The study of the large-scale structure, history, and future of the universe. Cosmology is about asking and answering questions about the "big picture" - the extent, origin, and fate of everything we know.

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5
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4answers
70 views

Why doesn't the light from galaxies appear stretched? [duplicate]

Maybe it's my ignorance of astrophysics/cosmology, but I have been wondering this: Why do galaxies not appear stretched when we observe them? Assuming a galaxy that we observe is 100,000 light years ...
3
votes
2answers
120 views

How does the gravity well change as space expands? [duplicate]

How does the gravity well change as space expands? If we assume that the Earth's gravitational field curves flat space to create a gravity well then how does the gravity well change as space expands ...
1
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1answer
72 views

Is there an absolute center of mass exist in the universe? [duplicate]

By the "absolute center of mass" I mean the center of all the matter that exists in a given moment. My friend asserted that today and it intrigued us very much but since we only have a CS degree and ...
12
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3answers
811 views

Is “now” or “the present moment” properly defined in GR?

My question is about the extent to which "now" is defined in GR. In Minkowski spacetime, it's possible to define a "now" for an inertial observer by finding a spacelike 3-plane such that, in the ...
3
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1answer
96 views

The FRW universe is NOT asymptotically flat? Its mass?

The Friedman-Robertson-Walker (FRW) metric in the comoving coordinates $(t,r,\theta,\varphi)$ which describes a homogeneous and isotropic universe is $$ ds^2\,= -dt^2+\frac{a(t)^2}{1-kr^2}\,dr^2 + ...
0
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0answers
28 views

extreme heat to extreme cold (define the endstate)

Contemporary cosmology frequently has space temperatures just after the 'big bang' in the regions of millions of degrees and with inflation and expansion of the universe this is now down to a couple ...
3
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3answers
113 views

Does or should the metric expansion of space imply a locally observable increase in kinetic energy?

The title is the question. Here's why it seems like local kinetic energy should increase: Numerous questions and answers here and elsewhere suggest that the reason the metric expansion of space is ...
0
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0answers
38 views

How much would the orbit of the earth change if we launched all 'garbage' into outer space

I know it's not economically feasible, but is there a simple equation telling you the ellipse earth would follow if you theoretically launch all of the landfills, i.e. garbage, beyond our orbit, thus ...
4
votes
1answer
151 views

Is the observable universe homeomorphic to $B^3$?

Is the observable universe homeomorphic to $B^3$? Where $$B^3=\{x\in \mathbb{R}^3 : |x|\leq 1 \}$$ Or is it even sensible to talk about space (rather than spacetime) as a 3 manifold?
3
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1answer
45 views

Spacial curvature and expanding space

If we take the analogy that in an empty space the space is just a flat sheet then if there is a single planet or a star then the flat sheet will curve below the planet leaving a curvature shaped like ...
0
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2answers
72 views

What is entropy of the universe? [duplicate]

What is entropy of the universe? What is the change in entropy of our universe? Is this change increases or decreases? If the entropy of universe keeps on increasing then what are the consequences? Is ...
2
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1answer
66 views

How is CMB related to the temperature of the universe

As I understand it, CMB (cosmic microwave background) is the radiation emitted when matter decoupled at the early stages of the big bang. The thing I don't understand is do all stars emit this kind of ...
2
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2answers
417 views

Wouldn't the presence of dark matter slow the expansion of the universe?

If there is a huge element of dark matter in the universe, wouldn't this extra gravity prevent the accelerated expansion of the universe?
0
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0answers
32 views

Space within galaxies. Is it stuck by the gravity of the galaxy or expanding and “slipping past”? [duplicate]

I understand that the inter-galactic space is expanding but galaxies themselves are not. What is happening to the space within a galaxy? Is it fixed by the gravity of the galaxy or is it expanding and ...
1
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1answer
95 views

The universe appears to have a lower bound in the time dimension, why not an upper bound?

The Big Bang looks like a lower bound to the "size" of the universe in the time dimension. Could it also have an upper bound, some furthest point in time from the Big Bang?
3
votes
1answer
158 views

How far can something travel in a straight line?

