How AI can make us understand our universe better, faster

 International collaborations using telescopes in Europe, India (uGMRT, the country’s largest telescope, is operated by the Pune-based National Centre for Radio Astrophysics) Australia and China independently reported similar results.

But what are gravitational waves and why study them? To study the universe, scientists have typically relied on electromagnetic (EM) radiation (visible light, X-rays, radio waves, microwaves, etc.) while some have also used subatomic particles called neutrinos. But EM astronomers find it very tough to detect things like colliding black holes because EM radiation can be absorbed, reflected, refracted, or even bent by gravity.

Gravitational waves, which interact very weakly with matter, do not face these problems and hence do not distort information as they travel through space. They were predicted by Albert Einstein in 1915 in his General Theory of Relativity that describes space and time as a fabric, which will sense ‘ripples’ if any object dents it.

In 1993, two astronomers—Russell Hulse and Joseph Taylor—received the Nobel Prize in Physics “for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation". On 14 September 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO), supported by the National Science Foundation and operated jointly by Caltech and the Massachusetts Institute of Technology (MIT), reported the first detection of gravitational waves generated by two colliding black holes 1.3 billion light years away.

The gravitational waves that LIGO detects is the release of energy caused by cataclysmic events in the Universe—colliding black holes, merging neutron stars, exploding stars, and possibly even the birth of the Universe itself. You may read more about this here (https://www.ligo.caltech.edu/page/gravitational-waves).

But what has artificial intelligence (AI) got to do with gravitational waves? The humongous amounts of data gathered by telescopes around the world need to be analyzed speedily to be leveraged by the scientific community, and it’s here that AI models are being used. For instance, the Gravitational-Wave Open Science Center (GWOSC) provides public access to released LIGO/Virgo data. The site includes tools and tutorials for analyzing LIGO data.

But AI models can do so much more. AI algorithms can speedily identify and filter out noise signals from the data, significantly accelerating the process of discovering and confirming new gravitational wave events.

In December 2017, Eliu A Huerta and Daniel George, theoretical astrophysicist and computational astrophysicist at the University of Illinois at Urbana-Champaign’s National Center for Supercomputing Applications, respectively, proposed the use of deep convolutional neural networks (CNNs) to detect and characterize gravitational wave signals in real time as opposed to conventional techniques that could take several days to narrow down the features of gravitational events from detector data. Their new method called Deep Filtering was demonstrated using simulated LIGO noise. They published their findings (https://arxiv.org/abs/1711.07966) in the journal Physics Letters B.

Four months later, in April 2018, researchers at the UK-based University of Glasgow explored the use of supervised (involves human moderation) deep learning to improve the efficacy of the the process of detection of gravitational waves. The idea was to develop an AI model capable of accurately identifying gravitational wave signals buried in noise from thousands of simulated datasets which they created. The study was published in the journal Physical Review Letters.

In July 2021, Argonne National Laboratory computational scientist Eliu Huerta partnered with the University of Chicago, the University of Illinois at Urbana-Champaign, and technology companies NVIDIA and IBM, to develop a new AI model to detect gravitational waves. The new AI model, according to a paper in Nature (https://www.nature.com/articles/s41550-021-01405-0), is orders of magnitude faster and can run on graphic process units (GPUs) to process data in real-time.

In their paper, the researchers explained that they developed a workflow that connects the Data and Learning Hub for Science--a repository for publishing AI models--with the Hardware-Accelerated Learning (HAL) cluster, using (funcX) a universal distributed computing service. “Using this workflow, an ensemble of four openly available AI models can be run on HAL to process an entire month’s worth (August 2017) of advanced LIGO data in just seven minutes, identifying all four binary black hole mergers previously identified in this dataset and reporting no misclassifications".

In their paper, the researchers explained that they developed a workflow that connects the Data and Learning Hub for Science--a repository for publishing AI models--with the Hardware-Accelerated Learning (HAL) cluster, using (funcX) a universal distributed computing service. “Using this workflow, an ensemble of four openly available AI models can be run on HAL to process an entire month’s worth (August 2017) of advanced LIGO data in just seven minutes, identifying all four binary black hole mergers previously identified in this dataset and reporting no misclassifications".

It’s only a matter of space and time before AI increasingly partners with humans to help us understand more of the universe.

