidden signals of the universe’s first gravitational waves. The pattern and properties of these primordial waves could then reveal clues about the early universe, such as the conditions that drove inflation.

“If the strength of the primordial signal is within the range of what next-generation detectors can detect, which it might be, then it would be a matter of more or less just turning the crank on the data, using this method we’ve developed,” says Sylvia Biscoveanu, a graduate student in MIT’s Kavli Institute for Astrophysics and Space Research. “These primordial gravitational waves can then tell us about processes in the early universe that are otherwise impossible to probe.”

Biscoveanu’s co-authors are Colm Talbot of Caltech, and Eric Thrane and Rory Smith of Monash University.

A concert hum

The hunt for primordial gravitational waves has concentrated mainly on the cosmic microwave background, or CMB, which is thought to be radiation that is leftover from the Big Bang. Today this radiation permeates the universe as energy that is most visible in the microwave band of the electromagnetic spectrum. Scientists believe that when primordial gravitational waves rippled out, they left an imprint on the CMB, in the form of B-modes, a type of subtle polarization pattern.

Physicists have looked for signs of B-modes, most famously with the BICEP Array, a series of experiments including BICEP2, which in 2014 scientists believed had detected B-modes. The signal turned out to be due to galactic dust, however.

As scientists continue to look for primordial gravitational waves in the CMB, others are hunting the ripples directly in gravitational-wave data. The general idea has been to try and subtract away the “astrophysical foreground” — any gravitational-wave signal that arises from an astrophysical source, such as colliding black holes, neutron stars, and exploding supernovae. Only after subtracting this astrophysical foreground can physicists get an estimate of the quieter, nonastrophysical signals that may contain primordial waves.

The problem with these methods, Biscoveanu says, is that the astrophysical foreground contains weaker signals, for instance from farther-off mergers, that are too faint to discern and difficult to estimate in the final subtraction.

“The analogy I like to make is, if you’re at a rock concert, the primordial background is like the hum of the lights on stage, and the astrophysical foreground is like all the conversations of all the people around you,” Biscoveanu explains. “You can subtract out the individual conversations up to a certain distance, but then the ones that are really far away or really faint are still happening, but you can’t distinguish them. When you go to measure how loud the stagelights are humming, you’ll get this contamination from these extra conversations that you can’t get rid of because you can’t actually tease them out.”

A primordial injection

For their new approach, the researchers relied on a model to describe the more obvious “conversations” of the astrophysical foreground. The model predicts the pattern of gravitational wave signals that would be produced by the merging of astrophysical objects of different masses and spins. The team used this model to create simulated data of gravitational wave patterns, of both strong and weak astrophysical sources such as merging black holes.

The team then tried to characterize every astrophysical signal lurking in these simulated data, for instance to identify the masses and spins of binary black holes. As is, these parameters are easier to identify for louder signals, and only weakly constrained for the softest signals. While previous methods only use a “best guess” for the parameters of each signal in order to subtract it out of the data, the new method accounts for the uncertainty in each pattern characterization, and is thus able to discern the presence of the weakest signals, even if they are not well-characterized. Biscoveanu says this ability to quantify uncertainty helps the researchers to avoid any bias in their measurement of the primordial background.

Once they identified such distinct, nonrandom patterns in gravitational-wave data, they were left with more random primordial gravitational-wave signals and instrumental noise specific to each detector.

Primordial gravitational waves are believed to permeate the universe as a diffuse, persistent hum, which the researchers hypothesized should look the same, and thus be correlated, in any two detectors.

In contrast, the rest of the random noise received in a detector should be specific to that detector, and uncorrelated with other detectors. For instance, noise generated from nearby traffic should be different depending on the location of a given detector. By comparing the data in two detectors after accounting for the model-dependent astrophysical sources, the parameters of the primordial background could be teased out.