Suppose you have an object some distance from you and moving at a velocity different to the Hubble velocity you'd expect at that point. How does the motion of this object change with time? Does it ...
1
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0answers
79 views

How to test that a flat metric represents a global three-torus geometry

When introducing Robertson-Walker metrics, Carroll's suggests that we consider our spacetime to be $R \times \Sigma$, where $R$ represents the time direction and $\Sigma$ is a maximally symmetric ...
1
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1answer
86 views

Could the center of the combined mass of the entire galaxy change if there were no external forces acting on that galaxy?

Everything in the galaxy orbits the center of the combined mass of the entire galaxy. So could the center of the combined mass of the entire galaxy change if there were no external forces acting on ...
3
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2answers
488 views

What trajectory has the Cosmic Microwave Background radiation taken to get to earth?

I have a few related questions: Where is the CMB coming (emitted/reflected/remitted) from? When CMB hits the earth, is that the first thing those photons hit since they were emitted 400 thousand ...
2
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2answers
74 views

Rectifying incomplete popular notions in cosmology

In looking at the answers to this question regarding light from distant galaxies ever being visible to us: Expansion of the Universe, will light from some galaxies never reach us? I came across a ...
1
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0answers
60 views

How to prove the Hubble law is the unique expansion law compatible with homogeneity and isotropy?

In the book physical foundations of cosmology, it saids that Hubble law is unique and a problem seems to be a hint of proving that. In order for a general expansion law,v=f(r,t), to be the same ...
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2answers
116 views

Has a theory of a small Big Bang in a much larger universe been proposed?

I have a theory I think might explain why the universe has accelerated in a way that doesn't require dark energy. I'm wondering if someone has proposed this theory before (did some research and ...
3
votes
4answers
145 views

What experiment would disprove Friedmann model of cosmology?

What experiment would disprove Friedmann model of cosmology? As a layman, I have read a lot of articles and threads in specialized forums. I am probably wrong, but I developed an impression that that ...
7
votes
4answers
691 views

Expansion of the Universe, will light from some galaxies never reach us?

Is it true that the light from some galaxies will never reach us? The explanation for that is that the Universe expanding faster than the speed of light. But, if the speed of light is constant in ...
2
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0answers
21 views

Why the initial spectrum of perturbation of inflation must be Gaussian?

Why the initial spectrum of perturbation of inflation must be Gaussian? I heared it was because of Central Limit theorem. But I don't know how use this theorem explicitly in this question to get this ...
2
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0answers
55 views

What's the difference between correlation functions and S-matrix, and between in-in formalism (or “closed time path formalism”) and in-out formalism?

I was reading the "in-in" formalism (or "closed time path formalism" used in condensed matter physics) in cosmology created by Schwinger in 1961, and there is a saying: "they care about correlation ...
1
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1answer
95 views

Infinite number of universes? Black holes are guilty? [closed]

I was thinking, if the space is infinite, what if there are infinite number of spaces, inside our universe? I mean, everyone knows that black holes exist, but nobody knows what happens when you get ...
8
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2answers
938 views

Will the CMB ever stop shining?

Will we ever reach a point where all the light that was set free by recombination finally reaches us from our point of view and the CMB stops "shining" for observers on Earth?
2
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2answers
323 views

What does a galaxy orbit?

Moons orbit planets, planets orbit stars, and stars orbit the center of a galaxy. So, my question is what does a galaxy orbit? The center of universe? (I know that the universe has no center)
3
votes
0answers
59 views

The universe could have created itself? [duplicate]

So in this Youtube video http://youtu.be/ZjaaSUHG7Xo The guy says, around 6:15, that the total energy of the universe is zero because of gravitational potential (or something like that i dont ...
1
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1answer
104 views

Calculating Hubble's constant at earlier times [duplicate]

I want to calculate Hubble's constant at some redshift $z$. I have found the following formula: $$H^2=H_0^2\left(\Omega_m\left(1+z\right)^3+\Omega_{\Lambda}\right)$$ Now it's obvious that at higher ...
3
votes
1answer
95 views

What is the difference between matter & spacetime? [duplicate]

If the universe is expanding why doesn't the matter in it expand proportionally making it seem as if the universe is static? Alternatively, as spacetime expands why does it not just slide past matter ...
2
votes
1answer
85 views

Adding matter to make galaxies bigger

What are the actual factors that play a role in the accretion of matter into galaxies? I read about Accretion Disks but I don't quite understand how they work yet.
1
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1answer
109 views

How did the universe get so big so fast? [duplicate]

The universe started at the big bang around 15 billion years ago. The universe is now at least 92 billion light-years in diameter. Together, don't these mean that the universe, at some time in the ...
0
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1answer
44 views

Gravitational lens to illuminate a planet?

