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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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
131 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 ...
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4answers
162 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 ...
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4answers
794 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 ...
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0answers
26 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 ...
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0answers
78 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 ...
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1answer
108 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 ...
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2answers
984 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?
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2answers
1k 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)
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0answers
61 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 ...
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1answer
127 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 ...
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1answer
113 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 ...
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1answer
88 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.
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1answer
128 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 ...
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1answer
48 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 ...
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0answers
71 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 ...
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1answer
342 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 ...
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2answers
123 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 ...
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0answers
43 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 ...
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2answers
111 views

Why was theory of cosmic aether discarded? [duplicate]

Can cosmic aether be considered analogous to higgs field? Higgs field remains (almost) consistent throughout.
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1answer
191 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. ...
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1answer
65 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 ...
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1answer
521 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 ...
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2answers
226 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 ...
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1answer
78 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 ...
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0answers
63 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 ...
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123 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. ...
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0answers
42 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 ...
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1answer
174 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?
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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). ...
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0answers
57 views

Is the Baum Framptom a valid alternative to Big Bang?

In the Baum Frampton model, proposed in 2007, because $\epsilon\lt-1$, after the Big Rip the universe starts again empty. The problem of this model is that inverting the arrow of the time, after a ...
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4answers
1k views

Is topology of universe observable?

There is an idea that the geometry of physical space is not observable(i.e. it can't be fixed by mere observation). It was introduced by H. Poincare. In brief it says that we can formulate our ...
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2answers
563 views

What is “Energy” of a vacuum in the context of quantum theory?

I'm not a physicist or cosmologist, so I hope I am asking the right question with the right words here. My question regards the word "energy" as it pertains to quantum vacuum states, and to concepts ...
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2answers
209 views

Frame dragging — is there a “non-tiny” example?

Now. As I understand it, in fact, the earth (10^25 kg) creates a very small, very tiny, frame dragging effect. Indeed, we have measured this using satellite experiments. So, the Earth (10^25 kg) ...
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0answers
71 views

Avoiding Pseudo-tensors when addressing global conservation of energy in GR

Discussions about global conservation of energy in GR often invoke the use of the stress-energy-momentum pseudo-tensor to offer up a sort of generalization of the concept of energy defined in a way ...
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1answer
47 views

Peaks and dips position in the Planck CMB

I am currently looking for the values of the firsts peaks and dips (at least the first 3) in terms of l and angular size for the CMB angular power spectrum measured from Planck. But the only thing I ...
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1answer
76 views

How has the age of the Universe been derived from the observations made by the Planck mission?

The parameters of $\rm\Lambda CDM$ model have been determined to an amazing high precision from the measurements made by the Planck mission. In particular, the Hubble "constant" (the value of Hubble ...
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3answers
266 views

Could Dark Matter particles that don't couple to quarks or leptons have been produced?

With what we know about physics, is it possible that when the universe 'began', around when quarks and leptons were produced, another particle, which doesn't couple to either quarks, leptons or ...
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2answers
171 views

If dark energy is accelerating the expansion of the universe, why can't I feel it?

Based on the Lambda-CDM cosmological model, our universe is not only expanding, but is accelerating in its expansion. However, the Equivalence Principle would suggest that inertia manifests itself in ...
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0answers
41 views

Future of a universe with scalar field

Assume that $\phi=\phi_0\gg M_{Pl}$, what can you say about the future of a universe in a model with $V(\phi)=\frac{1}{2}m^2\phi^2+\epsilon \phi$, where $|\epsilon|\ll m^2M_{Pl}^2$, is a small ...
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2answers
209 views

What was the density of the universe when it was only the size of our solar system?

What was the density of the universe when it was only the size of our solar system? Did it approach neutron star density? Is it physically correct to even ask such a question?
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0answers
36 views

Is the expansion accelerating or decelerating? [duplicate]

I have already asked this on Astronomy.SE but I couldn't understand the answer there. According to Hubble's Law, the farther a galaxy is, the farther it is moving away. But do we take into account ...
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0answers
88 views

Maybe photon energy is constant as the Universe expands?

This is a question following on from my previous post Time-like Killing vector in FRW metric? For simplicity I take the spatially flat FRW metric in cartesian co-ordinates given by: $$ds^2 = -dt^2 + ...
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1answer
103 views

Why is an electric and magnetic field present in all universe?

the Earth has its fields due to the motion of molten metals in the core but how does this whole universe has its fields without any discontinuity? And we can say for sure that fields are present in ...
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0answers
21 views

Lee Yang force and cosmology

The text I am reading (Stars and Relativity by Ya. B. Zel'dovich) discusses the possible existence of a repulsive force proportional to total baryon number. At the time of the book's publication it ...
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3answers
89 views

If empty space has energy, and space is expanding, is this energy equally distributed as space expands?

The cosmological constant (dark energy) is often described in terms of empty space having a non 0 energy value and this energy being the source of the accelerated expansion of the universe. If space ...
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2answers
188 views

When was the cosmic background radiation in the visible spectrum?

From what I understand about the Cosmic Microwave Background Radiation is that it was from the big bang, and since space has streched and become bigger since then the wavelength of the CMBR has ...
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0answers
54 views

Should a radiation-filled Universe be scale invariant?

Imagine a spatially flat Universe, without cosmological constant, filled only with EM radiation. As Maxwell's equations without charges or currents are scale invariant then should this Universe be ...
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1answer
59 views

Sign of Hubble slow-roll parameters

Given the Hubble slow-roll parameters $\epsilon=-\frac{\dot{H}}{H^{2}}$ and $\eta=\frac{\dot{\epsilon}}{H\epsilon}$, can they assume negative values? For inflation to occurr they are required to be ...
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1answer
113 views

Estimating age of the universe by Hubble's law?

Hubble's law states that $v=Hx$ Age of the universe is calculated by $T= \frac{x}{v} = \frac{1}{H}$ but the velocity is not constant; it changes with distance, so I think that this equation cannot ...