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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20 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 ...
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3answers
62 views

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

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 ...
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0answers
25 views

Expansion Of Universe : On the Basis Of Aether Theory [duplicate]

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 ...
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0answers
41 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 ...
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0answers
101 views

Intuition behind deriving the FRW metric

I am studying the FRW metric and am looking at a motivation for it here. The motivation seems to use four spatial dimensions. Why do we need the fourth spatial dimension in this? This doesn't seem ...
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1answer
30 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 ...
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1answer
32 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, ...
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2answers
50 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 ...
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1answer
37 views

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

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 ...
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1answer
69 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 ...
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1answer
39 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 ...
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1answer
62 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?
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2answers
403 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, ...
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1answer
84 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" (...
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1answer
36 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\...
2
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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 ...
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1answer
69 views

Why do we believe 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 ...
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1answer
93 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 ...
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1answer
79 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 ...
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1answer
85 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 ...
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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 ...
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1answer
50 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 ...
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1answer
72 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, ...
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1answer
61 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 ...
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2answers
74 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 ...
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1answer
51 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 ...
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1answer
58 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 ...
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0answers
46 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 ...
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2answers
54 views

Why do galaxies in the Local Group not obey the Hubble law?

The way I understand it is that the galaxies in the Local Group have a gravitational attraction that causes a relative velocity inconsistent with the recessional velocity predicted in the Hubble law. ...
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0answers
22 views

Does anybody know how to derive the Bolzman equation for photon distribution function in the presence of tensor perturbations?

Does anybody know of how to derive the Bolzman equation for photon distribution function in the presence of tensor perturbations? Ok, this is actually a problem (4.13) in Dodelson's book. I got stuck ...
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4answers
114 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 ...
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1answer
58 views

What is Hawking's, “No Boundary Conditions”? [closed]

In his "No Boundary Conditions", is Hawking stating that time is eternal? And what is the difference between Real Time and Imaginary Time? Is he saying there are two different arrows of time, and ...
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1answer
44 views

Does the FRW metric imply spacetime scales?

The FRW metric can be written in conformal co-ordinates to give: $$ds^2=a^2(\eta)(-d\eta^2+d\mathbf{\Sigma^2}),$$ where $\eta$ is the conformal time and $\mathbf{\Sigma}$ ranges over 3-dimensional ...
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2answers
68 views

Is the effect of mass density on the expansion of the universe linear?

The average expansion rate for large scales parsec and above measured is $\approx 73\,{\rm km}\,{\rm s}^{-1}\,{\rm Mpc}^{-1}$. This expansion is calculated for average densities of space volume across ...
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2answers
54 views

Friedmann equations with $w <-1$

Let's consider a flat universe with an FRW metric with scale factor $a(t)$, with some matter content. The continuity equation $\nabla_\mu T^{\mu\nu}=0$ combined with assumptions of isotropy and ...
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0answers
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What is the relationship between an expanding universe, an increase in entropy and the the creation of mass?

What is the relationship between an expanding universe, an increase in entropy and the the creation of mass? Could the creation of mass be driven by or in equilibrium with the creation of space?
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2answers
116 views

Is a spacetime of constant positive curvature just a 4-hypersphere?

In discussions of basic cosmological models, I don't see "spacetime of constant positive curvature" described more simply as a "4-hypersphere". What am I missing?
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1answer
42 views

How do we filter out radiation from galaxies from the cosmic background radiation?

When a whole sky view is done to get a picture of the cosmic background radiation the radiation from our Galaxy is filtered out, and we are left with a picture which show radiation coming, more or ...
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2answers
91 views

Vacuum energy and supersymmetry [closed]

Let us assume that dark energy is the energy of the vacuum. The density of this vacuum energy has been measured to be about $10^{-26}$ $kg/m^3$. Using the Compton wavelength formula, $\lambda=h/mc$, ...
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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 ...
3
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0answers
60 views

Which quantum gravity theories do predict a growing number of space dimensions?

In this 2013 Nature article there is a short presentation of the causal dynamical triangulations approach to quantum gravity, where it is said Intriguingly, the simulations also hint that soon ...
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0answers
27 views

Observationally distinguishing a galaxy of antimatter from a galaxy of matter [duplicate]

I was just wondering how, observationally, we would distinguish a distant galaxy of "normal" matter from one of antimatter. Maybe there is a simple answer but I don't see it. Once I started thinking ...
2
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1answer
46 views

Do satellite galaxies have the same proportion of dark matter as “ordinary” galaxies

My question is relatively straightforward: Do we know if satellite/ dwarf galaxies contain the same proportion of dark matter to ordinary matter as "regular" sized galaxies? The Milky Way, for ...
2
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1answer
100 views

If the universe has no center, how it can collapse?

After reading Does the universe have a center? and trying to find a place in my mind for "There exists no center", I asked myself: If there is no center, in what direction our universe will collapse, ...
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0answers
75 views

World's Largest Radio Telesope : How accurately does it need to be figured?

In Guizhou Province, China, the worlds largest radio telescope is almost complete, measuring 500 metres across. I am aware that todays largest optical telescope require very accurate figuring and ...
2
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1answer
120 views

How much noise is in the Cosmic Background Radiation, especially from Cosmic Rays

Do we have an estimate of how much noise, if any, say caused by cosmic rays in particular, is present in the CMB datasets and the maps based upon them? Can we extrapolate a figure from the cosmic ray ...
5
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2answers
71 views

On the expansion of space on small distances

After reading this post :Why does space expansion not expand matter? and after posting some comments to Peter Diehr, I was invited to make my argument to a question. The main things one needs from the ...
4
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1answer
52 views

Which 'annihilation' processes dump energy to the cosmological plasma?

I have been considering a generalisation of the cosmological process involved in computing the temperature of the cosmic neutrino background. It is well-known that (simplistically) this temperature $...
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What is the estimator that used to de-rotation of CMB polarization? [duplicate]

For de-rotation of CMB polarization we need to use of estimator for rotation angle. and used many estimator for this process.
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0answers
36 views

Where is the origin of cosmic expansion? [duplicate]

Perhaps, a rudimentary or dumb question, but nevertheless am curious. There is the idea that space is expanding at an exponential rate. From where? If everywhere, why doesn't this rapid creation of ...