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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If the universe didn't expand faster than light, would our nights brighter like day?

There's a common QnA which has amused and inspired many kids: There are billions of Stars in the sky. If we combine lights received from all stars, wouldn't it beat Sun? Why is night dark really? ...
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2answers
58 views

Radiation domination and the expansion of the universe

I have a problem understanding the ideas behind a basic assumption of cosmology. The Friedmann equations follow from Newtonian mechanics and conservation of Energy-momentum ($E_{kin}+E_{pot}=E_{tot}$) ...
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2answers
88 views

How does measuring redshift make us conclude that rate universe expansion is accelerating?

The universe is expanding in all directions. We can detect this by the redshift of electromagnetic radiation from other galaxies. We detect a larger redshift in galaxies which are further away, and ...
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1answer
42 views

Speed=mass=time? [on hold]

I was wondering why objects gain mass with speed? How can we measure speed if there is no truly stationary point to measure from? And if an object comes to a stand still will it loose its mass? ...
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2answers
220 views

Did the singularity really existed in the begining of the universe?

I read in Hawking's brief history of time the following: The final result was a joint paper by Penrose and myself in 1970, which at last proved that there must have been a big bang ...
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1answer
154 views

Baryon masses in Wetterich's new cosmology

Christoph Wetterich has put out a paper in which the universe isn't always expanding; it can be static or expanding just some of the time or even shrinking. And then there is an interaction which ...
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1answer
56 views

Why can't we detect redshift of Galaxies/Clusters moving parallel to us?

If a moving Galaxy/Cluster doesn't have radially outward motion component from us, we are unable to detect its redshift. The answer may look obvious: There's no wavelength squeezing/expanding ju-ju. ...
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0answers
14 views

Question on Equipartition at Reheating?

Apologies in advance if this is a silly question. I am fimiliar with the idea of Reheating in cosmology, as discussed briefly here. I have been reading several papers which make the assumption of ...
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2answers
55 views

CMB curly B-modes and dark matter

I raised a question a while ago regarding weak gravitational lensing of galaxies and the CMB. With all the fuzz with the BICEP2 data, I think it is time to raise even more questions about this amazing ...
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1answer
37 views

Qualtitative explanation for the link between low magnitude of high z supernova and accelerated expansion

Is there any explanation on a qualitative level why we can see in the observed magnitude vs. redshift z plot that the universe is expanding accelerated? See for example here: ...
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1answer
181 views

Can parallel universes constitute the missing mass aka dark matter?

I was pondering about this question for a while now. And with my limited knowledge of physics I can answer myself - if parallel universe theory is true and if indeed there is gravitation leakage from ...
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0answers
53 views

Please give some suggestions about choosing univeristies [closed]

I applied for the undergraduate transferring program. Now I have these offers: Stony Brook U Ohio state U Michigan State U U of Minnesota U of Illinois at Urbana-Champaign. Actually, the most ...
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1answer
25 views

Experimental evidence for the relic neutrinos

What are the experimental (indirect) evidence for the cosmic neutrino background? Where can I read more about this? The discussion on the wikipedia page about the C$\nu$B seems to me to be more ...
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2answers
112 views

Questions on redshift [closed]

What are different kinds of redshifts and are they related to each other? If yes, how? Is redshift a measure of distance or time or etc? For example when we say a supernova with a redshift of $z=0.9$ ...
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1answer
36 views

'There shall be reheating' and other inflation-related questions [closed]

I've some basic questions about inflation: What is reheating exactly? What triggered the reheating? Is it something that one pulls out of a hat? Through which force did the decay of the inflation ...
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0answers
15 views

Information Preservation and Weather Forecasting for Black Holes

Referencing Stephen Hawking's recent paper Information Preservation and Weather Forecasting for Black Holes and this question. I understand concept of holding the information on the apparent horizon ...
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2answers
94 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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53 views

why is universe neutrally charged?

I've read that the universe is neutrally charged. Therefore the number of protons and electrons of the universe are equal. But why the universe is neutral? What problem is caused if we assume that the ...
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0answers
16 views

Instabilities in the CDMT

Could anyone explain or refer to references on why the CDMT f(R) gravity model suffers from Instabilities any why the sign of ${\mu}^{4}$ matters.
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2answers
1k views

Why aren't we Boltzmann brains in an infinite universe?

Either space is compact, or it is infinite. If space is infinite in extent, either it is thermal over an infinite volume, or it is in the vacuum state for most of it. If it is thermal, infinity is a ...
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1answer
26 views

Calculating Hubble Radius using relativistic effects

I am trying to calculate the Hubble Radius, or the distance from which an object will recede from an observer at the speed of light. I initially tried using $v=H_0d$ which gives $d=1.424\times10^{26}$ ...
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1answer
1k 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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1answer
55 views

Is the Higgs field part of space?

So I looked for some explanations, how I understand it so far is that the Higgs field causes particles to have mass by drag, like drag force in water, which prevents objects from moving at the speed ...
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2answers
203 views

What is the entropy of the universe today?

What's the entropy of the universe today? How does one go about calculating this? I've heard the statement that black holes account for the bulk of the entropy in the universe today, but don't know ...
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64 views

Dark energy and conservation of energy [duplicate]

With accelerated expansion of universe which is same in all direction we know that dark energy increase with time because space between any two point in space time increases with time. So after some ...
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1answer
520 views

Particles entangled after the big bang

Is that true that the big bang caused the quantum entanglement of all the particles of the universe so every particle is entangled to each other particle of the universe?
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2answers
96 views

How could mass exist before Big Bang?

