Gravitational wave astronomy is still in its youth. Because of this, the gravitational waves that we can observe come from powerful catastrophic events. Black holes that consume each other in a violent twittering of spacetime, or neutron stars that collide in a huge explosion. Soon we might be able to observe the gravitational waves of supernovae or supermassive black holes merging billions of light years away. However, under the cacophony there is a very different gravitational wave. But if we can discover them, they will help us solve one of the deepest cosmological riddles.
They are known as primordial gravitational waves, and while they were formed in the heart of the Big Bang, they have faded in intensity to a faint hum. It is similar to the cosmic microwave background seen from anywhere in the universe, but is virtually invisible compared to the energetic light sources we see every day.
Map of the sky by BICEP2. Photo credit: BICEP2 Collaboration
Because these gravitational waves are so weak, most of the effort to discover them has focused on their effect on light. According to the standard model of cosmology, original gravitational waves should slightly twist the orientation of light on its way through space. Therefore light from the cosmic microwave background should have a B-mode polarization. The problem with this is that other things like dust can also induce B-mode polarization in the CMB. It's easy to get the two mixed up, as seen when the BICEP2 collaboration claimed they discovered them and then had to go back on their results a bit.
While the CMB can still capture primal waves, now that we can capture gravitational waves, it would be nice to capture primal waves directly. A new research group believes they have found a way to do this. Their results have been published in Physical Review Letters and show how we can extract the signal from the enormous noise.
There are many sources of gravitational waves. Photo credit: Christopher Moore, Robert Cole, and Christopher Berry
Their process is the opposite of what is normally done with sound recording. If you have a persistent background hum, you usually pick up the ambient sound of the room and then subtract it from your recording. To detect gravitational waves, the team suggests removing the loud signals in order to hear the faint hum. They built a model of an average total signal from events such as supernovae and black hole fusions. If we subtract this from the gravitational wave data we collect, there should be a bunch of random noise left. Most of this noise would be caused by random fluctuations in the gravitational wave detector itself. But we now have several gravitational wave observatories, and the random noise from each of them is different. The team therefore suggests comparing the noise data from multiple observatories and subtracting any noise that is not common between them. Since primordial gravitational waves should have the same signal across all observatories, the common "noise" should be the primordial signal.
The team has shown that this can work in simulations. The only problem is that the current observatories are so noisy that this method cannot be used. As new, more sensitive observatories go online, this method could be used to capture primordial gravitational waves.
If this method is successful, it would be an enormous boon to astronomers. Currently, the Standard Cosmological Model assumes that there was a period of rapid inflation in the early universe. This assumption solves many of the problems in early cosmology, but it remains hypothetical. However, if the inflation model is correct, the original gravitational waves would carry their signature. Their discovery would either confirm our suspicions about the Big Bang or point us to amazing new theories.
Reference: Biscoveanu, Sylvia, et al. "Measurement of the original gravitational wave background in the presence of astrophysical foregrounds." Physical Review Letters 125.24 (2020): 241101.