# If we could only see gravitational waves, how “bright” would the sky be?

I feel like we actually are not sure about the answer (as there have not been many detections so far, so our knowledge about gravitational waves is probably not very deep), so this might be too speculative as a question. Even a qualitative explanation would help me a lot in this - I'm trying to picture "how the sky would look" for a hypothetical artificial being that can detect gravitational waves.

I'd also like to know the estimated energy density of gravitational waves in the Universe (I tried searching for it, but wasn't very successful in finding the answer quickly, but I might have searched poorly) and if that is relevant in the cosmological context (if that can even be calculated with current data). By the way, my understanding of gravitational waves is very close to 0, so being a bit didactic would be advisable - not saying you can't use equations, but I'd be more comfortable with numerical comparisons.

• "The cosmic energy inventory", arxiv.org/abs/astro-ph/0406095 estimates that of the total energy in primaeval gravitational waves is $<10^{-10}$, the contribution from binary stars is $10^{-9\pm 1}$ and from black holes $10^{-7.5\pm 0.5}$. For comparision, optical starlight is $10^{-5.8\pm 0.2}$. So the gravity wave sky is pretty dark. – Anders Sandberg Oct 26 '17 at 17:21
• @AndersSandberg seems like a pretty good answer to me, rather than a comment! – CDCM Oct 26 '17 at 17:37
• Pursuing the speculation: if a planet-based life-form could evolve to "see" only gravitational waves, wouldn't the sky seem "bright" to it? OTOH, it would likely soon become prey to anything nearby with more conventional senses... ;-> – iSeeker Oct 31 '17 at 18:24
• Hah. Well, maybe they wiped out all competition in their search to develop their sight; maybe we are not much unlike such hypothetical beings. – Vendetta Nov 1 '17 at 15:13
• I think you can estimate this: GW150914, LIGO's 1st observation was 1 billion light years away, and 3 solar masses were converted to gravitational waves. Compare with the Sun (mag -26.74), and report back. Also: consider the $\nu$ flux from SN1987A: $1.3 \times 10^{14}\,m^{-2}$ (at 4.2 MeV each): 83 Joules per square meter over a few seconds. – JEB Jan 26 '18 at 18:04

The cosmic energy inventory estimates that of the total energy in primeval gravitational waves is $<10^{−10}$, the contribution from binary stars is $10^{−9\pm 1}$ and from black holes $10^{−7.5\pm 0.5}$. For comparison, optical starlight is $10^{−5.8\pm 0.2}$. So the gravity wave sky is pretty dark.