REMAIN SOME POSSIBILITIES FOR ALTERNATIVE THEORIES OF GRAVITY?

Image: Numerical simulations of the gravitational waves emitted by the inspiral and merger of two black holes. The colored contours around each black hole represent the amplitude of the gravitational radiation; the blue lines represent the orbits of the black holes and the green arrows represent their spins. Credit: C. Henze/NASA Ames Research Center

The observation of gravitational-wave signal by LIGO and VIRGO, corresponding to the inspiral and merger of two black holes, are consistent with the Einstein theory of gravity with high accuracy limited mainly by the statistical error.

In a recent paper (Konoplya & Zhidenko, 2016) the authors suggest that there is a number of alternative theories of gravity which produce the same black-hole behavior at far distances from their surfaces, but lead to qualitatively different features near the event horizon.

LIGO and Virgo data provide the angular momentum and mass of the final black hole with rather large allowance of tens of percents. The authors show that this indeterminacy in the range of the black-hole parameters allows for some not negligible deformations of the Kerr spacetime leading to the same frequencies of black-hole ringing. This means that at the current precision of the experiment there remain some possibilities for alternative theories of gravity.

Image: Simulation of a black hole merger. Credit: NASA's Goddard Space Flight Center/P. Cowperthwaite, Univ. of Maryland

Thus, they claim that there might exist a strongly deformed Kerr-like black hole, corresponding to an alternative theory of gravity, such that its behavior in the post-Newtonian regime is quite similar to Kerr black hole, while its near-horizon behavior is different.

The authors conclude that in order to disprove the above proposal, one needs to determine black-hole parameters with high accuracy. In the future this could be done either by improving the accuracy of detection of the gravitational-wave profile or with complementary observations of black holes in the electromagnetic spectrum, which could potentially give us an image of a black hole.

Konoplya & Zhidenko 2016 - Detection of gravitational waves from black holes: Is there a window for alternative theories? (arXiv)
Simulation of a black hole merger - (NASA)

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