All Questions
Tagged with cosmology cosmological-constant
248
questions
10
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526
views
Age of a dark energy dominated universe
In a flat universe that is dominated by dark energy (or cosmological constant), the Friedmann equation can be written as:
$H^2 = (\frac{\dot a}{a})^2 = \frac{8\pi G\varepsilon_{\Lambda}}{3c^2}$
Where $...
- 411
0
votes
1
answer
55
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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
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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
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1
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If space has a positive curvature, is the expansion of the universe caused by time, not "dark energy"? [closed]
Ok, I will assume that space has a positive curvature, where space is the "surface" of this sphere, and time is the radius from the center, so the universe is a 4D hypersphere. Under these ...
- 403
0
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1
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128
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Do we really know the universal gravitational constant?
We've all heard $$F_g=\frac{gm_1m_2}{r^2}.$$ However, since I took physics, we've discovered "dark energy," which if I have any concept of the current thinking is caused by space being ...
0
votes
0
answers
70
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Different types of Dark energy and conservation of energy
According to this Sean Carroll article, and other threads in here, depending on your definition of energy, dark energy does not violate conservation of energy. My question is if this is true ...
- 117
1
vote
1
answer
94
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Why do people say that the $\Lambda$-CDM model has six independent parameters?
The Wikipedia article on the $\Lambda$-CDM model says that the model has six "independent parameters". It also says that the model has several "fixed" parameters and several "...
- 48.4k
1
vote
0
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42
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Model of Quintessential Inflation
I have a question about how the potential for an inflaton field is selected. It is clear that there are limitations associated with number of e-folds, scalar-to-tensor relation and scalar spectral ...
- 11
0
votes
1
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70
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Cosmological implications of String theory compactification?
Is the process of compactification of hidden dimensions in string theory equivalent to an increasing dilaton field?
Would one expect the compactification process to continue indefinitely?
Could the ...
- 5,925
4
votes
1
answer
101
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Sign and physical meaning of the cosmological constant
I've heard that a cosmological constant can be used to model dark energy (e.g. $\Lambda$-CDM model), and that the constant $\Lambda$ should be positive. But my (quite small) understanding of dark ...
- 163
0
votes
1
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67
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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
2
votes
1
answer
148
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Negative $\Lambda$ FLRW spacetimes as infinite black holes?
Consider the Friedmann equation:
$$H^2+\frac{k}{a(t)^2} = \frac{\Lambda}{3}+\frac{8 \pi}{3}\rho$$
and set the parameters for dust in either flat euclidean or open hyperbolic spatial slices with a ...
- 125
0
votes
0
answers
94
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Measurement of the Cosmological Constant
Is there some way to measure Lambda, the cosmological constant, independent of $H_o$,
the Hubble constant and omega_lambda, the Dark Energy density? A standard equation for calculating Lambda, ...
- 149
2
votes
0
answers
24
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Why is there an infinite supply of energy in slow-roll inflation?
The physical model of inflation includes a metastable false vacuum, or a slow-roll field on a flat potential. In either case, I just realized how this is completely insane. With the exponential growth ...
- 619
2
votes
3
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189
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Will cosmic microwave background become invisible in the future?
If my understanding of CMB and Hubble's Law is correct, then CMB photons emitted from more than ~14.4 Glyr during Recombination Epoch would not reach us. The reason is this would correspond to Hubble'...
- 131