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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44 views

How can I compute the scale factor for an open universe?

I'm struggling in obtaining an analytical expression for an open-universe $a(t)$. I know it is usually calculated from the second Friedmann equation, and with respect to the image I know how to ...
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0answers
74 views

Does $G$ effectively change in an expanding metric?

Assume that we have a flat FRW metric expressed in conformal time $\eta$ so that the line element is $$ds^2=a^2(\eta)(d\eta^2-dx^2-dy^2-dz^2)\tag{1}$$ where $a=1$ at the present time $\eta=0$ and the ...
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50 views

How come the universe is considered flat if zero point energy is infinite?

If quantum field theory calculates that the vacuum energy is infinite and Einstein's theory of gravity implies this energy should produce a curvature of space-time then why shouldn't the universe be ...
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30 views

Has the Strong Anthropic Principle any equivalent specific model or theory in theoretical physics? [on hold]

The anthropic principle is a philosophical consideration that observations of the universe must be compatible with the conscious and sapient life that observes it. There are two main types of ...
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1answer
28 views

Scale factor for flat universe filled with radiation and cosmological constant

I am trying to solve problem 1.20 in the book 'Physical Foundations of Cosmology' from Mukhanov. In the problem, we should show that the scale factor $a$ for a flat universe filled with cosmological ...
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1answer
44 views

What's the advantage of Conformal Time?

I'm trying to follow the analysis of CMB Acoustics and several charts are done using conformal time instead of chronological time. Conformal time corresponds to the quotient of the particle horizon ...
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0answers
40 views

Why does Penrose expect visible Hawking Points in CMB?

I've only just started reading into Conformal Cyclic Cosmology. My understanding of the hypothesis is that, with some conformal transformation of space, the very end of the universe at $t=\infty$ ...
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0answers
34 views

What is Dark Energy? [on hold]

Preface So I would to preface with the fact that I merely have an undergraduate degree but I like to entertain cosmological ideas and concepts. While an engineer, I find that an undergraduate ...
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0answers
42 views

What was the source of energy at the moment of big bang? [duplicate]

We know that our universe emerged from the point of singularity, right after the big bang, 13.7 billion years ago. Singularity is the point of infinite density and gravity i.e. all energy and mass ...
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1answer
70 views

Could decaying dark energy solve the Hubble tension? [on hold]

The redshift-based distance ladder values of the Hubble constant are higher than that inferred by the cosmic microwave background. This discrepancy in the last few years has become highly ...
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1answer
42 views

Thermodynamics of early universe - proton and neutron ratio [duplicate]

So, I have been reading about connection of particle physics and thermodynamics of early universe. Like after big bang, and quark - gluon plasma etc.. Anyway, after 1 second after big bang, the ratio ...
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1answer
61 views

Are longer lives more likely in Everettian multiverse?

There are some interesting statistical thought experiments related to observers and the Anthropic Principle such as the Doomsday Argument formulated by a physicist Brandon Carter and later improved by ...
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66 views

Is there any validity to the claims of Dr. Jason Lisle? [closed]

Dr. Jason Lisle has made claims, like anisotropic synchrony convention, that have solved the starlight problem. I know that Lisle is a creationist, but he does have a PhD in astrophysics. Is there any ...
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44 views

Universe heat death? [closed]

Will parallel universes exist after heat death?
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1answer
48 views

Is this the Melvin metric (magnetic universe) in disguise?

I'm solving the Einstein equation assuming a cylindrical symmetry and found something interesting which I never saw elsewhere. I now feel that I may have found the Melvin magnetic universe solution ...
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1answer
65 views

Why is the measure problem an issue if the universe is always finite?

As a follow-up from this question, I was wondering how the measure problem applies to the idea of eternal inflation. First, why is it even a problem? As time progresses, more universes are created in ...
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2answers
94 views

How necessary is inflation given the laws of physics?

How deeply is cosmic inflation required for the laws of physics? Is inflation required for a universe remotely like ours, or is it simply a contingent on the starting conditions of the universe? For ...
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1answer
42 views

Do post-big-bang quantum fluctuations determine star and planet formation?

It is my understanding that cosmology simulations largely implement general-relativity, taking quantum-effects into account merely by setting the fluctuations in the initial conditions. In the real ...
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1answer
44 views

The expanding Universe [closed]

Is the universe expanding or is the vacuum of space declining. Could the vacuum of space moving towards absolute zero give the same observations?
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2answers
104 views

Is the speed of light in a vacuum constant?

