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Features: Faculty Insights


To understand the world around us it's best to look and listen. That's as true in physics as it is in everyday life and it's one of the reasons why physicists at DAMTP are excited about the first detection of gravitational waves emitted by the collision of two neutron stars.

As well as causing gravitational waves — ripples in the fabric of spacetime — a merger of neutron stars also produces electromagnetic waves that can be seen by telescopes. The different signals are produced by different features of the system and observing them both can tell you a lot more than observing just one.

What I am most excited about, personally, is a completely new way of measuring distances across the Universe. Ulrich Sperhake

Last week scientists world wide, including researchers from the Department of Applied Mathematics and Theoretical Physics (DAMTP), announced the detection of gravitational waves emitted by the collision of neutron stars — the remnants of once gigantic stars collapsed down to approximately the size of a city. The gravitational waves reached Earth at 1.41pm UK time on Thursday 17 August 2017, and were recorded by the twin detectors of the US-based Laser Interferometer Gravitational-wave Observatory (LIGO) and its European counterpart Virgo.

While gravitational waves that stem from the merger of  two black holes produce "chirps" lasting a fraction of a second in the LIGO detector's sensitive band, the August 17 chirp lasted approximately 100 seconds and was seen through the entire frequency range of LIGO — about the same range as common musical instruments. Scientists could identify the chirp source as objects that were much less massive than the black holes seen to date. In fact, "these long chirping signals from inspiralling neutron stars are really what many scientists expected LIGO and Virgo to see first," said Christopher Moore, member of the DAMTP/Cambridge LIGO group and researcher at CENTRA, IST, Lisbon. "The shorter signals produced by the heavier black holes were a spectacular surprise that led to the awarding of the 2017 Nobel Prize in Physics." (See here to find out more about an earlier detection of gravitational waves.)

A few seconds after the detection of the gravitational waves an electromagnetic signal in the shape of a gamma-ray burst from the collision was also recorded by two specialist space telescopes. Over following weeks, other space- and ground-based telescopes recorded the afterglow of the massive explosion. Studying the data confirmed scientists' initial conclusion that the event was the collision of a pair of neutron stars. "This is a spectacular discovery, and one of the first of many that we expect to come from combining together information from gravitational wave and electromagnetic observations," said Nathan Johnson-McDaniel, researcher at DAMTP, who contributed to predictions of the nature of the collision.

There are a number of "firsts" associated with this event. The announcement confirmed the first direct evidence that short gamma ray bursts are linked to colliding neutron stars. The shape of the gravitational waveform provided a direct measure of the distance to the source, and it was the first confirmation and observation of the previously theoretical cataclysmic aftermath of this kind of merger - called a kilonova. By combining gravitational-wave and electromagnetic signals together researchers also used — for the first time — a novel technique to measure the expansion rate of the Universe.

New insights into the nature of neutron stars are also in the offing. "These objects are made of matter in its most extreme, dense state, standing on the verge of total gravitational collapse," said Michalis Agathos of DAMTP. "By studying subtle effects of matter on the gravitational wave signal, such as the effects of tides that deform the neutron stars, we can infer the properties of matter in these extreme conditions." Research papers on the aftermath of the collision event have also produced a new understanding of how heavy elements such as gold and platinum are created by supernova and stellar collisions and then spread through the Universe. More such original science results are still under current analysis.

And this isn't all. "What I am most excited about, personally, is a completely new way of measuring distances across the Universe through combining the gravitational wave and electromagnetic signals," said Ulrich Sperhake, Head of Cambridge's gravitational wave group in LIGO. "Obviously, this new cartography of the cosmos has just started with this first event, but I just wonder whether this is where we will see major surprises in the future."

Kip Thorne, one of the recipients of the 2017 Nobel Prize in Physics for his contribution to the detection of gravitational waves, agrees. "At some point there will be some giant surprises," he said at a recent lecture at DAMTP. "But I can't tell you when and I can't tell you what." Given the deep insights gravitational waves have already provided since they were first detected in 2016, those  upcoming surprises might well change our view of the Universe.

You can find out more about gravitational waves on Plus magazine and you can watch Kip Thorne's lecture 'LIGO and Beyond - Exploring the Universe with Gravitational Waves' here.

This article has been adapted from a University of Cambridge press release and is reproduced under a Creative Commons Attribution 4.0 International Licence. Image: Artist's impression of merging neutron stars. Credit: ESO/L. Calçada/M. Kornmesser.