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Scientists Find First Evidence Of Gravitational Waves At Very Low Frequencies, Notice A 'Hum' Linked To Cosmic Clocks

 

ABP Live spoke to Professor Michael Lam, an astronomer at the SETI Institute, and one of the scientists involved in the research, and asked him about the cosmic hum. 

In a breakthrough achievement, scientists have found the first evidence of gravitational waves at very low frequencies. While Albert Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity, these ripples in space-time caused by violent and energetic processes in the universe were discovered in 2015. The Laser Interferometer Gravitational-Wave Observatory (LIGO), which is operated by the California Institute of Technology and the Massachusetts Institute of Technology, physically sensed undulations in space-time caused by gravitational waves on September 14, 2015. About eight years later, researchers confirmed the first observation of very low frequency gravitational waves. 

Scientists speculate that these gravitational waves are responsible for undulations in the radiation emitted by pulsars, which are cosmic clocks. These gravitational waves across multiple frequencies have been described as a background "hum". 

The results recently appeared in a set of papers published in The Astrophysical Journal Letters. Fifteen years of data collected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) Physics Frontiers Center (PFC) has helped determine the occurrence of the gravitational wave signal. The collaboration involves more than 190 scientists from the United States and Canada who use pulsars to search for gravitational waves. Other international collaborators include researchers from India, Europe, Australia and China. 

The gravitational waves are associated with changes in pulsar signals

NANOGrav had earlier found a mysterious timing signal common to all the pulsars they observed, but the signal was too faint to reveal its origin. The fifteen-year-data collected by NANOGrav has shown that the timing signal is consistent with slow gravitational waves passing through the Milky Way. This means that these gravitational waves are responsible for a change in the timings of the radiation emitted by the pulsars observed. 

NANOGrav had earlier found a mysterious timing signal common to all the pulsars they observed, but the signal was too faint to reveal its origin. The fifteen-year-data collected by NANOGrav has shown that the timing signal is consistent with slow gravitational waves passing through the Milky Way. This means that these gravitational waves are responsible for a change in the timings of the radiation emitted by the pulsars observed. 

Quoting Dr Stephen Taylor, the current chair of NANOGrav PFC, a statement released by the collaboration said the 15-year-data on pulsars is key evidence for gravitational waves at low frequencies. 

How were the low-frequency gravitational waves observed?

While LIGO, a ground-based observatory, observed high-frequency gravitational waves, continuous low-frequency signals could be perceived only with a detector much larger than Earth. Therefore, astronomers used pulsars to make a huge gravitational-wave antenna. Over 15 years, NANOGrav has collected data from 68 pulsars. Together, these exotic cosmic clocks formed a detector known as a pulsar timing array, NANOGrav said.

Observing these pulsars has been possible with the help of the Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia, and the Very Large Array in New Mexico, among other telescopes and observatories. 

In the statement, Dr Maura McLaughlin, the co-Director of the NANOGrav PFC, said since pulsars are very faint radio sources, thousands of hours a year on the world's largest telescopes are required to carry out the experiment. 

How can gravitational waves affect pulsar signals? 

The theory of general relativity accurately predicts how gravitational waves can affect pulsar signals. Gravitational waves can increase or decrease the timings of the pulses of a pulsar by stretching and squeezing the fabric of space in a small but predictable manner. The distance between two stars can determine how the timings of the pulses are affected. 

In the statement, Dr Xavier Siemens, co-Director of the NANOGrav PFC, said it has been possible to see the first signs of the correlation pattern predicted by general relativity due to the large number of pulsars used in the NANOGrav analysis.

The researchers who are a part of NANOGrav looked for the pulsars precise enough to help search for low-frequency gravitational waves. 

What is the cosmic “hum” described in the papers?

NANOGrav researchers started to see hints of the cosmic hum in 2020. "Hum" has been used to describe gravitational waves which affected the timing behaviour of all the pulsars in the array. With the help of 15 years of pulsar observations, astronomers have shown the first evidence for the presence of very low-frequency gravitational waves, which have periods of years to decades. 

ABP Live spoke to Professor Michael Lam, an astronomer at the SETI Institute, and one of the scientists involved in the research, and asked him about the cosmic hum. 

