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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3
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2answers
28 views

Why is there a matter-dark matter asymmetry?

It is said generally that nature is symmetric. For example if light behaves as both a particle and a wave, then matter must also do so, which turns out to be true. But we find that the Universe ...
0
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0answers
14 views

Does the Bekenstein entropy bound present problems for unitarity in cosmological models that invovle a collapse or bounce?

If we expect the Bekenstein bound, or something like it, to hold in a collapsing universe, will that not eventually force us to accept some loss of information, or is there something I'm missing?
0
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0answers
16 views

complete local thermal equilibrium and conserved quantum numbers

I am reading Weinberg's Cosmology book and I am confused of the statement "complete local thermal equilibrium in which all conserved quantum numbers vanish". My question is: How is complete local ...
-1
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0answers
55 views

Intuition behind deriving the FRW metric [on hold]

Consider a 3D sphere embedded in a 4D “hyperspace”: $$(x^1)^2 + (x^2)^2 + (x^3)^2 + (x^4)^2 = a^2,\tag{74}$$ where $a$ is the radius of the 3D sphere. The distance between two points in 4D space ...
-2
votes
3answers
42 views

How can we look back at our Universe 13Billion years ago?

I read this article on Space.com and I quote "As far as we know, that's as far [back] as we can see — we get an image of the universe as it was when it was about 389,000 years old," So what I wanna ...
-4
votes
0answers
24 views

Expansion Of Universe : On the Basis Of Aether Theory

I would like to ask that in the recent experiment of discovering Gravitational Waves scientists have also found or believe that they have found the evidence of Dark Matter. So if dark matter is made ...
-4
votes
1answer
32 views

Cosmological Significance of Uncertainty [on hold]

Is it fair to say that quantum uncertainty is the most fundamental and primordial characteristic of the universe? Wouldn't uncertainty have predated the initial expansion of spacetime, and not be ...
-6
votes
0answers
36 views

The origin of the universe and its current movement [duplicate]

Our universe started with the big bang, which was not within our universe, since it didn't exist. Let's assume it was the result of a quantum fluctuation in time. I have 2 questions: The common ...
3
votes
2answers
48 views

Could a pocket universe be birthed out of the negative pressure in our universe?

I have just recently started learning about inflationary cosmology so I don't know much about it. But from my understanding, the theory says the big bang was started by the expansion of a small region ...
0
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1answer
26 views

Energy loss of photons crossing a dielectric medium

How much energy do photons lose when crossing an "isotropic" dielectric medium? What would be possible mechanisms. I assume the straight forward answer would be that the photon loses zero energy and ...
0
votes
1answer
28 views

What is the fate of a hyper-sphere universe?

This PBS video postulates a positively curved universe which is in the shape of something the narrator calls a "hypersphere". http://youtu.be/AwwIFcdUFrE?t=6m33s Such a universe would have no edge, ...
0
votes
1answer
32 views

Any good statistics books under the context of astronomy? [on hold]

I just started a position as a research aide with the cosmology group at Argonne National Lab, and am working on creating and finding correlations within velocity dispersions of galaxy clusters. After ...
0
votes
1answer
65 views

“Big Crunch” Ending of Universe?

So, I am reading Introduction to Modern Cosmology by Andrew Liddle and have just learned that the universe is cooling as it is expanding. Now, I am mathematician knowing very little about physics, but ...
3
votes
3answers
246 views

Big Freeze or Heat Death?

I can't seem to find this on the Internet.... Which is most likely to occur to "end" the universe, the big freeze scenario? Or the heat death scenario? P.s. When, if it does at all, will the ...
26
votes
3answers
958 views

Do intergalactic magnetic fields imply an Open Universe?

According to a paper on the arXiv (now published in Phys Rev D), they do. How credible is this result? The abstract says: The detection of magnetic fields at high redshifts, and in empty ...
0
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1answer
37 views

Direction of travel in universe

When observe the universe we see that galaxies are much closer than they are at this very moment because of the time it takes for the light to reach us. However do we see this phenomenon in every ...
29
votes
6answers
2k views

Why isn't dark matter just matter?

