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Questions tagged [cosmology]

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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120 views

How inflation creates a universe from nothing?

I have a basic, mostly purely conceptual understanding of Quantum Field Theory, and after lots of Youtube (thanks PBS Spacetime!) I have an idea of how inflation works to turn the vacuum into a ...
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0answers
19 views

entropy and redundancy of data

I have a quick question about the redundancy of data and the information that it represents. A means of evaluating the amount of information contained in the data is evaluated with a notion of entropy....
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0answers
42 views

Is every object in the universe really expanding? Even small scale objects like humans? [duplicate]

So I was asked a question by someone that "if everything in the universe is expanding, then the lens we use to look at stellar objects is expanding as well, meaning we shouldn't be able to tell the ...
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1answer
54 views

Earliest existence of particles and forces

I'm trying to understand the relation between particles and forces during the very early universe--the Planck epoch and subsequent stages as the fundamental forces separated. At what point did the ...
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3answers
63 views

Is a vacuum the same as 'outer space'?

Is a vacuum the same as 'outer space'? When The Big Bang is described the terminology used is that 'space (itself) is expanding'. If that's true what happens when the volume of a container, '...
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2answers
119 views

How could we know if we are within 10 years of a Big Rip?

The expansion of the universe might cause the Big Rip. The mathematical physics of the possible Big Rip has been worked out beyond my mathematical abilities. I want to know if we can calculate if we ...
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0answers
60 views

According to Cosmological Redshift, How long does it take for 1nm to be lost? [closed]

According to the rules of Cosmological Redshift, how long does it take for one nanometer to be lost? For example: I beam 450nm (blue) to the other side of the universe, how long will it be until it'...
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0answers
78 views

The very first phase of our universe, are we missing something? [closed]

We have no really secure information from the very first phase of our universe, because we can neither observe nor generate the corresponding energies experimentally. Therefore, all these initial ...
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1answer
87 views

How can I describe a body in motion into a Einsteinian Universe?

I'm trying figure out a way to describe bodies in a motion over a Einsteinian Universe. My question is: how is it possible? Is there some formula to describe the gravitational interaction in terms of ...
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15 views

Forecast in cosmology survey : loss or gain of information with cross-correlations

In the context of a forecast in cosmology, I am currently working on Fisher's formalism which is part of a more general theory, that of information. My problem applies to estimating cosmological ...
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2answers
121 views

Where does the 379,000 year recombination prediction of the Big Bang theory come from?

The Big Bang theory postulates that recombination happened 379,000 years after the Big Bang. However, I have never seen a single statement as to how this number came about. I don't imagine they ...
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57 views

Is This Actual Quantum Physics or Quantum Flapdoodle (re Dr Amit Goswami) [closed]

Can anyone make sense of this supposed “science” from Dr Amit Goswami, previous theoretical physicist at the University of Oregon? I have my sense but there are quantum physicists in our community ...
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43 views

Linearly expanding vacuum solves cosmological constant problem? [duplicate]

The flat FLRW metric written in terms of conformal time $\eta$ is given by $$ds^2=a^2(\eta)(d\eta^2-dx^2-dy^2-dz^2)\tag{1}$$ where the scale factor $a(0)=1$. The interval of proper time $\tau$ ...
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1answer
138 views

Why does the universe require relativity? [closed]

Are there any known compelling reasons why our universe is governed by relativity instead of Newtonian physics? I recognize that an infinite $c$ would be a problem, so I'm not asking about that; I ...
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0answers
31 views

Why is momentum divergence from energy release not considered in Big Bang models?

From the formation of subatomic particles to the formation of atoms to the nuclear fusion in stars to the explosions of supernovae, the process of conversion of rest-mass energy from fields into ...
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0answers
66 views

Higgs Field before or after Cosmological Inflation?

