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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114 views

Is Gerard 't Hooft's Cellular Automaton Interpretation of Quantum Mechanics background independent?

In Gerard 't Hooft in his Cellular Automata Interpretation of Quantum Mechanics (https://www.researchgate.net/publication/...
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2answers
128 views

What exactly is the problem that Inflation solves?

There's copious documents about how Inflation solves the problem that General Relativity predicts a lumpy CMB. That influation 'smooths' out the curvature fluctuations and, so, predicts a CMB at ...
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50 views

Is the highest photon energy currently possible a Planck photon or based on space-time quantinization?

It makes sense that there could be an upper limit to the frequency/energy for individual photons if the universe as we know it is quantized. But, the highest energy photons I've heard about have a ...
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27 views

What does the Hartle-Hawking state imply?

According to a book I read, the Hartle-Hawking state is a wave function that is meant to explain how the universe started. In the book the author explained that according to Hartle and Hawking’s ...
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1answer
78 views

Does CMB rule out that the universe is infinite?

If the universe were infinite, the energy of the big bang would have been long dissipated, and very little or nothing would hit us. Does the fact that CMB still comes roughly the same from every ...
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2answers
63 views

Can we calculate the expansion rate of the Universe at each moment in time?

Do we know the expansion rate of the Universe at each moment of time $t$ of its history? We only measured the expansion rate for a few decades (let's say), which isn't much compared to the age of ...
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2answers
85 views

How is observable universe so big if the universe is so young? [duplicate]

The diameter of observable universe is 93 billion light years but the age of universe is only estimated to be 14 billion years. So how does light have 46.5 billion years to travel from the boundary of ...
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1answer
45 views

What is the proof of constant energy in the universe? [duplicate]

It might sound a bit naive but, How is it that conservation of energy is never broken, how is it that we know that always and for sure the energy in our universe is constant and we cannot create a ...
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3answers
147 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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24 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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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
55 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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73 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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122 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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1answer
90 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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17 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
127 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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45 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
146 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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34 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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69 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
65 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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31 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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161 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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36 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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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
34 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
47 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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65 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
35 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
72 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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47 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
50 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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53 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
108 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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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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1answer
63 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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38 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
47 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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85 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
122 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
42 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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55 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
149 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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35 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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60 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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34 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 ...