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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11
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2answers
1k views

Is dark matter repulsive to dark matter? Why?

I think I saw in a video that if dark matter wasn't repulsive to dark matter, it would have formed dense massive objects or even black holes which we should have detected. So, could dark matter be ...
2
votes
2answers
839 views

What is meant by positive and negative gravity/energy/spactimecurvature?

I have recently come across some cosmological assertions (based on empirical data) about the universe being self contained in the sense that it is entirely capable of coming into existence from a ...
12
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2answers
1k views

What exactly is meant by the “Gaussianity” of CMBR?

What does it mean when we say that the CMBR is mostly gaussian? What are non-gaussianities in CMBR? How does evaluation of 3-point correlation functions of the inflaton field tells us that there is ...
11
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6answers
3k views

How many bits are needed to simulate the universe?

This is not the same as: How many bytes can the observable universe store? The Bekenstein bound tells us how many bits of data can be stored in a space. Using this value, we can determine the ...
2
votes
2answers
327 views

Have CMB photons “cooled” or been “stretched”?

Introductory texts and popular accounts of why we see the "once hot" CMB as microwaves nearly always say something about the photons "cooling" since the Big Bang. But isn't that misleading? Don't ...
2
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0answers
356 views

Calculation of the non-Gaussity parameter for primordial cosmological perturbations by the ADM Formalism

Maldacena has used the ADM Formalism in one of his papers (http://arxiv.org/abs/astro-ph/0210603) in computing the the three point correlation function (i.e the non-Gaussianity) parameter for ...
3
votes
1answer
904 views

Mathematical justification of Hartle-Hawking “no boundary” proposal

In Hartle-Hawking "no boundary" proposal it is proposed that Riemannian spacetimes rather than Lorentzian dominated the path integral near the big bang. Moments after the big bang however spacetimes ...
10
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2answers
749 views

Can the Big Rip really rip apart an atomic nucleus?

Some scenarios describing the fate of the matter vs dark energy tug of war on the universe involve the acceleration of the universe increasing to the point that it ends up ripping apart even atoms. ...
6
votes
2answers
1k views

on causality and The Big Bang Theory

With the notion of causality, firmly fixed by GR, we derived the concept of a singular point from where space-time begun. Causality alone gives us the possibility to talk about a known past (i.e. ...
2
votes
3answers
253 views

Distance in cosmology

How can we calculate the change in distance over time between (say) two galaxies that they are at rest in respect to each other? In other words, how do we modify the "classical" equation: $$ ...
0
votes
1answer
403 views

Does quantum mechanics prohibit extraterrestrial life? [closed]

There is a known thought experiment, connected to quantum immortality: a duel between physicist and a philosopher. Each turn physicist and philosopher fire at each other with a pistol. The quantum ...
5
votes
3answers
703 views

If the universe were a fractal

Inflation seems to solve many of the problems of cosmology like horizon problem, flatness problem etc. Now suppose, I am a devil's advocate and tries to find holes in this beautiful theory. I argue ...
1
vote
1answer
247 views

Shear, viscosity and expansion of universe

What is the meaning of expansion, shear and viscosity in context of universe? How can we conclude a result after getting a numerical value of above terms?
8
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1answer
818 views

On Flatness problem, Inflation etc

I have a couple of naive questions from the topic of the title. We know \begin{eqnarray} \Omega-1=\frac{k}{a^2H^2}-\frac{\Lambda}{3H^2} \end{eqnarray} Now I read that from the standard big bang ...
4
votes
2answers
1k views

Entropy of an empty universe

After watching the first episode of wonders of the solar system, one question came up which is not explained. Bryan Cox says that ultimately the universe will be devoid of matter, so not even a ...
29
votes
2answers
1k views

Are modified theories of gravity credible?

