The following is an extract from our monthly Launchpad newsletter, in which resident space expert Leah Crane journeys through the solar system and beyond. You can sign up for Launchpad for free here.
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 whatās known as the gravitational wave background. If youāre familiar with the cosmic microwave background, this is similar, but instead of remnants of the light that radiated out from the big bang, itās 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? Letās get into it.
For one thing, this could be the first time weāve 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.
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Iāve 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 āchirpā. 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 theyāre like a cosmic metronome. When gravitational waves pass through the galaxy, they ever so slightly upset the metronomeās timing, and that tiny change is what the NANOGrav researchers measured. If you want to read more about the mechanics behind the discovery, my colleague Alex Wilkins wrote a great piece about it here.
The detection took 15 years of data. It was a feat of patience, technology and large-scale data analysis. But it wasnāt really a surprise. āWeāve known for a long time that most galaxies have these supermassive black holes at their centres, and weāve known that galaxies merge,ā says Chad Hanna at Penn State University. āWhat hasnāt been known is whether these black holes actually get close enough together to create a gravitational wave thatās observable.ā
Weāve never actually observed supermassive black holes merging together before and, in fact, cosmologists arenāt 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, weāre 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 isnāt the case, the black holes could just hurtle around one another forever ā or close enough to forever that it doesnāt 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. Itās 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 itās not from black holes. Some theories argue that the background could be left over from the very early universe, just after the big bang. āIf we were to find that even some component of this background was coming from a different source than these supermassive black holes, thatās likely to have a huge impact on fundamental physics and cosmology,ā says Hanna. āWe do expect it to be mostly from supermassive black holes, but thereās lots of good reason to suggest that, at some level, there will be some signal from early universe processes.ā
Gravitational waves are pretty much the only way we can probe the universe before the era of recombination ā 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 thereās 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 thereās a lot that we just donāt know about them. Even the most agreed-upon ideas that we do have tend to stand on shaky ground because of the difficulty ā sometimes impossibility ā of getting direct observational proof. If weāve spotted gravitational waves from the early universe, though, all that could change.
āItās 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,ā says Hanna. This could mark the first tug on the thread of the early universe ā the beginning of a great unravelling of the cosmic mysteries there.
