NANOGrav’s exciting discovery is likely to keep cosmologists busy for years.
In a Universe-shaking announcement, an international consortium of astronomers have presented evidence that the fabric of the cosmos itself is constantly vibrating with light-years-long gravitational waves.
These low-frequency disturbances in spacetime were seen with the help of a galaxy-spanning network of rapidly spinning neutron stars (the remains of massive stars post-supernova explosion) called pulsars.
The regular pulses of radio light that give pulsars their name serve as precise cosmic metronomes, hitting us once per stellar rotation, up to hundreds of times per second.
Because gravitational waves stretch and squeeze the space they’re passing through, slightly changing the distances to the pulsars, their presence can be inferred if those metronome beats arrive a little early or late for sources in a specific pattern across the sky.
While astronomers are extremely excited about this latest development, it’s understandable if you are experiencing a bit of deja vu. Haven’t we already detected gravitational waves? Didn’t we know these were out there? It’s true that pulsar timing arrays (PTAs) are not bringing us the first evidence of the existence of gravitational waves.
That honour goes to the LIGO experiment, which, in late 2015, detected a violent disturbance in space due to two roughly 30-solar-mass black holes colliding in a distant galaxy.
Since then, LIGO, in collaboration with partner observatories VIRGO and KAGRA, has detected a total of nearly 100 merger events involving black holes and neutron stars.
The big difference is one of frequency. Just as electromagnetic radiation — light — exists on a spectrum, from high-frequency (short-wavelength) gamma rays to visible light to low-frequency (long-wavelength) radio waves, gravitational waves are also generated at different frequencies by different kinds of cosmic events.
When it comes to mergers, the main factors are the masses of the objects and the speed at which they’re rotating around each other.
Black holes with stellar masses (those produced from the collapse of massive stars) whirl around each other hundreds of times per second during the final inspiral and create a burst of gravitational waves at the moment of collision.
These waves consequently have high frequencies and short wavelengths, matching the sensitivity range of instruments we have on Earth.
Supermassive black hole pairs, on the other hand, emit appreciable gravitational waves when orbiting with periods of years, creating light-year-long waves and necessitating a detector the size of a galaxy, collecting data over decades.
The difference between what LIGO shows us and what we get from a PTA is like the difference between an image from an optical telescope and one from a radio antenna. Not only are the data and observational methods completely different, so are the lessons we learn about the Universe from them.
What is all of the fuss about?
So, what exactly has the PTA collaboration found? So far, the main result, which was seen most clearly by the NANOGrav collaboration, is that low-frequency gravitational waves exist in the Universe all around us.
Even though this release includes 15 years of data, it’s not yet enough to show us specific sources. But the signal is consistent with a cosmic chorus of contributions produced by the final orbits of pairs of supermassive black holes merging when their galaxies collide.
Assuming the background is primarily due to supermassive black hole collisions, the information contained in it is staggering. With just these first preliminary results, there are already indications that the final stages of galaxy mergers may be more exciting than we anticipated.
For one thing, the signal is somewhat louder than astronomers thought it would be, based on the most straightforward calculations of how galaxies and their supermassive black holes collide.
This might indicate that supermassive black holes are, on average, more massive, or colliding more often than expected. There is also some indication that collisions are helped along by the astrophysical environments in which they occur.
That is the central regions of galaxies are a bit messy and the combined effect of all the stars and gas and maybe some unexpected things hanging around them are jostling the supermassive black holes enough to make them come together sooner.
With just a few more years of observations, and the combination of data from all the PTAs, we should start to be able to see hints of individual sources. This raises the prospect of multi-messenger astronomy, in which a gravitational wave event could alert us to a merger in progress that we might also observe with a conventional telescope.
Whereas the LIGO experiment gives us a new view of star formation and evolution by capturing the final moments of stellar remnants, PTAs will be able to show us the evolution and build-up of galaxies: the basic units of the large-scale structure of the Universe.
Of course, this all depends on the signal truly originating in supermassive black hole collisions. We may be entirely wrong about that (the data are as-yet ambiguous) – which would be the most exciting possibility yet.
Violent processes that may have shaped the early Universe can also produce a background of low-frequency gravitational waves, as can certain kinds of dark matter and some hypothetical exotic Big Bang relics.
Detecting any of those, either as the dominant signal or as some contribution to it, would completely revolutionise our understanding of our cosmic history.
It is frequently said that every time we open a new window on the Universe, we find something completely new and unexpected. In the coming years, the full power of PTAs to illuminate our spacetime environment will be revealed.
Perhaps they will give us a better view of the build-up of cosmic structure; perhaps they will take us all completely by surprise. Personally, I can’t wait to find out.
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