Mapping the Gravitational Wave Universe

Gravitational waves are everywhere, all the time. Rippling through the Galaxy, stretching and squeezing space-time at the Earth, even passing through you and me. There’s an entire spectrum of gravitational waves, and in June last year, several pulsar timing teams around the world found the strongest evidence yet for the gravitational-wave background, and since then we have been paying close attention to the signal.

Now there's no need to worry, these signals are tiny! They stretch the space across the length of your body by around a femtometre, which is less than the width of a proton. Measuring these tiny distortions is an impressive feat and requires some exceptionally sensitive equipment. 

We measure the gravitational-wave background with pulsar timing arrays. We use pulsars, and their exceptionally regular pulsations, to carefully measure the space between us and the pulsar. When we see coordinated distortions in space in many pulsars across the Galaxy, we can be reasonably sure these are caused by the choir of gravitational waves that make up the gravitational wave background.

Using the MeerKAT radio telescope, we created an array of 83 pulsars to do just this. Not only did we measure the gravitational wave background in half the time of other collaborations, but we also created detailed maps of the signal. This is the first time maps of this nature have been established, and they have the potential to unlock many answers to important physics questions about our Universe. 

Pulsars are incredibly precise timekeepers. 

Gravitational waves are ripples in space-time, the fabric that makes up our Universe. Imagine a 2D analogy of our Universe being the flat surface of a lake. Once you throw a pebble in, it creates concentric rings of ripples that spread from the source. These ripples, represent radiating gravitational waves, moving across space-time. These waves were first measured in 2015 and since then, scientists have been slowly building our catalouge of gravitational wave events. These ripples in space-time have opened a whole new window through which to view our Universe. They are generated when incredibly dense, massive objects, like neutron stars or black holes, spiral inwards towards each other and eventually collide. The energy from the inspiral motion, as well as the explosive events once these massive objects merge, is radiated away as gravitational waves, stretching and squeezing space-time as the waves move through the Universe at the speed of light. 

The gravitational wave background is the background hum from many of these events. Supermassive black holes - gargantuan black holes million to billions of times the mass of the Sun - create low-frequency gravitational waves. There are likely many of these systems across the Universe and when you add their signals together you get a background hum. 

Just like light, gravitational waves come in a spectrum. Ground-based detectors like LIGO measure high-frequency energetic events. The gravitational wave background is expected to be at nanohertz frequencies. This translates to a wave period of years. This necessitates a Galactic-scale gravitational wave detector, something humans can not physically build. Instead, we utilise what nature has already given us - the pulsars - which are located in all different directions across the Galaxy. 

In our work, published today in the journal MNRAS, we created the most detailed maps of the gravitational wave background to date - and they show some intriguing results. The extraordinary sensitivity of the MeerKAT radio telescope allowed us to create the one of the largest gravitational wave detectors, in turn, allowing us to produce the highest-resolution maps. The array of 83 pulsars also allows us to have unprecedented spatial resolution when looking at the sky. 

These maps, which measure the directional power of the gravitational wave signal, reveal some intriguing results. Astrophysicists generally expect the gravitational wave signal to be approximately the same in all directions. This arises from key cosmological assumptions that the Universe is homogeneous and isotropic, generally evenly distributed in all directions and evenly mixed. 

Surprisingly our map shows a ‘hot spot’ - an area of stronger gravitational wave signal in the Southern Hemisphere. This hot spot could suggest that a nearby, extraordinarily massive black hole binary is drowning out others, but it may also just be a statistical anomaly.

Pulsar timing arrays are complex detectors. Each pulsar has its quirks, radio telescopes require precise calibration and many other complexities impact a photon on its journey from the pulsar, through the interstellar medium and finally arriving at the Earth. For this reason, it is too early to say if this hot spot is a genuine signal or statistical noise. 

These maps of the gravitational wave background are the key to unlocking the source of the signal. While astronomers think that the gravitational wave background likely comes from colliding supermassive black hole binaries, other possible causes could be events that occurred in the early Universe, soon after The Big Bang.

The early Universe was very hot and dense, akin to a pot of boiling water. Collisions between ‘bubbles’ in this early Universe soup may generate gravitational waves. If the gravitational wave background does in fact come from this cosmic soup shortly after the Big Bang, we will need to re-write our fundamental understanding of physics.

So how can we differentiate between sources? A gravitational wave background from early Universe physics would generate an almost uniform background due to the early Universe being much smaller than it is now. We would not expect much directional variation in the resulting signal. 

However, a background generated by supermassive black hole binaries should have slightly more variation, as the location and distance to the gravitational wave-emitting binary systems will affect the directional power of the signal. In such cases, however, we would not expect large deviations in the signal due to the cosmological principles introduced earlier. 

While supermassive black hole binaries are the expected source of the gravitational wave background, there is still exciting science to be done if we find this is the case. We know that huge black holes - billions of times more massive than the sun - live in the centre of galaxies, but how to form black holes this massive remains an elusive mystery and exciting area of research. By studying these binaries in the gravitational wave background we may be able to understand how these black holes grow. 

While the maps released today were not able to decisively determine the source of the gravitational wave background, radio telescopes around the world, including MeerKAT, are continuously timing pulsars. In just 4.5 years we (the MeerKAT pulsar timing array) were able to produce similar gravitational wave background evidence to other arrays with decades of data, and generate the most detailed maps of the background. This analysis takes time, and MeerKAT has now been timing pulsars for 6 years. Imagine what we will find in that data set. 

Additionally, many pulsar timing array campaigns around the world come together under the International Pulsar Timing Array which is currently combining data sets from collaborations across the globe. Combining the long-baseline data from older telescopes, with the higher sensitivity data from modern telescopes should offer deeper insights into the gravitational wave background.

Observational techniques and analysis of the gravitational wave background are in their infancy, and this signal still holds many secrets about the Universe.