All Questions
Tagged with space-expansion density
22
questions
0
votes
1
answer
42
views
Friedmann Equation and a contracting universe
Consider a universe with a nonzero curvature and matter.
One can write the Friedmann Equation in this universe as such:
$$\frac{H(t)^2}{H_0^2} = \frac{\Omega_0}{a^3}+\frac{1-\Omega_0}{a^2}$$
Where $H(...
- 411
1
vote
1
answer
250
views
How to get the density parameter of $\Lambda$CDM as a function of time?
For flat universe, the Friedmann equation is given by:
$$
H^2 = \frac{8\pi G}{3} ~~ \left( \rho_m + \rho_r + \rho_\Lambda \right).
$$
From this thread:
What is the equation for the scale factor of ...
- 395
-1
votes
2
answers
43
views
Is this right that the fate of space expansion depends on matter density inside the universe?
Is it right think that a right amount of matter density inside the universe could eventually stop the expansion of the universe or the expansion of space is something intrinsic only to space so the ...
- 2,255
0
votes
1
answer
102
views
The average density inside the particle horizon (observable universe) $ρ(t)$?
Does anyone know the average density of our universe and the particle horizon function as a function of time?
The average density inside the particle horizon(observable universe) : $ρ(t) $?
Radius ...
- 117
0
votes
1
answer
42
views
$Ω = ρ/ρ[c]$, so although near to 1, for an accelerating expansion $Ω$ must be below 1. What's its value?
$Ω$ is taken to have different components - ordinary matter, dark matter, dark energy. But because it is expressed in relation to the critical density for attractive gravity, it seems that omega is ...
- 85
1
vote
2
answers
205
views
Are galaxies and clusters of galaxies much denser when we observe the 'past' far universe than the 'present' closer part of the universe?
Are galaxies and clusters of galaxies much denser when we observe the 'past' far universe?If the universe is expanding, the galaxies should have been much closer in the past so should we see a larger ...
- 1,815
1
vote
0
answers
73
views
Negative mass density in cosmology?
I came across a problem in Chapter 6 of An Introduction to Modern Cosmology by Andrew Liddle that reads:
Identify a sufficient and necessary condition that must be satisfied by the equation of state ...
- 77
1
vote
1
answer
204
views
What is the jerk parameter in terms of the Density parameters and equation of state?
In cosmology the deceleration parameter defined as the
$$q_0 = \frac{1}{2}\sum_i\Omega_{i,0}(1+3w_i)$$
Is there a similar expression for the jerk parameter ($j_0$)?
- 3,122
1
vote
1
answer
80
views
Seeking a plot of the energy history of the Universe
How can we plot the energy density $\rho$ as a function of the age $t$ of the universe (or temperature $T$) from the time of radiation domination to the time of cosmological constant domination with ...
- 11.8k
0
votes
1
answer
441
views
How do you calculate curvature from the density?
We know that this density results in a flat universe.
$$\rho_c=\frac{3 H^2}{8 \pi G}$$
And we know that if the universe isn't flat, the density as a proportion of critical density can be expressed as ...
- 584
2
votes
1
answer
473
views
What is cosmological critical density?
I'm taking a first course in cosmology and I'm struggling to understand exactly what critical density is. I've been told "Critical density is the energy density of the universe that corresponds to the ...
- 87
0
votes
0
answers
94
views
Cosmology - Demonstration for equation of the evolution of the density contrast
In a context of cosmology, I need help about a differential equation that I can't get to demonstrate:
The growth of density fluctuations obeys a second order differential equation. At early enough ...
user87745
1
vote
1
answer
148
views
What is the density of a static dust sphere in an accelerating universe?
Consider a largish but finite sphere of pressure-less homogeneous dust late in the $\Lambda$CDM cosmology. The outside metric is presumably Schwarzschild-de Sitter with no matter, the interior a ...
- 32.9k
6
votes
2
answers
160
views
How do we know that dark matter energy density scales as $\rho\propto a^{-3}$?
How do we know that dark matter energy density scales as $\rho\propto a^{-3}$?
- 26.8k
0
votes
2
answers
534
views
How can the Friedman Equation produce negative pressure or density?
As I understand it, the Friendman Equation provides the driving mechanism for Inflation.$$\frac{\ddot a}{a}=-\frac{4\pi G}{3}\left(\rho+\frac{3p}{c^2}\right)$$If $\rho$ or p is negative enough, you ...
user32023