The researchers tested the new method by first simulating 400 seconds of gravitational-wave data, which they scattered with wave patterns representing astrophysical sources such as merging black holes. They also injected a signal throughout the data, similar to the persistent hum of a primordial gravitational wave.

They then split this data into four-second segments and applied their method to each segment, to see if they could accurately identify any black hole mergers as well as the pattern of the wave that they injected. After analyzing each segment of data over many simulation runs, and under varying initial conditions, they were successful in extracting the buried, primordial background.

“We were able to fit both the foreground and the background at the same time, so the background signal we get isn’t contaminated by the residual foreground,” Biscoveanu says.

She hopes that once more sensitive, next-generation detectors come online, the new method can be used to cross-correlate and analyze data from two different detectors, to sift out the primordial signal. Then, scientists may have a useful thread they can trace back to the conditions of the early universe.

## Primordial Black Holes and Origin of the Universe

The experimental device could detect and capture the small black holes through the gravitational waves they emit in the process of merging, which turns out to contain higher levels of frequencies than the currently available ones, reports SciTech Daily.

The gravitational wave antenna that detects the small-scale black holes is comprised of a metal cavity combined with an external magnetic field. The device generated electromagnetic waves in its cavity when the gravitational wave passed through the magnetic field. This entire process lets the gravitational wave push the cavity and emit reverberating microwaves instead of a soundwave, comparable to a hiss.

The device scales to a few meters but is enough to capture fusions of primordial black holes. The detection of the smaller versions of the black hole located millions of light-years from Earth is much more effective and compact in contrast with the presently available detectors, including KAGRA, LIGO, and Virgo, all of which are massive in size.

The patented idea for the device is now in the advanced theoretical modeling phase. Even if it is not complete yet, the device's necessary pieces of equipment are now ready along with the additional studies needed for a more concrete phase as it undergoes prototype construction, reports EurekAlert.

Once the device is completed, it will be a gateway to fundamental research on the origins of our universe through the detection of the gravitational waves emitted by primordial black holes during the Big Bang.

Check out more news and information on Space on Science Times.

## Big Boost for the Big Bang

T ime was, a picture of an infinitely tiny point could have been described with a simple caption: “The universe, actual size.” That’s clearly not the case anymore, and it’s close to unanimously accepted that what changed everything was a primal explosion known as the Big Bang, which occurred 13.8 billion years ago. Now a single observation has all but nailed down the Big Bang, eliminating the few other remaining scientific theories about how the universe began. In the bargain, it has also at last confirmed the existence of what are known as gravitational waves and the inflationary universe.

Gravitational waves were first described by Albert Einstein, who 99 years ago envisioned all space-time as a sort of cosmic fabric that could be warped and jiggled the way a trampoline can be set shaking by a dropped bowling ball. It was an elegant theory, but no one in the past century had been able to prove it. The inflationary universe was theorized in the 1980s by physicists who calculated that in the first billionth of a trillionth of a quadrillionth of a second after the Big Bang, the universe expanded so rapidly, it actually exceeded the speed of light.

If the right kind of jiggling could be spotted, it would prove both gravitational waves and the inflationary universe and buttress the Big Bang in the process. That’s exactly the observation made by a team of researchers headed by astrophysicist John Kovac of the Harvard-Smithsonian Center for Astrophysics.

“When I got the call, I had to ask if it was real,” says Marc Kamionkowski, a theorist at Johns Hopkins University who was not part of the Kovac group. Avi Loeb, chair of the Harvard astronomy department and also not involved in the study, believes that if the discovery holds up, “it’s worth a Nobel.”

Kovac and his colleagues didn’t see jiggling. What they saw instead, with the help of the Background Imaging of Cosmic Extragalactic Polarization 2, or BICEP2, instrument at the South Pole, was a distortion in the microwave radiation that pervades the cosmos. That seems innocuous, but it’s like seeing ripples in a pond, and these particular ripples were powerful enough that they likely came from an inflationary universe (check) that produced gravitational waves (check), which were set in motion by the Big Bang (check).

