Gravitational Wave Background

If you read last week’s Spacing Out Newsletter, you saw the bit about the discovery of the Gravitational Wave Background. I didn’t have a lot of room to go into it there, and it’s a mix of a whole lot of really cool science and clever detection methods, so I wanted to get into it in more detail.

The TL;DR in case you don’t read the newsletter: an international team of astrophysicists have detected a background hum in the Universe that is most likely being caused by the faint leftover gravitational waves from supermassive black hole mergers (and how they did it was really cool, but we’ll get to that farther down).

Black Holes and Gravitational Waves

If you’re not already familiar with this material, that last sentence had a whole lot of “huh, what?” in it, so let’s break it down a bit. You’re reading a blog about space, so I’m going to make an assumption that you already are aware of the existence of black holes, those weird, dark, super-dense objects that form from the collapsed cores of really massive stars when they die epically in supernovae. At the centers of galaxies are the Big Boys, the supermassive black holes with masses millions or billions of times the mass of our Sun.

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How do they get from normal-sized black holes to the Big Boys? I’m so glad you asked! Except the answer is “we don’t exactly know”. We know how the Little Guys form, those ones from collapsed stars and have a mass usually 4-10 times the mass of our Sun. But there’s a huge difference between a few solar masses and a few billion solar masses, and we’re not quite sure how you get from one to the other, or if they even do—it’s possible the Big Boys form in a completely different way. Certainly they seem to have been very large very early in the Universe’s history.

One thing we do know is that black holes can merge whenever they encounter one another. This can happen on the small scale, when the remnants of two stars come together, or on the large scale when galaxies collide and their supermassive black holes squoosh together (not the technical term, but it should be). When this happens, the merger generates gravitational waves. (Side note, “gravitational waves” and “gravity waves” are not actually the same thing. Gravity waves are actually vertical movements of air in an atmosphere. No, I don’t know why they’re called that either.)

Gravitational waves are actual ripples in spacetime. Spacetime isn’t just a cool sci-fi concept—space and time are actually intertwined and the way something moves through space will also affect how it moves through time. It’s a whole thing and it’s fascinating and I may take a deep dive into it at some point or another, but for the moment let’s just go with the idea that gravitational waves are actual ripples in space itself. Technically these waves happen whenever anything with mass moves through space, but it takes something really massive moving really fast (like, say, when two black holes are spiraling around each other, about to merge) to generate gravitational waves big enough to detect. We’ve been able to spot these waves from smaller black hole mergers since 2015, with our gravitational wave observatories LIGO and VIRGO. 

But big things are merging all the time, and have been throughout the history of the Universe, spewing their gravitational waves out into the void where they just kind of keep going. There isn’t really anything to stop them after all. So, astrophysicists theorized, the Universe should be filled with the echoes of old mergers, including the ones from supermassive black holes that are too low-frequency for LIGO or VIRGO to detect. And of course they were right, this is what was just discovered, the Gravitational Wave Background (GWB for short). But I just told you that our existing detectors can’t find this background noise of the Universe. So how did we go about discovering it?

The Extra Cool Part

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This was the shiny part for me—to find the GWB we used pulsars that are scattered throughout the Milky Way. Pulsars are another thing that can happen to massive stars when they die. When a big star goes supernova, if it’s not quite massive enough for its core to collapse to a black hole it will form something tiny and dense (but not so tiny and dense as a black hole) called a neutron star. These objects spin rapidly, and some of them have focused beams of radiation coming out of them. If those beams happen to be pointing in the direction of Earth then as the neutron star spins we see this light (which is in the radio part of the electromagnetic spectrum) appear to flash on and off, similar to how a lighthouse looks. When we can see that, we call the neutron star a pulsar, for pulsating radio source.

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Pulsars are among the most accurate timepieces in the Universe. Their spin time is precise enough to make a Swiss watchmaker weep with envy. So if the timing of a pulsar’s spin gets knocked slightly off, we know something had to happen to disrupt it. Fun fact: this is actually how we discovered the first planets outside of our Solar System, when we found a pulsar whose spin was off because it was being affected by the gravity of the planets going around it.

And gravitational waves can also affect our view of a pulsar. If light from a pulsar travels through a gravitational wave on its way to Earth, the wave affects the timing of the light’s arrival, making the pulsar’s spin time look off to us. So astronomers used an array of radio telescopes around the world (including the dear departed Arecibo Observatory. Still sad about that) to monitor the timings of pulsars over 15 years. 15 years. Being an astronomer sometimes takes a lot of patience. But over that time they detected exactly the patterns of changes in pulsar timings that we expect to see when gravitational waves from supermassive black hole mergers long past are sweeping across the Milky Way.

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So let’s put this into perspective: we used the remnants of dead stars as cosmic lighthouses to create an observatory the size of the galaxy and detect the faint echoes in the fabric of spacetime itself left behind by billions of years of collisions by some of the most massive and mysterious objects in the Universe.

Whoo boy. I can’t believe circumstances allowed me to write that. I need a moment.

What Does It Mean?

Okay, so besides being beyond cool, what does this new discovery mean for astrophysics? At this moment, not a ton. But in the future, now that we know it’s there? Maybe a lot. There’s a lot we don’t know about how supermassive black holes interact with each other. They are, after all, actually impossible to see, and their interactions generally happen over long timescales, so at best we get to see single snapshots of the process. But the imprint of these ginormous mergers is there, stamped in the waves they sent out into the Universe. And now we know how to find those waves. I for one, am looking forward to what we can learn from our GWB in the coming years.

But also how freaking cool is this stuff??