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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145 views

Were fermions ever massless?

In a discussion of the Standard model and Higgs mechanism it was claimed that accordingly: "During an early phase of the cosmos all fermions were massless." I wonder whether this claim can be ...
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330 views

Never ending inflation

There are a lot of different models of inflation in cosmology, with plenty of different features and issues. I read that the problem with some models is that they introduce a never-ending inflation, ...
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152 views

what is the radial extent of the last scattering “shell”?

At CMB recombination (z=1090), what is the radial extent of the last scattering "shell"? a) Delta(z) = .... b) Delta(comoving angular distance)= ....Mpc The WMAP first-year parameters give Delta(z) =...
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112 views

Closed linear cosmology implies $\frac {GM} R = c^2$?

I have a question about a linear FRW cosmology with $k=+1$. Assuming zero cosmological constant the first Friedmann equation can be written: $$\left(\frac{\dot R}{R}\right)^2 + \frac{kc^2}{R^2}=\...
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125 views

Evolution of black holes ensemble

Background: I’ve read many times that arrow of time can be explained from extremely low entropy of the Universe at the Big Bang (http://preposterousuniverse.com/eternitytohere/faq.html). The argument ...
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256 views

Questions about classical and quantum scale invariance

This is kind of a continuation of this and this previous questions. Say one has a free "classical" field theory which is scale invariant and one develops a perturbative classical solution for an ...
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108 views

When spacetime expands to the point where galaxy clusters are not observable, will there by any interaction?

It's my understanding that in a few billion years, clusters of galaxies won't be able to directly observe one another due to the expansion of spacetime overcoming gravity between those clusters. ...
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59 views

In the αβγ cosmology paper, how do the authors assume the integral of density over time in the early universe?

In the famous Alpher-Bethe-Gamow paper, the authors say: "it is necessary to assume the integral of $\sigma_n dt$ during the building-up period is equal to $5 \times 10^4 \frac{\text{g sec}}{\text{cm}^...
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514 views

How is the poincare conjecture(and perelman proof) helpful in studying the properties of the universe?

Can someone tell me how the poincare's famous conjecture or its proof by perelmen can be helpful in deciding some properties like the shape of the universe?
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1answer
48 views

Does the Bekenstein entropy bound present problems for unitarity in cosmological models that invovle a collapse or bounce?

If we expect the Bekenstein bound, or something like it, to hold in a collapsing universe, will that not eventually force us to accept some loss of information, or is there something I'm missing?
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695 views

Is there any evidence against the Cosmological Principle?

I have been slowly getting more into math, but haven't gone into differential geometry or anything like that yet, so this question might be basic. Trying to get a deeper understanding of the ...
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56 views

Is Higgs cosmology a largely ignored theory, and if it is, why?

(I am not a cosmologist and instead currently working on a master thesis about Feynman integrals, so sorry if I'm missing something obvious. This question is merely out of interest.) I was wondering ...
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29 views

Fluctuations in the cosmological equation of state during the radiation dominated era

The radiation dominated era is described as a universe full of photons and ultra-relativistic particles with little slow moving matter. Ideally it has w = P/ρ = 1/3. But didn't the actual value of w ...
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74 views

Why is there matter when all of it should have annihilated with antimatter?

The theory is that after the Big Bang, photons turned into matter and antimatter particle pairs, but they should have all annihilated with each other. Since there would be an equal amount of matter ...
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How smooth does reheating (after inflation) need to be?

Let's estimate the size of perturbations in the very early (GUT scale or earlier) universe required for consistency with CMB. The CMB fractional temperature (and hence density) perturbations are of ...
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51 views

Does the Sachs-Wolfe effect depart from $1/3$ for large $k$?

In the Sachs-Wolfe effect (the one at last scattering, not the ISW), the standard result is $$ \frac{\Delta T}{T} = \left( 1 - \frac{2}{3} \right) \Delta \Phi = \frac{1}{3} \Delta \Phi . $$ To ...
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17 views

Silk damping in CMB power spectrum

Can someone explain Silk-damping in a conceptual way? I understand that it is the effect causing the power spectrum to decrease in amplitude at smaller angular sizes because the effect is apparently ...
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50 views

Formula to get scales of the two components (radial and perpendicular) of Baryonic Acoustic Oscillations (BAO)

In an astrophysics context about BAO (Baryonic Acoustic Oscillation), it is written on the following paper BAO paper : "BAO are much smaller in amplitude than the CMB acoustic peaks, and are ...
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66 views

Operatorial CTP / in-in formalism for sourced GW production?

Introduction / context - The inflationary paradigm of modern cosmology is a very powerful and predictive framework in which we can trace the primordial cosmological perturbations as originated by zero-...
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1answer
64 views

Scale factor in conformal time

I'm having trouble getting the scale factor for conformal time. In de Sitter we've $$a(t)=\sinh(H_It), -\infty <t <\infty$$ and the relation: $dt=ad\eta.$ I don't see how this relation ...
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85 views

Does Olber's paradox imply a bright sky or a divergent one

Doing Olber's paradox with specific systems is simple enough. Take stars on a grid of length $\ell$, then the total luminosity is just \begin{equation} L = \sum_{i,j,k} \frac{L_0}{4\pi\ell^2(i^2+j^2+...
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64 views

Would the nightsky be bright (filled with starlight) without accelerating space expansion?