I'm a statistician with a little training in physics and would just like to know the general consensus on a few things. I'm reading a book by John Moffat which basically tries to state how GR makes ...
0
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2answers
142 views

Star trace in the sky

If the space of universe is expanding at fast rates..(even faster than light) Why we do not see the trace of the stars? A possible answer could be that the stars are only moving away but mantaining ...
1
vote
0answers
164 views

can someone review a 8 page essay for me for accuracy? essay is in cosmology [closed]

I am a bio-chemist by profession and recently written an essay for a book which is related to the subject of cosmology. This is not my area of expertise. Is there a cosmologist out there who will be ...
2
votes
2answers
152 views

Mathematical Elegance in a Cosmological Theory Considered Necessary?

In general, it seems cosmological theories that encompass more and more of the phenomena of the universe are expected to be more and more mathematically elegant, in conception if not in detail. Our ...
4
votes
2answers
333 views

Is our universe a 2+1D superconformal field theory?

If our universe comes from a flux compactification of string theory over 6 dimensions with a nonzero flux, then it can't be continually deformed to another compactification with zero flux as the total ...
1
vote
1answer
257 views

Inflation and string theory in general relativity

How can we relate inflation and string theory? We study some inflationary cosmological model or some string cosmological model under some specific assumptions or conditions. Is their any relation or ...
11
votes
1answer
321 views

Is almost all entropy in our universe entanglement entropy?

Our observable universe, or a subregion of our universe many times larger than the observable universe, originated from inflating from a very tiny inflationary patch. Being so small, the initial ...
2
votes
2answers
132 views

Observation of cosmological topological defects

Are there any projects/experiments running for the observation of topological defects (domain walls, monopoles, etc)? Are there any past/canceled or future such projects?
8
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3answers
375 views

Does MOND make good predictions?

Well, it does according to this preprint for certain scales. What would be a simple way to explain MOND to a layman? Does it ignore mainstream physics? How much?
1
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1answer
193 views

Gravitational effect of the distant universe

When we observe at close to the cosmological horizon (as close as we currently can) we see the universe as a much younger, denser place. As we feel the gravitational forces from these objects distant, ...
8
votes
5answers
894 views

Dark matter and dark energy references

I've been looking for questions about dark matter, and I've read some very interesting answers. However, I desire too look into it deeply. This is not actually a question. I'm asking the community ...
3
votes
1answer
401 views

Quantum death like heat death possible?

Quantum decoherence is an irreversible process which is the result of interaction of the system with its environment. It prevents interference due to lack of coherence. Environment acts just like a ...
3
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4answers
1k views

Maximum size of the observable universe?

The size of our observable universe must have grown over time in the early universe. Conversely with the accelerated expansion, I have heard that eventually our observable universe will shrink down ...
7
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4answers
662 views

de sitter cosmologic limit

It has been said that our universe is going to eventually become a de sitter universe. Expansion will accelerate until their relative speed become higher than the speed of light. So i want to ...
-2
votes
1answer
522 views

History of Man Acquired knowledge of solar system [closed]

I know the basics of solar system like how earth moves around sun, and that we have so many planets, elliptical orbit of earth, and how far is sun from earth etc etc. I want to take a step back and ...
4
votes
4answers
954 views

Question regarding the validity of the big bounce

I have several questions regarding the "big bounce" theory. It appears to be popular among LQG researchers. My questions are as as follows. 1) How one reconciles it with the fact that it is now ...
9
votes
5answers
2k views

Did spacetime start with the Big bang?

Did spacetime start with the Big Bang? I mean, was there any presence of this spacetime we are experiencing now before big bang? And could there be a presence/existence of any other space-time before ...
6
votes
3answers
591 views

The Big Bang in an infinite universe

If the universe is spatially infinite (and assuming, if it makes a difference, that we don't have eternal inflation), what actually happened 13.7 billion years ago? Was the energy density infinite (or ...
2
votes
0answers
224 views

Singularities in Bianchi models in general relativity ( physical science)

what are the conditions to check point type singularity in a bianchi type model ? bianchi type model are of Type I,II,III,IX,IV or u can say we use different Bianchi type models having some specific ...
8
votes
1answer
2k views

Where does CMB come/emit from?

