One of the things about being a scientist is you have to accept that periodically throughout your career, things you learned as “This is certain” sometimes get flipped to, “Oh, that was completely wrong.” 

Sometimes it’s just an assumption: back in 1998, we learned that our assumption that the constant lambda, which we’d happily set to zero for most of a century, was actually a real number thanks to the overwhelming amount of dark energy that no one expected would be populating the universe. 

Sometimes it’s the immutable nature of an isolated particle that is only thought to change by decay processes. No, particles can change wildly, or at least, as we learned in 2001, neutrinos can change wildly as they oscillate between different flavors.

Today, I find myself confronted with a new “We never thought we could” kind of moment. 

For the past many years I have sworn up and down that we would never be able to measure anything that happened in the universe prior to the formation of the cosmic microwave background (CMB). That wall of light came into existence about 400,000 years after the Big Bang and was created in the moment the universe went from a soup of free electrons and fully ionized atomic nuclei to a neutral gas of atoms, with every atom emitting light as it glommed onto electrons. Prior to that moment, our universe was opaque, and there is no observational way to see what happened. We can only see the shadows of past physics in the distribution of that released light.

In 2015, however, we developed a new way to explore our sky: for the first time our equipment became sensitive enough, and our software good enough, to allow the LIGO and Virgo systems to detect gravitational waves. These oscillations in space-time come in many kinds and sizes that are driven by all kinds of events. So far, we’ve only been able to detect the mergers of neutron stars and small to intermediate-sized black holes, but this is a technological limitation rather than a physics limitation.

In a new paper in Physical Review Letters, researchers led by graduate student Sylvia Biscoveanu, have developed a potential technique for separating out sounds of nearby gravitational wave sources and sort through the background noise to detect those gravitational waves that originated in the early universe. If this can be done, it will allow us to get data from a time before the formation of the CMB.

According to Biscoveanu: If the strength of the primordial signal is within the range of what next-generation detectors can detect, which it might be, then it would be a matter of more or less just turning the crank on the data, using this method we’ve developed. These primordial gravitational waves can then tell us about processes in the early universe that are otherwise impossible to probe.

This isn’t easy, and so far this team is only working with simulated data. But their technique is recovering their simulated background of gravitational waves, and this is no easy task. As Biscoveanu explains: The analogy I like to make is, if you’re at a rock concert, the primordial background is like the hum of the lights on stage, and the astrophysical foreground is like all the conversations of all the people around you. You can subtract out the individual conversations up to a certain distance, but then the ones that are really far away or really faint are still happening, but you can’t distinguish them. When you go to measure how loud the stage lights are humming, you’ll get this contamination from these extra conversations that you can’t get rid of because you can’t actually tease them out.

So, it turns out, we may be able to sense signals back to the beginning of the universe. We just won’t be able to see them. Light isn’t the answer for exploring the early universe, but gravity might just get us there.

More Information

MIT press release 

“Measuring the Primordial Gravitational-Wave Background in the Presence of Astrophysical Foregrounds,” Sylvia Biscoveanu, Colm Talbot, Eric Thrane, and Rory Smith, 2020 December 9, Physical Review Letters.