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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How has the observable radius and mass within it varied over time?
How have the values for the radius of the observable universe and observable mass within this radius varied over the time of the universe?
Was the observable radius bigger or smaller in the past? Was ...
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Why is everything in the universe spinning? [duplicate]
What is the science behind all planets,stars, galaxy and maybe Black holes too spinning?
Is it only due to force of attraction or gravity or something else.
Isn't there any exception like the universe ...
AROHI GUPTA
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Verification of electron variable mass and charge from universe observations
Recently I have skimmed a very interesting article "On the same origin of quantum physics and general relativity from Riemannian geometry and Planck scale formalism", which tries to unite ...
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Existence of tachyons [closed]
I know that none of the experiments which have been conducted by physicists to conclusively prove the existence of tachyons have been successful.
This got me thinking. What if, we cannot find tachyons ...
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Are time and matter really impossible to be compatible or not? [closed]
**time and matter ** really impossible to happen? I went to chatgpt to clear this doubt with the AI, but my theory was intriguing to him, imagine a basin of water, the basin would be infinite, see it ...
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Universe: A Living Organism? [closed]
Is it possible for the universe to be a living entity within a larger system, similar to how a human body contains living cells? If so, could the expansion of the universe be analogous to the growth ...
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Laplace's demon and quantum mechanics paradox of not influencing our world? [duplicate]
We may regard the present state of the universe as the effect of its past and the cause of its future. An intellect which at a certain moment would know all forces that set nature in motion, and all ...
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Does Heisenberg's Uncertainty principle Imply the universe is finite/periodic? [closed]
Does Heisenberg's Uncertainty Principle imply the universe is either finite or periodic?
Heisenberg's uncertainty principle says your position can't be a "known" because the Fourier ...
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Time-temperature relation in matter dominated and dark energy dominated era
Can we derive a cosmological relation between time and temperature in matter dominated era and in dark energy era? Something like the radiation era expression here
$$T= \frac{t^{-1/2} 1.15}{k_B}$$
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Assuming that proton decay occurs, will all types of particles be dispersed without exception?
Proton decay is a theoretical possibility, as it is predicted to happen by some GUTs, so the proton may not be stable and could decay into lighter particles considering enormous time scales.
If proton ...
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Why is the universe depicted as elliptical in all images? [duplicate]
Is there a physical equation that describes the ellipse of the Universe? It seems to me that this is not the case, as most of the motions within it are chaotic (with the exception of the many motions ...
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The collapse of the wave function in the early universe
If the wave function collapse is true then what in the early Universe observed things to create all of the different particles?
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Could there be new particles after the universe will reach heat death?
Apparently, the universe will reach heat death in the extremely far future, after the last black holes would have evaporated. Even more, it is possible that all objects in the universe will turn into ...
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Is the universe accelerated expansion slowing down?
Some months ago there were several articles like this one (https://bigthink.com/starts-with-a-bang/physicists-question-fate-universe/) claiming that the universe's accelerated expansion was found to ...
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Is it possible that prior to the Big Bang, space and time were separate and were joined as a result of the Big Bang? [closed]
Is it possible space and time were separate prior to the Big Bang and the explosion was a result of their collision?
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Cosmological perturbation theory
I am reading the paper by Mukhanov, Feldman and Brandenberger (1991) on cosmological perturbation theory. Here is the corresponding text snippet:
For reference here is the text from the paper:
I have ...
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Why did scientists believe in an eternal universe before Lemaitre, even if the Kelvin paradox was already known since the 19th century?
Why did scientists believe in a static and eternal universe in Einstein times, before Lamaitre showed that the Einstein equations are compatible with a universe which started from a primeval atom, ...
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What is the relative density contrast?
I was reading a paper talking about the KBC Void, a local underdensity in the Universe which we are also part of.
The authors calculated: "[...] the observed relative density contrast δ ≡ 1 − ρ/...
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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 [closed]
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 ...
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Question about light and distance
If we see into the past with light and distance travelling so we can’t see things how they are currently, only how they were in the past; and James Webb took a photo from the beginning of the universe ...
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An unusual calculation of our universe's age? [closed]
Does the following make sense? And has anyone else come
across this odd ~’cosmological coincidence’ before?…
…If we posit that our total universe mass is:
(1) $$M_{U}=\frac{{M_{pl}}^4}{{M_{p}}^...
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How big was the universe after inflation? [duplicate]
If we assume the universe is a closed 3-sphere, how big was the universe after inflation, compared to nowadays? Was, relatively, most space already there? If we envision the universe as a 2-sphere, a ...
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