The work still has to be replicated by other researchers, which is always the case with science–especially science of this magnitude. But that validation should come quickly. Cosmologists at Princeton, Berkeley, the University of Minnesota and elsewhere were already doing similar work, and they plan to stay at it, now trying to confirm the Harvard-Smithsonian findings as opposed to making the discovery on their own.

Whether it’s correct or incorrect “will be known very quickly,” says Kamionkowski. When it is known–and when the findings are likely confirmed–the world will not change in the same way it did when smallpox was eradicated or the airplane was invented. But the universe–the entire 13.8 billion-year-old universe–will all at once become a more rational and fathomable place. Not a bad haul for a single observation.

A telescope at the south pole, called Bicep (Background Imaging of cosmic Extragalactic Polarisation), has been searching for evidence of gravitational waves by detecting a subtle property of the cosmic microwave background radiation. This radiation was produced in the big bang. It was originally discovered by American scientists in 1964 using a radio telescope and has been called the "echo" of the big bang. Bicep has measured the large-scale polarisation of this microwave radiation. Only primordial gravitational waves can imprint such a pattern, and only then if they have been amplified by inflation.

The big bang was originally hypothesised by Belgian priest and physicist Georges Lemaître. He called it "the day without yesterday" because it was the moment when time and space began.

But the big bang does not fit all astronomers' observations. The distribution of matter across space is too uniform to have come from the big bang as originally conceived. So in the 1970s, cosmologists postulated a sudden enlargement of the universe, called inflation, that occurred in the first minuscule fraction of a second after the big bang. But confirming the idea has proved difficult. Only inflation can amplify the primordial gravitational wave signal enough to make it detectable. If primordial gravitational waves have been seen, it means that inflation must have taken place.

## New technique could uncover gravitational echoes of the Big Bang

Researchers can now sift through the astrophysical noise to see the conditions of the early Universe.

Published: 10th December, 2020 at 15:52

First detected in 2015, gravitational waves have helped scientists understand the development of our Universe from the Big Bang onwards. Gravitational waves are oscillations in spacetime which move like a ripple in water when you skim a stone – if you could skim a stone at the speed of light.

Now researchers at MIT have created a technique which could help detect the fainter signals thought to represent the Universe’s earliest gravitational waves, created only moments after the Big Bang.

While the ripples from more recent history are detected daily, the primordial waves are too faint for current detectors to pick up. Interpreting these early ripples will help us piece together the beginning of the Universe, improving our understanding of the Big Bang.

“These primordial gravitational waves can then tell us about processes in the early Universe that are otherwise impossible to probe,” said Sylvia Biscoveanu, the graduate student who led this development.

Currently, these faint signals are hidden beneath the noise of the “astrophysical foreground” which includes noisy events like colliding black holes and neutron stars from later in the Universe’s history. To uncover the hidden hum of these signals, previous methods have attempted to subtract the drowning noise of more recent signals, but Biscoveanu recognised they were limited by crude estimations of the foreground noise.

“The analogy I like to make is, if you’re at a rock concert, the primordial background is like the hum of the lights on stage, and the astrophysical foreground is like all the conversations of all the people around you,” Biscoveanu said.

“You can subtract out the individual conversations up to a certain distance, but then the ones that are really far away or really faint are still happening, but you can’t distinguish them. When you go to measure how loud the stagelights are humming, you’ll get this contamination from these extra conversations that you can’t get rid of because you can’t actually tease them out.”

Biscoveanu’s team at MIT created models which simulate these ‘conversations’ and predict their weaker gravitational wave signals. They quantified the uncertainty in their measurements and included this in the characterisation of each pattern. When testing their simulation, they were able to distinguish between the foreground and the softer echo of early gravitational waves.

