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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197 views

What is behind the cosmological horizon barrier?

I'm wondering what is behind the cosmological horizon barrier?
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3answers
338 views

Can extra dimensions be too large to be observable

String theory postulates 6 extra dimension, all too small to be observed. The best description of a small dimension is that of an ant walking on a flagpole: The ant observes that the flagpole allows ...
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2answers
1k views

Have red shifted photons lost energy and where did it go? [duplicate]

I think the title says it. Did expansion of the universe steal the energy somehow?
3
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2answers
111 views

Cosmic radiation cutoff at LOW energies?

The energy spectrum of the cosmic radiation (not CMB) is limited to both sides. I know about the GZK-cutoff at high energies. Basically the interaction probability for photons of energies above 10^20 ...
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2answers
317 views

Quantum physics before the observer and relic interferences

It is known that the concept of an observer plays important role in quantum mechanics. In interpretations with collapse only the observer can trigger the wavefunction collapse. From this naturally ...
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0answers
57 views

Status of large-scale structure formation within cosmology today

Since the CMB results of the past decade, would it be fair to say that the consensus among cosmologists is that cosmic strings are no longer considered as a (major) source for density perturbations? ...
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2answers
132 views

How to test whether galaxies are moving away from each other in a static background or whether space is being created beween them?

Observations show that galaxies are moving away from one another on the macroscopic scale. Now, scientists interpret this by saying this happens not because galaxies are really moving away from each ...
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4answers
497 views

Did really everything begin with a state with very low entropy?

As emphasized by Penrose many years ago, cosmology can only make sense if the world started in a state of exceptionally low entropy. The low entropy starting point is the ultimate reason that the ...
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2answers
440 views

What will we see between the CMB and the current oldest object seen?

The cosmic background radiation (CMB) is estimated to be from 13.7 billion years ago (BYA), and very shortly after the big bang compared to that time frame. The oldest coherent objects we've detected ...
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1answer
581 views

What are the theoretical explanations behind the “no-boundary condition” in cosmology?

In his book "The Grand Design" on page 167-173 Stephen Hawking explains how one can get rid of the so called "time zero issue", meaning by this the very special initial state needed for inflation to ...
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1answer
398 views

Was the universe a black hole at the beginning?

Big bang cosmology, as far as I understand it, says that the universe was super hot and super dense and super small. It looks like that all the current matter, seen and unseen, were compressed to ...
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1answer
2k views

What did Hawking mean? 'Time started at the big bang'. Book suggestions please [closed]

After writing down this question, I have come to realize that. What I really want is reading materials on the questions below. Before the big bang there was no such thing as 'time' (Steven Hawking on ...
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1answer
280 views

Does the dark matter halo rotate with the galaxy?

If the dark matter halo is stationary related to the arms of the galaxy then tidal effects should slow the galaxy rotation. If it rotates with the normal matter in the galaxy then shouldn't it ...
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2answers
316 views

How to concile flat spacetime and big bang?

After reading How do we resolve a flat spacetime and the cosmological principle? I still remain perplex. Please excuse my ignorance and try explaining to me : I thought that basically, when we ...
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2answers
678 views

Can the expansion space time reverse itself and contract the same way?

If there's a mechanism for space-time expanding faster than the speed of light, is there an example of It contracting in the same manner? If whatever mechanism is causing it to expand, can the ...
2
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1answer
601 views

Superluminal expansion of the early universe how is this possible?

Is this a postulate? I get the expansion of the universe, the addition of discrete bits of space time between me and a distant galaxy, until very distant parts of the universe are moving relative to ...
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2answers
222 views

What are the limitations of the FLRW metric?

I was wondering, given how in any other area of life making an explosion spherically symmetric is more or less impossible is there any reason to expect that the universe is? I appreciate that the FLRW ...
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1answer
99 views

Do Galactic Black Holes and Extra-Solar Planets change the mass balance of the universe?

With all the recent discoveries of supermassive black holes being at the center of nearly every galaxy, and the proposed "at least one planet" around (probably) every star. Does this affect the ...
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1answer
314 views

Hydrostatic equilibrium of a star derivation

I've been told to revise the derivation that proves $\frac{\mathrm{d}P}{\mathrm{d}r} =\frac{GM(r)p(r)}{r^2}$ where brackets indicate a function of, P is pressure and p is density. Rather helpfully he ...
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2answers
313 views

Why is it concluded that the cosmos is expanding when in fact the observations are for events further back in time?

Why is it concluded that the cosmos is expanding at an ever increasing rate when looking further out and therefore further back in time? Surely, if the time were reversed to play forwards, the ...
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2answers
324 views

How is causal patch complementarity compatible with behavior during inflation?

Causal patch complementarity is the conjecture that in de Sitter space with a positive cosmological constant, the states within the causal patch are sufficient to fully describe the universe with the ...
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1answer
132 views

Why is cosmological acceleration expressed in terms of an energy density?

In the articles that I have (tried to) read, acceleration ends up being expressed as a dimensionless constant (omega-lambda) or else occasionally in terms of a "dark" energy density. Presumably one ...
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0answers
69 views

How is it possible to measure expansion of universe [duplicate]

Possible Duplicate: Why does space expansion not expand matter? If according to Hubble's law space the expanding then shouldn't everything else along with the space expand and we should not ...
2
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1answer
83 views

Will the universe reach zero mass?

I was reading Phil Plait's 'Death from the skies' where it describes the fate of the universe. It describes the period from 10^90 years to infinity as the "dark age" when all the of the black holes ...
2
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1answer
437 views

How is the expansion of space itself explained in LQG?

To explain the expansion of space I have often heard people saying that space is continuously created. This picture is usually applied to cosmological scales but I`m nevertheless curious if some ...
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1answer
334 views

how many times have the first photons lapped the universe?

