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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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Is there any reliable place where i can put my theory of universe (it explained/predicted the behavior of almost everything i came across) [on hold]

Is there any reliable place where i can put my theory of universe (it explained/predicted the behavior of almost everything i came across), so that people can ask question and i can try and explain ...
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1answer
52 views

Solution of Friedmann equation in terms of conformal time

We know that for the flat geometry of lamda CDM model, the solution for Friedmann equation is $$ a(t) = \left\{ \frac{Ω_{m,0}}{Ω_{Λ,0}} \sinh^2 \left[\frac{3}{2} \sqrt{Ω_{Λ,0}} H_0(t - t_0)\right] \...
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1answer
58 views

Size of universe if the Sun was one millimetre big [on hold]

If the Sun was one millimetre big and you drew this on a piece of paper, how big would the universe be relative to this?
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1answer
70 views

Did the observable universe come from nothing?

Does the big bang theory hold that there is a possibility that the whole of the observable universe came out from NOTHING? I mean if all of this universe was occupying an extremely small size of a ...
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0answers
18 views

Total mass-energy of a system in GR [duplicate]

the Gravitation book confirm that "for closed universe the total mass-energy and angular momentum is undefined and undefinable" why many cosmologists claim that the total energy of a closed universe ...
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0answers
23 views

Applying information content limit to simulated universes [on hold]

In simulated universe hypothetical scenario. It is often just considered that parent universes can simulate child universes, and those child universes can simulate further on etc... Yet, it seems ...
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0answers
29 views

What would happen if all fundamental force of every particle is increased by 1%? [on hold]

I've read that the everything in existence is made out of fundamental particles and the properties of molecules and everything that is made from molecules are based of the interactions between it's ...
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2answers
98 views

If the universe is random, why does it contain structure? [closed]

Ive read that the scientific community agrees on that the universe works in a non deterministic way. However, because we have some kind of structure in the universe I assume (from a super novice ...
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1answer
69 views

Experimental status of the Big Bang theory

Take the most successful version of the Big Bang theory I'm aware that the Big Bang theory have unanswered questions related to the very first moments of the Big Bang. I also know that for our ...
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1answer
67 views

Can our universe be a true vacuum bubble?

The paper "Spontaneous creation of the universe from nothing" by Dongshan He, Dongfeng Gao and Qing-yu Cai claims that our universe was created by the quantum fluctuations in the metastable false ...
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1answer
13 views

Pridictions and Observational evidences of No Boundary Condition of S.Hawking

Reference: http://www.hawking.org.uk/the-beginning-of-time.html Predictions of No Boundary Condition proposal: 1) Irregularities in the current universe same as the Big Bang theory predicts and it ...
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19 views

Energy quantization in macroscopic world [duplicate]

Is energy quantized in macroscopic world according to principle of correspondence ? Can an object have quantized kinetic or potential energy ?
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Is the Universe an isolated system? [duplicate]

I am not a physicist but this question has bugged me for a while now. The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. However, if the ...
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0answers
29 views

Was the filament structure of the universe due to the same process as in the continuity equation?

In this long Twitter thread, Dr Phil Metzger suggests that the large-scale filament structure of the universe is due to the same process that determines the continuity equation. The entire tweet ...
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0answers
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The universe, as a whole? [duplicate]

If the current visible universe looks the same no matter which direction you look, how can it be described as flat? Since the big bang is the current theory of the beginning of the universe, there is ...
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0answers
32 views

Photon will deform through time? [duplicate]

Is it possible that photon will deform through time? so after millions of years a single photon will get redshifted? Can we test this possibility by current technology? If this is true, what ...
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1answer
53 views

Gravitation inside spherical hole in universe

I was doing a little thought experiment and ran into a paradox. How to compute the gravitational field inside a spherical hole in a universe that is otherwise filled with uniformly distributed mass? ...
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0answers
53 views

Why the large-scale structure is hierarchical? [closed]

Reading a lot after asking my question, I understood that it is too broad. So, I asked only the second part of my question.
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5answers
82 views

Does the Universe itself move to any direction on top of expansion?

