New source of gravitational waves discovered

Numerical relativity simulation of an neutron star black hole binary merger, showing the disruption of the neutron star

For the first time, scientists have picked up the ripples in space-time caused by the death spiral of two celestial juggernauts, a neutron star and black hole.

Scientists picked up the reverberations from these two objects using a global network of gravitational wave detectors, the most sensitive scientific instruments ever built.

This brand new source of gravitational waves was detected by an international team of scientists, with the UK taking a lead role.

Echoes in space-time

Gravitational waves are produced when celestial objects collide.

The ensuing energy creates ripples in the fabric of space-time which carry all the way to the detectors we have here on Earth.

On 5 January 2020 gravitational waves from this entirely new type of astronomical system were observed by:

  • Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detector in Louisiana in the US
  • Advanced Virgo detector in Italy.

LIGO and Virgo picked up the final throes of the death spiral between a neutron star and a black hole as they circled ever closer and merged together.

Multiple signals

Remarkably, just days later, a second signal was picked up by Virgo and both LIGO detectors in Louisiana and Washington state.

Again, the signal was coming from the final orbits and smashing together of another neutron star and black hole pair.

This is the first time scientists have seen gravitational waves from a neutron star and a black hole.

Previous gravitational wave detections have spotted black holes colliding, and neutron stars merging, but not one of each.

The role of the UK

The UK has contributed greatly to the field of gravitational wave astronomy, from helping to design and build the incredibly sensitive detectors to continuing to search the universe for these signals.

The UK’s contribution to the collaborations is funded by the Science and Technology Facilities Council (STFC).

The LIGO scientific collaboration comprises over 1,400 scientists from 19 countries, and includes researchers from 12 UK universities:

  • Glasgow
  • Portsmouth
  • Birmingham
  • Cardiff
  • Strathclyde
  • West of Scotland
  • Sheffield
  • Edinburgh
  • Cambridge
  • King College London
  • Southampton
  • Lancaster.

Analysing and interpreting data

UK scientists have contributed to the analysis and interpretation of the data collected from all the observing runs, including the data leading to this result.

Professor Sheila Rowan CBE, director of the University of Glasgow’s Institute for Gravitational Research, said:

These are our first robust detections of neutron-star-black-hole mergers.

This important new source of gravitational waves could help us to answer crucial questions about how, and where, neutron stars and black holes form in the cosmos.

Proving a theory

For several years, since the first ever direct detection of gravitational waves in 2015, astronomers have predicted that this type of system, a black hole and neutron star merger, could exist.

But there hasn’t been any compelling observational evidence.

Now that gravitational wave scientists have finally witnessed the existence of this new type of system, their detection will bring important new clues about how black holes and neutron stars form.

Dr Vivien Raymond, from Cardiff University’s Gravity Exploration Institute, said:

After the detections of black holes merging together, and neutron stars merging together, we finally have the final piece of the puzzle: black holes swallowing neutron stars whole.

This observation really completes our picture of the densest objects in the universe and their diet.

Professor Alberto Vecchio, Director of the University of Birmingham’s Institute for Gravitational Wave Astronomy:

We have been anxiously waiting to discover this kind of mixed systems for a long while.

Now that the wait is over, these new observations provide us with yet another glimpse of the surprisingly tangled paths of formation and evolution of neutron stars and black holes.

What comes next?

When gravitational waves are detected, astronomers can point their telescopes to look deep into space searching for an accompanying signal in the form of light.

Such a light signal was observed in 2017 when LIGO and Virgo observed two neutron stars colliding for the very first time.

It could also occur when neutron stars and black holes collide.

No light was observed from these two new events, however, possibly meaning that the neutron stars were ‘swallowed whole’ by their more massive black hole companions.

In future, as the LIGO and Virgo detectors are made even more sensitive, the team hope to detect many more neutron star black hole collisions.

They hope to observe a black hole tearing apart the neutron star in both gravitational-waves and light. This will help scientists to find out more about what neutron stars are made of.

Ground-breaking science

STFC has today allocated £9.4 million to UK universities and institutes for gravitational wave research, with hopes to continue the ground-breaking science.

Professor Grahame Blair, STFC Executive Director of Programmes, said:

Today’s exciting news from the gravitational wave community shows that space still holds secrets to be uncovered in this dynamic field of research.

From first witnessing these ripples in space-time a few short years ago, we are now seeing a rich harvest of new observations coming in.

This is the first time scientists have ever witnessed an event of this kind, and it goes to show why continuing to fund this research is vital in enhancing our understanding of the Universe.

Further information

The projects funded under the STFC gravitational waves grants include:

  • a consortium including the universities of Glasgow, Strathclyde, West of Scotland and Lancaster
  • Cardiff University
  • University of Birmingham
  • University of Cambridge
  • University of Nottingham
  • University of Portsmouth.

Themes the projects will be working on:

  • exploiting gravitational wave data to answer fundamental questions about:
    • the black holes and neutron stars in our universe
    • massive star evolution
    • the physics of compact object formation
    • the unknown properties of infant clusters and ultimately unveil the processes that govern the formation and evolution of stars in the Universe
  • realising the full scientific potential of the O4 and O5 observing runs at the Advanced LIGO and VIRGO gravitational wave detectors, including:
    • substantial increases in detector sensitivity
    • subsequent increases in the rate of observed events such as black hole mergers
  • fundamental research on suspension systems or materials and dielectric coatings at room and cryogenic temperatures. This is essential for successful operation of the Advanced LIGO+ and Virgo+ detectors and associated planned upgrades, and strategically critical for any future generation of detectors
  • support for Advanced LIGO operations, underpinning UK involvement in the LIGO Scientific Collaboration and access to observing data
  • exploiting gravitational wave data to answer fundamental theoretical questions about the nature of the universe and expanding on the standard model of particle physics
  • using gravitational wave data to produce modelling on possible nature of dark matter and dark energy.

Top image:  Image from a MAYA collaboration numerical relativity simulation of a neutron star black hole binary merger, showing the disruption of the neutron star (credit: Deborah Ferguson (UT Austin), Bhavesh Khamesra (Georgia Tech), and Karan Jani (Vanderbilt)).

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