On the fourth day of advent, we remember one of our most exciting cosmological events – watching the announcement that gravitational waves had been detected for the first time on 11 February 2016, surrounded by some of the most eminent cosmologists in the world.

"Ladies and gentlemen, we have detected gravitational waves!" David Reitze's announcement that afternoon was greeted by cheers and applause across the globe, including the common room upstairs from our office where we watched the press conference with physicists and mathematicians from the University of Cambridge. Reitze, the executive director of LIGO, the Laser Interferometer Gravitational-wave Observatory, announced that, on September 14 2015, they had finally detected ripples in the fabric of spacetime that were predicted by Albert Einstein all the way back in 1916. This was one of the most important events in physics over the last few decades.

Gravity is the manifestation of the curvature of space and time. Image courtesy NASA.

As their name suggests, gravitational waves are connected to the force of gravity we are all familiar with. As we saw behind door #2, one of the consequences of Einstein's theory of gravity is that when gravitational monsters such as black holes shunt their weight around, they should create ripples that can be felt across space and time. "Near black holes the curvature of space-time is extremely high," explained gravity expert Bangalore Sathyaprakash (in How does gravity work?). "Now imagine two black holes moving around each other: the curvature is large but also changing. It's a bit like taking a stick and moving it around in a pond. That's going to generate ripples in the water. Only in the case of black holes, we're talking about ripples in the very fabric of spacetime."

That these ripples should exist is important for theoretical reasons: not only is Einstein's theory correct in predicting those waves, tests performed on the gravitational waves that have been detected since 2016 have also confirmed that all gravitational waves travel at the speed of light. This rules out alternative theories of gravity in which this isn't the case.

But gravitational wave are also important for another reason: they give us a new way of listening to the Universe, and finding out about the objects that lurk within it. You can find out more behind the next couple of doors of the Plus advent calendar.

This year's advent calendar was inspired by our work on the documentary series, Universe Unravelled, which explores the work done by researchers at the Stephen Hawking Centre for Theoretical Cosmology and is available on discovery+. Return to the 2020 Plus Advent Calendar.