Third gravitational wave detection gives hints on dark matter and black holes

Latest observation by Ligo brings scientists closer to goal of using gravitational waves to see ancient events invisible to optical and radio telescopes

Ripples in the fabric of spacetime triggered by the cataclysmic merger of two black holes more than 3bn years ago have been detected by physicists.

The observations mark the third occasion that scientists have spotted gravitational waves – the compression and stretching of space itself that was first predicted by Einstein.

The latest detection, by the Laser Interferometer Gravitational-Wave Observatory, or Ligo, has produced intriguing hints on the nature of black holes and, potentially, dark matter.

The faint hum, picked up by Ligo’s twin instruments on 4 January 2017, could be traced back to the violent collision more than 3bn years ago of two black holes with masses of more than 31 and 19 times the mass of the sun, respectively. The two combined to form a single black hole with a mass slightly less than 49 times that of the sun and the leftover energy spilled out as deformations that spread outwards across spacetime like ripples across a pond.

Ligo scientists made the first historic observation of gravitational waves in September 2015, using a pair of 4km-long perpendicular pipes, one in Hanford, Washington state, the other in Livingston, Louisiana. A second detection occurred three months later.

The discovery provides new insights into the mysterious nature of black holes and, potentially, dark matter.
The discovery provides new insights into the mysterious nature of black holes and, potentially, dark matter. Photograph: LSC/OzGrav

With the third detection, scientists are beginning to close in on their goal of using gravitational waves as a way of observing ancient events that would be invisible to optical or radio telescopes.

Christopher Berry, of the University of Birmingham and a researcher on the project, said: “We’re not just in this business to detect gravitational waves. We’re really interested in doing a new type of astronomy.”

The detectors are sensitive enough to pick up distortions as tiny as one thousandth of the diameter of a proton in laser beams sent down Ligo’s tubes, and can even crudely recreate the dynamics of the collisions.

The astronomers believe their latest finding “provides clues about the directions in which the black holes are spinning”. As pairs of black holes spiral inwards, heading towards a collision, they also spin on their own axes. The latest observations hint that these spins are misaligned. This could indicate that the pair of black holes formed not from a binary star system, but from two independent black holes that randomly came together in a dense primordial star cluster.

Black holes and dark matter

Intriguingly, the observations are consistent with one of the more exotic explanations for the nature of dark matter – that dark matter haloes observed around galaxies are made up of a smattering of small black holes.

The theory would require these black holes to have been formed in the early universe, from patches of material– rather than stars – collapsing inwards. The latest observations reveal the existence of black holes in a “mass window” that would be consistent with this theory. However, the theory is still viewed as “a little exotic”, according to Berry.

Why discovering gravitational waves changes everything

The study once again puts Albert Einstein’s theories to the test. The researchers looked for an effect called dispersion, which occurs when light waves in a physical medium such as glass travel at different speeds depending on their wavelength; this is how a prism creates a rainbow.

Einstein’s general theory of relativity forbids dispersion from happening in gravitational waves as they propagate from their source to Earth. Ligo did not find evidence for this effect.

“It looks like Einstein was right, even for this new event, which is about two times farther away than our first detection,” said Laura Cadonati of Georgia Tech and a project scientist. “We can see no deviation from the predictions of general relativity, and this greater distance helps us to make that statement with more confidence.”

The research is published the journal Physical Review Letters.


Hannah Devlin Science correspondent

The GuardianTramp

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