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The FLRW energy equation $$\left(\frac{\dot a}{a}\right)^2~=~\frac{8\pi G\rho}{3c^2}$$ gives a solution for the scale factor $a(t)~=~a_0(t)exp(t~\sqrt{\frac{8\pi G\rho}{3c^2}})$. We have here that the density $\rho$ is the density of energy in the vacuum of space. This is most often thought of as due to the zero point energy of quantum mechanics. The ...

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I think there is no common explanation for this. Some people try to build some theories about that, but nobody can prove them. Every "beyond standard model" theory probably has it's own explanation for dark energy. One of them is still pretty interesting at least. I remember that Stanley Brodsky talked about that during the NED/TURIC meeting in 2014. He ...

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The particle horizon is at $$\eta(t)~=~\int_0^t\frac{dt'}{a(t')},$$ where for the de Sitter cosmology $a(t)~=~exp(t\sqrt{\Lambda/3})$. Here $\Lambda$ is the cosmological constant or parameter. The cosmological horizon is at $r_h~=~\sqrt{3/\Lambda}$. It is not hard to see that if I perform this integration that the particle horizon and the cosmological ...

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That's because light outside the particle horizon has started travelling while it was still inside, but has yet to arrive. The particel horizon has retracted beyond the starting point since then, but the light was already underway and is still inside the particle horizon. This also means that the observable universe will diminish to become as big as the ...

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As the general details of reheating are quite complicated, I won't say much about it, instead I'll just focus on answering the simpler part: given a Lagrangian, how to compute the decay rate (or, in the case of the first term, the cross section)? Now, the decay rate for a general reaction in which one initial particle decays into $n$ final ones is given by: ...

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I don't know if negative pressure (but see my added edit below) , more importantly there is a theory of inflation, and some good evidence for it. It was caused by a yet unknown inflation field, with its parameters somewhat matching what the cosmic microwave background (CMB) measurements show. [edit added: The field is a quantum field that rolled from a high ...

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Pocket universes have arisen in different theories. Just to name you two, one is alan Gut's inflationary theory idea that Eternal inflation produces pocket universes with all physically allowed vacua and histories. Another is that from sean carroll, who claims that inside every black hole there is an entirely new universe.

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