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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Is Λ-CDM and no inflaton field compatible with the observed value of the scalar spectral index?

From the cosmic microwave background, one can extract the scalar spectral index $n_s$. It is measured to be smaller than 1 by several standard deviations. Wikipedia says that it is a parameter of the ...
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1answer
33 views

Cosmological pressure and the expansion of the universe

I have a problem understanding the thoughts behind a basic assumption of cosmology. The Friedmann equations follow from Newton mechanics (probe mass at the surface of a sphere) and conservation of ...
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3answers
153 views

What is the scientific view of creation? [closed]

Can space be created or destroyed? Is space conserved? I am not asking for matter,energy and time. Its just a question about conservation of space PS: I am asking for what was there before big bang. ...
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1answer
86 views

What do the names “E mode” and “B mode” mean? Where do they come from?

This has been bugging me a bit since the BICEP announcement, but if there are any resources that answer my question in a simple way, they've been buried in a slew of over-technical or over-popularized ...
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1answer
101 views

BICEP2 and e-foldings during inflation

After the BICEP2 results, we now know that $n_s = 0.96$ and $r = 0.2$. From what I understand, this fits extremely well with the basic chaotic inflation model given by $V(\Phi) = \lambda \Phi^4$. We ...
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0answers
19 views

Connection between BAO und CMB Spectrum

I have a problem understanding the connection between the accoustic peaks in the CMB spectrum and the baryon oscillation picture. On the one hand it is stated, that the odd accoustic peaks (1,3,5..) ...
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2answers
143 views

BICEP2 experiments

How was the polarization experimentally measured in the BICEP2 experiments and why did they look specifically at B-modes? Why is it implying the existence of gravitational waves and the need to ...
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1answer
201 views

What do the BICEP2 results mean for string gas cosmology and the ekpyrotic universe?

The imprint of gravitational waves created shortly after the big bang may offer direct evidence for inflation theory, according to a discovery by the BICEP2 experiment at the South Pole and released ...
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2answers
222 views

What experiments compete with BICEP 2, and when are their results expected?

The recent results of the BICEP 2 experiment published on March 17th 2014, has generated a lot of media attention, with the general consensus being that "this is a major discovery" perhaps leading to ...
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1answer
473 views

What do the line segments on the BICEP2 B-mode polarization map mean?

The first image of BICEP2 visuals shows the "BICEP2 B-mode Signal", described as follows: Gravitational waves from inflation generate a faint but distinctive twisting pattern in the ...
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5answers
5k views

What was the major discovery on gravitational waves made March 17th, 2014, in the BICEP2 experiment?

The Harvard-Smithsonian Centre for Astrophysics held a press conference today to announce a major discovery relating to gravitational waves. What was their announcement, and what are the implications? ...
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200 views

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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132 views

On Group Theory: Symmetry Groups and Our Interest

Over the past few years, I've been doing a lot of self education in the Quantum Mechanics and General Relativity, and of course, there are mathematical elements of both doctrines that are matrices. ...
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0answers
87 views

General formula to compute the redshift (first order perturbations)

Consider an expanding universe with the following metric in conformal time/co-moving coordinates: ...
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0answers
26 views

What is the perturbative vacuum?

Will someone please supply me with a good definition of the perturbative vacuum vis a vis quantum cosmology?
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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
60 views

Is the universe a perpetual motion machine? [closed]

Is the universe a perpetual motion machine?
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1answer
127 views

Can we have a static point in the universe? [closed]

Big bang theory states that the universe began from a single point (which we may consider as a central frame of reference for the objects in the Universe). In 1929, Edwin Hubble confirmed that the ...
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1answer
85 views

Stress-Energy Tensor

As of recent, I've been doing a bit of self education in GR, equipped with a working knowledge of the key elements of the differential geometry in GR, and in looking at the Einstein-Rosen bridge, I ...
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4answers
243 views

Was Einstein wrong when he said nothing can go faster than the speed of light? [duplicate]

If the universe is constantly expanding faster than the speed of light, how could Einstein be right?
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0answers
49 views

Object that causes spacetime to expand?

Is there any thing that will cause spacetime to expand, so that particles are pushed away from them rather than pulled towards it. I know things such as black holes and planets causes dips and curve ...
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2answers
43 views

Approximation of the energy for low $T$ in the early universe

In Perkins `Particle Physics', to compute the baryon-antibaryon-ratio, he uses that for $Mc^2\gg kT$: $$E= Mc^2 + \frac{p^2c^2}{2m}.$$ I realize that the approximation comes from $E^2=M^2c^4 + p^2c^2$ ...
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1answer
50 views

How many years of expansion occurred during inflation?

During inflation, expansion happens at a very rapid rate. How many years of expansion did it fast forward through? Meaning, if it weren't for inflation, how many years would it take for the ...
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0answers
25 views

Physical difference between the derivative of the Hubble parameter and the second derivative of the scale factor?

In Carroll's GR book, he discusses the difference between $\dot H$ and $\ddot a$, stating that they are the answers to two different questions, pg. 349. He seems to imply that if one set up two ...
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0answers
24 views

How is the scale factor from the FLRW equation used with Volume?

