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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Independant verification [on hold]

Historically, new theories were developed to explain observed phenomena and it was expected that the theory would make predicitions that could be used as a test of the theory and provide independant ...
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2answers
159 views

Are massive gravitons as the dark matter fluid more correct than the notion of WIMPs?

Accepted at APS Physical Review D: Massive gravitons as dark matter and gravitational waves Precursor: Bigravitons as dark matter and gravitational waves The massive graviton is a viable ...
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10 views

What is signifcance of density of the universe if it changes?

So theories of expansion of the universe (before dark matter and energy) said that the ultimate fate of the universe is based on the density of the universe and this being compared to the critical ...
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1answer
43 views

Before Dark Energy was discovered

I'm really confused on this topic. Pardon my French if my questions seem really stupid. Before Dark Energy was discovered in 1998 did scientists know that the cosmological constant calculated, I ...
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2answers
225 views

Is there any problem with the idea that gravity is repulsive at long distances?

If gravity is repulsive at long distances, would this suffice in explaining "dark energy"? Was there any reason why a new concept (dark energy) was introduced rather than just assuming that gravity ...
3
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1answer
29 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?
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3answers
54 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 ...
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0answers
19 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
51 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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24 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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1answer
39 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 ...
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0answers
38 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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86 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
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
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1answer
29 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, ...
3
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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 ...
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1answer
34 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 ...
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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 ...
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1answer
38 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
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?
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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
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1answer
83 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
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\...
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
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 ...
2
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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 ...
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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 ...
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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 ...
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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
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 ...
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45 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
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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, ...
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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
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 ...
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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
55 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 ...
2
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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 ...
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2answers
53 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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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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46 views

Is Dark Radiation Superluminal in speed?

In recent news http://www.kolkata24x7.com/tech-news/universe-expanding-faster-than-expected scientist have observed that the universe is expanding 5 to 9 percent faster than their calculations. ...
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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 ...
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1answer
53 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 ...
4
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2answers
62 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
51 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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28 views

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?
3
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2answers
115 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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89 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 ...