Astronomers Might Have Detected the Background Gravitational Waves of the Universe

In February 2016, Gravitational Waves (GWs) were detected for the first time in history. This discovery confirmed a prediction made by Albert Einstein over a century ago and triggered a revolution in astronomy. Since then, dozens of GW events have been detected from various sources, ranging from black hole mergers, neutron star mergers, or a combination thereof. As the instruments used for GW astronomy become more sophisticated, the ability to detect more events (and learn more from them) will only increase.

For instance, an international team of astronomers recently detected a series of low-frequency gravitational waves using the International Pulsar Timing Array (IPTA). These waves, they determined, could be the early signs of a background gravitational wave signal (BGWS) caused by pairs of supermassive black holes. The existence of this background is something that astrophysicists have theorized since GWs were first detected, making this a potentially ground-breaking discovery!

As Einstein predicted with his Theory of General Relativity, gravitational waves are generated when two or more massive objects merge (black holes, neutron stars, etc.), causing ripples that are detectable many light-years away. In some cases, these ripples may result from galactic mergers, including the Supermassive Black Holes (SMBHs) at their cores or from events occurring soon after the Big Bang. Ever since the first GW event was detected, scientific consortiums worldwide have been looking for signs of this gravitational wave background (GWB).

For example, the International Pulsar Timing Array (IPTA), the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array in Australia (PPTA) use Millisecond Pulsars (MSPs) as a system of Galactic clocks. These stellar remnants are neutron stars that spin hundreds of times per second and have tremendously powerful magnetic fields – which focus their electromagnetic emissions along the poles.

This energy is emitted as pulsing beams of radio waves (hence their name) that sweep across space to create a strobing (or “lighthouse”) effect. For years, astronomers have used this effect for time-keeping since their pulses are extremely consistent over long periods. At the same time, their strobing light has been used to measure astronomical distances and probe the interstellar medium (ISM). With the birth of GW astronomy, these consortia are now using pulsars to probe the Universe for signs of background GWs.

This comes down to using their observatories to look for disturbances in the sweeps of pulsar beams, which are attributed to passing gravitational waves. Recently, these consortia have come together to combine data sets, including the IPTA’s new data release – Data Release 2 (DR2). This consists of precision timing data from 65-millisecond pulsars, neutron stars that spin hundreds of times per second.

“The GBT contributes to the IPTA as one of the most important telescopes used by c,” said Dr. Ryan Lynch, a Green Bank Observatory scientist and a NANOGrav member. “The combination of the GBT’s excellent sensitivity, instruments, and ability to see so much of the sky make it a critical part of the IPTA’s efforts,”

Analysis of the IPTA DR2, combined with independent data sets from the other collaborations, revealed strong evidence for this low-frequency gravitational wave signal – as indicated by many pulsars. The characteristics of this signal were consistent with what astrophysicists expected to see from a gravitational wave background (GWB). This background is formed by many overlapping GW signals caused by a cosmic population of supermassive black holes that orbit each other (binary SMBHs) and eventually merge.

This GWB is analogous to background noise in a crowded room and is reminiscent of the Cosmic Microwave Background (CMB), the remnant radiation left over from the Big Bang. These results not only strengthen the case for the existence of a GWB, something astronomers have been predicting for some time. It also demonstrated the effectiveness of the observatories and instruments involved and strengthens the case for similar signals found in the individual data sets from the participating collaborations.

As Lynch indicated, The Green Bank Observatory is developing new technology to enhance the GBT’s capabilities for this research:

“The IPTA is a great example of scientists and instruments from around the world coming together to advance our understanding of the cosmos. New instruments, like our upcoming ultrawideband receiver [funded by the Moore Foundation], will ensure that the GBT continues to make essential contributions to NANOGrav and the IPTA. If what we are seeing here is indeed the signature of gravitational waves, then the next few years are going to be really exciting.”

The Laser Interferometer Space Antenna (LISA) will study the mergers of supermassive black holes, test Einstein’s theory of general relativity, probe the early Universe, and search for gravitational waves. Credit: NASA

However, the scientific collaborations caution they don’t have definitive evidence for the GWB yet. While the case for it has been bolstered by these latest findings, the contributing consortia are still gathering information and looking into what else this signal could be. The ultimate goal for studying GWs is to find evidence of a unique relationship in the signal strength between pulsars in different parts of the sky. These “spatial correlations” are yet to be found, but the existing signal is consistent with scientists’ predictions.

Looking ahead, the IPTA will be analyzing more recent data hoping that this will confirm that this new signal is evidence of a GWB. In addition, many new instruments and scientific collaborations will begin gathering data in the coming years – like the MeerKAT array in South Africa and the India Pulsar Timing Array (IPTA). There’s also NASA’s Laser Interferometer Space Antenna (LISA), a proposed mission that will consist of three satellites scheduled to launch sometime in the late 2030s and the first dedicated space-based gravitational wave detector.

Said Dr. Maura McLaughlin, a researcher of West Virginia University who uses the GBT for data collection for NANOGrav:

“If the signal we are currently seeing is the first hint of a GWB, then based on our simulations, it is possible we will have more definite measurements of the spatial correlations necessary to conclusively identify the origin of the common signal in the near future.” 

Header image: Artistic impression of a Double Pulsar system, © Michael Kramer/MPIfR

Further Reading: Green Bank Observatory