I'm not really interested in the actual number of times, but rather an explanation of a closed Universe. How do we still measure the background radiation of the Big Bang? Those electromagnetic waves ...
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0answers
13 views

Topics in particle cosmology [duplicate]

Possible Duplicate: Topics in particle cosmology I am interested in learning more about this interdisciplinary approach. 1) What are some of the top questions in particle cosmology (e.g ...
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0answers
89 views

Topics in particle cosmology [closed]

I am interested in learning more about this interdisciplinary approach. 1) What are some of the top questions in particle cosmology (e.g nature of dark matter, inflationary structure, topological ...
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1answer
162 views

The range of light

It occurs to me that the empirical evidence shows that there is a point out in space where light stops coming from. Putting aside the expansion of the universe for a second, and focusing strictly on ...
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5answers
163 views

Could there be more universes?

In the documentary: "Curiosity - Did God Create the Universe (on YouTube)", theoretical physicist and cosmologist Stephen Hawking states that time did not exist before the big bang. The first ...
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3answers
2k views

Value of the Hubble parameter over time

There is something I don't understand about the Hubble parameter $H$, as it seems to clump two concepts together that I can't quite unify in my head. On the one side, we have $$V = D H$$ which means ...
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1answer
287 views

Vanishing Ricci flow on a curved manifold

If I understand this right the Ricci flow on a compact manifold given by $\partial g_{\mu \nu} = - 2R_{\mu \nu} + \frac{2}{n}\!R_{\alpha}^{\alpha} \,g_{\mu \nu}$ tends to expand negatively curved ...
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2answers
142 views

What are some examples of how the discovery of dark energy can impact other, seemingly unrelated, branches of physics?

We know that dark energy is leading to the accelerating expansion of the universe and therefore determines the ultimate fate of our universe, but what other implications might it have on physics and ...
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2answers
195 views

Can the implications of dark energy be used to bridge the gap between Quantum Mechanics and General Relativity?

Can the findings of the Physics Nobel Laureates of 2011, namely the overpowering existence of dark energy (vacuum energy) have any implications in the quest the combine Quantum Mechanics and General ...
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3answers
535 views

How scientists estimated that our universe is 4% normal matter, 21% dark matter, and 75% dark energy?

Is there a simple way to understand how scientists estimated/calculated the following percentages?
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1answer
316 views

Is there more mass in stars or interstellar medium?

From what I gather the interstellar medium has about about 1 atom per cubic centimeter. But on the other hand, as they say, "Space is big, really really big" So if it is known (or at least theorized ...
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3answers
237 views

Universe is expanding at enormous speed

I'm not an expert but I've come to understand that the universe is expanding at enormous speed. That means that all of the visible galaxies are moving away from us at great speed. I also came to ...
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2answers
88 views

Is there any chance we could eventually observe the supernova of the first (Population III) stars

In an answer to my previous question about the first stars it was stated they probably formed at Z=20 to Z=60 and may have had a mass between tens to 100s to 1000s of times the mass of the Sun. Given ...
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10answers
275 views

In astronomy what phenomena have theory predicted before observations?

As far as I know, astronomy is generally an observational science. We see something and then try to explain why it is happening. The one exception that I know of is black holes: first it was thought ...
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2answers
419 views

This Expansion-of-the-Universe-Diagram Confuses Me

The following blue-cone Wikipedia diagram confuses me. At any point of cosmological time the encircling horizontal lines in the diagram are of finite circumference. That is indicative of a closed ...
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1answer
102 views

What is the best evidence/theory about the first generation of stars

In particular, during what range of years after the Big Bang did the stars form? What is the expected range of masses of these stars and what is the expected lifetime before they supernova? I assume ...
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2answers
2k views

Physical laws prior the big bang (quantum fluctuations)

A theory among scientists says that quantum fluctuations caused the big bang and created the universe. This seems plausible to me. What I can't grasp yet is how a quantum fluctuation can even start ...
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1answer
204 views

Dark matter and mass-energy conservation

I have been thinking recently about dark matter, and it lead me to the following question. Consider a hypothetical particle which, like the photon, is chargless and massless. However, unlike the ...
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4answers
3k views

How do we know Dark Matter isn't simply Neutrinos?

What evidence is there that dark matter isn't one of the known types of neutrinos? If it were, how would this be measurable?
2
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1answer
169 views

Violation of Lorentz symmetry on cosmological distances

This question is about the domain of validity of Lorentz symmetry. As far as I know, general relativity is a generalization of special relativity. Does that mean that Lorentz symmetry is violated on ...
3
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2answers
391 views

Big Bang and Cosmic microwave background radiation?

One of the experimental evidence that supports the theory of big bang is cosmic microwave background radiation (CMBR). From what I've read is that CMBR is the left over radiation from an early stage ...
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0answers
112 views

What is the meaning of colors in deep space pictures? [closed]

Are the telescopes really capturing these colors in deep space or are they just "artist's impression". What could make such vast space "red" or "green"?
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2answers
489 views

Is eternal inflation and the multiverse compatible with causal patch complementarity?

The argument for eternal inflation is we have some patch of metastable vacuum with positive cosmological constant, and so it expands exponentially a la de Sitter. Most of the patch decays to something ...
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3answers
294 views

Why every galaxy is moving away from every other galaxy on astronomical scale? because of an explosion or because of creation of space?

Why galaxies move away from each other in general on the astronomical scale? Which answer is correct of the following? It is because of the big bang theory, everything is just moving away from ...
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1answer
85 views

Are there stable string theory vacua with non-minimal cosmological constant?

Naive reasoning suggests that a string theory vacuum with cosmological constant Lambda1 is always unstable as long as there is a string theory vacuum with cosmological constant Lambda2 < Lambda1 ...