Explaining the contributions of different international collaborators to the detection of low-frequency gravitational waves, Professor Lam said: "Each collaboration analyzed their data sets individually. We report on various levels of significance towards low-frequency gravitational waves". 

He explained that we cannot "see" gravitational waves as we can electromagnetic waves but they can still be directly observed, and that the evidence of gravitational waves across multiple frequencies is as if astronomers are listening to a background hum. "We find evidence of gravitational waves across multiple frequencies and sustained across the timespan we are observing. Therefore, it is as if we are listening to a background hum."

What generated these low-frequency gravitational waves?

According to NANOGrav researchers, the possible source of the gravitational waves is a pair of supermassive black holes, whose masses are millions or billions of times that of the Sun, and are found at the centres of the largest galaxies in the universe. In a black hole binary, the two black holes orbit each other, and produce low-frequency gravitational waves. Such binaries are formed when two galaxies with supermassive black holes at their centres merge, and cause the two black holes to reach the centre of the newly-combined galaxy. After several years, the two black holes will combine. Due to the interaction between the two black holes before the eventual merger, gravitational waves are generated. It is speculated that this is how very low-frequency gravitational waves reached the Milky Way, and were detected by the pulsar timing array. 

Professor Lam told ABP Live that supermassive black hole binaries are the most probable cause behind these very low-frequency gravitational waves because a supermassive black hole is present at the centre of every major galaxy in the universe, and galaxies merge. "We know that supermassive black holes exist throughout the Universe, we know that every major galaxy has a supermassive black hole at the center, and we know that galaxies merge. Therefore, supermassive black hole binaries are the most probable cause."

Gravitational wave signals may overlap, leading to a background “hum”

According to NANOGrav, similar to voices in a crowd or musical instruments, gravitational wave signals from supermassive black hole binaries are expected to overlap, resulting in an overall background "hum". It is due to this hum that astronomers observed a unique pattern in pulsar timing data, something NANOGrav researchers have been seeking for about 20 years. 

Quoting Dr Luke Kelley, chair of NANOGrav's astrophysics group, the statement said that at one point, scientists were concerned that supermassive black holes in binaries would orbit each other forever, never coming close enough together to generate a signal like this, but now they finally have strong evidence that many of these extremely massive and close binaries do exist. Dr Kelley explained that once the two black holes get close enough to be seen by pulsar timing arrays, nothing can stop them from merging within just a few million years. 

According to NANOGrav, the supermassive black hole binaries at the cores of major galaxies produce electromagnetic waves ranging from radio waves to gamma rays. Telescopes on Earth and in space can detect these electromagnetic waves. The gravitational waves emitted by black hole binaries can be observed through their effects on pulsars.

Significance of the study

Scientists study the subatomic world with the help of the Standard Model of particle physics. The last missing particle of this model, the Higgs Boson, was discovered in 2012. Though the Standard Model of particle physics successfully describes all known subatomic particles, it is unable to explain key properties of the universe, including the characteristics of dark energy, a good particle candidate for dark matter, and the origin of the observed asymmetry between matter and antimatter. These three observations indicate that new physics beyond the Standard Model (BSM) exists.

Scientists are testing some proposed BSM models in high-energy laboratories around the world, at particle accelerators such as CERN's Large Hadron Collider (LHC). Several BSM models predict that gravitational waves were generated in the early universe. 

The hum detected by NANOGrav may also contain a contribution from gravitational waves produced in the early universe right after the Big Bang, the collaboration said on its website. 

As part of the research, the scientists considered the BSM models that predict the generation of a gravitational wave background only fractions of a second after the Big Bang. This primordial gravitational wave background has propagated more or less freely through the universe since it was produced. 

The aim of the research was to determine to what degree the primordial gravitational wave background could explain the hum observed by NANOGrav after collecting data from pulsars for 15 years. According to NANOGrav, the gravitational wave background can be thought of as the gravitational analogue of cosmic microwave background radiation, the difference being that the latter, which refers to electromagnetic radiation produced when the universe was as young as 3,80,000 years, was generated much after the gravitational wave background, which was generated much closer to the Big Bang. 

Therefore, by detecting a primordial gravitational wave background, scientists can obtain a direct glimpse into the processes which occurred in the early universe, and which cannot be understood through other means. 

