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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202 views

Can the entropy of a subsystem exceed the maximum entropy of the system in quantum mechanics?

Quantum mechanics has a peculiar feature, entanglement entropy, allowing the total entropy of a system to be less than the sum of the entropies of the individual subsystems comprising it. Can the ...
6
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6answers
991 views

Are many-worlds and the multiverse really the same thing?

Are many-worlds and the multiverse really the same thing? Not too long ago, Susskind and Bousso uploaded the article "The Multiverse Interpretation of Quantum Mechanics" with the thesis that the ...
8
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1answer
752 views

Size of the universe

This is really a follow up to the Shape of the universe question. In the first answer to the question, Ted Bunn says: However, the best available data seem to indicate that the Universe is very ...
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1answer
627 views

Has Cosmological Natural Selection been disproved?

I've been reading Lee Smolin's Life of the Cosmos. Great book and it makes a lot of sense that the conditions in black holes are the same as conditions at the big bang. Question is, has his theory ...
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3answers
2k views

Do the laws of physics evolve?

Hubble's constant $a(t)$ appears to be changing over time. The fine stucture constant $\alpha$, like many others in QFT, is a running constant that varies, proportional to energy being used to measure ...
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1answer
516 views

Inflation factor and doubling time

I get the general idea about cosmic inflation, but the numbers associated with it seem to be pulled out at random. For example, in The Elegant Universe, Brian Greene says that the universe doubled ...
6
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1answer
163 views

What are the viable non-anthropic explanations for the tininess of the cosmological constant?

The cosmological constant is 10-120 times its natural value, but it is yet nonzero. Even TeV-scale supersymmetry breaking can't save it. The renormalization group would seem to imply it ought to be at ...
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2answers
226 views

If dark matter is a new type of particle, what does that imply?

My understanding is that dark matter cannot be (or is at least highly unlikely to be) an exotic form of any known particle. On the other hand, articles about particle accelerators seem to say that the ...
7
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3answers
385 views

Feedback on the paper, 'CCC-predicted low-variance circles in CMB sky and LCDM' by V. G. Gurzadyan and R. Penrose [closed]

Ref: CCC-predicted low-variance circles in CMB sky and LCDM To all cosmology / theoretical physics / related or similar researchers and academics, Are there some updates concerning the issue of ...
27
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3answers
1k views

The Pioneer anomaly finally explained?

Pioneer 10 & 11 are robotic space probes launched by the NASA in the early 1970's. After leaving our solar system, an unusual deceleration of both spacecrafts has been measured to be approximately ...
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1answer
267 views

How does inflation drive Ω close to 1?

I'll keep it simple. How does inflation drive Ω close to 1?
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2answers
3k views

How can something finite become infinite?

How can the universe become infinite in spatial extent if it started as a singularity, wouldn't it take infinite time to expand into an infinite universe?
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1answer
671 views

Lawrence Krauss' Theory of Origin

I don't understand how a vacuum, the absence of matter, can hold energy. How can it hold energy when Einstein proved that matter is energy? And a second related question; how does the energy in a ...
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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
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2answers
827 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 ...
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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 ...
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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 ...
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2answers
317 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
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1answer
896 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
735 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
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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
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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: $$ ...
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1answer
401 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 ...
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3answers
701 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 ...
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1answer
245 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?
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1answer
804 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 ...
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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 ...
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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 ...
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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 ...
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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
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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
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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 ...
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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
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1answer
319 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 ...
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2answers
131 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
374 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?
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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, ...
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5answers
854 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 ...
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1answer
400 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 ...
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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
646 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 ...
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1answer
517 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
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4answers
923 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
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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
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3answers
586 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
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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
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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 ...
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4answers
314 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
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2answers
280 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 ...