New answers tagged

0 votes

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 ...
0 votes

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 ...
  • 6,171
0 votes

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 ...
0 votes

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 ...
  • 2,592
16 votes

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 ...
  • 17.2k
11 votes

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 ...
  • 36.6k
2 votes

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.
  • 70.7k
0 votes

What tells us the speed of light is constant in another galaxy?

The short answer is the cosmological inflation theory. The more analytical: The speed of light $c$ in a vacuum is given by the equation: $$ c=\frac{1}{\sqrt{\varepsilon_0 \mu_0}} $$ where $\...
  • 3,291
2 votes
Accepted

Explaining inflation in $\Lambda$CDM

$\Lambda$CDM is a model primarily intended to describe the universe only after its earliest moments. By "after earliest moments" I mean, roughly, after the Planck era and after a rapid ...
0 votes

Why is the flatness problem a problem?

The flatness problem is akin to asking: why does the universe have these initial conditions? This is a question that, by definition, cannot be answered in the framework of physics, because every ...
  • 2,651

Top 50 recent answers are included