I've been involved in a Internet debate: Is it possible to have a special cosmic configuration consisted of multiple bodies such that a planet's surface can be lit up by light focused by ...
3
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0answers
50 views

Could the singularity that gave rise to our universe be past-eternal?

Are there any compelling models that include a past eternal singularity that ultimately gave rise to our universe? Does the "no-boundry" hypothesis that utilizes imaginary time have past eternal ...
2
votes
1answer
325 views

Why does earth cool? [duplicate]

This question is boggling me for some time. We know that heat can be transferred from matter to matter and heat is nothing more than tiny atoms vibration intensity (correct me if I'm wrong). But space ...
1
vote
2answers
118 views

Could the universe have evolved WITHOUT the non-determinism of quantum mechanics? [closed]

(I'm going to make a few conjectures here - please answer the question in light of them as if they were true, even though of course they may be overly simplistic or wrong) Assuming that: the ...
3
votes
0answers
42 views

Active-sterile mixing for KeVins

People sometimes talk about KeV mass sterile neutrinos as a warm dark matter candidate. I think they call them KeVins (horrible name btw). Now, In order for it to be a good dark matter candidate it ...
1
vote
2answers
102 views

Why was theory of cosmic aether discarded? [duplicate]

Can cosmic aether be considered analogous to higgs field? Higgs field remains (almost) consistent throughout.
0
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1answer
111 views

What is the difference between a low vs. high entropy plasma just after and before a singularity occurs?

Although it appears that the universe is not closed (since it is accelerating) I have been reading different websites on how the universe would look like if it was closed and the big crunch did occur. ...
1
vote
1answer
54 views

Number of Stars vs Value of Omega (Crtitical Density of the Universe)

I may be badly mixing things up here. If I am, please kindly correct me. As I understand it, if the universe was too dense at the start of the big bang, it would have collapsed back in on itself. Too ...
0
votes
2answers
318 views

Does the universe have an edge/boundary/barrier? If yes, what is at the edge? [duplicate]

My question related kind of to asking what the shape of the universe is. Say a hypothetical alien civilization built an faster-than-light spaceship. If they keep flying would they end up where they ...
7
votes
2answers
174 views

Do the standard cosmology models spontaneously break Lorentz symmetry?

In standard cosmology models (Friedmann equations which your favorite choice of DM and DE), there exists a frame in which the total momenta of any sufficiently large sphere, centered at any point in ...
3
votes
1answer
74 views

Time Energy symmetry in General Relativity (not asking about energy conservation)

In General Relativity is there a TE symmetry similar to CPT symmetry in the Standard Model ? It's pretty easy to understand that by flipping charge and parity you merely get a time reversed equivalent ...
3
votes
0answers
50 views

Which cosmological dark matter theory is most compatible with current particle physics?

This occurred to me while I was writing Why hasn't warm dark matter replaced cold dark matter as the standard model of cosmology? but I think this deserves a question of its own. The background ...
6
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0answers
80 views

Why hasn't warm dark matter replaced cold dark matter as the standard model of cosmology?

The $\Lambda\rm CDM$ (cold dark matter with cosmological constant) is the current standard model of cosmology because the model comes with a long list of phenomena successfully explained by it. ...
3
votes
0answers
35 views

How do inflationary models predict the generation of gravitational waves during the inflationary period?

Recent results from the BICEP2 experiment have produced a lot of talk about the primordial gravitational waves produced during the inflationary period. I would like to have some explanation about how ...
2
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1answer
165 views

Redshift of Cosmic Microwave Background

Does the cosmic microwave background radiation have a red shift parameter z? If so what is the value for z?
0
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0answers
38 views

Cosmic Expansion - Why aren't we ripped off yet? [duplicate]

According to Hubble's law, the universe expands exponentially ever since the big bang. If the space-time expands, what effects does it have upon us, (1.Earth, 2.solar system and 3. Milky Way). ...