So the universe expands right, but does this expansion move to any direction in hyperspace as an expanding cluster of matter somewhere? It's like if you would explode a bucket of paint under the. ...
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2answers
134 views

Do free particles slow down in expanding universe?

My favorite toy model of cosmic microwave background in the expanding universe is a cube with totally reflective walls. If CMB is a black-body radiation, it must be isotropic and homogeneous - the ...
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1answer
40 views

What will happen with the cosmic microwave background radiation if the Big Rip theory is true?

There are three things I can imagine happening: It appears one light year further away every year, which would be a bit weird, because it would be behind the universal event horizon, but still ...
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2answers
678 views

Are Mirrored Universes With Opposing Directions of Time Theoretically possible? [closed]

I found an article that talks about this here, entitled "Big Bang May Have Created a Mirror Universe Where Time Runs Backwards". I know any notion of backwards time is probably a dead horse in this ...
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0answers
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Boundary conditions of temporal progress [duplicate]

If one wanted to assume time as a sequence of smallest units, what would be their physically possible sizes ?
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1answer
44 views

Would any continuous model of the universe have/be based on hypercomputational laws?

I've read that when Turing-Church thesis is applied to the universe and physics, one of the three interpretations that we can use and is defended by some important physicists is that: "The universe ...
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0answers
22 views

How Can the Universe Be Bigger Than It is Old? [duplicate]

My question is this: If we can look back to roughly 500m years after the big bang and have estimated the age of the universe at 13.7B years, why is the galaxy we are looking at not a part of the non-...
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1answer
44 views

Would time still exist after Heat Death of the Unverse?

From my (honestly limited) knowledge of what the Heat Death of the Universe is, I understand that HDotU is the point when energy levels across the entire Universe reach equillibrium, thus energy doesn'...
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0answers
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Why does a universe with total positive energy eventually collapse in a crunch?

While reading about negative energy in the following Wikipedia page, I came across a statement that, if in a universe positive energy dominates, it will eventually collapse upon itself in a crunch. ...
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1answer
62 views

What were early photons travelling through

Please excuse the use of incorrect terminology. I've been trying to wrap my head around the fact that the universe has no edge and if you travel in a straight line you would loop back to your original ...
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1answer
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How would a difference in electron and proton charge be detectable in astronomy?

How would a deviation in neutrality of matter affect astronomy? Obviously this would introduce a lot of repulsive force but how, exactly, would that affect the universe? If the repulsive force was ...
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1answer
60 views

Spacetime diagram of LCDM

I have a question concerning the LCDM spacetime diagram https://i.stack.imgur.com/Uzjtg.png published on the Physics Forum Stack Exchange website Can space expand with unlimited speed? How are the set ...
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1answer
63 views

Universe inside atom [closed]

It's not my personal theory. Many scientists raised questions about this: https://news.softpedia.com/news/Top-Physicists-Ponder-On-the-Idea-of-Universe-In-An-Atom-93428.shtml https://en.wikipedia....
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0answers
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How would primordial black hole forms if expansion of space overwhelms gravity? [duplicate]

I read that density is quite high in the early universe and many speculate that primordial black hole can form and would probably have a mass similar to a human ovum(egg cell). Usually black hole only ...
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0answers
34 views

Does the energy of the mass of the universe equal it's gravitational potential energy? [duplicate]

Does the positive energy equivalent of the mass of the universe derived from E= mc^2 equal the universe's negative gravitational potential energy, if Omega equals 1? It seems that converting matter ...
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3answers
114 views

If there was but one object in the universe, can it be accelerating or not accelerating?

If there is only a single (material) object in the universe does it still make sense to speak of it as accelerating or not accelerating? I believe it might be an equivalent question to ask whether it ...
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1answer
94 views

How far away are distant objects really?

