The Rhythm of the Night

NANOGrav / Olena Shmahalo

At the dawn of time, so the theory goes, the cosmos underwent a flash of rapid expansion. Almost instantly the visible Universe grew from a volume smaller than a proton to a spherical region nearly two meters across. It’s a moment known as early cosmic inflation. Although Inflation solves several cosmic problems, such as the homogeneity of the Universe and the ratio of hydrogen to helium we observe, we haven’t been able to prove it. But we might be able to prove it by looking for a particular type of gravitational wave.

If Inflation is true, then it would have caused a ripple of gravitational waves throughout the Universe, kind of like how striking a bell causes it to ring with sound. These gravitational waves would be far too faint for us to study with our current gravity observatories such as LIGO, so astronomers have tried to observe them indirectly through pulsar observations.

Wikipedia user Cmglee CC BY-SA 4.0

The idea is quite clever. Pulsars emit radio flashes at extremely regular rates. Except for a bit of slowdown over time and the occasional timing glitch, pulsars act as precision cosmic clocks. By observing lots of pulsars, astronomers can see small fluctuations in their timing due to their relative motion to us. With a bit of statistics, they can observe the effect of long-wavelength gravitational waves as they move through our galaxy. Projects such as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) have already started doing this.

But simply detecting long-wavelength gravitational waves isn’t enough to prove Inflation, as a recent study points out.¹ Cosmic Inflation isn’t the only thing that would produce a uniform background of gravitational waves. Supermassive black hole binaries produce them as well, so the challenge will be to distinguish between the two sources.

As the authors point out, one particularly strong signal from a supermassive binary might be indistinguishable from inflationary gravitational waves, but binaries from multiple nearby galaxies would each produce signals with slightly different frequencies. Their gravitational waves could interfere with each other to create beats in the gravitational waves, just as two similar notes can create a warbling sound to our ears. As the authors point out, by listening for the gravitational beats, astronomers could identify binary black hole sources and might even be able to filter them out to find the cosmic ringing of Inflation.

We don’t have enough observational data to do that yet. Right now the NANOGrav data isn’t strong enough to be a conclusive detection of gravitational waves. But that will change in the future, and if we continue to listen to the radio beats of pulsars, we may eventually discover the Inflationary rhythm of the night.