There's more gravitational force in our galaxy (and others) than can be explained by counting stars. So why not lots of dark planetery systems (ie without stars) ? Why must we assume some undiscovered ...
0
votes
1answer
57 views

Why does most galaxies resemble a 2 dimensional plane? [duplicate]

Older galaxies mostly tend the revolve forming a disk. Why are the stars not revolving around the center of the galaxy like a sphere instead similar to electron in an atom?
3
votes
1answer
81 views

Exotic differentiable structures in physics

When reading a bit on exotic spheres and exotic $\mathbb{R}^4$'s, I came across some papers of Carl H. Brans and Torsten Asselmeyer-Maluga: "Exotic differentiable structures and general relativity" (...
5
votes
2answers
393 views

Why did our universe become transparent to light approx. 300,000 years after the Big Bang started?

Most books say that at age 300,000 years, our universe had cooled to approx. 3000 degrees Kelvin, which allowed free-electrons to bind with atomic nuclei, which allowed light to travel, unimpeded, ...
3
votes
1answer
141 views

Distance between two galaxies of different redshift

Let $Q_1$ and $Q_2$ two different objects in the Universe (we can think to two galaxies or quasars), that we observe from the Earth at different angular position $(\alpha_1,\delta_1)$, $(\alpha_2,\...
2
votes
1answer
90 views

Two conflicting forms of equation of state of non-relativistic gas

I've run into two conflicting derivations of the equation of state of a non-relativistic gas. However, the derivations of the relativistic equation of state of both sources agree. I think maybe the ...
1
vote
1answer
33 views

Formula for cosmic variance

I was reading this page: Sample and Cosmic Variance. The section states that The multipoles $C_\ell$ can be related to the expected value of the spherical harmonic coefficients by $$ \Bigg\...
-1
votes
0answers
25 views

Soup of observable universe in a un-observable universe medium? [closed]

Can we assume that our observable universe is expanding in a medium of un-observable universe? then we can predict that same volume of universe’s mass does not change even after expansion of the ...
5
votes
3answers
127 views

How did steady-state universe deal with entropy?

Until the 1960's the general consensus was that the universe is infinite years old (steady-state). The second law of thermodynamics says that entropy always increases, so we'd expect that within a ...
-2
votes
1answer
56 views

Reading a Paper on the Cosmological Constant Problem [closed]

My professor wants to give me (and another kid) a problem in quantum cosmology. To that end, he asked me to read through his recent paper that appeared in the Physical Review Letters. He said that I ...
2
votes
1answer
51 views

Age of universe from Hubble's constant

Assume the Robertson-Walker metric: $$g = -d\tau^2 + a^2(\tau)\gamma$$ where $\gamma$ is the flat, spherical or hyperbolic spatial metric and $a$ is the scale factor. Wald seems to calculate the age ...
1
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1answer
52 views

A question about baryon asymmetry

The question is that how can we make sure that our universe is baryon asymmetric? I mean, is it possible that there are antimatter domains beyond some very large scale. Yes, if that kind of domains ...
5
votes
2answers
666 views

What is the equation for the scale factor of the universe, a(t), for the best fit of data to the $\Lambda CDM$ Model of Cosmology?

Ideally I like a single formula or multiple formulas for different time ranges that would cover the time from the end of inflation through 100+ billion years after the big bang using the $\Lambda CDM$...
0
votes
1answer
54 views

Qualitative picture or reference for a Lemaître's Cold Big Bang theory

Warning: please, consider this question to be motivated by historical curiosity or as an exercise in model-building. I believe this cannot be considered non-mainstream physics as it was very much ...
0
votes
1answer
76 views

Why isn't mass still being created in the universe? [closed]

I was watching a lecture by Alan Guth and wished I was in the classroom to be able to ask a question. He explained that the total energy in the universe could be very close to zero since mass and ...
13
votes
5answers
3k views

Hubble's law and conservation of energy

If all distances are constantly increasing, as Hubble's law say, then lots of potential energies of form ~$\frac{1}{r}$ changes, so how is the total energy of the Universe conserved with Hubble's ...
5
votes
5answers
747 views

If the size of universe doubled

My question is silly formulated, but I want to know if there is some sensible physical question buried in it: Suppose an exact copy of our universe is made, but where spatial distances and sizes are ...
1
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1answer
81 views

Is cosmological constant really constant?

