Gravitational wave astronomy is still young. Because of this, the gravitational waves we can observe come from powerful cataclysmic events. Black holes consume each other in a violent chirping of spacetime, or neutron stars collide in a terrible explosion. Soon we might be able to observe the gravitational waves of supernovae, or supermassive black holes merging billions of light-years away. But beneath the cacophony is a very different gravitational wave. But if we can detect them, they will help us solve one of the deepest cosmological mysteries.
They are known as ancient gravitational waves, and as they formed within the heart of the primordial explosion, they faded in intensity to a faint hum. It is similar to the cosmic microwave background visible from all over the universe, but it is virtually invisible compared to the energetic light sources we see every day.
Because these gravitational waves are so weak, most of the effort to detect them has focused on their impact on light. According to the standard model of cosmology, ancient gravitational waves must twist the orientation of light slightly as it travels through space. Thus, light from the cosmic microwave background must have B-mode polarization. The problem with this is other things such as dust can also induce B-mode polarization in the CMB. It is easy to confuse the two, as seen when the BICEP2 collaboration claimed to have detected them, then had to step back slightly from their results.
While detecting primordial waves through the CMB is still possible, now that we can detect gravitational waves, it would be good to detect primordial waves directly. A new research group thinks they have found a way to do that. Their results were published in Physical Review Letters, and it shows how we could pull its signal out of a terrible noise.
Their process is the opposite of what usually happens in a sound recording. If you have a constant background buzz, you usually record the ambient sound of the camera, then subtract it from your recording. To detect gravitational waves, the team proposes to remove the loud signals to hear the faint hum. They created a model of average total signal of events such as supernovae and black hole fusions. Subtracting this from the gravitational wave data we collect, what would remain is a lot of random noise. Most of this noise would be caused by the random fluctuations of the gravity wave detector itself. But we have several gravitational wave observatories now, and the random noise of each of them is different. So the team proposes to compare noise data from many observatories and subtract all the noise that is not common among them. Because primordial gravitational waves must have the same signal across all observatories, the common “noise” must be the primordial signal.
The team has shown that this can work through simulations. The only problem is that current observatories are so noisy that this method cannot be used. As new more sensitive observatories go online, this method could be used to detect primordial gravitational waves.
If this method is successful, it would be a tremendous benefit to astronomers. Right now the standard cosmological model assumes that there was a period of rapid inflation in the early universe. This assumption solves many problems in early cosmology, but it remains hypothetical. But if the inflation model is correct, ancient gravitational waves would bear its signature. Detecting them would either confirm our suspicions of the big bang or point us to amazing new theories.
Reference: Biscoveanu, Sylvia, and others. “To measure the ancient gravitational background in the presence of astrophysical foregrounds.” Physical Review Letters 125.24 (2020): 241101.