All Questions
Tagged with universe cosmological-constant
39
questions
0
votes
1
answer
55
views
How to find critical density?
In Cosmology critical density is defined as the minimum density for a flat universe to keep expanding, by Friedmann Equation:
${\left({\frac {\dot {a}}{a}}\right)^{2}={\frac {8\pi G}{3}}\rho -{\frac {...
- 411
1
vote
1
answer
61
views
How to understand critical density?
In Cosmology, critical density is given by setting $\Lambda = 0$ and $k = 0$, in other words, a universe without dark energy and zero curvature. According to my understanding and Wikipedia, this ...
- 411
0
votes
1
answer
67
views
Solve Friedmann equation for non-zero curvature and non-zero cosmological constant
I tried to find an elegant way to solve (without approximating for low densities)
$$\dot{R}^2=\frac{8 \pi G}{3 c^2} \rho R^2-k c^2+\frac{c^2 \Lambda}{3} R^2$$
for $k=\pm 1$ and $\Lambda \neq 0$ (one ...
- 181
0
votes
0
answers
48
views
Big Bang as stretching space time?
I am still new to researching the big bang so please be patient. I am having trouble envisioning the expansion. As I understand under current theory it is not to be thought of as a singularity ...
- 101
5
votes
2
answers
415
views
Properties of anti-deSitter space
I have some questions about anti-deSitter space, (note: I am not a physicist)
When describing deSitter space it is almost always mentioned that it has a positive cosmological constant and is therefore ...
- 191
2
votes
1
answer
161
views
DeSitter cosmological horizon stability?
If the universe keeps expanding at an accelerated rate (given by the cosmological constant) then the universe would approach a DeSitter spacetime where there would be a cosmological horizon that would ...
- 2,462
0
votes
0
answers
40
views
What exactly do astrophysicists mean when they say that the universe is expanding at an accelerated rate? [duplicate]
What exactly do astrophysicists mean when they say that the universe is expanding at an accelerated rate? Assuming that the universe is a sphere, do they mean that the radius of the universe increases ...
- 23
1
vote
0
answers
33
views
The General form of the Friedmann equation written in another way
Using the general form of the Friedmann equation:
$$H^2 =H_0^2(Ω_{m0}(1+z)^3+Ω_{r0}(1+z)^4+Ω_{k0}(1+z)^2+Ω_Λ)$$
and taking $a_0=1$, How can I derive that the Friedmann can be writing in the following ...
- 11
2
votes
1
answer
94
views
About a hypothetical universe
Imagine a universe dominated by matter, but it is balanced with a cosmological constant $\Lambda=4\pi G\rho$ so the universe is static ($H=0$). However, what would happen if some of that matter turns ...
20
votes
1
answer
869
views
Did Einstein really invent the cosmological constant to make the universe static in his 1917 paper?
The popular account of Einstein inventing the cosmological constant goes like this:
Einstein finds that the Einstein Field Equations predict an expanding universe
Unable to accept this, Einstein adds ...
- 565
0
votes
1
answer
117
views
Absence of matter stretches the fabric of space?
If matter attracts matter, because it curves the space, could it be that the absence of matter stretches the space,
and that's why the universe expansion is accelerating?
I mean that could be ...
0
votes
0
answers
35
views
What is the dark energy & dark matter? [duplicate]
Can anyone explain to me simply what is the dark energy or what is the dark matter. I have been trying to understand it deeply. but I somehow failed.
1
vote
1
answer
463
views
Einstein's Static Solution with $\Lambda =0$ to The Friedmann Equations?
Is it possible to show the universe is static (i.e., $a=a_*=\rm{const}$) without assuming $a=\rm{const}$ to begin with, and using a mix of $w=0$ and $w=-1$? Let $\Lambda=0$ in the Friedmann equations, ...
- 61
2
votes
1
answer
897
views
Static solution to the Friedmann equation [duplicate]
I want to find the static solution for the Friedmann equation:
$$ \bigg(\frac{\dot{a}}{a}\bigg)^2=H_0^2\bigg(\Omega_m\bigg(\frac{a_0}{a}\bigg)^3+\Omega_v+\Omega_k\bigg(\frac{a_0}{a}\bigg)^2\bigg) $$
...
- 135
1
vote
0
answers
51
views
Can a cosmological constant model inhomogeneities?
Consider the following zero-order approximation to the universe:
Spacetime is perfectly homogeneous, and
The cosmological constant is exactly zero.
This doesn't quite work. Neither assumption is ...
