# Could Dark Energy be a “Cosmic Gravity Background”

Could dark energy (the accelerating expansion of the universe) be the result of gravity from the dense past universe "pulling" outward from all directions? The universe theoretically started out extremely dense and hot, and we can see the light reaching us as the cosmic microwave background. I wonder if that same region and denser portions beyond are exerting a gravitational effect that is stretching out everything within toward it, effectively being a cosmic gravity background. If so, is this the explanation for dark energy?

• Gravity on a universal scale has to be treated in the context of General Relativity and the Robertson-Walker metric. Your question does not make any sense because every place causally connected to us in the universe was coincident in the past. – Rob Jeffries Apr 21 '17 at 14:15

There is in fact a cosmic gravitational wave background. These waves are expected to be stochastic, having originated in the early universe (much earlier than the cosmic microwave background). Random fluctuations were subsequently stretched during inflation, making them observable over many wavelengths. A good and reasonably up-to-date introduction I read is Lasky et al. (2016), which gives some of the most recent constraints on the parameters of the frequency spectrum $\Omega_{\text{GW}}(f)$. The great thing about the wide spectrum is that a number of different tools (pulsar timing arrays, interferometers like LIGO, etc.) can be used to constrain different parts of it, both directly and indirectly.

That said, dark energy is almost certainly not a result of the gravitational wave background. First, there's no theoretical mechanism to explain how such an effect would arise. Second, the background is probably not strong enough to have an influence on the current expansion of the universe (see the graph here to see how weak it is). Finally, dark energy is a relatively recent phenomenon, arising several billion years ago. The background was around long before then, and yet had no effects.

• Are we sure the dark energy effect wasn't around earlier? It would seem that the inflation near the very beginning would be a strong presence of dark energy. Thank you for your detailed answer! – Jonathan Apr 21 '17 at 2:38
• @Jonathan Inflation and dark energy are almost certainly two completely different things. – HDE 226868 Apr 21 '17 at 2:57
• @Jonathan Let me just clarify HDE's statement. Dark energy is certainly a recent phenomenon, but the idea is that the strength of dark energy (whatever that means) is related to the total volume of space. As space expands, dark energy gets stronger, making space expand faster, ad infinitum. Dark energy has always existed (hypothetically) but it's only recently (i.e., the past several billion years) that it has "built up enough" to have a noticeable effect on our universe. As such, it cannot be tied to inflation in the early universe and that inflation was caused by some as-yet-unknown source. – zephyr Apr 21 '17 at 16:02

Could dark energy (the accelerating expansion of the universe) be the result of gravity from the dense past universe "pulling" outward from all directions?

No, because gravity alters the motion of light and matter through space. It doesn't make space fall down in some Chicken-Little fashion. You may hear that it does, as per the waterfall analogy, but it doesn't. In similar vein the "reversed" gravity that you're suggesting doesn't make space fall up, as it were.

The universe theoretically started out extremely dense and hot, and we can see the light reaching us as the cosmic microwave background.

I think most people are fairly happy with the CMBR.

I wonder if that same region and denser portions beyond are exerting a gravitational effect that is stretching out everything within toward it, effectively being a cosmic gravity background.

It's good to think for yourself, and it's good to wonder. But I have to say I think this idea is a non-starter. We have galaxies receding faster than light (see this paper). That means it's space expanding, not gravity pulling.

If so, is this the explanation for dark energy?

Sorry, no. If I were you I'd have a look at vacuum energy.

• "We have galaxies receding faster than light." A more correct statement is that we predict some galaxies are receding faster than light. We have no direct observational evidence for this and thus you can't use this as prove that space is doing the expanding instead of gravitational influences. – zephyr Apr 21 '17 at 15:52
• Good point about galaxies receding faster than the speed of light. I think that might still be plausible in my proposed idea, as I believe it would possibly "stretch space", but it does make me think. – Jonathan Apr 22 '17 at 1:34