After detection of gravitational waves in 2015, what are it's implications on quantum gravity theories? Are there any quantum gravity predictions validated through GW detection?
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$\begingroup$ arxiv.org/abs/1907.02489 $\endgroup$– ProfRobCommented Apr 28, 2020 at 6:45
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The observation of gravitational waves allows us to probe physical regimes that thus far had been inaccessible to us. This allows us to search for deviations from general relativity caused by quantum gravitational effects. Many of such test are mediate through the use of extensions of general relativity (such as scalar-tensor theories) that can appear as the low energy effective theories for fundamental theories of quantum gravity (such as string theory).
Thus far such tests have only yield further constraints include: See https://arxiv.org/abs/1903.04467
Many alternative theories of gravity would lead to the graviton acquiring a mass. If that were the case gravitational waves of different frequencies would travel at different speeds, causing the shape of a gravitational wave signal to change as it travels through space. No such deformation of the signal is observer allowing us to put an upper bound on the graviton mass of $4.7\times10^{-23} eV/c^2$.
Alternative theories of gravity would also effect how a BH binary evolves, changing the waveform produced. For example, scalar-tensor theories could lead to production of dipole radiation leading to a faster merger process (and shorter waveform). The data has been search for various such deviations, and all mergers observed to date have been found to be consistent with plain GR. This provides a constraint on alternative theories of gravity (and by extension quantum gravity), albeit not very strong ones.
The near simultaneous observation of gravitational waves and electromagnetic waves from the merger of a neutron star binary tells us that gravitational waves travel almost exactly the same speed as light with the relative difference being at most $10^{-13}$. (This is smaller than accuracy with which the speed of light can be related to other measures of length under lab conditions!) This has eliminated large classes of alternative theories that would lead to gravitational waves travelling at another speed.
Finally, if the black holes involved in a merger do not actually have an event horizon, then generically the merger should be followed by gravitational wave echoes caused by the waves bouncing around in the potential well of the formed object. Such echoes could be observable even if the deviations from GR happen only a Planck length away from the (would be) horizon. Some groups have claimed to have found some evidence of such "echoes" in some of the public LIGO date. However, these claims have been generally refuted by more thorough analyses of the data by (among others) the LIGO Scientific Collaboration.
