Boran, S and Desai, Shantanu and Kahya, E O and Woodard, R P
(2018)
GW170817 falsifies dark matter emulators.
Physical Review D, 97.
pp. 1-6.
ISSN 2470-0010
Abstract
On August 17, 2017 the LIGO interferometers detected the gravitational wave (GW) signal
(GW170817) from the coalescence of binary neutron stars. This signal was also simultaneously seen
throughout the electromagnetic (EM) spectrum from radio waves to gamma rays. We point out that this
simultaneous detection of GW and EM signals rules out a class of modified gravity theories, termed “dark
matter emulators,” which dispense with the need for dark matter by making ordinary matter couple to a
different metric from that of GW. We discuss other kinds of modified gravity theories which dispense with
the need for dark matter and are still viable. This simultaneous observation also provides the first
observational test of Einstein’s weak equivalence principle (WEP) between gravitons and photons. We
estimate the Shapiro time delay due to the gravitational potential of the total dark matter distribution along
the line of sight (complementary to the calculation by Abbott et al. [Astrophys. J. Lett. 848, L13 (2017)]) to
be about 400 days. Using this estimate for the Shapiro delay and from the time difference of 1.7 seconds
between the GW signal and gamma rays, we can constrain violations of the WEP using the parametrized
post-Newtonian parameter γ, and it is given by jγGW − γEMj < 9.8 × 10−8.
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