As the Universe expands, the dark energy in it also increases. I heard that the cosmological constant $\Lambda$ represents dark energy, so that constant must change as time passes, right? Correct me ...
3
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2answers
151 views

General expression of the redshift: explanation?

In some papers, authors put the following formula for the cosmological redshift $z$ : $1+z=\frac{\left(g_{\mu\nu}k^{\mu}u^{\nu}\right)_{S}}{\left(g_{\mu\nu}k^{\mu}u^{\nu}\right)_{O}}$ where : $S$ ...
6
votes
1answer
48 views

Equation of state in cosmology

In cosmology we usually assume that the matter follows an equation of state given by $$ P=w \rho. $$ Given that we have a lot of non-standard theoretical proposals for field theories and exotic ...
-1
votes
0answers
44 views

What is the significance of the so called Planck force? [duplicate]

So, what is the significance of the so called Planck force, and why is it called Planck force, when it has nothing to do with any of the Planck's discoveries? The Maxwell’s wave equation is $\nabla^...
3
votes
1answer
69 views

What does freeze out mean?

I'm taking a cosmology course this semester, and I don't really understand the concept of freeze out. Here is a short paragraph from our lecture. During the initial phase of the Universe, ...
9
votes
3answers
273 views

Reference request for low entropy big bang

There is a somewhat widely accepted argument that the second law of thermodynamics exists because the universe began in a low-entropy state. I'm writing a paper that mentions this (and must be ...
0
votes
0answers
38 views

Could dark matter annihilation prolong the stellar era?

I wonder, if we assume that dark matter actually exists, and it is WIMPs, and it's annihilate to fermions and W/Z bozons, which decay/combine into protons, neutrons and electrons and finally recombine ...
0
votes
2answers
191 views

Is it possible for the Higgs Field strength, to increase with time? [closed]

Magnetic fields are created when an electric current flows: the greater the current, the stronger the magnetic field. What influences could affect the Higgs Field? If the Higgs field grew in ...
2
votes
2answers
2k views

How long did inflation take to happen?

If inflation happened, it ended about 13.7 billion years ago. But how long was inflation going on before that? Was it just a short burst of time so that it's really okay to say that the age of the ...
1
vote
4answers
113 views

Why is acceleration of expansion of space unexplained?

I didn't know how to better phrase the question so here we go. It says that the farther we see from our galaxy the faster the other object seems to be going away (accelerating). My common sense ...
1
vote
2answers
66 views

In the classical inflation and big bang theory how can the universe grow to infinity?

Some astronomers make the conjecture that the universe is actually infinite in extent (which is a bad manner of expressing, I know). If so, how could it grow to this size in the classical theory of ...
0
votes
1answer
60 views

Relativity on a universal scale

Imagine there was a clock on a planet the same size as earth, travelling at the same speed through space, and that this planet was at the most distant part of the universe from earth. If we had a ...
-4
votes
1answer
49 views

Can reality be explained in 2 dimensions? [closed]

First, I apologize if this question is too long or too incoherent to respond to on your website. Second, I am no mathematician or physicist, only a thinker. Third, what follows is all part of my ...
6
votes
2answers
484 views

Does the current acceleration of universe imply that our universe is open?

Does the current acceleration of universe imply that our universe is open? If the universe is closed, from the Friedmann's equation, the acceleration of universe wouldn't be possible, would it be? (Of ...
2
votes
0answers
41 views

Dark energy and gravitation force

This might be a silly question to ask but still I had to ask. we know that gravitational force between two galaxies are quite less as compared to the dark energy. so two galaxy are separated at speed ...
2
votes
2answers
218 views

Cosmology: proper evolution of energy density ratios with time?

Solving Einsteins equations for the FLRW metric with ideal quantum gases for matter and radiation, assuming that pressure is proportional to the energy density $p=w\rho$ and deriving $w$ from ...
6
votes
1answer
229 views

The cosmological constant as a Lagrange multiplier?

The cosmological constant $\Lambda$ can be introduced into the gravitational action like this : \begin{equation} S = \frac{1}{2 \kappa} \int_{\Omega} (R - 2 \Lambda) \sqrt{-g} \; d^4 x + \text{matter ...