According to NANOGrav, the collaboration's search for primordial gravitational waves allows the researchers to test ideas such as grand unification, which refers to the unification of all subatomic forces in one common super force at extremely high energies. 

By studying the background hum in detail, researchers can also obtain insights into how supermassive black holes grow and merge. The strength of the hum indicates that there could be hundreds of thousands or millions of extremely massive black hole binaries in the universe. 

Analysing the signal can also help scientists understand how the universe evolved, how it was formed, how often galaxies collide, and what factors cause black holes to merge. They may also discover new kinds of exotic particles that exist in our universe. 

NANO Gravitational hopes that in the future, the collaboration will be able to detect gravitational waves emitted by relatively nearby, individual supermassive black hole binaries.

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Gravitational Wave Background: A Very Simple Explanation

 By now you’ll have seen the news about the ground-breaking discovery of a low-frequency gravitational wave background caused—scientists think—by the supermassive black holes that orbit each other for a short while before merging.

We’re told this is big news—a new window to the universe, no less—but to many it will seem both complex and have little meaning to our lives. A tempting conclusion, but this really is incredible—and it’s worth five minutes of your time.

Here’s everything you need to know about the gravitational wave background in simple language:

Gravitational Waves Explained

A gravitational wave is a ripple in space-time caused by a violent event somewhere in the universe. They were predicted by Albert Einstein’s theory of general relativity in 1916. That theory posits that gravity is a symptom of spacetime being warped, twisted and curved by the presence of massive objects, such as stars and planets. However, it also predicted that massive accelerating objects would disrupt space-time in waves traveling out into the universe, in all directions, at the speed of light. The 2016 detection of gravitational waves proved that prediction when Laser Interferometer Gravitational-Wave Observatory (LIGO) physically sensed gravitational waves generated by two colliding black holes 1.3 billion light-years away.

What’s now been discovered is a cosmic background—a web—of long-wavelength gravitational waves. They’re thought to come from supermassive black hole binaries—two supermassive black holes orbiting each other.

A black hole is a region in space where gravity is so intense that nothing can escape—even light. A supermassive black hole weighs billions of times the mass of our sun.

How This Discovery Came About

It’s not just what we know, but how we know it that makes astronomy such a fascinating subject. For the gravitational wave background, that goes double.

Let’s start with a really simple visualization.

Imagine the universe as a lake with kayakers rhythmically paddling around it. Two motor boats are going around in a circle, following each other and sending out ripples that cause the kayaks to wobble—and the kayakers’ rhythmic paddling to be slightly altered. Now have dozens of pairs of speed boats all doing the same thing.

This is akin to what the scientists have seen. They monitored the light from 25 pulsars, the cores of dead stars that rotate every millisecond, sending out highly predictable radio signals. Astronomers call them cosmic clocks. They’re the kayakers’ paddles—and astronomers have noticed slight variations in their rhythms that reveal a web of ripple-like disruptions.

The speed boats, of course, are the supermassive black holes orbiting each other, creating the ripples. What the scientists want to do next is to trace these cosmic ripples—each of which carries information about their origins and clues about the nature of gravity itself—back to the supermassive black hole binaries they’re searching for.

After all, astronomers have yet to confirm the existence of a single supermassive black hole binary.

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Astronomers find evidence of universe's 'background noise', first predicted by Albert Einstein

Astronomers across the world announced on Thursday that they have found the first evidence of a long-theorised form of gravitational waves that create a "background hum" rumbling throughout the universe.

The breakthrough -- made by hundreds of scientists using radio telescopes in North America, Europe, China, India and Australia after years of work -- was hailed as a major milestone that opens a new window into the universe.

First predicted by Albert Einstein more than a century ago, gravitational waves are ripples in the fabric of the universe that travel through everything at the speed of light almost entirely unimpeded.

Their existence was not confirmed until 2015, when the US and Italian observatories detected the first gravitational waves created by two black holes colliding.

These "high-frequency" waves were the result of a single violent event that sends a strong, short burst rippling towards Earth.

But for decades scientists have been searching for low-frequency gravitational waves, thought to be constantly rolling through space like background noise. Joining forces under the banner of the International Pulsar Timing Array consortium, scientists working at gravitational wave det ..