Where exactly does CMB come from. I've seen it in documentaries as a huge sphere with Earth in the middle. But if all this radiation was ejected from the start of the universe some time after the big ...
1
vote
4answers
315 views

Cosmic background radiation

Where/what is emitting cosmic background radiation, and when did it come into existance, was immediately after the big bang? I know that the universe isn't 3D in the traditional sense, and I don't ...
3
votes
2answers
285 views

asymptotic curvature of the universe and correlation with local curvature

There is not-so-rough evidence that at very large scale the universe is flat. However we see everywhere that there are local lumps of matter with positive curvature. So i have several questions ...
1
vote
1answer
484 views

Can the Universe create itself?--Is Gott's Use of CTC in Planck time or less valid?

My question is the title of a 1991 paper by Richard Gott and Li-Xin Li: http://arxiv.org/abs/astro-ph/9712344 and is also a subject of his popular book, "Time Travel in Einstein's Universe" Ultimately ...
9
votes
3answers
3k views

Why does the low entropy at the big bang require an explanation? (cosmological arrow of time)

I have read Sean Carrol's book. I have listened to Roger Penrose talk on "Before the Big Bang". Both are offering to explain the mystery of low entropy, highly ordered state, at the Big Bang. Since ...
9
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3answers
2k views

How many bytes can the observable universe store?

Is the number of states in the Universe countable? What framework could be used to answer the question in the title?
3
votes
1answer
604 views

What is a maximal analytic extension?

Can someone explain (as rigorously as possible) what is involved in analytically continuing, say, the Schwarzschild solution to the Kruskal manifold? I understand the two metrics separately but I'm ...
0
votes
1answer
304 views

Volume of Matter in the Universe [closed]

What would the size of the universe be if it were physically possible to remove all of the empty space, leaving only matter?
4
votes
3answers
672 views

Do singularities have a “real” as opposed to mathematical or idealized existence?

I was thinking of, for example a Schwarzchild metric at r=0, i.e. the gravitational singularity, a point of infinite density. I realise that there are different types of singularities--timelike, ...
15
votes
5answers
2k views

How many times has the “stuff” in our solar system been recycled from previous stars?

Is there a cosmologist in the house? I've got a basic understanding (with some degree of error) of some simple facts: The Universe is a little over 13 billion years old. Our galaxy is almost that ...
0
votes
1answer
148 views

Effect of Cosmological Expansion at the Atomic Level [duplicate]

Possible Duplicate: Why does space expansion not expand matter? Does cosmological expansion have any effects at the atomic level?
3
votes
1answer
214 views

If Fine Structure Constant in not uniform in space what would that imply for cosmic background?

Recently J.Webb submitted paper which has extraordinary claim - that Fine Structure Constant is different in different directions in space! He (with others) measured $\alpha$ using quasar spectrum ...
1
vote
2answers
426 views

Expansion of the space-time metric [duplicate]

If the space-time metric is expanding with the expansion of the universe, if I could travel back in time, would I be less dense than the matter in that previous era?
2
votes
3answers
288 views

Is it physically meaningful to talk about the arrow of time in other universes?

The arrow of time is a concept that has been around for a while. In modern cosmology, it has been argued that there may be different arrows of time in other universes. Since these are also ...
4
votes
3answers
533 views

References for ADM formalism and cosmological perturbation theory [closed]

What would you consider the best online resources for learning the 3+1 ADM formalism and gauge invariant perturbation theory in cosmology? (Assuming intermediate level GR and QFT familiarity)
11
votes
4answers
682 views

Big Bang snuffed by a black hole?

Wasn't the density of the universe at the moment after the Big Bang so great as to create a black hole? If the answer is that the universe/space-time can expand anyway what does it imply about what ...