Improvements made on current detectors will increase their sensitivity to hopefully detect these further-off waves. Biscoveanu says she hopes these will be used alongside this new technique to tease out the details of early gravitational waves and colour our understanding of the Universe’s early stages.

#### Reader Q&A: What would happen if a very strong gravitational wave passed through us?

Asked by: Thomas S Marcotte, USA

Gravitational waves spread out from any violent event involving matter – such as, say, the collision of two black holes. Like gravity, however, they’re incredibly weak, so you’d have to be extremely close to their source in order to feel their effects.

It would definitely feel weird, as they’d create a rhythmic stretching and squashing sensation on the body. But you’d have to be so close to the cataclysm itself that you’d never live to describe it.

## The Search for Gravitational Waves

Think of it as a low hum,  a rumble too deep to notice without special equipment. It permeates everything—from the emptiest spot in space to the densest cores of planets. Unlike sound, which requires air or some other material to carry it, this hum travels on the structure of space-time itself. It is the tremble caused by gravitational radiation, left over from the first moments after the Big Bang.

Gravitational waves were predicted in Albert Einstein’s 1916 theory of general relativity. Einstein postulated that the gravity of massive objects would bend or warp space-time and that their movements would send ripples through it, just as a ship moving through water creates a wake. Later observations supported his conception.

The imprint of this type of radiation on the oldest light in the universe—the cosmic microwave background (CMB)—is one prediction of inflation theory, which was first proposed in 1979. That theory states that the universe, originally chaotic quantum noise made of unstable particles and space-time turbulence, expanded at an unimaginable rate, creating these gravitational waves, smoothing out the chaos, and leaving the orderly cosmos we see today.

“Gravitational waves allow us to see all the way back to the start to the universe,” says Katherine Dooley, a postdoctoral researcher at the California Institute of Technology in Pasadena. “The early universe was too dense such that standard electromagnetic waves”—light—“would get scattered off of all the material, and could not travel to us today.” Observing these gravitational waves might confirm what we know about general relativity, or they might give us new insight into the nature of the universe, like whether the Big Bang was the beginning of all time, or if another universe preceded ours. The story of the universe’s origin is best told through this primordial rumble…if we can figure out how to detect it. A few gravitational wave observatories have been built—none has yet detected a wave—and more are planned over the next few decades. It’s an exciting time for astronomers, who may soon have real evidence on which to ground this new branch of one of the oldest scientific disciplines.

A team in Germany stands in front of LISA Pathfinder, a test mission scheduled to launch this year. (ESA) When gravitational waves pass through a tunnel-like observatory, they should change the distance between mirrors at the tunnel’s opposite ends. (Enrico Sacchetti) A ski-equipped LC-130 passes observatories in Antarctica, such as BICEP2 (center), looking at the first light from the universe. (Steffen Richter/Harvard) The Einstein Telescope (artist rendering) is a proposal for a gravitational wave observatory that will be 10 times more sensitive than the current U.S. observatories. (CERN) A 2.5-mile arm stretches out from the gravitational-wave observatory in Washington state. (LIGO Hanford Observatory) How gravitational waves might emanate from a white dwarf binary. (Sterl Phinney/CalTech) At LIGO’s second facility, in Louisiana, engineers inspect the tunnels. (LIGO Livingston Observatory)

Practically every action makes gravitational waves—you can create them by waving your arms—but it takes serious astronomical doings to generate anything powerful enough to be detected. Earth orbiting the sun produces them, but they are low energy (which is good for the long-term stability of our solar system) two pulsars, the ultra-compact remnants of massive stars, locked in binary orbit produce far more substantial waves. As those bodies sweep around each other, they compress and expand the structure of space-time itself, creating a disturbance that travels out at the speed of light.

