Questions tagged [friedmann-equations]

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3 votes
1 answer

Size of the whole universe if it were spherical

Suppose the universe is spherical and its density ratio is $\Omega \leq 1.00125$ $\Omega = 1.00125$ is approximately the maximum possible value of the density ratio according to the Planck Mission ...
  • 801
1 vote
1 answer

How can a time-like trajectory be calculated using the Friedmann-Robertson-Walker metric?

Using the following FRW metric, $$ds^2=-c^2dt^2+R^2(t)(d\chi^2+f(\chi)d\Omega)$$ and considering $d\Omega=0$ from now on, it is clear that for a light-like trajectory, the equation must read $ds^2=0$ ...
  • 147
2 votes
1 answer

Strange plot in Max Tegmark book, Our Mathematical Universe

In Max Tegmark's book, Our Mathematical Universe, we can find (in chapter 5, figure 5.3) the following (horrible and poor quality) plot that is supposed to highlight the extreme sensitivity of the ...
2 votes
1 answer

Why are galaxies thought to get bigger with distance in an expanding universe model?

In an expanding universe model. In addition to galaxies getting fainter with distance, they are also thought to get bigger. Thus the surface brightness of the object should decrease with distance. ...
  • 29
1 vote
0 answers

Does the Schwarzschild metric solution require two (A and B, or g11 and g44) or four components (g11, g22, g33 and g44)?

Different places on the web imply that Schwarzschild's metric uses four components or separate equations, similar to how Einstein's full set requires 6 or 10, or they say only two are needed. Is it 2 ...
  • 4,161
0 votes
1 answer

If the ratio of actual total mass to critical mass (Ω) in the universe is only about 0.3, why do cosmologists believe it is flat, as far as we see?

What about astronomical observations makes scientists believe our universe is flat, at least as far as they can tell? Despite the critical Friedmann density being less than one, plus the existence of ...
  • 4,161
5 votes
0 answers

How did pressure evolve in the (early) universe?

I am trying to derive how the cosmological pressure $p(t)$ evolved over time in the universe, especially in the radiation and matter dominated epochs. There are some very nice explanations how $H(t)$ ...
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