The news site is titled, “Speed of Light May Not Be Constant, Physicists Say.” It talks about how the speed of light in a vacuum is not constant and that estimates of the size of the universe might be ...
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1answer
24 views

Thermal History: Electron-Positron Annihilation

At the moment I am studying for my cosmology exam and I am having a problem understanding the following from Baumann's comsology lecture notes: Shortly after the neutrinos decouple, the temperature ...
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1answer
80 views

Cylindrical universe cosmology in general relativity

Is there a compact cylindrical universe solution to the Einstein equation, with space homogeneity, without using "artificial" periodic boundaries? I'm expecting a metric of the following shape: \...
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1answer
83 views

Inflation v.s. Big Bang

It is said that that transition or certain quantum rotor field, and “rolling” down into the valley (of some quantum potential), is what causes inflation to come to an end, and create the hot Big Bang. ...
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3answers
106 views

COSMIC ACCELERATION

Please forgive a probably naïve question from a lay person. I understand that galaxies are all moving away from us in every direction we look. And for any given distance the speed is the same. And ...
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0answers
23 views

Pressure-velocity dispersion relation $p=\rho(r)\sigma^2$

The relation is $p=\rho(r)\sigma^2$. Where $\rho(r) $ is density distribution and $\sigma$ is velocity dispersion(RMSE/root mean square error value). This relation is given for a isotropic velocity ...
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5answers
3k views

Ambiguity in applying Newton's shell theorem in an infinite homogeneous universe

Newton's shell theorem has two corollaries: The gravitational attraction of a spherically symmetric body acts as if all its mass were concentrated at the center, and The gravitational acceleration ...
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0answers
84 views

Is it a coincidence that Big Bang nucleosynthesis lasted about one free neutron half-life?

The free neutron half-life is about 10 minutes. Big Bang nucleosynthesis, which mostly involves protons and neutrons interacting, lasted about 20 minutes. Question: Is it a coincidence that these ...
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0answers
32 views

Justification for “running” cosmological constant

I am looking for papers which seek out to justify that the cosmological "constant" could be time-dependent. I am aware that within the framework of GR the cosmological constants really has to be a ...
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2answers
76 views

How will Andromeda collide with Milky Way in spite of Hubble's law? [duplicate]

According to Edwin Hubble our Universe is expanding because he noticed that other galaxies are moving away from us. But then how Andromeda galaxy will collide with Milky Way in ~4.5 billion years, as ...
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0answers
31 views

Why are we in 3 spatial dimensions? [duplicate]

I am very curious to find if there is any reason that why are we in 3 dimensions not in any other like 1D would be hard almost impossible but 2D could have been possible, or what about 4D or in ...
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3answers
64 views

Is the expansion of the universe uniform? Are there parts which are expanding faster than others?

Is the expansion of the universe uniform? Are there parts which are expanding faster than others? The very fabric of the universe is expanding..but is it expanding at a constant rate? If it is not ...
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0answers
24 views

Is it possible that the acceleration of the growth of our universe is driven by the overall gravitational pull of neighboring universes [closed]

I am painfully unequipped when I think about my my knowledge of physics. Still, I am enthralled, as many of us are, by the mysteries and complexity of the universe, both known and unknown. I was ...
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1answer
55 views

Does curvature affect the change in the acceleration/deceleration of the Universe?

Suppose I'm modelling a Universe with non-zero curvature, filled with matter, radiation and dark energy (further described by quintessence). The appropriate Friedmann equation would be of the form: $$...
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2answers
84 views

Could the matter for a Big Bang come from the Hawking radiation of a super fast expanding universe?

If I understand it right, the universe will at some point in time expand so fast, that from any particular point in space, an observer will see an event horizon around himself. This event horizon ...
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0answers
33 views

Formula for the bias of galaxies

From this article : https://arxiv.org/pdf/1611.09787.pdf , I try to deduce the equation that my teacher told me which links 2 quantities : 1) the global number density of galaxies 2) the local ...
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0answers
20 views

Tianqin project vs LISA

How does Tianqin space-based gravitational wave detector compare to LISA? Will it be able to detect gravitational waves from e.g. inflation (considering possible upgrades)?
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0answers
19 views

Spectral index in Planck 2018 report

In the "Planck 2018 constraints on inflation" paper, I came across two types of results for the scalar spectral index. For one, they reported $n_{s}=0.9649\pm0.0042$ with a small running $dn_{s}/d\ln ...
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0answers
32 views

Co-moving distance and universal time

I am reading about Cosmology, in particular, the concept of co-moving distance. I am trying to derive the equation for calculating the co-moving distance with the following argument. If the source ...
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0answers
24 views

Demonstration about large-scale bias of galaxies

From this article : https://arxiv.org/pdf/1611.09787.pdf , I try to deduce the equation that my teacher told me which links 3 quantities : 1) the global number density of galaxies 2) the local ...
1
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0answers
41 views

Different duality-correlations in holographic principle?

I found an interesting article "Surface/State Correspondence as a Generalized Holography" (https://arxiv.org/abs/1503.03542) If I understood it well, the authors proposed this model to generalize the ...
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2answers
77 views

Why Does the amount of gravitational potential energy in the universe increase as it expands?

Ok so this is my thought process... please bear with me I understand that as the universe expands, there is supposed formation of matter due to dark energy,and with an increase in mass comes and ...
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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
23 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
54 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 ...
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0answers
37 views

What do marginalized or marginalized 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 ...
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1answer
185 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
45 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 ...
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1answer
52 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
54 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 ...