Question: When exactly did the Higgs Field begin to exist (before cosmological inflation or after cosmological inflation)?
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1answer
55 views

Field equations with cosmological constant

Is it enough to add the cosmological constant in the einstein field equations to guarantee that the obtained solution describes an expanding universe?
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28 views

Killing Tensor of Friedman-Robertson-Walker Metric

I would like help showing that the tensor, $$K_{\mu\nu}=a^2(g_{\mu\nu}+u_\mu u_\nu),$$ where $u^\mu =(1,0,0,0)$, is a Killing tensor of the spatially flat FRW metric, $$ds^2=-dt^2+a(t)^2\left(dr^2+d\...
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3answers
144 views

How does the ratio of hydrogen to helium help prove the big bang theory?

From what I have read one of the key pieces of evidence for the big bang theory is the ratio of hydrogen to helium in the universe. I have not seen any explanation as to how the big bang theory ...
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0answers
35 views

Does an expanding universe assume a space within which the expansion is happening? [duplicate]

When I read about the expanding universe and inflation I often see pictures like this: (Source: https://en.wikipedia.org/wiki/Inflation_(cosmology)) or explanation like this: For getting an idea ...
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0answers
10 views

Coherence length of velocity field in cosmology

While reading a paper I came across a term 'coherence length of velocity field in cosmology' .....I have searched for this ,but didn't get any result...can anyone please help me getting this ...
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1answer
32 views

How would distant objects in a contracting universe look?

Our current universe is expanding, and distant galaxies are redshifted, with a redshift that increases as the distance to the galaxy increases. After a certain distance, the galaxies move faster than ...
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1answer
43 views

On universe homogeneity. Why “shouldn't be” so? [closed]

I start by apologising if I have took sparse sentences to literally. If I didn't understand something that's is why I am asking. In more or less divulgative literature, here, and also in the few ...
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0answers
59 views

$c$ slowing down rather than universe expanding?

Can the Friedmann equations be reformulated with taking the universe as being rigid and instead have the vacuum-speed of light in the void decrease over time? So, not changing any equations, just ...
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1answer
27 views

Deriving the continuity equation for a perturbation from the continuity equation for the full density

I am attempting to derive the continuity equation for a density perturbation $\delta$, given the continuity equation for the full density $\rho(\mathbb{x}, t)$. This is in the context of cosmological ...
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1answer
64 views

Multiverse and the anthropic principle? [closed]

In this paper (https://arxiv.org/abs/hep-th/0211048) by physicist Andrei Linde, the multiverse concept, the anthropic principle, quantum cosmology and inflationary cosmology are analyzed. In the ...
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44 views

Friedmann equations and the age of the universe

We have Friedmann equations of the form \begin{equation} \left(\frac{a'}{a}\right)^2 = \frac{8\pi G}{3}\rho + \frac{\Lambda}{3}. \end{equation} We also assume that the stress-energy tensor has $T^{00} ...
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1answer
47 views

Universe expanding - Interstellar space, Intergalactic space or Intersupercluster space? [duplicate]

When we say that the universe is expanding, the space between everything is not. The space between atoms, the space between my house and school, and the space between Earth and Sun remains the same, ...
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46 views

Matters Including Resolution of Olbers' Paradox

Is there evidence for galaxies receding from us faster than the speed of light? If so,that might help resolve Olbers' paradox, though other factors must also be taken into consideration. Could such a ...
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2answers
99 views

Is the acceleration of the expansion of the universe caused by expansion of space time itself?

When considering the accelerating expansion of the universe, it is often stated that dark energy is required to cause acceleration of expansion in order to overcome the pull of gravity. However, it ...
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1answer
41 views

As the universe expands wouldn’t the affect of gravity between large distances decrease and so allow expansion to accelerate?

I have read that gravitation in a homogenous scale of the universe should cause expansion to decelerate. However, that seems to be the opposite of my logical conclusion. It seems that if spacetime ...
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0answers
44 views

Newton vs. Einstein in universe evolution description

The equation for the universe evolution in the presence of a perfect homogeneous fluid, namely $$\dot{R}^2+k=\frac{8\pi G}{3}\rho R^2$$ can be obtained by solving Einstein equation with some effort. ...
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1answer
58 views

Is there an intrinsic speed of time? [closed]

If time and speed is relative, then we could double the speed of every particle in the universe, as this increases the clocks rate we shouldnt be able to tell any difference. But my question is, in ...
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36 views

The Great Attractor

Somewhere in the Universe (I forget where) there is a mysterious object dubbed by astronomers The Great Attractor, which is drawing a gigantic cluster of galaxies towards it. Have astronomers figured ...
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1answer
43 views

why do quantum fields exist? [closed]

Why did the big bang give rise to the quantum fields it did, could the big bang have created the universe any other way? and why not more elementary particles?
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1answer
81 views

What theories purport to describe “physical-level understanding” of dark energy?