As I currently understand, if a photon was emitted from a far away point in the Universe, beyond the event/particle horizon, so that the space inbetween the emitter and us is expanding faster then ...
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33 views

How is Universe expansion measured with CMB

There are plenty of articles about measuring the universe with standard candles, parallax, certain types of stars, etc. CMB measurements are usually introduced into the article to bring attention to ...
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90 views

Is the universe expanding, or is everything in it getting smaller?

I was recently reading some of the nice answers to the (related) question: If space itself is expanding, how could we ever tell? From many of the answers their appears to be a symmetry between: A ...
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1answer
62 views

Jeans Instability in an Expanding Universe, Understanding the perturbation

I am trying to derive the equation for a case, where we have a dust(zero-pressure) in an expanding universe. There are 4 equations but I think exercising on one of them would be helpful for me. So ...
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1answer
61 views

Age of universe

It's said that at the moment of the Big Bang, space expanded so that to an objective observer matter moved at thousands of times the speed of light. If so, how can we say the universe is 10-30 billion ...
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Contribution of gravitational waves to a maximum-entropy big bang

Our universe did not have a maximum-entropy big bang. If it did, then by the second law of thermodynamics we wouldn't see the universe today in a non-equilibrium state. I've seen this explained by ...
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63 views

Temperature dependence of the Higgs VEV

Presumably, The Higgs field vacuum expectation value has a strong temperature dependence close to the electroweak symmetry breaking phase transition. Does this have known consequences for the ...
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45 views

Why potential energy of a non-relativistic particle in an expanding universe is $ma^2\phi$?

I have been reading a paper on neutrino clustering in an expanding universe: see section 3, particularly 3.3.2 of this paper. In this paper for non-relativistic neutrinos, which travel through a ...
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71 views

Can I detect my own rotation by looking at the cosmic background radiation?

This is a simple question. Nevertheless it might be interesting to others. If I move with constant speed across the cosmic background radiation, I see a "dipole": a blue shift of the CBR looking ...
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56 views

What caused the first measurement to the universe?

I've been bumbling with a question recently. People postulate that with the give model of big ban, particles forms and breaks and eventually became what we saw as of today. However, if we put ...
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62 views

Exact solution for (simple) Reheating Boltzmann equations?

Let's say we have the Universe which is exiting Inflation, with the inflaton field $\phi$ decaying into relativistic particles (radiation $R$). Without any other assumption, the equations describing ...
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46 views

Increasing the view of observable universe

I have read that the maximum observable universe is about 92 billion light years in diameter by measuring the microwave background waves emitted after some few million years ago from the big bang and ...
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96 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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294 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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82 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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51 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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26 views

How to show no boundary wave function satisifies Wheeler-de-Witt equation?

I've seen various places it says the no boundary wave function (Hartle Hawking state) satisfies the Wheeler-de-Witt equation. But I don't think I've seen a proof. Is there a proof of this somewhere? ...
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1answer
56 views

Scale factor at surface of last scattering

Is there an exact theoretical scale factor associated with the CMB? More specifically, since the CMB is associated with a temperature of around 3000K (as I understand), is there an exact thermodynamic ...
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69 views

How is the entropy of the universe defined?

My understanding is that to define the entropy of a system what you have to do is as follows: Define the boundaries of your system. Define a set of "microstates" of the system. Define a partition of ...
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82 views

How could Proca equation be written in NP formalism

I am trying to do some calculation of fields in curved background. Is there anyone know the complete form of Proca equation $$\nabla_{\mu}F^{\mu\nu}+m^{2}A^{\nu}=0$$and$$F_{\mu\nu}=\nabla_{\mu}A_{\nu}...
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40 views

Long-term fate of hydrogen atoms in a rogue planet

Imagine a rogue planet drifting in intergalactic space. For the sake of specificity, suppose that it is the size of Jupiter and composed of pure hydrogen. What will happen to it over the next $10^{...
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54 views

Would Bekenstein bound disappear in some holographic models?

In Holographic principle models there's a limit to the information that the system can store known as the "Bekenstein bound". In physics, the Bekenstein bound is an upper limit on the entropy S, or ...
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53 views

Can I decompose a compact spacetime into conformal and $SL(2,C)$ transformations?

I've been thinking about compact spactimes lately. My understanding of the Yamabe problem is that one can always conformally transform a (compact) spacetime to one of constant scalar curvature, ...
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96 views

Meaning of time and distance in general relativity?

I have just started studying cosmology and I have realised that certain statements (that made perfect sense before I studied GR) no longer make sense: -The universe is 13.2 billion years old. What ...
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83 views

Antiparticles, CPT and leptogenesis

When people are being careful they'll tell you that antiparticles are the CPT conjugates of particles. You can't say that they are C conjugates or CP because these, while they do reverse the charge, ...
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89 views

Are deductions from CMB much less accurate than claimed, owing to approximations in the FLRW model?

My question concerns "timescape cosmology" and measurements of dark energy based on CMB radiation. The background to the question is Lawrence H. Dam, Asta Heinesen, David L. Wiltshire, "Apparent ...
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72 views

Is information conserved in an expanding universe?

From the classical and quantum mechanical viewpoint, is information conserved in an expanding universe or not?
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65 views

Could the horizon problem be explained by a finite universe instead of inflation?

Suppose the universe is finite (either closed or open with a non-trivial topology), any point could be within the horizon of another.
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118 views

Total number density of galaxies and problematic expression in practise

I am asked to give the formal expression of the total number density of galaxies and explain why is this expression problematic in practice. From what I saw from my research and into my lectures, I ...

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