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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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20
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3answers
4k views

Why is quantum gravity non-renormalizable?

The book The Ideas Of Particle Physics contains a brief treatment of quantum gravity, in which the claim is asserted that if one attempts to construct a model of gravity along the same lines as QED, ...
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0answers
11 views

How does the cosmological constant dominate cold matter perturbations?

I have been studying inhomogeneous universe in physical cosmology, whereupon I arrive at cold matter perturbations which are dominated by various background fluids. I understood the solutions for the ...
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0answers
39 views

What is the relationship between Loop Quantum Gravity and Conformal Cyclic Cosmology?

I have heard/read of both of these "theories" or concepts by Roger Penrose (et al.?) - pardon my possible misuse of the word "theory". Is CCC just one possible model of the universe based on the ...
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0answers
58 views

Is it there any theory or model in theoretical physics that is akin to Tegmark's Mathematical Universe Hypothesis?

Physicist Max Tegmark proposed a hypothesis that asserts that all mathematical structures do exist as universes. (https://en.wikipedia.org/wiki/Mathematical_universe_hypothesis) But this hypothesis ...
3
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0answers
57 views

What do marginalised or marginalised error mean? Contours and posterior

I am curently working on Forecast in cosmology and I didn't grasp very well different details. Forecast allows, wiht Fisher's formalism, to compute constraints on cosmological parameters. I have 2 ...
7
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1answer
199 views

Does the Standard Model have texture defects?

In the standard classification of topological defects, in a theory with vacuum manifold $\mathcal{M}$, $\pi_0(\mathcal{M})$ corresponds to domain walls, $\pi_1(\mathcal{M})$ corresponds to strings/...
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0answers
49 views

If universe is expanding then relative to what? [duplicate]

I am very keen about the fact that universe is expanding.If something expands, it expands relative to something else. If the universe is expanding, then it must expand relative to something else. What ...
0
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1answer
61 views

What relation exists between Wolfram's model of the Cellular Automata universe and the Holographic Principle and the Bekenstein Bound?

Are computational models of the universe, like Stephen Wolfram's cellular automaton model of the universe (proposed in his book A New Kind of Science), somehow related to the Holographic Principle and ...
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1answer
57 views

What if there was no baryon-antibaryon asymmetry in our universe?

We know that the universe goes through different evolutionary phases: radiation, matter and later dark energy epoch. What would happen if there was no baryon-antibaryon asymmetry? How would the ...
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1answer
123 views

Isn't Big Bang theory a violation of law of conservation of linear momentum?

What Big Bang theory assumed in the formation of universe? Answer:-At its simplest, it says the universe as we know it started with a small singularity, then inflated over the next 13.8 billion ...
0
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1answer
34 views

Hubble Parameter in terms of Density Parameters

So I have the equation (derived from the Friedman equations with $a(t_0)=1$ and the powers of $a(t)$ quoted)$$H^2 = H_0^2\left(\frac{\Omega_{R_0}}{a^4(t)}+\frac{\Omega_{M_0}}{a^3(t)}+\Omega_{\Lambda_0}...
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0answers
153 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/...
6
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2answers
163 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 ...
0
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2answers
56 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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0answers
37 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 ...
1
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2answers
160 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 ...
0
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2answers
65 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 ...
2
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2answers
100 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
47 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
164 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
26 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....
2
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0answers
43 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
56 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
75 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, '...
1
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2answers
124 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 ...
2
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1answer
94 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 ...
0
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0answers
18 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 ...
1
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2answers
139 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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0answers
47 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 $\tau$ ...
1
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1answer
148 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
39 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
75 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
100 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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0answers
36 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
561 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 ...
1
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0answers
50 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
12 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
36 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 ...
0
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1answer
49 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 ...
2
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0answers
73 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 ...
0
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1answer
54 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 ...
-1
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1answer
82 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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0answers
52 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
56 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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0answers
59 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 ...
0
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2answers
148 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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1answer
43 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
46 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
77 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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0answers
44 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 ...