Breaking astronomy news: The background hum of the Universe is finally revealed!

June 29, 2023   Issue #584

Astronomy News

It’s a big Universe. Here’s a thing about it.

[Note: Given the importance of this news, I’ve made this issue free to all subscribers.]

The Universe is very softly humming to itself.

It’s a cacophonous tune, wildly varying in pitch, from a chorus scattered throughout the cosmos. And if we had to classify these performers, they’d be basso molto profundissimo: Such a low pitch that it would take years to hear a single note.

And now, for the first time, astronomers have detected convincing evidence of this song.

This hum is the cosmic gravitational wave background, predicted decades ago. And to find it, astronomers needed a detector the size of a galaxy.

There’s quite a bit of background, so to speak, to go into understand this amazing development, so:

<tl;dr> Astronomers have convincing evidence that, using an array of dozens of pulsars scattered around the Milky Way, they have found the background of gravitational waves permeating the Universe. The source for these waves isn’t certain, but it’s most likely thousands of pairs of gigantic black holes circling each other, slowly spiraling together before they catastrophically merge and release catastrophic amounts of energy. </tl;dr>

Gravitational waves are literally the expansion and contraction of space itself. Think of spacetime as a fabric, a framework in which everything is embedded. If you accelerate any object that has mass — so, anything like you, me, orcas, stars, or, say, black holes — they will create these ripples, similar to those you see in the surface of a lake if you throw a rock in. I talk about this in detail in an article on The Old Blog™.

When a gravitational wave passes, space contracts and expands on an extremely tiny scale. These waves are so weak, in general, that you need extremely massive objects accelerated at an extremely high rate to make waves that are detectable. That’s where black holes come in.

Black holes are massive, at least several times the mass of the Sun. They have ridiculously strong gravity, too, so each can be accelerated quite strongly by the gravity of the other. If two black holes are in a close orbit around one another they’ll emit decently strong gravitational waves. Sending these waves out steals energy from their orbit, so they slowly spiral together, whizzing around each other faster and faster. As they get closer and faster they emit stronger gravitational waves, so this is a positive feedback loop. Eventually they get so close together that, when nearly touching, they move at very nearly the speed of light, sending out very strong and sharp waves, and then kaPOW! They merge into a single, more massive black hole.

For black holes with a few or a few dozen times the Sun’s mass, these gravitational waves have a very short wavelength, or, if you prefer, a high frequency. We can build observatories on Earth to detect them, like LIGO, which are a few kilometers in size. And it works! We have detected dozens of stellar-mass black holes merging, and that was a very big deal indeed.

But what if the black holes are bigger? A lot bigger?

At the center of every big galaxy lies a monster: A black hole with millions or billions of times the mass of the Sun. These are supermassive black holes. They’ve been observed in galaxies that are so far away we see them as they were less than a billion years after the Big Bang, when there wasn’t much time for them to grow so huge. How they got so big so rapidly is a mystery.

We know that galaxies collide. Our own Milky Way grew big because it ate a lot of smaller galaxies. If a galaxy like ours collided and merged with another big one, the two supermassive black holes will fall to the center of the maelstrom. They can fall into orbit around one another and slowly spiral together, but the physics of the situation shows that this will stop when they’re still a few light-years apart, before they can emit strong enough gravitational waves to shrink their orbits. This is called the Final Parsec Problem (a parsec is 3.26 light-years, a historical unit astronomers like to use) and we assume that it can be overcome somehow because it’s probably how supermassive black holes get so, well, supermassive. There are some ideas how this might happen, but no close binary supermassive black hole pair has been seen.

But, if they do exist, then they will emit gravitational waves as they orbit. Those orbits can take a year or more, so the waves they emit will have a wavelength of a light-year or longer. In terms of frequency, this would be measured in billionths of a Hertz, which is super low frequency. If it were translated into sound, it would be something like 35 octaves lower than middle C. Profundissimo, indeed.

There are hundreds of thousands if not millions of such supermassive black hole binaries in the Universe, scattered throughout. Together, their combined gravitational waves would create a sort of background vibration in the fabric of spacetime, a hum made up of all their different monotones at slightly different pitches, depending on their orbital period. This is the gravitational wave background, and it was predicted decades ago. Finding it, though, is another matter entirely.

To observe such waves you’d need an incredibly sensitive detector because the waves would be faint, and it would also have to be immense because the wave crests are light-years apart. We can’t build anything on a scale like this, of course!

But we don’t need to. Nature did it for us.

Neutron stars are the superdense cores of massive stars that exploded as supernovae. The core of the star collapses, and if it has less than about three times the mass of the Sun it forms a mind-crushingly dense ball of neutrons (if it’s more massive than three solar masses it forms a black hole). This neutron star will spin pretty rapidly, usually in mere seconds, and have an incredibly strong magnetic field. As it spins, it sends out beams of energy like a lighthouse that sweep across space. From Earth, we see this as a pulse of energy, a blip, every time the beam passes over us. We call neutron stars like this pulsars.

