Questions tagged [universe]
The universe refers to the cosmos; all of space-time and that which exists as part of it. Alternatively, it can refer to the observable universe, which only contains the part we can see. Questions tagged with this should ask about physics at scales the size of the universe or specific properties of the universe
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Dark matter in the time window between freeze-out and kinetic decoupling
Background
After the freeze-out, when all annihilations have stopped, the abundance ($Y=\frac{n}{s}$) of thermal dark matter species no longer changes with time. However, it is still kept in kinetic ...
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Why is all matter in the universe, not found at its lowest state of potential energy? [closed]
Preface: it may be of interest that I am a second year Biology student, with no experience in studying Physics and a very basic understanding of Mathematics. ...
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What is meant by "spontaneous creation" in this paper?
I have some questions in regard to the paper "Spontaneous creation of the universe from nothing". If I am not mistaken it is akin to Alexander Vilenkin's proposed cosmological model that has the ...
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Simple question about finite Universe [duplicate]
If, by Big Bang, Universe was created from initial singularity, with finite "speed" of expansion of matter, shouldnt it be finite as well?
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Does the universe have a center? [duplicate]
If the big bang was the birth of everything, and the big bang was an event in the sense that it had a location and a time (time 0), wouldn't that mean that our universe has a center?
Where was the ...
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Is the observable universe analogous to a white hole?
My instinct is no, but my lack of understanding with respects to white holes doesn't tell me why. My thinking is this: The universe is expanding and the further away from us the faster it is expanding....
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Are there universes where "symmetry breaking" went differently? [closed]
What have happened with other possible variants of asymmetry?
Are there other universes being run in parallel to our universe where the ball is not at C, but at B?
Stephen Wolfram told
I have found ...
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Is the whole universe a closed quantum system?
By the whole universe I mean everything besides $|0\rangle$, if not what is the environment then? How do they interact?
If the whole universe is a closed system, can we assign a single Hamiltonian to ...
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Age of universe vs Hubble time in Milne universe
Consider an empty universe where energy density $\varepsilon = 0$, thus the Friedmann Equation can be reduced into:
$\dot a^2= -\frac{kc^2}{R_O^2}$
$k$ is the curvature of space, $R_0$ is the radius ...
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Negative Horizon distance
Consider a flat universe, here, proper distance can be given by R-W Metric:
$$d_p (t_0) = c\int_{t_e}^{t_0}\frac{dt}{a(t)},$$ $t_e$ is the time when a photon is emitted from a distant galaxy, $t_0$ is ...
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Dark energy and conservation of energy in General relativity [duplicate]
i know that conservation of energy in general relativity has been discussed multiple times here at PE, a popular explanation on the topic is Sean Carroll's blog "Energy is not conserved" ...
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Curves on LCDM spacetime diagram
I have a question concerning the LCDM spacetime diagram on this Physics Stack Exchange post.
How are the set of curves for $z = \text{constant}$ (1, 3, 10, 50, etc) constructed or calculated, i.e. ...
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Origin of electromagnetic radiation in the universe
I don't know if the total electromagnetic radiation in the universe is finite or not.
But either way: what is the origin of the electromagnetic radiation in the universe? Can we say that all of it ...
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According to Hubble's Law, how can the expansion of the Universe be accelerating?
Scientists today think the expansion of the universe is accelerating.
According to Hubble's law, objects further away are moving faster than objects closer to us. The further away an object is, the ...
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Why are spherical shapes so common in the universe?
I have a simple question. Why are most objects in the observable universe spherical in shape? Why not conical, cubical, cuboidal for instance? I am furnishing a few points to justify this statement:
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