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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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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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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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
48 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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$c$ 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
43 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
75 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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50 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
54 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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58 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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141 views

Is the acceleration of the expansion of the universe caused by expansion of spacetime 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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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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45 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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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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42 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
50 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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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
126 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
43 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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56 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
156 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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36 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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62 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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36 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
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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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31 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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60 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
80 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
246 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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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.
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1answer
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Contractions between big bang and gravitational law [duplicate]

If big bang happened then the centre of the universe must be place of big bang. Let the mass of whole universe be $M$ and mass of small planet or any Astro object be $m$. Then the Gravitational ...
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Examples of theories that assume the existence of an “External Reality”?

In this paper written by physicist Max Tegmark (https://arxiv.org/pdf/0704.0646.pdf) it talks about "External Reality Hypothesis". Specifically, he says: Although many physicists subscribe to the ...
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37 views

Can the shape of our Universe be a Mobius strip? [duplicate]

The Friedmann Equations describe three possibilities for the shape of our using General Relativity, I read in a book that the shape of our Universe is a 3-sphere such that in any direction if you ...
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3answers
124 views

Cosmological Inflation: If Photon expands, why not other matter?

I keep repeatedly reading in many Stack Exchange and Quora questions that space of universe expands but particles (matter) don't, see e.g. this Phys.SE post. The reason given is that particles are ...
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2answers
128 views

What came first, neutrons or electrons? [closed]

I wonder if electrons were first (at an early stage of the cosmos) embedded into neutrons, making it easier to understand why they would fit so well with protons later, or if the genesis of electrons ...
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2answers
611 views

Could one theoretically use the expansion of the universe to travel through it? At least in one direction?

Could one theoretically use the expansion of the universe to travel through it? At least In one direction? That’s it that’s my question. I’m not a physicist but I do get ideas. I also wonder if one ...
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1answer
91 views

How do they know the numbers of the energy pie chart of the universe?

They say that the total energy of the universe consists of 4.9\% ordinary matter, 26.8\% dark matter and 68.3\% dark energy. I hear this pie part in virtually every lecture in cosmology. How do we ...
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73 views

Could two atomic clocks really be used to detect gravitational waves from a distant source? If so, how?

Three articles report on the recent paper in Phys Rev. D: Flanagan, Éanna É. et al. 2019 Persistent gravitational wave observables: General framework (also ArXiv): Phys.org: Gravitational waves leave ...
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70 views

How do we know that dark matter energy density scales as $\rho\propto a^{-3}$?

How do we know that dark matter energy density scales as $\rho\propto a^{-3}$?
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27 views

Is the universe be expanding at different rates depending on its temperature and density?

As I understand it, when observing different regions of the universe we get a different rate of expansion (the two Hubble constants), specifically the more distant observation seems to return a lower ...
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2answers
155 views

Newton's First Law and things that are very old [closed]

Recently I've been revisiting physics text books, and books by Feynman, and others. A curious thought has arisen, while I was reading about Mach's principle, and it seems to gnaw on my mind. I hope ...
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1answer
57 views

How far was the surface of last scattering at the moment of recombination?

If I understand correctly, then the surface of last scattering (the last particles off of which CMB photons scattered before traveling for 13 billion years and reaching us) is currently at a distance ...
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1answer
75 views

Is Hubble's Law Only Valid Nearby?

I am looking this paper by LIGO, and it claims at the very beginning, At nearby distances $(d \lesssim 50 \text{ Mpc})$ [the Hubble constant] is well approximated by the expression $$v_H = H_0d, \...
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1answer
36 views

If the universe stopped expanding, would there be an infinite amount of reachable negentropy?

Given that the Universe's expansion is accelerating, stars are constantly leaving the reachable Universe. However, if the Universe wasn't expanding (nor contracting), we could forever reach further ...
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39 views

Horizon entry, Meszaros suppression and start of perturbation growth

I thought that the onset of perturbation growth was determined by horizon entry of the perturbation (because there won't be a gravitational collapse of an over dense region not causally connected to ...
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2answers
101 views

Existence of things outside the observable universe

I've read multiple physicists talk about stuff (e.g. galaxies, black holes, etc.) "existing" outside the observable universe, but it's unclear to me in what sense these things "exist". By definition ...
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1answer
135 views

Is universe symmetric about a point?

We have a good amount of discussion and theories on the formation of universe. I want to ask is universe symmetric about a point? I think that the answer should depend upon the uniformity of ...
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1answer
4k views

Why can’t you see at the start of the Big Bang?

I’m quite confused with regards to photon emission throughout the creation of the universe. From what I’ve heard, there was no light (of any frequency) in the universe until 300,000 years after the ...
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1answer
257 views

Is space itself expanding or is it just momentum from the Big Bang carrying things apart? [duplicate]

So, we observe that galaxies are moving apart (on average) and the further away from us they are the faster they are moving away from us. If space was expanding that would explain this. However, I ...