Gravitational waves from binaries like this are regular, like a pure note from a single string of an instrument. In principle we could trace such a signal back to its source, though, as with sound, triangulation is less precise than for light. Primordial radiation, on the other hand, comes from every place at once, since it was produced everywhere, when the universe was much smaller, and traveled in all directions from where it was created. The ultimate sources were tiny fluctuations in the quantum chaos that was the cosmos right after the Big Bang the gravitational ripples created by the fluctuations stretched out when the universe expanded rapidly into large, solar system-spanning waves.

In the pipe organ that is the gravitational-wave universe, inflation would be the longest, largest pipes, producing sounds so low-pitched they are felt rather than heard. Binary pulsars would lie toward the middle register, and violent catastrophes like supernovas or cosmic collisions would be the short, piccolo pipes. “Hearing” each type of wave requires equipment tuned to the appropriate register.

The principle of detection is simple: As gravitational waves pass by, they massage matter, squeezing and stretching it along the waves’ crests and troughs. The effect of the wave is recorded as its “strain” on the detector, though that strain is tiny by everyday standards. So far, nobody has detected gravitational waves directly, though indirect detections abound. The most famous is the Hulse-Taylor binary pulsar, named for the two researchers who discovered it 40 years ago. Russell Hulse and Joseph Taylor earned a Nobel Prize in physics for their observation that the two pulsars were orbiting closer and closer together, and the energy leaving the system as the orbit decayed was the same as what the general theory of relativity predicted would be lost due to gravitational waves.  Since then, other astronomers identified even stronger gravitational wave sources, including a pair of white dwarfs—the last life stage of stars less massive than the sun—which take just 12 minutes to orbit each other.

But this indirect detection isn’t satisfying: Astronomers want to detect the waves themselves. “As with all new windows you open on the universe, there’s going to be things we’re going to find [with gravitational waves] and we have no idea what the hell they are,” says Matt Benacquista of the University of Texas at Brownsville. Historically, every new type of astronomical observation, from radio waves to gamma rays, has led to unexpected discoveries, and gravitational waves are likely to be no different. “That in many ways is the most exciting part about doing this,” says Benacquista.

Yet for a number of reasons, the problem of direct detection is vexing. Like sound, gravitational waves are comparable in size to whatever produces them. Large systems, like big black holes orbiting each other at the centers of galaxies, will make very-long-wavelength, low-frequency waves, which require suitably huge detectors. Even relatively small sources, such as a pair of pulsars very close to collision, require detectors measuring more than a mile across. If they exist, primordial gravitational waves from inflation would exist at a wide range of wavelengths, but only extremely long ones—those with a wavelength comparable in size to Earth’s orbit around the sun—would provide a large enough signal for astronomers to detect.

Technically we are bathed in the “sound” of gravitational radiation all the time, but the sound is faint and usually too low-pitched. Gravity is by far the weakest of the four fundamental forces of nature, and its influence grows smaller with distance, so when a gravitational wave—even a relatively powerful one—passes by, very little energy gets transferred. To make matters more difficult, since the effect on matter is to push it around, detectors on the ground must deal with interference by anything that could make them vibrate, from earthquakes to big trucks rumbling by.

So observatories must be large, sensitive to faint signals, and isolated as completely as possible from any stray vibrations. That’s a tall order, solved best by building multiple observatories or launching detectors into space. Scientists, being resourceful creatures, are doing both.

Katherine Dooley got hooked  on gravitational radiation research during a summer undergraduate fellowship at Caltech, which, with the Massachusetts Institute of Technology, operates the two Laser Interferometer Gravitational-wave Observatories (LIGO) in the United States: one in Richland, Washington, and the other in Livingston, Louisiana. For her doctoral dissertation, she spent four years designing the apparatus in Livingston to be more sensitive through the use of more laser power. She moved to Hannover, Germany, to do her postdoc research at GEO600, a gravitational wave observatory that began operation in 2002. She’s now back working with LIGO, just in time for the inauguration of the upgrades she helped institute.