What theories propose to close the gap (as suggested by http://arXiv.org/abs/1210.6008) between 'phenomenological behavior of dark energy' and 'physical-level understanding of dark energy' and to what ...
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20 views

Horizons not comoving with cosmic fluid

Ive been reading papers on inhomogeneous cosmological models like the McVittie spacetime. The apparent horizons of such spacetimes are time dependent. In almost every paper, he authors always make it ...
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2answers
114 views

Why physical / cosmological models that have been falsified weren't abandoned? [closed]

Before anything, I'd like to say that I'm a layman (non physicist) and english is not my main language; I apologize if my choice of words make me sound rude / arrogant, that is not my intent. I often ...
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2answers
41 views

Speed of at expansion at the start of the big bang [duplicate]

If I understand the basics of the big bang correctly it starts with a very small dense point. At the moment of the big bang, less then microseconds after, the universe is already the size of our solar ...
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2answers
54 views

In what planetary system can we have arbitrarily long winters?

Unlike on Earth where the length of seasons are almost of the same duration each time, the hypothetical planet on which Game of Thrones took place has seasons of arbitrary length i.e. some winters ...
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2answers
132 views

How do Graviton-based theories of gravity explain the expansion of the universe?

In General Relativity, the expansion of the universe is modeled using the Friedmann–Lemaître–Robertson–Walker metric, and the expansion itself is a metric expansion by which the scale of space itself ...
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0answers
34 views

Is the location of everything relative? [duplicate]

I have heard that everything in our physical reality is relative. But I wonder: Does the universe have an edge? In an expanding sphere, there would be an edge is it like that? If the above is true ...
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0answers
56 views

Number of e-folds in phase space for the inflaton field

In José Gálvez's paper "Observational constraints on Constant Roll Inflation" it's said that, using the equation of motion for the inflaton field \begin{equation} \ddot{\phi}+3H\dot{\phi}+\frac{\text{...
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33 views

What is the relationship between the Cosmological Constant and the Cosmological Principle?

I believe I've misunderstood a relationship between the cosmological constant and the cosmological principle having read: Einstein introduces his cosmological constant which attributes, in the ...
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1answer
25 views

Volume of dark energy in space [duplicate]

As I understand it, Dark Energy is believed to reside in empty space. As the Universe exapnds, more empty space exists inside the Universe and so the small energy per unit of empty space adds up to a ...
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0answers
30 views

Is this a good clock for quantum cosmology?

I was thinking about Quantum Cosmology and the wave function of the universe $\Psi[g,\phi]$ in the no boundary proposal. I was thinking about what would make a good clock. It would be some function ...
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0answers
57 views

How does the direction of time work with timeless wave functions?

In thermodynamical theory, if we have a set of states for example: A) gas all in top left corner of box. B) gas spread out near the left. C) gas spread out to fill entire box. We can give the ...
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1answer
69 views

The relationship between time, relativity and entropy

I came across a discussion about the nature of time and whether or not time is an illusion on a physics forum. I'm not so much interested in the philosophical issues regarding time, but the following, ...
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2answers
215 views

How bright was the universe after Big Bang during photon epoch?

After 10 seconds of Big Bang, most of the leptons and antileptons annihilate each other leading to an outburst of photons. The universe was said to be dominated by photons resembling a glowing fog. ...
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0answers
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What is the shape of the universe? [duplicate]

If it's flat then how a volume can be flat? And I've read that it's actually not flat .....it's a "Poincaré dodecahedral space". Any suggestions for books posts or articles are highly appreciated.