Tuesday, October 16, 2018

New Astronomy Constraints On Extra Dimensions And Graviton Lifetimes

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, dGWL, with the inferred distance to the electromagnetic counterpart NGC 4993, dEML. We constrain dGWL=(dEML/Mpc)γ with γ=1.01+0.04−0.05 (for the SHoES value of H0) or γ=0.99+0.03−0.05 (for the Planck value of H0), where all values are MAP and minimal 68% credible intervals. 
These constraints imply that gravitational waves propagate in D=3+1 spacetime dimensions, as expected in general relativity. In particular, we find that D=4.02+0.07−0.10 (SHoES) and D=3.98+0.07−0.09 (Planck). Furthermore, we place limits on the screening scale for theories with D>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 of t>4.50×10^8 yr.
Pardo et al. 2018, "Limits on the number of spacetime dimensions from GW170817"

In many beyond the Standard Model theories, all particles and forces except gravity are confined to the 3+1 dimensions of General Relativity, but gravity can escape those dimensions to higher dimensions, which partially explains its relative weakness as a force. This result disfavors theories of that class.


neo said...

since most string theorists seemingly prefer compact spacetime dimensions,

do these results constrain compact spacetime dimensions, since couldn't gravitons leak into compact spacetime dimensions also resulting in attenuation?

Marnie said...

Thanks! This was fun to skim over. Nice to read about what the GW170817 signal (https://en.wikipedia.org/wiki/GW170817) means for GR.

"According to GR, the GW amplitude decreases inversely with luminosity distance. However, extra-dimensional theories of gravity with noncompact extra dimensions generally predict a deviation from this relationship."

"In summary, we find that GW170817 is fully consistent with GR."


Marnie said...


regarding your question "do these results constrain compact spacetime dimensions"

see page 4 of the paper:

"We stress that our results do not hold for extradimensional theories with compact extra dimensions (e.g. string theory or the ADD model). The extra di
mensions need to be at least on the order of the wavelength of the gravitational waves (∼ 100 km) in order to have a damping effect."

neo said...

Hello Marnie

there was a paper iirc that talked about "breathing" which also covers compact dimensions via gravitational waves.

neo said...

ACME II experimenthttps://www.nature.com/articles/s41586-018-0599-8

Marnie said...

Thanks, neo. The ACME II paper was written up in phys.org:


Also, I found the "Signatures of extra dimensions in gravitational waves" paper you were talking about:


Don't know about the massless breathing thing, but at least in the observability section at the end of the paper, the authors do discuss what it would take to observe shorter frequency gravity waves that cannot currently be measured by LIGO. Thanks for pointing out this paper.

andrew said...

Great discussion. Thanks everyone for the information brought to the table.

neo said...

"Signatures of extra dimensions in gravitational waves"

yes that was the paper i was alluding to, and it discusses "breathing"

i wonder whether the LIGO data + E&M could explore that, the combination, as the paper in the OP

ACME II paper seemingly suggests that the 100TEV collider won't find SUSY