I'm trying to put a spreadsheet together that shows the co-moving volume of the universe from the time soon after the Big Bang through the present and then as predicted into the future. I am pretty ...
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0answers
37 views

Entropy bounds and the size of the universe

The principle that the maximum amount of information or entropy a volume of space can hold is proportional to its surface area apparently applies to all space, not just black holes. Since volume grows ...
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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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2answers
114 views

How can space expand?

How can space expand when it is only a perception of the separation between at least 2 objects. Isn't saying "space expands" implying it has properties?
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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
64 views

A way to determine the exact location of the Big Bang?

See What is our location relative to the Big Bang?, which contains the following: The conclusion is that the Big Bang happened everywhere, all at once. Phys.SE users Ali and WernerCD reached this ...
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1answer
69 views

If we were able to double the mass of the ordinary matter in the universe it will recollapse?

What will happen if we suppose to be able to add mass at the universe? For example doubling the mass of the ordinary matter?
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1answer
58 views

Is conformal time observable?

The standard FRW metric with cosmic time is $$ ds^2 = -dt^2 + a^2(t)(\gamma_{ij}dx^i dx^j),$$ and we can measure $t$ as the proper time for comoving observers. Thus it makes sense to talk about the ...
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0answers
33 views

Meaning of $k$ in Sachs-Wolfe formula for angular power spectrum

I've seen the formula for the angular power spectrum of the CMB written as $$C_\ell = \frac2\pi \int\left|\Theta_\ell(k) \right|^2 k^2dk, $$ where $\Theta_\ell(k)$ is the temperature contrast at a ...
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0answers
39 views

Why can't Wetterich's “Universe without expansion” be tested compared to standard cosmological redshift?

I have read on several sites that this is a theory that cannot be tested. Why is this a theory that can not be tested? It seems to me that if the mass of particles is continually increasing, then the ...
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1answer
64 views

why does the CMB change so slowly

If anisotropies of the CMB originated from "quantum fluctuations of matter in a very tiny space that expanded to the size of the observable universe we see today" (quote from Wikipedia) would they ...
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2answers
62 views

Is there any mathematical structure modelling parallel universes (multiverse)?

Is there any mathematical structure modelling parallel universes (multiverse)? If we established such structure, is it science or science-fiction?
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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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1answer
64 views

Is this a possible big-bang scenario? [closed]

Assume an universe with a single black hole that contains all the universe's mass and then let's wait for quite some time. What happens if by Heisenberg's uncertainty principle a virtual ...
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1answer
32 views

Cosmological horizon, energy and information

If something crossed the horizon of the observable Universe, does this mean that all information about it is lost forever? If yes (or no), how this fits with the information-cannot-be-lost law? ...
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3answers
121 views

Space and the size of infinity [duplicate]

If you could build a spaceship and keep travelling in one direction, what would finally happen? One answer is that you would never ever reach the end. But this sounds purely platonic space and comes ...
2
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1answer
92 views

Difference between horizon and flatness problems & how inflation solves flatness (w/out math)

Laymen here so I'm hoping for an answer for my query that doesn't involve math. I'm reading about inflation and how it solves the flatness and horizon problems. I get that the horizon problem deals ...
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1answer
49 views

Horizon problem in the standard cosmological model

The horizon problem is usually stated as follows: Causally disconnected parts of the universe that we see today have the same temperature, so how could that be so if no information travels faster than ...
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2answers
709 views

Can Flow of time become still?

According to theory of time dilation, flow of time slows down significantly at the speed of light.Is there any conditions practically or theoretically when flow of time is reduced to zero means it ...
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2answers
81 views

Why doesn't everything expand when the universe expands? [duplicate]

Everybody has been taught at one point, "oh the universe expands, but that doesn't mean that everything is expanding uniformly, since that means we can't detect the expansion, but only that huge ...
7
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2answers
194 views

Do photons and cosmic rays radiate energy through gravitational waves? If not, why not?

Due to the mass-energy equivalence, both matter and EM radiation bend spacetime, and both are capable of forming singularities (black hole, white hole/kugelblitz). In light of this, why do photons ...
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1answer
113 views

Would a collapsing Universe have the density of water?

I understand that the typical density of the super massive black hole is close that of the water. It is also my understanding that this density is not true matter density because the volume used to ...
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0answers
77 views

Is there any place for teleology in physics? [closed]

Most physicists absolutely hate the idea of teleology. They take it as an unquestionable article of faith that causality only runs in one direction, that is, from the past to the future. Is there any ...
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2answers
95 views

What is the best explanation for the fine tuning of the Universe? [closed]

In my view, this is one of the most profound unsolved mysteries of Science. How to explain the fine tuning of the Universe without recourse to saying that a God/Designer did it. Examples of fine ...
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0answers
64 views

Modelling a matter dominated universe collapsing into a black hole

With the FLRW equations we can get solutions for a matter dominated closed universe in which the finale is an ultimate collapse, but this is only in terms of $a$ (the scale factor) and $t$ (time) and ...
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2answers
128 views

In what way are the Mathematical universe hypothesis and A New Kind of Science connected

The Mathematical universe hypothesis, mainly by Max Tegmark and A new Kind of Science, mainly by Stephen Wolfram both claim (as least as I understand it) that at its innermost core reality is ...