Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

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

2
votes
2answers
59 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 "...
1
vote
2answers
71 views

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 ...
-3
votes
0answers
54 views

What would happen if the universe had no gravity? [closed]

My question is what would the universe be like without gravity.
1
vote
1answer
87 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 ...
0
votes
0answers
32 views
+50

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 ...
1
vote
1answer
68 views

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 ...
0
votes
1answer
23 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'...
0
votes
0answers
23 views

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=\...
1
vote
1answer
66 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 ...
-1
votes
1answer
53 views

Age of empty universe vs actual universe [closed]

Can anyone please explain the following question for me? I know how to get to the formula to find age from Friedmann equation but I am not sure how to answer this question.
0
votes
2answers
62 views

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 ...
0
votes
0answers
35 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 ...
1
vote
1answer
49 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?
1
vote
1answer
89 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(...
0
votes
2answers
74 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 ...
1
vote
2answers
54 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 ...
1
vote
2answers
74 views

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 ...
1
vote
1answer
44 views

Negative work done by dark energy

If I lift a book up, I applied a force opposite its weight so there's positive work done. Dark energy is said to be pushing galaxies apart and the energy is coming from this negative work done. ...
1
vote
1answer
28 views

Areal radius, cosmology

In cosmology, what areal radius means exactly? Is defined as \begin{equation} R(t,r)=a(t) \, r \end{equation} Where $a(t)$ is the scale factor.
1
vote
1answer
68 views

What is the age of a universe with positive, negative, and zero curvature?

I am trying to calculate the age of universes with different curvatures using the Hubble constant and Friedmann equation. What does it mean when we say that the universe started out at equipartition ...
6
votes
1answer
601 views

Why do physicists think that the dark matter is cold?

Most of the dark matter in the Universe is cold or nonrelativistic which rules out neutrinos as dark matter candidate as they are hot or relativistic. But why do physicists think that the dark matter ...
0
votes
0answers
16 views

How come everything in the universe except the universe itself are spinning? [duplicate]

The planets and stars are spinning, galaxies and clusters are spinning so shouldnt the universe also spin? I think objects spin is to preserve angular momentum but it must also implied that in the ...
1
vote
0answers
57 views

Could the horizon problem be explained by a finite universe instead of inflation?

Suppose the universe is finite (either closed or open with a non-trivial topology), any point could be within the horizon of another.
0
votes
0answers
27 views

Is universe infinite or not? [duplicate]

What do you guys think of Hawking's last paper "A Smooth Exit from Eternal Inflation?" https://arxiv.org/pdf/1707.07702.pdf according to him there's no such thing as infinite multiverses so he ...
2
votes
0answers
30 views

Would Left and Right Weyl spinor components mix to become massive in an expanding space?

Sorry, this might be a dumb question. I was just reading a very old paper by Schrodinger where he's talking about different frequency modes mixing in an expanding universe. Basically he says if the ...
3
votes
2answers
46 views

Intuition for why matter dominated expansion is faster than radiation domination?

In a matter dominated universe $a_{\rm mat.}(t)\sim t^{2/3}$, while in a radiation dominated universe, $a(t)_{\rm rad.}\sim t^{1/2}$. Therefore, a matter dominated universe is expanding more quickly, ...
4
votes
2answers
62 views

Is gravitational lensing affected by dark energy?

Is gravitational lensing affected by dark energy? I mean, despite the effect if cause on the expanding Universe, could dark energy cause gravitational or anti-gravitational lensing?
1
vote
0answers
27 views

Where I can learn Science as a whole? From nothing to relativity, thermodynamics, to quantum [duplicate]

I'm a Computer Science Engineer, and I live in India. I'm taking interests in Physics and Science, and I am much more curious about it. But I don't know where to start and where to finish (although I ...
0
votes
1answer
58 views

Is this how vacuum works?

Vacuum exerts a net zero force, or i guess pressure? on what's inside it, right? So if the universe is expanding at an accelerating rate, is the vacuum of outer space is growing? Like space is ...
1
vote
0answers
34 views

Entropy and observers

Struggling to understand the stance Bousso takes in his paper, The Entropic Landscape, https://arxiv.org/abs/1001.1155 Is he saying that because only by observing an increase in available state we ...
-3
votes
2answers
57 views

Can general theory of relativity predict the scale of the universe (finite or infinite ) [closed]

Can GTR predict boundary of the universe . GTR is too difficult for me to understand its main purpose and its contribution in cosmology . i have another question to be asked -I have been struggling ...
4
votes
2answers
155 views

Big Bang: what is it exactly that is expanding?

