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

Michael Lam, an astronomer at the SETI Institute, told ABP Live that the evidence of gravitational waves across multiple frequencies is as if scientists are listening to a background hum.

In a breakthrough, 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. 

A pulsar is a rotating neutron star, or an ultra-dense remnant of a massive star's core after the star's death in a supernova. This rotating neutron star emits regular pulses of radiation at its spin rate, including radio waves. These waves appear to "pulse" when seen from Earth. Millisecond pulsars are the fastest pulsars. These spin hundreds of times each second, and are very stable. Therefore, they act as cosmic timepieces. 

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.

Professor Lam explained that based on what scientists know about the universe, supermassive black hole binaries are the most likely candidate for the source of gravitational waves, but there is not much statistical evidence to say what the sources are. “We find evidence of the gravitational wave background but cannot definitively say what the source(s) of that background are. While the most likely candidate is from merging supermassive black hole binaries based on what we know about the Universe, we do not have enough statistical power in our data to say what the sources are, just that it is there.”

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. 

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