# Tag Info

### Particle production in an expanding universe?

You get an approach to a finite de Sitter temperature $T=\hbar H/2\pi k_B\approx 2.67\cdot 10^{-30}$ K in a universe expanding forever due to a cosmological constant, and this does produce a very ...
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### Particle production in an expanding universe?

This was the hypothesis behind the steady state cosmology. Observations (particularly of the cosmic microwave background) do not agree with its predictions, so it is no longer accepted by most ...
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### How can the Friedman Equation produce negative pressure or density?

You could imply a negative pressure from $\rho + 3P$ Which features in the Friedmann equation under some strict stipulations, but an alternative way is to just consider $dE = d\rho V + \rho dV$ Which ...

### Why is the monopole problem a problem?

But we have no evidence that such exotic particles even exist! Exactly. We don't see them, despite the fact that they are predicted by a lot of beyond-the-Standard-Model theories. This means there is ...
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### Why do we need inflation?

There are three commonly-given reasons to believe in cosmic inflation, and they're all given in the Wikipedia article. The horizon problem. The universe is homogeneous on large scales when there is ...
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### Why do we need inflation?

Cosmic inflation is an expansion of the metric of space itself, not an expansion or explosion into space like a supernova. As such, the rate of expansion is not limited by the speed of light. The ...
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### Is the speed of light $c$ invariant with the vacuum space energy density?

In SI units the speed of light is fixed at exactly 299792458 m/s. It cannot vary, by definition. Even during inflation, if you are talking about c in SI units then it is fixed by definition.
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