I've done some research on this site and others and I'm having a bit of trouble. References: Why is the observable universe so big? Does the speed of light change? https://www.skyandtelescope....
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2answers
72 views

Questions regarding the popular “finite but unbounded” universe

Regarding the finite but unbounded universe, according to my understanding some people come up with this idea because of the limitation of tools to detect what's "outside" the cosmic horizon (or "...
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2answers
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Charge of multiverse

The charge of the universe is said to be zero. The most general argument I hear about this is that: since there was no net charge before the universe there must not be any after. Hence the universe is ...
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1answer
104 views

Zero-energy universe - What is nothing?

I am a layman, so excuse me in advance for the stupidity of my questions, and I hope you can answer them in a way that I can understand. I have read, here and there, that the Universe might have a ...
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0answers
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From the expansion to temperature and recovering equilibrium number densities from the scale factor

For a particle species of mass $m$, when the temperature $T$ of the Universe is $T>m$, the equilibrium number density $n$ falls as $$n\propto T^3\tag{1}$$ and for $T<m$, it falls as $$n\propto ...
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1answer
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Is space continuous? [duplicate]

Take a particle, it has a position $(x, y, z)$ maybe it is $(0.231, 8.962, 10.567)$. Is there a maximum precision to this? Is the space discrete or continuous? If it is discrete, how thin the ...
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1answer
28 views

Relationship between Energy density and Curvature

I don't know GR so while answering the question so keep in mind that. In the Friedmann Equations, is energy density has an effect on curvature or vice versa? Or they are separate things and they don'...
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0answers
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Why Empty universe have to obey the Negative Curvature? [duplicate]

For empty universe it seems to me that we can have two solutions. $$H^2=\frac {8\pi G\epsilon} {3c^2}-\frac {\kappa c^2} {R^2a^2(t)}$$ For an empty universe when we set $\epsilon=0$ we get $$H^2=\...
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1answer
86 views

Why is the heat death, rather than a Big Crunch, the most accepted theory of the ultimate fate of the Universe?

The Heat Death is accepted by most as the end of the Universe, but how can that be? Wouldn't the Big Crunch make a lot more sense? I mean, even if everything in the Universe is spread out uniformly ...
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2answers
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What if humans doubled size… and everything else… could we notice? [duplicate]

After the big bang, everything expanded from a small mass. That expansion is said to be still happening. Imagine if everything observable constantly grew in size. EG. Everything slowly doubled in ...
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0answers
37 views

Must the whole universe have the same entropic arrow of time?

Theoretically, could it be possible for one galaxy to have low entropy in the past, but another nearby to have low entropy in the future? I understand that, cosmologically, there almost certainly are ...
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1answer
50 views

How do we measure the age of the universe? [duplicate]

How do we measure the age of the universe if time is relative to the observer? What is the reference frame we use to measure the age of the universe?
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1answer
104 views

number density of photons and the total energy of the universe

I was reading over some assignment solutions that my lecturer put up and one of them said that instead of calculating the number of photons in the universe with the familiar method of $\int^{\infty}_0(...
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2answers
82 views

If the universe is flat, does that imply that the Big Bang produced an infinite amount of energy?

Too much density and the universe is closed, analogous to a sphere in four dimensions: you travel in a straight line and you end up where you started. Too little and you have a saddle: not sure about ...
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2answers
62 views

entropy in isolated system

Ok, so the question is about the concept of increasing entropy. We obtain the result (by utilizing the Clausius inequality theorem) $dS = \frac{dQ_{rev}}{T} \geq \frac{dQ_{irr}}{T}$. Then it's stated ...
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2answers
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What is the biggest structure in the Universe?

The CMB cold spot (if it exists) may imply a supervoid of approximately 1 billion light years across. If the diameter of the Observable Universe is 46 billion light years this is about 2% the ...