Sometimes there’s another star orbiting the pulsar. The fierce gravity of the pulsar — a billion or more times the gravity you feel standing on Earth — draws matter off that normal star, and it spirals down to slam into the pulsar’s surface. This speeds up the pulsar’s spin. Sometimes a lot. We’ve found dozens of pulsars that spin hundreds of times per second, faster than the blades of a garbage disposal or a blender! These are millisecond pulsars.

They make incredible cosmic clocks, because their spin is very stable and they send out a lot of blips. Using radio telescopes, where a lot of the pulsar energy is emitted, we can measure the time between blips (the star’s rotation rate) with phenomenal accuracy, down to millionths of a second.

And here is where the fun part is.

Remember, gravitational waves compress and expand space on a tiny scale. So when a gravitational wave passes over the pulsar, it changes the distance between that pulsar and Earth. This in turn changes the timing of the pulses’ arrival at our telescopes. In principle, if you can measure that change you can detect those waves.

In practice, though, it’s very very very very hard. You have to look at a lot of pulsars — astronomers call this collection a pulsar timing array — and then compare the changes in the pulse timing of each one against the changes in every other one. This is incredibly challenging on its own, but there are a lot of sources of noise and other effects for which astronomers need to compensate to be able to see these minute changes. It’s a Herculean task.

But now it’s been done. [link to paper]

If you like this article and think this stuff is super cool — as you should — then feel free to share this article with a friend!

The North American Nanohertz Observatory for Gravitational Waves, or NANOGrav, is a consortium of 190 scientists at 70 institutions, and its job is to look for exactly these pulse timing variations. Over the past 15 years they’ve used radio telescopes — the Green Bank Telescope in West Virginia, the (now, sadly, defunct) Arecibo Telescope in Puerto Rico, and the Very Large Array in New Mexico —to monitor 68 different millisecond pulsars. They observe each of them roughly once a month or so for quite some time to determine the period of the pulses as accurately as possible.

Over the years they’ve released their data to astronomers at large. The last one was about three years ago. In that data, using fewer than 50 pulsars, there was a hint of something in the data, a whisper that might be the gravitational wave background, but the data were a little too noisy to be sure.

Since then an additional 21 pulsars have been added to the list, ones newly discovered in surveys, and these were observed for several years, bringing the total to 68. This addition more than anything else strengthened the observations.

And what they found looks very much like the expected background signal of gravitational waves! Statistically speaking, for the math nerds, they’re in the three to four sigma range, which means a greater than 99% chance the signal is real and not some random noise. That’s really good, but in general scientists like to see a five-sigma detection (so, 99.99994% certainty) to claim a discovery.

So the NANOGrav astronomers have been careful to say this isn’t a discovery, but instead is very strong evidence they’ve found it. I’m OK with that.

Why is this a big deal? For one thing, it’s one of the few cosmological predictions of General Relativity that hasn’t been tested observationally. The existence at all of gravitational waves was a prediction of GR, so the first black hole merger announced by the LIGO team in 2016 was a huge accomplishment. This is actually a far harder detection to make.

Also, the 2 - 10 nanoHertz frequency of the background hum is what you’d expect for supermassive black holes that are very close together, from roughly a tenth to a hundredth of a light year apart (so, less than a trillion kilometers). We haven’t seen any supermassive black holes this close together! If this detection holds up, it shows that nature has solved the Final Parsec Problem. That’s pretty cool.

For another, if this background hum is real than this is just the beginning. Further observations, using other observatories and more pulsars, will be able to hone in on this signal and see it more clearly. As details emerge it will help us understand how these black holes behave, possibly how they grew so big so quickly, and even tell us their universal size distribution — literally the range of sizes of these monsters. These are all key steps in better understanding how the Universe works.

I’ll also note there are other things that could create this background hum, but they all involve somewhat exotic physics. For example, we think that when the Universe was a teeny tiny fraction of a second old it underwent a period of rapid inflation, briefly expanding faster than the speed of light. When this inflation stopped it could’ve sent out gravitational waves that have been dimmed by time, creating the background. Sound weird? This is the least bizarre of the other ideas the astronomers tested. All of them also fit the data seen, but are pretty out there, whereas we know supermassive black holes exist and should be common enough to create the hum.

Another fun bit: when two supermassive black holes merge, it’s the single biggest and most violent event in the Universe. A fraction of the mass of the two black holes is converted into energy in the form of those gravitational waves, and it’s colossal: The energy emitted is hundreds of millions of times the energy emitted by every single star in the entire Universe combined.

So yeah. These are a big deal. It’s amazing to me to think that these immense eruptions of energy dwindle to a murmur so quiet that we need to use a natural detector thousands of light-years across to observe them at all. It’s incredible that we can do this!

… or perhaps I need to choose my words more carefully. It’s entirely credible. I am deeply, deeply impressed with the level of work that’s been done here, not just in the analysis but also the calibration and processing of the data. I can’t adequately describe how difficult this task was, and several times reading the journal papers I was slack-jawed as I took in the details. They did a phenomenal job.

So congratulations to the entire NANOGrav team! This was an amazing result.

But your work has just started. It’s a big Universe, filled with these binary beasts, and now it’s time to take a closer look at them.

Et alia

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