# Questions tagged [friedmann-equations]

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### Luminosity distance in a FRW Universe

In the following presentation: https://www.slac.stanford.edu/econf/C0307282/lec_notes/pinto/pinto.pdf we have the relation for the luminosity distance in a FRW universe: \begin{aligned} D_L & =\...
• 189
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### How long has it been since dark energy started to reverse our slowdown and accelerate the expansion of the universe? 4 billion years ago? 4.5? 5?

Or has it been as long as six billion years? Over the last few years I keep seeing longer and longer numbers... Have researchers noticed something different recently? Or found a mistake in their ...
• 5,177
1 vote
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### Critical density of universe with intrinsic spatial curvature

$H^2 = \frac{8 \pi G}{3}\rho-\frac{kc^2}{a^2}$ Shown above is the first Friedmann equation. I understand that the curvature parameter, k, represents the spatial curvature of the universe which is ...
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### 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 ...
• 2,182
1 vote
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### 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 $ds^2=0$ ...
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### 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 ...
• 189
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### 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. ...
• 129
1 vote
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### 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 ...
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### 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 ...
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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 $H(t)$ ...