An international group of astronomers has presented evidence that the fabric of the cosmos itself is constantly vibrating with gravitational waves that are light-years across in an announcement that shakes the universe.
Pulsars, the remnants of massive stars after the supernova explosion, are a galaxy-spanning network of rapidly spinning neutron stars that were used to observe these low-frequency disturbances in spacetime.
Pulsars’ regular radio light pulses, which give them their name, are precise cosmic metronomes that strike us up to hundreds of times per second and once per stellar rotation.
If those metronome beats arrive a little early or late for sources in a particular pattern across the sky, their presence can be inferred because gravitational waves stretch and squeeze the space they are passing through, which slightly changes the distances to the pulsars.
Even though astronomers are very excited about this new development, you might be feeling a little bit of déjà vu. We haven’t yet discovered gravitational waves, have we? Were we not aware that these existed? It is accurate to say that pulsar timing arrays (PTAs) are not providing us with the initial proof of gravitational waves.
This honor goes to the LIGO experiment, which in late 2015 detected a violent space disturbance caused by the collision of two black holes with masses of about 30 solar masses in a faraway galaxy.
Nearly 100 mergers of neutron stars and black holes have been observed by LIGO and its partner observatories VIRGO and KAGRA since then.
The primary distinction lies in frequency. Gravitational waves are also produced at various frequencies by various kinds of cosmic events, just as electromagnetic radiation, or light, has a spectrum that ranges from high-frequency (short-wavelength) gamma rays to visible light to low-frequency (long-wavelength) radio waves.
The masses of the objects and the rate at which they are rotating around each other are the primary factors in mergers.
During the final inspiral, black holes with stellar masses, which are formed when massive stars collapse, whirl around each other hundreds of times per second and produce a burst of gravitational waves when they collide.
As a result, these waves have short wavelengths and high frequencies, matching the instrument sensitivity range on Earth.
On the other hand, supermassive black hole pairs emit significant gravitational waves when they orbit for years. These waves are light-years long and require a galaxy-sized detector to collect data for decades.
Like the difference between an image from an optical telescope and one from a radio antenna, the difference between what LIGO shows us and what we get from a PTA is similar. Not only are the observations and data collected in different ways, but also the lessons we gain about the universe from them are also very different.
What’s all the commotion about?
So, precisely what has the PTA collaboration discovered? The NANOGrav collaboration’s most notable finding thus far is that low-frequency gravitational waves are present everywhere in the universe.
Even though this release contains data from 15 years ago, it is not yet sufficient to demonstrate specific sources. However, the signal is consistent with a cosmic chorus of contributions made by the final orbits of supermassive black hole pairs whose galaxies collide, which result in their merging.
The information in the background is staggering if supermassive black hole collisions are the primary cause. Already, these initial preliminary findings suggest that the final stages of galaxy mergers might be more exciting than we anticipated.
Firstly, based on the simplest calculations of how galaxies and their supermassive black holes collide, the signal is somewhat louder than astronomers anticipated.
This could suggest that supermassive black holes are typically larger or that they collide more frequently than expected. There is likewise some sign that impacts are assisted along by the astrophysical conditions in which they with happening.
That is, the central regions of galaxies are a little messy, and the supermassive black holes are being jostled enough by all the stars, gas, and possibly some unexpected things that are around them to come together sooner.
We should begin to see hints of individual sources with just a few more years of observations and the combined data from all PTAs. This makes multi-messenger astronomy more likely, in which a gravitational wave could let us know that a merger was happening and that we could also see it with a regular telescope.
PTAs will be able to demonstrate the evolution and formation of galaxies, whereas the LIGO experiment provides a fresh perspective on star formation and evolution by capturing the final moments of stellar remnants: the fundamental components of the universe’s massive structure.
Naturally, the signal truly emanating from supermassive black hole collisions is the only thing that matters here. Since the data are still hazy, it’s possible that we’re wrong about that, which would be the most exciting possibility to date.
Low-frequency gravitational waves can also be generated by violent processes that may have shaped the early universe, as can certain types of dark matter and some speculative exotic Big Bang relics.
Our understanding of our cosmic history would be completely transformed if any of those could be detected, either as the dominant signal or as a contribution to it.
It is often said that when we look through a new window into the universe, we discover something brand-new and unexpected. The full potential of PTAs to illuminate our spacetime environment will become apparent in the coming years.
Perhaps they will provide us with a clearer perspective on the formation of cosmic structure; Perhaps they will completely surprise us all. I can’t wait to find out for myself.