Not all gravitational waves start with a bang

You’ve probably heard of gravitational waves. They’ve been in the news a lot recently because of two experiments called LIGO and Virgo, which have detected gravitational waves coming from some of the most violent events in the Universe – black holes merging and an enormous explosion known as a kilonova. But did you know there are other kinds of gravitational waves which haven’t been detected yet and which don’t start out so spectacularly?

Objects moving around in space create gravitational waves, which travel outwards in all directions like waves from a pebble landing in a lake. Like the pebble pushing water out of the way, really massive objects such as black holes, neutron stars, and white dwarfs can stretch and squeeze space as they move. Stars, planets, moons, and smaller objects create very faint gravitational waves, but only their larger and denser cousins create waves which are strong enough for us to study.

Physicists such as Henri Poincaré, Albert Einstein, and Arthur Eddington predicted gravitational waves in the early twentieth century. Indirect evidence that they existed was seen in the 1970s when two pulsars were discovered orbiting each other, getting closer and closer exactly as you’d expect if they were losing energy through gravitational waves. In 2015 LIGO finally directly detected gravitational waves, winning last year’s Nobel Prize in Physics for Rainer Weiss, Barry Barish, and Kip Thorne.

LIGO and Virgo both work by splitting a laser beam in two and firing the two beams at right angles to each other, to precisely measure the distance in two directions. When a gravitational wave passes by, those distances change and the lasers can measure it. That measurement is done by looking at the changes in the pattern the beams make when they’re put back together – a process called interferometry; “LIGO” stands for Laser Interferometer Gravitational Wave Observatory.

LIGO has two identical detectors in the US; one in Livingston, Louisiana, and one in Hanford, Washington. At least two detectors are required to filter out background noise; the LIGO team know they’ve seen a gravitational wave when both detectors see the same thing at the same time, but if they see something only in one detector it’s probably just something like a truck passing by, or a distant earthquake, or anything else that can cause small vibrations in the ground. Recently the two LIGO detectors have been joined by a third detector called Virgo (pictured above) in Pisa, Italy, which works in much the same way as LIGO. Adding a third detector allows astronomers to measure the direction of gravitational waves much more precisely, meaning they can narrow down the source of the waves to a smaller part of the sky.

There are a few different types of gravitational wave; some are created by big, violent events, while others have more low-key beginnings. They can all be detected the same way even though they’re created in very different ways. The type of gravitational wave that’s made the news recently is the compact binary inspiral wave. Those are waves created by really massive objects (e.g. black holes and neutron stars) colliding or merging with each other. When these objects collide, they don’t simply hit each other like balls on a pool table. Instead they spiral around each other, sending out gravitational waves in all directions which carry away energy and slow down the objects. Slowing down makes them move closer together, which makes them spiral faster, which sends out more waves, which slows them down more… and you get the picture. After a few thousand years they get close enough that they’re spiralling so quickly, and sending out such strong gravitational waves, that LIGO and Virgo can detect them. At that point there’s only a few seconds left before the objects finally merge and the detectors record what astronomers refer to as a “chirp”.

Burst gravitational waves are like compact binary inspiral waves, in that they are sudden, big events, but they’re not caused by objects spiralling together. No burst waves have been detected so far, but they could tell us a lot if we do eventually see them. That’s because no-one knows exactly how they might be created, but that should change once we see what they look like. Astronomers could detect a burst wave coming from a supernova, for example, and based on how the wave looks they might be able to figure out exactly what was happening inside the supernova when the wave was created.

The beams of a neutron star sweep past us, appearing as a pulsar. Image credit: Michael Kramer (JBCA, U. Manchester)

Next are continuous gravitational waves. These are waves which are there all the time (as the name suggests) and not big, one-off events. They should be created by any big, rotating objects which happen to be a bit lopsided. For example, a neutron star with lumps on its surface will send out a continuous gravitational wave as the lumps sweep around. Continuous gravitational waves could allow us to detect some neutron stars which would normally be invisible to us. It’s pretty hard to see neutron stars, except when they’re producing beams of radiation which happen to be lined up just right for us to see them as a pulsar (see left). At the moment the detectors aren’t sensitive enough to detect continuous gravitational waves, but soon they should be and that should allow astronomers to catalogue a lot of currently unknown nearby neutron stars.

The final type of wave is also the most common: stochastic gravitational waves. “Stochastic” just means they have a random pattern, and this category refers to small, faint waves that happen all the time. They’re background noise, like the static pattern on an old-fashioned TV or radio, and individually they don’t tell us much. Some of them could be left over from the Big Bang, though, so the overall pattern of stochastic waves could tell us something about the origin of the universe. This is especially important, because until now the earliest time astronomers have been able to study is roughly 300,000 years after the Big Bang (remember that light from far away takes a long time to reach us, so when you see something really far away you’re seeing how it looked a long time ago). For the first 300,000 years, the Universe was so hot and dense that light couldn’t travel freely; the Universe wasn’t transparent like it is now. That means we can’t use light to observe anything earlier. With gravitational waves, however, we might be able to see things much earlier – a tiny fraction of a second after the Big Bang – and that could tell us a lot.

While there’s been a lot of news in the last few years, gravitational wave astronomy is really only just getting started. LIGO and Virgo finished their latest observing run in August, and their next run is due to start later this year with better sensitivity, so you can expect to hear a lot more about gravitational waves very soon.

Further reading

Virgo detector image credit: The Virgo collaboration/CC0 1.0

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