With her experience in detector design, Dooley understands the challenge of gravitational wave observation better than most people. Ground-based observatories like LIGO and GEO600 are similar: powerful laser beams travel down two “arms,” at the ends of which the light strikes a mirror, which reflects it back to its source. When a gravitational wave passes by, the mirror should move, changing the distance the light travels ever so slightly. By running its two observatories simultaneously, LIGO can better eliminate local disturbances—when a gravitational wave passes through Earth, both observatories should feel it. The size of gravitational wave sources necessitates long arms: At the LIGO facilities the arms are four kilometers (two and a half miles) long GEO600’s are 600 meters (hence the name).

Benacquista is the kind of gravitational wave astrophysicist who prefers to take notes with a fountain pen. Like Dooley, he has worked with two observatory projects, one of which was LIGO, from 2006 to 2013, albeit from the theoretical side: He characterizes the sources of gravitational waves that detectors might see. In 1995, a summer research program at NASA’s Jet Propulsion Laboratory connected him with an exciting project just getting started, the Laser Interferometer Space Antenna (LISA). As the name suggests, LISA will be a space-based observatory designed to orbit the sun, made of three small spacecraft in a V-formation, each separated by a million kilometers. The basic operation is similar to LIGO: By measuring the distance between each spacecraft using laser light, researchers can detect a gravitational wave as it compresses or expands the space-time between the spacecraft.

Conceived as a joint project between NASA and the European Space Agency, LISA was originally projected to launch between 2012 and 2016. However, NASA withdrew participation in 2011, leaving the very expensive project entirely up to ESA. By cutting back on the ambition a bit, the project survived as European LISA, or eLISA, but now the launch date is 2034, which is far enough in the future to make any forecasts doubtful.

“I’m still kind of pessimistic about LISA,” Benacquista says. He’s hopeful that when LIGO detects the signal from colliding neutron stars in the next five years or so, the LISA launch might get pushed up by a few years, but that still doesn’t place it in the next decade. “Hopefully I’ll still be alive!” he laughs ruefully.

On track, however, is the LISA demonstration mission slated to launch later this year. Called LISA Pathfinder, it will test the instrumentation and physical concepts the observatory will use: lasers and masses, which are, like the mirrors on LIGO, designed to move independently of the spacecraft. Additionally, the GRACE Follow-On (Gravitational Recovery And Climate Experiment) mission, targeted to launch in 2017, will observe tiny fluctuations in Earth’s gravitational field by measuring the distance between two independently flying spacecraft, just as LISA will do. The mission is a follow-up to the previous GRACE and GRAIL (Gravity Recovery And Interior Laboratory) probes, which performed the same duty for the moon in 2012. To gravitational wave researchers, the successes of these missions, coupled with the delays on LISA, are a simultaneous joy and frustration.

“Bicep2 is an experiment that aims  to do just one thing and do it well,” says Walt Ogburn, a cosmologist at Stanford University who spent the summer of 2009-2010 at the South Pole installing the telescope. That one thing BICEP2 was designed to do: measure the polarization of the cosmic microwave background. The CMB comes from a time when the universe cooled enough to become transparent, about 380,000 years after the Big Bang. Similar to the way polarizing sunglasses reduce glare by filtering light, various cosmological objects—big galaxies, cosmic dust molecules, and gravitational waves—filter the cosmic background radiation in interesting ways.

“Since these fluctuations [waves] are in space-time, they stretch or compress space—and particles—as they pass,” says Renée Hložek, a postdoc at Princeton University involved with the Atacama Cosmology Telescope polarization project, or ACTPol, in Chile, another experiment to measure the polarization of the CMB. The particles Hložek refers to include photons, the particles of light. Because gravitational waves squeeze space-time in one direction and stretch it in the other (something known as tensor modes), Hložek says, “the pattern of polarization induced from these gravitational waves is very specific.” Experiments like BICEP2 and ACTPol are trying to confirm inflation theory by discovering the primordial gravitational waves such rapid expansion would have created they are observing the light—the CMB—the waves should have polarized.