I heard that before Big Bang the temperatures were so high that mass and energy constantly converted to each other. After big bang the temperature became less and the conversion stopped that is what ...
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2answers
853 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. ...
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1answer
38 views

Could the universe be a series of Big Bangs? [closed]

Imagine an eruption of energy/mass $E$ from a singularity $O$, as in a Big Bang. After the energy/mass $E$ is all at more than a distance $d$ from $O$, is it for some value of $d$ possible that there ...
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4answers
67 views

If you shoot a light beam behind the event horizon of a black hole, what happens to the light?

I have a couple of questions about light here, and sorry of they are silly.. So since anything that goes beyond the event horizon can't go out, so what if a light beam was pointed somewhere behind ...
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1answer
32 views

Does the universal expansion affect the gravitational constant?

If the universe was not expanding, would gravity be stronger? since the expansion happens everywhere and can overcome gravity at long distances? If not, then are there any changes that would happen to ...
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3answers
260 views

Why doesn't dark energy vary with time?

Trying to teach myself the basics of cosmology. I can see why, in an expanding universe, radiation and matter density, and radiation and matter pressure vary with time. But why is it assumed that dark ...
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3answers
343 views

Could the acceleration of universe expansion be caused by gravity itself?

Dark energy is suggested to be a repulsive force in the universe causing an accelerated expansion. If the amount of mass outside our observable universe is greater than inside (higher mass density), ...
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0answers
33 views

Is Dark Energy Pushing Us Or Pulling Us? [duplicate]

This may seem silly, but how do they know that the effects we are seeing from dark energy, (which we only assume must be there due to observed acceleration), are not from gravity pulling us out from ...
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2answers
117 views

Cosmological constant of standard model of cosmology and observational data

I am curious whether the current Lambda-CDM model of cosmology matches well with observational data, especially expansion of the universe. How well does Lambda-CDM defend its established status from ...
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1answer
33 views

Open Big Bang-less universe?

This came up in discussion around a class I'm taking. For a Universe with $\Lambda$ and matter contributions to energy density (and implicitly curvature, but no radiation), can you have a universe ...
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3answers
97 views

Is the universe flat?

There are more than one way to view the description of the universe as flat. There is the description of an open, flat or closed universe in terms of it's fate, expansion forever away from gravity, or ...
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1answer
55 views

Acceleration of the Expansion of Universe

Is the acceleration of the expansion of the universe (AEU) uniform in direction? Meaning that if a particular direction in the sky is chosen and the AEU is measured and then other directions are ...
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0answers
19 views

Link for resource on cosmology [duplicate]

I have recently just begun studying cosmology. I have a background in physics. I would like a link to download video resources on Stephan Hawking's theories on physics and/or cosmology that have not ...
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0answers
33 views

Book suggestion for Theoretical Physics with easy maths [duplicate]

I am a Computer Scientist with literature interest in theoretical physics. I have already read books such as A Brief History of Time and Physics of the Impossible, and I am looking for suggestion for ...
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3answers
562 views

Whats left at the center of the Universe after Big bang?

If you consider big bang. According to that What's left at the center of universe where the Big bang occured?
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1answer
32 views

Is the Universe Past-Eternal?

Does the Borde-Guth-Vilenkin theorem definitively demonstrate that the Universe cannot be past-eternal, whatsoever? Does it not assume a classical space-time while the real world requires Quantum ...
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1answer
52 views

Size of the Universe: Curved vs flat? Finite vs infinite?

I have recently heard the theory that the Universe may be smaller than observed but may be curved to the extent that light rays may have looped past us once already and hence appear to have originated ...
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1answer
269 views

How to obtain the field equations in Brans-Dicke theory from the action?

The action for the Brans-Dicke-Jordan theory of gravity is $$ \\S =\int d^4x\sqrt{-g} \; \left(\frac{\phi R - \omega\frac{\partial_a\phi\partial^a\phi}{\phi}}{16\pi} + \mathcal{L}_\mathrm{M}\right). ...
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2answers
797 views

Is 2.5x speed of light possible between two objects?

These news are in Finnish: http://www.hs.fi/ulkomaat/Kaukaisin+havaittu+galaksi+et%C3%A4%C3%A4ntyy+maasta+valoa+nopeammin/a1305618680897 The main excerpt is: "Distant galaxy moves away from us as ...
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1answer
24 views

Does the rate of expansion of the universe exceed the rate at which the local group is being pulled toward the Great Attractor?

I understand that the expansion rate of the universe driven dark energy exceeds the rate at which the local group is being pulled toward The Great Attractor (Hydra-Centaurus Supercluster). Does this ...
7
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1answer
101 views

In the B mode power spectrum, what is the relationship between the multipole number and the wavelength of the seed gravitational waves?

One of the key datasets of the recent BICEP2 results is the B mode power spectrum shown below. The existence of these B modes implies the existence of gravitational waves prior to inflation. My ...
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1answer
19 views

Inflation in a closed universe or a stage 1 multiverse?

With the discovery of gravitational waves, Max Tegmark has been using this promote his level 1 multiverse in that the universe is flat and everything duplicates. My question is, are there any ...
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2answers
56 views

Conservation of stress-energy and the fluid equation in cosmology

I'm trying to derive the equation for the cosmological fluid: $$\dot \rho + 3 \frac{\dot a}{a}(\rho +P)=0$$ by starting from the conservation of the stress-energy tensor: $$\nabla^\mu T_{\mu \nu} = ...
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2answers
61 views

How does “space” expand? [duplicate]

According to Big Bang theory and The Red shift theory "space" is "expanding". Keeping this notion of "space" as physically something in and of itself(space -time did not exist before The Big Bang); ...