I have read a bit about the Big Bang over the years, but being no physicist I have never been able to really understand what it is about. As far as I know, starting with Hubble we have been able to ...
2
votes
0answers
54 views

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 ...
-3
votes
2answers
93 views

Would large scale quantum computing slow the universe. If so, how could we detect that? [closed]

I'm not a physicist. It seems to me that a quantum computer needs to store and access all its quantum states. Currently the universe is concerned with calculating the interaction of its particles ...
5
votes
1answer
162 views

What is the current measured (i.e. model independent) density of hydrogen $H$ in the universe?

The question came up in this thread: What is the current value for the temperature at which Recombination took place? I was under the impression that we had an independent measurement of Hydrogen in ...
2
votes
1answer
58 views

Can holography be applied to every model?

Can all models have a holographic representation? I was talking with a physicist and he told me: "Holography is not a property you can just apply to any model of the universe willy-nilly. It either ...
2
votes
1answer
59 views

How is distance between galaxies and rate of expansion of universe calculated?

I have recently read in book 'Theory Of Everything' that rate of expansion of the universe is changing 5%-10% per 1000 million years, I would like to know how such calculation has been made. Edit: I ...
0
votes
1answer
75 views

Where was the farthest object 13 billion years ago?

Asumme that we see a shape of the gallaxy which reached to us in 13 billion LYs. I understood that the object right now maybe in different shape and is >40 billon LYs away. But, as the universe is ...
3
votes
1answer
117 views

Is the Universal Wavefunction a sensible ontological concept?

It seems to me that several quantum interpretations rely on the idea that there exists a wavefunction that completely specifies the state of the universe. Of these, the Many Worlds Interpretation is ...
1
vote
1answer
43 views

General question about dimensionality of the universe [duplicate]

I am a high school math teacher with some basic physic/lay person knowledge. Each year after the AP test, I show the movies Flatland and Sphereland to my calculus classes and talk about the ...
0
votes
1answer
107 views

Can the universe build itself back together after the Heat Death? [closed]

We all know that due to the 2nd Law of Thermodynamics the universe after a certain long period of time will eventually have all its energy spread out homogeneously and reach a Heat Death where nothing ...
1
vote
1answer
43 views

Do we have enough time for understanding space? [closed]

May be there more big bangs out there... while studding about space i some times think about a bacteria living on my body that may have a life span of 5 seconds if it has logical understating even ...
0
votes
1answer
55 views

Can somebody tell me what was the size of the universe and the expansion rate at the big bang? [duplicate]

I want to know the exact numbers and preferably the calculations that gives the size of the universe at the Big Bang and the initial rate of expansion at the moment of big bang. Does somebody know ...
0
votes
0answers
23 views

Universal Entropy [duplicate]

I’m a first-time poster... Could entropy possibly be the reason the universe is expanding and even why its expansion is accelerating? If space is not surrounded by anything at all, it would seem to ...
0
votes
1answer
29 views

What does the matter density in density parameter mean?

When we write $$\Omega_m = \frac{\rho_M}{\rho_{\mathrm{crit,o}}}$$ what density does $\rho_M$ refer to? As in, suppose our universe was made with just hydrogen. Then would $\rho_M$ mean just the ...
0
votes
3answers
60 views

Travelling at lights speed of light as universe expanding

I have read that the object, the light of which is reached us in 13.8 billion years is actually about 46 billion light years away from us now, due to the expanding Universe. Lets assume that we ...
0
votes
1answer
129 views

What is the definition of gravitational horizon $R_h$ in the $R_h=ct $ universe?

Melia's $R_h=ct $ universe proposal suggests that our gravitational horizon $R_h(t)$ should be equal to $ct$ for all cosmic time $t$, not just its present value $t_0$: "several unpalatable ...
1
vote
0answers
47 views

Why is the observable universe 90 billion light years in diametre? [duplicate]

The question is fairly simple, we know that the universe has been expanding for 13.8 billion years, so how did they determine the size of the universe that we can observe, and why can we not peek ...
0
votes
3answers
55 views

Time dilation and The Big Crunch [closed]

When/if the universe collapses in onto itself as described by the big crunch theory, would time slow down due to time dilation (relative to our experience) as all matter gets pushed together, or do I ...
2
votes
0answers
21 views

How the matter and radiation densities of universe can be calculated? [closed]

Barbara Ryden gave somehow detailed and intuitive calculation in Article 5.1 of Introduction to Cosmology but skipped detail on some values. I don't know, it's good or not but I'm adding snaps from ...