New answers tagged space-expansion
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How is Hubble's constant (the expansion rate) predicted from LCDM and the CMB?
Gabe, you are talking on the “relationship between angular size and physical size is familiar from standard trigonometry, rs=θsDA”: if rs is physical, and θs is the measured angle, then I think DA ...
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Does scalar field dark matter behave like dust in cosmology?
Thanks to @knzhou for clearing up my confusion in their comment. I'm not sure where I got the idea that $p=\rho$ for a scalar field, but this is not always the case. Looking at the energy density and ...
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Is the angular diameter distance to the surface of last scattering the product of a circular argument?
If the physical size of the sound horizon at the surface of last scattering is fixed (e.g., Jungman et al. 1995), then the angular scale associated with this depends on the geometry of the universe.
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Does inflation really answer why the universe is almost flat?
You are confused by the standard way FLRW metric is written where in 3d metric you keep $a^2(t)$ as a common factor multiplied on sphere or hyperboloid metric with unit curvature radius. However, in ...
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Can a closed universe become open?
In a closed universe the density $\rm \rho$ is higher than the critical density $\rm \rho_c=3 H^2/(8 \pi G)$, in a hyperbolic one it's lower and in a flat one it matches.
If you could somehow change ...
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Why don't cosmologists consider that the universe has been expanding at a constant rate since the big bang?
I can't see why you think this would be the simplest explanation. A universe expanding at a constant rate would have to be empty, since the presence of matter slows the expansion. And if you measure ...
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Horizon problem and the infinite whole universe
Yes the universe could be structured on scales larger than those we can observe.
The fact that the universe is close to homogeneous on very large scales (but within the scales we can observe) is an ...
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Horizon problem and the infinite whole universe
ABC asked: "Doesn't this mean outside that scale in other parts of the universe, beyond our observable universe, things were never in causal contact and thus may have different temperature?"
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Accepted
Computation of conformal Hubble parameter
cciopi.py wrote: "$\rm \mathcal H=a'(\eta)/a(\eta)$ where $\rm a'(\eta)$ is the derivative of the scale factor with respect to the conformal time."
This expression for $\mathcal H$ is just ...
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Why did dark energy play a subdominant role in the radiation and matter dominated era?
According to me this means that the expansion of the universe was all the time slowing down till dark energy came to play so the slowing down of the expansion of the universe was getting
diminished. ...
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Accepted
Initial condition for scalar perturbation in cosmology
This is the Bunch-Davies vacuum state. It is de Sitter invariant and corresponds to the zero-particle state as seen by a geodesic (free-falling) observer. In other words, at high momentum, this ...
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Does inflation imply the existence of dark energy?
There is both a nomenclature problem and a circular reasoning problem in your question.
The nomenclature issue is that "inflation" is not shown on your diagram. It is inferred to take place ...
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Does inflation imply the existence of dark energy?
This question seems to be asking that if we have the requirement that the universe is expanding as fast as we observe it to be today (yellow circle), and we have the requirement that the universe went ...
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