By Liam Mannix
In your everyday life, time and space know how to behave. A kilometre is a kilometre, and an hour is an hour – no matter how late you are running.
Perception is not reality. Einstein showed space and time were one thing, and it bends and warps. A decade ago, astronomers for the first time observed gravitational waves: universe-spanning ripples that move through space-time – and through us.
This discovery proved to be only part of the story. Astronomers are finding our universe is so permeated by gravitational waves, the fabric of reality is constantly in flux.
An artist’s impression of how space-time bends and foldsCredit: Carl Knox, OzGrav, Swinburne University of Technology
The picture that is coming into focus, says astronomer Dr Matt Miles, is one that shows the universe as a vast ocean, dark and powerful.
“The Earth is really just a ship, being bounced across the waves. The universe is constantly changing size. Time is constantly changing. The Earth is just riding that wave.”
In a series of papers published last week in the Monthly Notices of the Royal Astronomical Society, a group of Australian astronomers reveal the strongest evidence yet for what they call the “gravitational wave background”: the constant rippling of space-time.
Einstein imagined space like a piece of stretched fabric, with stars and planets and black holes scattered across it. Their mass bends and curves the fabric. The moon orbits the Earth because it is trapped in the depression in space-time created by the Earth’s mass – it spins like a coin in a funnel.
Now, on the stretched fabric of space, introduce a very heavy spinning object: a pair of super-massive black holes orbiting each other.
These spinning objects create ripples that spread through the sheet: gravitational waves.
As these waves pass through us, space is distorted just slightly. The Earth – and everyone on it – widens and then shrinks.
Why don’t we notice? Because a fish that lives in the ocean does not notice the waves.
“Waves and currents pass through the literal fabric they live in,” says Rowina Nathan, an astronomer at Monash University who co-wrote one of the papers. “It’s the same for us.”
Scientists have been spotting single waves passing through Earth since 2015. The new papers show waves are rippling throughout the entire universe.
The LIGO interferometer in Livingston, Louisiana, which is used to detect gravitational waves.
The team took observations of pulsars, rapidly spinning remains of supernovas, which emit bursts of radiation with such regularity they can be used as cosmic lighthouses.
Over almost five years, the team used the MeerKAT radio telescope in South Africa to measure the distance between Earth and 83 pulsars.
They watched as the distances widened and shrank – by up to the size of a tennis court – revealing what Miles describes as a “cacophony of noise” from the many waves they could see.
With this information, they have created the most detailed maps of gravitational waves across the universe to date.
Exotic explanations
The next question to answer: what is causing these waves?
The simple answer, if you can call it that, is super-massive black hole pairs. We know they produce single waves; it is likely there are enough of them out there to set the fabric of reality constantly in motion.
But there are other more exotic explanations. “The physics is a little wild,” admits Miles, an astronomer with OzGrav at Swinburne University who led two of the papers.
The first is known as “cosmological phase transition”. In the moments after the Big Bang, the universe was so hot the physical forces, like electromagnetism, that govern our reality had not yet formed.
Eventually, parts of the universe cooled enough for basic physics to start functioning. But – in theory – this did not happen everywhere at once.
This produced regions within the early universe operating on different physical laws. As these regions – or bubbles – came into contact with each other, the different physics collapsed into symmetry, releasing huge amounts of energy – and, possibly, gravitational waves. The gravitational wave background may be the leftover ripples from the bursting of those bubbles.
If that theory is not exotic enough, there’s a second: cosmic strings. When the regions of the early universe came into contact, rather than bursting, they may have created long fractures – imperfections in the fabric of space-time.
“They get frozen in when the universe cooled from its very hot state. And you can’t get rid of them,” says Professor Eric Thrane, an astrophysicist at Monash University. As these long fractures move, they could also generate gravitational waves.
“People really believe in these. We don’t know if we’ll ever be able to prove it,” says Miles.
Gravitational waves further tell us the universe is a strange and wonderful thing.
“The universe is this very exciting cosmic dance,” says Miles. “And the Earth is going along with the motions.”
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