Strong constraints on cosmological gravity from GW170817 and GRB 170817A

One of the many interesting scientific results to emerge from last week’s announcement of a gravitational wave source (GW170817) with an electromagnetic counterpart (GRB 170817A) is the fact that it provides constraints on departures from Einstein’s General Theory of Relativity. In particular the (lack of a) time delay between the arrival of the gravitational and electromagnetic signals can be used to rule out models that predict that gravitational waves and electromagnetic waves travel with different speeds. The fractional time delay associated with this source is constrained to be less than 10-17 which actually rules out many of the proposed alternatives to general relativity. Modifications of Einstein’s gravity have been proposed for a number of reasons, including the desire to explain the dynamics of the expanding Universe without the need for Dark Energy or Dark Matter (or other exotica), but many of these are now effectively dead.

Anyway, I bookmarked a nice paper about this last week while I was in India but forgot to post it then, so if you’re interested in reading more about this have a look at this arXiv paper by Baker et al., which has the following abstract:

The detection of an electromagnetic counterpart (GRB 170817A) to the gravitational wave signal (GW170817) from the merger of two neutron stars opens a completely new arena for testing theories of gravity. We show that this measurement allows us to place stringent constraints on general scalar-tensor and vector-tensor theories, while allowing us to place an independent bound on the graviton mass in bimetric theories of gravity. These constraints severely reduce the viable range of cosmological models that have been proposed as alternatives to general relativistic cosmology.

14 Responses to “Strong constraints on cosmological gravity from GW170817 and GRB 170817A”

  1. Peter, you have an astonishing knack of posting about something I was just wondering about!

  2. Yes, thanks for the compliment, Phillip.

  3. Two other papers I like on this are Creminelli and Vernizzi
    https://arxiv.org/abs/1710.05877
    and
    JEzquiaga and Zumalacárregui
    https://arxiv.org/abs/1710.05901

  4. Phillip, if I understand correctly, the backreaction posting is to a paper about DM constraints. I’m much more interested in constraints on DE!

    • It’s not obvious to me (having thought about it for 2 seconds) how this could work. Dark matter? By ruling out modified gravity designed to reproduce observations which dark matter can also reproduce. Since such modified gravity might predict different speeds of propagation, one can rule out some models. But dark energy? What effect could it have here? I’m not saying that there is none, just saying that it is not as obvious.

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