The gravitational waves that could shed light on the cosmic dark age

Some astrophysicists have said that the discovery of the gravitational wave background could shake the foundations of physics – why is it so momentous?

The cosmos is like a water balloon wibbling and wobbling mid-flight, and we can finally detect the wobbles. Earlier this year, the researchers of the NANOGrav collaboration made a big announcement: they have found what’s known as the gravitational wave background. If you’re familiar with the cosmic microwave background, this is similar, but instead of remnants of the light that radiated out from the big bang, it’s made up of ripples in space-time created aeons ago that permeate the entire universe. Astrophysicists are absolutely buzzing about the news, with some saying it could shake the foundations of physics. So why is this so momentous? Let’s get into it.

The Green Bank telescope in West Virginia, part of the NANOGrav collaboration

For one thing, this could be the first time we’ve observed supermassive black holes merging. These behemoths are found at the centre of almost every galaxy, and when two galaxies collide, their black holes should slowly spiral towards each other and smash together. This is the main hypothesis for what is causing the gravitational waves that were just detected rippling throughout the cosmos.

I’ve mentioned gravitational wave detectors, like the Laser Interferometer Gravitational-Wave Observatory (LIGO), before in this newsletter. LIGO has seen evidence of smash-ups between smaller black holes, which result in a relatively high-frequency wave that can be translated into a kind of high-pitched “chirp”. But the gravitational wave background is more of a low-grade thrum, with wavelengths measured in light years. If it is produced by colossal black holes colliding, then it is their sheer size that causes this low frequency and long wavelength.

To detect these ripples, researchers used a tool called a pulsar timing array. Pulsars are neutron stars that emit a beam like a lighthouse as they rapidly rotate, so, from our point of view, we see them flash at a regular interval. Their cadence is so even that they’re like a cosmic metronome. When gravitational waves pass through the galaxy, they ever so slightly upset the metronome’s timing, and that tiny change is what the NANOGrav researchers measured. 

The detection took 15 years of data. It was a feat of patience, technology and large-scale data analysis. But it wasn’t really a surprise. “We’ve known for a long time that most galaxies have these supermassive black holes at their centres, and we’ve known that galaxies merge,” says Chad Hanna at Penn State University. “What hasn’t been known is whether these black holes actually get close enough together to create a gravitational wave that’s observable.”

We’ve never actually observed supermassive black holes merging together before and, in fact, cosmologists aren’t really sure whether they actually can. This comes down to something called the final parsec problem: as two of these giants get closer and closer together, they should devour all the material around them or fling it away, which is crucial to slowing their speed. But once all the gas and dust and stars are gone, we’re not actually sure whether the black holes could slow down enough to actually collide. That collision would be what creates a gravitational wave strong enough for us to detect. But if this isn’t the case, the black holes could just hurtle around one another forever – or close enough to forever that it doesn’t actually make any practical difference.

If the gravitational wave background does come from supermassive black holes, that would be the first solid evidence that they actually can merge. It’s a big deal for the evolution of the black holes themselves and the galaxies in which they reside. It also could be a clue as to how supermassive black holes get so big so fast, which is a long-standing mystery in cosmology.

But an even more interesting outcome would be if it’s not from black holes. Some theories argue that the background could be left over from the very early universe, just after the big bang. “If we were to find that even some component of this background was coming from a different source than these supermassive black holes, that’s likely to have a huge impact on fundamental physics and cosmology,” says Hanna. “We do expect it to be mostly from supermassive black holes, but there’s lots of good reason to suggest that, at some level, there will be some signal from early universe processes.”

Gravitational waves are pretty much the only way we can probe the universe before the era of recombination – when the cosmos stopped being a roiling soup of radiation and the first atoms formed, about 380,000 years after the big bang. The time before that is commonly known as the cosmic dark age, because the universe was entirely opaque, so there’s no way for us to observe it using light. But gravity, as far as we know, has always been able to travel freely.

The early years of the universe are extraordinarily important to our understanding of physics in general, but there’s a lot that we just don’t know about them. Even the most agreed-upon ideas that we do have tend to stand on shaky ground because of the difficulty – sometimes impossibility – of getting direct observational proof. If we’ve spotted gravitational waves from the early universe, though, all that could change.

“It’s an understatement to say that gravitational waves have opened this new window on the cosmos, because it really is unlike any other way that we observe the universe,” says Hanna. This could mark the first tug on the thread of the early universe – the beginning of a great unravelling of the cosmic mysteries there.

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