But while these observatories are very good at measuring polarization, they can’t tell us exactly what is causing it. In March 2014, researchers with BICEP2 announced they detected the polarization—evidence of gravitational waves—and thus confirmed inflation theory. Stanford University even released a moving video showing professor and BICEP2 researcher Chao-Lin Kuo bringing news of the experiment’s results to Andrei Linde, one of the most influential authors of inflation theory. Chao-Lin surprised Linde at his home with champagne to toast the “smoking gun” that proved Linde’s life’s work to be true. The excitement turned out to be premature. More ordinary phenomena—such as dust particles in the Milky Way—can polarize light in a similar way, and after follow-up study by the European Space Agency’s Planck spacecraft, the BICEP2 team revised its initial findings to say that it’s possible the entire signal was caused by cosmic dust.

A successor experiment, BICEP3, installed in Antarctica early this year, along with ACTPol and other studies, will help by adding more data, but gravitational wave signals might still hide from such detection. That brings us back to the question of direct detection. As Walt Ogburn points out, the Big Bang Observer would be able to settle the issue of inflation for good. A possible follow-up to LISA, the project as initially proposed would consist of 12 satellites in three groupings that would orbit around the sun. The vast scale would provide the ability to measure gravitational radiation with wavelengths comparable to the size of the solar system—what we would expect from inflation.

Not only could the Observer confirm results from polarization observatories, it also could discover things about the fundamental structure of the cosmos. As Ogburn points out, these early waves “also represent new physics at energies a trillion times higher than what we can reach at the [Large Hadron Collider].” They could even help settle one of the looming conundrums in modern physics: understanding the quantum mechanical properties of gravity.

Even though LIGO and GEO600 are remarkably sensitive instruments, they are simply too small to observe primordial gravitational waves. Their mission is elsewhere: detecting waves caused by colliding black holes, neutron stars, and other relatively compact objects that pack a lot of energy. And as large as LISA will be, its million-kilometer arms will still be too short for the largest gravitational waves. The Big Bang Observer is currently the best hope we have, and it is far in the future.

Like other gravitational wave researchers, Katherine Dooley and Matt Benacquista are philosophical about the lack of direct detections so far. Gravitational wave research is difficult, and the waves that would be easiest to detect because they’re the most common—those from binary pulsars and black holes—could be detected only by bigger detectors than we can build on Earth’s surface.

For that reason, nobody in the field was really surprised when the first iterations of LIGO didn’t see anything. Each phase of LIGO was always intended to be a learning process, much as engineers build and test many rocket prototypes before settling on a design to launch valuable payloads aboard. The deepest concern now is “noise hunting,” says Dooley, identifying all the environmental and technical disturbances that could get in the way of being able to see a clear signal from a gravitational wave when one comes along.

Advanced LIGO, the version with Dooley’s upgrades that began operation last February, has ten times the sensitivity of the original experiment. In practical terms, that means it can “hear” ten times as far, which represents a volume of the universe that is a thousand times greater. In that pocket of space, says Dooley, signals from colliding neutron stars “could be as infrequent as once a year or once every other year, or even as frequent as almost every day.” If Advanced LIGO detected one gravitational wave signal per month, that would be enough to keep researchers busy and happy for some time.

When astronomers finally detect gravitational waves, what doors to our understanding of the universe will open? Benacquista doesn’t mind not knowing in advance: “That’s one of the things I really like about astrophysics. It’s like a game where you’ve been told the conclusion to a story, and now you have to invent the story that got you to that point.” With every new field in astronomy, scientists discovered something unexpected: radio astronomy led to pulsars, X-ray astronomers found black holes, microwave antennas detected the cosmic microwave background. If this first generation of gravitational researchers at last hears the rumble of the first moments of the universe, they may find themselves, thrillingly, at the beginning again.