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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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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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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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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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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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 {...
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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 ...
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In a universe with no photons, will everything necessarily be at absolute zero temperature?
Imagine a universe governed by the same physical laws as ours, i.e., the same fundamental forces, with the only caveat that there are no photons, hence no electromagnetic radiation in this universe. ...
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R-W Metric and null geodesic path of photon
I was reading through Introduction to Cosmology, on Chapter 3, it gives me the R-W Metric:
$ds^2 = -c^2dt^2+a(t)^2[dr^2+{S_κ}(r)^2dΩ^2]$
${S_κ}(r)$ is a function related to the curvature of space, κ ...
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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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How can baryonic CP violation contribute to the explanation of the large matter-antimatter asymmetry in our universe?
I have been doing baryonic CPV experimental search in the past few years. However, I never really get a good clue on this fundamental question.
The thing is that CPV has only been found in the meson ...
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Can glueballs and bosons survive indefinetely in space (forming structures)?
I have been recently interested in looking for possible structures (ranging from clumped structures like "stars" to diffuse clouds of gas or halos) made from standard model-particles other ...
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Matter density estimates in the 1980s
Liddle (2015, p.67) writes: "From the crude estimates that a typical galaxy weighs about $10^{11}M\odot$ and that galaxies are typically about a megaparsec apart, we know that the Universe cannot ...
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If we consider the spacetime of the universe to be four-dimensional, does the Big Bang lie in its center?
Apologies for the (hopefully now somewhat less) clickbait-y title. Now, of course, I know that the Big Bang did not happen at any point connected to a single point in our current $3$-dimensional ...
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If an area in 2D cannot be curved and finite is the same regarding the space of our pressumed 3D universe?
Is the sentence in the title right that our universe is infinite? And if so does it mean that stars are not evenly distributed along our universe but they all move from a populated centre to a fairly ...
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Noether's Theorem in relativistic cosmologies [duplicate]
Is Noether's Theorem valid within the context of relativistic cosmology? If not, does this mean that the universe does not conserve energy on cosmological time and distance scales?
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What would an inter-universal medium be like?
I don't really believe in the Multiverse Theory, but hypothetically, if universes were like soap bubbles in a foam, what might that foam be like? I know this seems like an opinion-based question but ...
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Horizon problem, what if our observable universe is roughly equal to the whole universe, especially in early times?
How do we know that at a time of 380.000 years, when CMB got free, the observable universe was not equal to the actual universe? Maybe they were roughly the same and couldn't that explain the horizon ...
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How does one draw the Penrose diagram for an FLRW universe with three different epochs?
Let's model the universe with the FLRW metric
$$ ds^2 =-dt^2 +a(t)^2\big(d\chi +R_k(\chi)^2 d\Omega^2\big)$$
where $a(t)$ is the scale factor and $R_k(\chi)$ is $\chi$ for a spatially flat ($k=0$) ...
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Does vacuum decay create/destroy energy?
From what I understand, vacuum decay involves the release of potential energy, but where does this energy come from? Is it created in the moment and how does it affect the energy already present in ...
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Can the universe be unpredictable but still have only one possible history? [closed]
This question will involve concepts in quantum mechanics.
So unless you believe in many worlds theory, certain outcomes out of a series of outcomes occur. But there seems to be an assumption that one ...
user353810
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Another universe due to a rogue wave fluctuation
Let us consider our Universe at its heat death state, and the rogue wave phenomenon that is due to improbable superposition of small waves. Is it possible that a rogue wave-like quantum fluctuation ...
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Could the universe have a form of a $T^3$-torus?
Cosmological measurements suggest that we live in a flat universe. However, what might be less clear is its topology. So could the flat universe have the form of a $T^3$-torus, i.e. the torus whose ...
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Was "flow of time" equally fast during the life of universe? Is Doppler Effect the only interpretation of "shift to red"? [duplicate]
I'm an IT developer and recently I created a project where I tried to send signals between two threads in a slowing down environment. I simulated two points with their own clocks and tried to send a ...
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Cosmic web shape
Does anybody know why the cosmic web is shaped like a web? I feel like it would be more likely that it is more like a galaxy with a supermassive black hole in the center, if that were true, what would ...
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Why does the total gravitational potential in the universe exactly equal the total mass energy RIGHT NOW? [duplicate]
In the zero energy universe model, the gravitational field has negative energy, and this negative gravitational energy of all the distant mass exactly balances and cancels the positive mass-energy in ...
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Does going to the other side of Milky Way galaxy mean seeing different observable universe?
What if there was a robot that got sent to the other side of the Milky Way by just traveling with a fusion engine and started orbiting a planet that has around Earth's gravity in order to avoid any ...
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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 ...
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Can we measure temperature in an isothermal Universe?
It is envisaged that ,in the future ,Universe can end in a big freeze, where there will be no energy gradient. It is also theorised that Universe was isothermal(with some irregularities I guess) in ...
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Are there any ways to conceptualize the relationship between gravity and space-time other than curvatures?
This might sound like a random question, but it came to me while I was trying to conceptualize the size of the universe and started thinking of entire galaxies resembling grands of sand floating ...
