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**Limits on the number of spacetime dimensions from GW170817**

Kris Pardo, Maya Fishbach, Daniel E. Holz and David N. Spergel

Published 23 July 2018 • © 2018 IOP Publishing Ltd and Sissa Medialab

Journal of Cosmology and Astroparticle Physics, Volume 2018, July 2018

http://iopscience.iop.org/article/10.1088/1475-7516/2018/07/048/pdf

Abstract

The observation of GW170817 in both gravitational and electromagnetic waves provides a number of unique tests of general relativity. One question we can answer with this event is: do large-wavelength gravitational waves and short-frequency photons experience the same number of spacetime dimensions? In models that include additional non-compact spacetime dimensions, as the gravitational waves propagate, they "leak" into the extra dimensions, leading to a reduction in the amplitude of the observed gravitational waves, and a commensurate systematic error in the inferred distance to the gravitational wave source. Electromagnetic waves would remain unaffected. We compare the inferred distance to GW170817 from the observation of gravitational waves,dLGW, with the inferred distance to the electromagnetic counterpart NGC 4993,dLEM. We constraindLGW = (dLEM/Mpc)γ with γ = 1.01+0.04−0.05 (for the SHoES value ofH0) or γ = 0.99+0.03−0.05 (for the Planck value ofH0), where all values are MAP and minimal 68% credible intervals. These constraints imply that gravitational waves propagate inD=3+1 spacetime dimensions, as expected in general relativity. In particular, we find thatD= 4.02+0.07−0.10 (SHoES) andD= 3.98+0.07−0.09 (Planck). Furthermore, we place limits on the screening scale for theories withD>4 spacetime dimensions, finding that the screening scale must be greater than ~ 20 Mpc. We also place a lower limit on the lifetime of the graviton oft> 4.50 × 108 yr.