Gravitational waves (GWs) were found for the first time in history in. This discovery proved a prediction that Albert Einstein made more than 100 years ago. It also started a revolution in the field of astronomy.
Since then, dozens of GW events have been found coming from different places, like black hole mergers, neutron star mergers, or a mix of both. As the tools used for GW astronomy become more advanced, scientists will be able to find more events and learn more from them.
For example, a group of astronomers from around the world recently used the International Pulsar Timing Array to find a series of low-frequency gravitational waves (IPTA).
They figured out that these waves might be the first signs of a background gravitational wave signal (BGWS), which is made by two supermassive black holes moving toward each other. Astrophysicists have thought about the existence of this background since the first GWs were found. This could be a very important discovery!
Einstein’s theory of general relativity said that gravitational waves are made when two or more massive objects (black holes, neutron stars, etc.) collide. These waves can be seen from many light-years away.
Some of these ripples may be caused by galactic mergers, including the supermassive black holes (SMBHs) at their centers, or by events that happened soon after the Big Bang. Since the first gravitational wave event was found, scientists around the world have been looking for signs of this gravitational wave background (GWB).
For example, Millisecond Pulsars (MSPs) are used as a system of Galactic clocks by 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).
Neutron stars, which are what’s left of these stars, spin hundreds of times per second and have very strong magnetic fields that concentrate their electromagnetic radiation at the poles.
This energy is sent out as pulsing beams of radio waves, which is how they got their name. They move through space in a way that looks like a flashing lighthouse.
Astronomers have used this effect to keep track of time for years, because the pulses are very consistent over long periods. At the same time, their flashing light has been used to measure the distances between stars and look into the space between them (ISM). Since GW astronomy came into being, these groups are now using pulsars to look for signs of background GWs.
This comes down to using their observatories to look for disturbances in the sweeps of pulsar beams, which are thought to be caused by passing gravitational waves.
Recently, these groups have come together to combine data sets, including the IPTA’s new data release, Data Release 2. (DR2). This is made up of precise timing information from 65-millisecond pulsars, which are neutron stars that spin many times per second.
“The GBT [Green Bank Telescope] contributes to the IPTA as one of the most important telescopes used by c,” said 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.”
The analysis of the IPTA DR2 and the other collaborations’ independent data sets showed strong evidence for this low-frequency gravitational wave signal. This is because many pulsars pointed to it. Astrophysicists expected to see the same things in a gravitational wave background as they did in this signal (GWB).
This background is made up of many GW signals that overlap. This is because there are a lot of supermassive black holes in the universe that orbit each other (binary SMBHs) and will eventually merge.
This GWB is like the noise in a crowded room and reminds me of the Cosmic Microwave Background (CMB), which is the leftover radiation from the Big Bang. These results not only make it more likely that there is a GWB, which is something that astronomers have been saying for a long time.
It also showed how well the observatories and instruments were working and backed up the idea that similar signals can be found in the different data sets from the different collaborations.
Lynch said that the Green Bank Observatory is working on new technology to improve the GBT’s research capabilities:
“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.”
But the scientific groups warn that they don’t yet have proof that the GWB is happening. Even though these latest findings make the case for it stronger, the contributing consortia are still getting more information and trying to figure out what else this signal could be.
The main goal of studying GWs is to find proof that the signal strength of pulsars in different parts of the sky is related in a unique way. Scientists have yet to find these “spatial correlations,” but the signal that is already there fits with what they think will happen.
In the future, the IPTA will look at more recent data in hopes of proving that this new signal is proof of a GWB. Also, in the next few years, a lot of new instruments and scientific groups will start collecting data, like the MeerKAT array in South Africa and the India Pulsar Timing Array (IPTA).
The Laser Interferometer Space Antenna (LISA), which is being planned by the European Space Agency, will be the first space-based gravitational wave detector. It will be made up of three satellites that will launch in the late 2030s.
West Virginia University researcher Dr. Maura McLaughlin, who uses the GBT to collect data for NANOGrav, said:
“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.”
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