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How Dark Matter Affects The Speed Of Gravitational Wave?

[Estimated read time: 3 minutes]
  • New calculations suggest that dark matter influence propagation of cosmic gravitational wave. 
  • The effects are too small to be measured by exiting or near-future detectors. 

For more than half a century, dark matter has remained one of the greatest mysteries in modern science. Dark matter and dark energy are two missing pieces of our universe that explain the large scale structure and observed expansion of the universe.

Recently, some direct observations of gravitational waves from distant sources (like a binary black hole merger) diverted scientists’ interests towards the propagation of these waves from source to detector.

Some scientists believe that properties of dark matter could be revealed by studying gravitational waves, in a much similar way seismic waves are used to analyze the interior structure of the Earth. But the question is how to measure the effect of cosmic matter on the propagation of gravitational waves.

In the previous study, scientists used Boltzmann equation to calculate the effect of collision-less neutrinos on gravitational waves. The outcomes of this measurement were further used to analyze the effect of the perturbation back on the wave. The effects were substantial when the method was applied to cosmic gravitational waves in the radiation-dominated era.

Now astrophysicists at the University of California and the University of Texas, Austin, have come up with new calculations that follow the same track, but the addition of non-negligible mass makes them quite complicated.

The Effects Are Undetectable

These new calculations suggest that gravitational waves could be influenced by the dark matter, however, the effects would be too small to be measured by exiting or near-future detectors.

The light slows down when it travels through vacuum into a material. This happens due to the interaction between material’s electromagnetic fields and light wave. The same thing could happen when gravitational waves meet dark matter: the speed of waves is reduced.

However, its effect is too small. According to researchers, dark matter would decrease the speed of gravitational waves with wavelengths equivalent to the size of our Universe by nearly one part per million.

In 2015, Laser Interferometer Gravitational-Wave Observatory detected a gravitational wave coming from a binary black hole merger. As per calculations, dark matter would reduce the speed of gravitational wave by just one part in 1045.

Reference: Physical Review D | doi:10.1103/PhysRevD.97.123506

Dark Matter Affects The Speed Of Gravitational WaveImage source: wikimedia

Researchers also discovered the effects of dark matter on primordial gravitational waves (not yet detected), which comes from cosmic changes in the early Universe.

In the early state of the Universe, dark matter is considered to have been bonded with neutrons and protons, and have travelled at relativistic speeds. Under these surroundings, dark matter would reduce the speed as well as the amplitude of gravitational waves.

Read: How Can We Find The Speed Of Dark Matter?

Neither of these effects has been detected yet. And, so far, scientists haven’t been able to figure out how to accurately calculate the reduced speed of gravitational waves.

However, they did demonstrate that the primordial gravitational waves’ spectrum contains a wealth of information about dark matter, such as interacting behavior of dark matter particles and temperature of kinetic decoupling.

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