"We now know that the universe is awash with gravitational waves," Michael Keith of the European Pulsar Timing Array told AFP.


- Using dead stars as clocks -

As gravitational waves travel through space, they very subtly squeeze and stretch everything they pass through. To find evidence of this squeezing and stretching at low frequencies, astronomers looked at pulsars, the dead cores of stars that exploded in a supernova.

Some spin hundreds of times a second, flashing beams of radio waves at extremely regular intervals, like cosmic lighthouses. This means they can act as "a very, very precise clock," Keith said.For the new research, radio telescopes around the world were aimed at a total of 115 pulsars throughout the Milky Way.

Scientists then measured the incredibly small differences in the timing of the pulses, searching for telltale signs of gravitational waves. French astrophysicist Antoine Petiteau said they were able to "detect changes of less than one millionth of a second across more than 20 years".

Maura McLaughlin of the US Pulsar Search Collaboratory programme said they were "awestruck" after first seeing evidence of the waves in 2020.

It was "really a magical moment," she told a press conference.

The early evidence was consistent with Einstein's theory of relativity and science's current understanding of the universe, the scientists said. But they emphasised they have not yet definitively "detected" the waves, because they have not reached the gold-standard five sig ..

"We're frustratingly just shy of the mark," Keith said, adding that there was a 99-percent probability that the evidence points to gravitational waves. Each country or group in the consortium published their research separately in a range of journals.

Steve Taylor, chair of North America's NANO Grav gravitational wave observatory, said that once all the data was combined, the five sigma mark could be reached in a year or two.

- 'Like sitting in a noisy restaurant' -

The leading theory is that the waves are coming from pairs of supermassive black holes sitting at the centre of galaxies that are slowly merging. Unlike those that caused the previously detected gravitational waves, these black holes are almost unimaginably huge -- sometimes billions of times bigger than the Sun.

Daniel Reardon, a member of Australia's Parkes Pulsar Timing Array, told AFP that -- if confirmed -- the waves would be "the sum of all of the supermassive black hole bina ..

Keith said the "background hum of all these black holes" was "like sitting in a noisy restaurant and hearing all these people talking".

Another theory is that the gravitational waves could be from the rapid expansion that came within a second after the Big Bang, a period called cosmic inflation that is hidden from the view of scientists. Keith said the galaxies between Earth and the Big Bang were likely "drowning out" such waves.

But in the future, low-frequency gravitational waves could reveal more about this early expansion and possibly shed light on the mystery of dark matter, the scientists said.

It could also help them understand more about how black holes and galaxies form and evolve.

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15 Years of Radio Data Reveals Evidence of Space-Time Murmur

 Scientists have found evidence of a universal background of gravitational waves, or ripples in the fabric of space-time.

The motion of black holes and other massive objects through space can create ripples in the fabric of the universe, called gravitational waves. On June 28 scientists announced the first evidence of a background of long-wavelength gravitational waves that fills the cosmos. These waves are thought to have been created over eons by supermassive black holes, up to billions of times the mass of our Sun, circling each other before they merge. Detecting the gravitational wave background is analogous to hearing the hum of a large group of people talking at a party, without distinguishing any particular voice.

The background ripples detected by NANOGrav could help scientists better understand how gravitational waves are created and what happens to them as they propagate through the universe. They could also be used to study supermassive black hole mergers, events that can last for millions of years. Scientists think these mergers happen in most galaxies and influence their evolution.

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) presented the evidence in a series of papers published in the Astrophysical Journal Letters. NANOGrav is a National Science Foundation-funded Physics Frontiers Center of more than 190 scientists from the United States and Canada, including scientists at NASA’s Jet Propulsion Laboratory in Southern California and other NASA centers. The collaboration has spent more than 15 years collecting high-precision data from ground-based radio telescopes, looking for these gravitational waves.

The discovery complements the first-ever detection of gravitational waves in 2015 by LIGO, the Laser Interferometer Gravitational Observatory. Those signals, at a much shorter wavelength than the new discovery, were from black holes about 30 times the mass of our Sun.

NASA is contributing to the ESA (European Space Agency)-led Laser Interferometer Space Antenna mission, a future space-based observatory that will detect gravitational waves that are in a wavelength range between those detected by NANOGrav and LIGO.

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