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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2answers
48 views

Can the wavelengths (sound, light) be infinite?

Can the wavelengths (sound, light) be infinite or are they limited by something?
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0answers
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Horizon problem: Angular size of causality regions on CMB

I'm having difficulties in calculating the angular size of the causally connected regions on the cosmic microwave background (CMB), as seen from Earth today. I read in several documents that this ...
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37 views

Cosmological or Doppler redshift in distance calculations

If I want to determine the distance to a far away galxy with a high redshift, then using relativistic doppler shift will not give the true distance as it does not take into account the expansion of ...
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How can the interior pressure of compact objects affect cosmology?

This paper suggests that dark energy concentrated in black hole interiors (they use an unconventional BH model) could act like a cosmological constant. Their claim is that to calculate the equation of ...
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1answer
26 views

Sterile neutrinos findings

There is a relatively new paper https://arxiv.org/abs/1805.12028 of MiniBooNE on measurements of neutrino oscillations that can only be explained by the postulate of a sterile neutrino. I also read ...
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Dark matter as an effect of spacetime [on hold]

The question got in my head for so long. we barely made any theoretical improvements to this day. now why can't we simply look for another explanation? for example what is wrong with this idea? dark ...
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Reparametrization invariance in FRW

It is usually pointed out that FRW metric is invariant under time reparametrization. Consider the flat case for simplicity $$ds^2=N(t)^2dt^2-a(t)^2dr^2-a(t)^2r^2d\Omega^2$$ The choice of function $N(...
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1answer
29 views

Increasing expansion rate and time dilation

Does the increasing dispersion of matter that occurs as the universe expands impact time (and therefore observed speed) for older/further compared to newer/closer events being observed simultaneously ...
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166 views

Why does nobody ever consider the possibility that the universe is not smooth?

Disclaimer: I'm not an astronomer, physicist, mathematician, etc. so this is a question from a complete newbie. One of the greatest mysteries of our age is "where is the dark matter?" The universe ...
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1answer
37 views

Vacuum decay and Coleman-de Luccia bubbles

Can someone suggest me some good and detailed books (or notes) on the problem of vacuum decay and Coleman-de Luccia bubbles?
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1answer
31 views

What is the density of a static dust sphere in an accelerating universe?

Consider a largish but finite sphere of pressure-less homogeneous dust late in the $\Lambda$CDM cosmology. The outside metric is presumably Schwarzschild-de Sitter with no matter, the interior a ...
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Could entropy be reversed with enough energy? [closed]

Could a vast amount of energy (such as all the energy in the accessible universe) be intentionally used to pull matter in the universe back together, thus reversing entropy? (edit: the previous ...
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The answer to this question is not close to the age of the universe, then why do we say it is?

I saw this question in our textbook A great physicist of the century (P.A.M. Dirac) loved playing with numerical values of Fundamental constants of nature. This led him to an interesting ...
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1answer
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What is the justification for Dirac's large numbers hypothesis?

Dirac stated that "Any two of the very large dimensionless numbers occuring in Nature are connected by a simple mathematical relation, in which the coefficients are of the order of magnitude unity." ...
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1answer
51 views

Strange anomaly in the calculation of the cosmological constant

I was trying to calculate the cosmological constant with two different methods proposed on the internet, that are apparently equivalent but they give different results and different dimensional ...
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51 views

Good references to start researching on dark energy knowing only graduate general relativity

I'd like to research on Dark Energy, but I only know (Schutz-Foster-Carroll) general relativity. Can anyone indicate to me some good books to start to research this topic? I'm interested in how to ...
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1answer
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What percentage of the mass/energy of the universe is in the form of electromagnetic waves? [duplicate]

Is there any theoretical framework or model that would lead to a prediction, either precise or approximate, about how much or what proportion of the universe's total mass is in the form of photons, or ...
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32 views

Is every observer equidistant in all directions from the point of origin of the universe? [duplicate]

I am interested in the origin and present structures of the dimensions of space and time. I do not think space/time can be correctly described by an ordered set of scalars: (x,y,z,t). So. If every ...
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1answer
45 views

Does cosmological expansion change the gravitational field around a massive body?

Do expanding universe affect the curvature of spacetime? If so, does Einstein's field equation account for the change in curvature (gravitational field) around massive object in the expanding universe?...
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The formula for the CMB Damping Tail makes no sense

The formula for the Damping Tail in the CMB BAO analysis generally has the form: $$\mathcal{D}(k)=\int_{0}^{\eta_0}\dot\tau \exp\left\{-\left[\frac {k}{k_D(\eta)} \right]^2\right\}\mathrm d\eta $$I ...
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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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1answer
80 views

Why did dark energy play a subdominant role in the radiation and matter dominated era?

After inflation, the Universe passed through radiation dominated (RD) and matter dominated (MD) era during both of which the evolution of the scale factor (or the Hubble parameter) was decreasing with ...
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1answer
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Given observer is in the center of the cosmic horizon, and in holography all objects are mapped, where he is then mapped on the horizon?

Since the observer is in the center of the cosmic event horizon sphere (of an approximately de Sitter space) and according the Holographic principle the event horizon has mappings of all inside ...
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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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2answers
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About the applicability of the FRW metric in the present epoch vs the time of last scattering

The form of the metric of the Universe, the FRW metric, is obtained from the principles of spatial homogeneity and isotropy which are supported by observation. If we probe the present Universe, it ...
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1answer
54 views

Why Cosmic Microwave Background is still present? [duplicate]

This is a novice question. It is theorized that Cosmic Microwave Background (CMB) is remnant of radiation from very early time of universe creation. As these were photons that have been traveling, ...
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3answers
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Why are primordial black holes considered non-baryonic? [duplicate]

A very important question that has been bugging me is why do people consider primordial black holes to be non-baryonic. I know that such a statement is then used to saying that they can be considered ...
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1answer
61 views

Could Schrödinger's universes have different fundamental laws?

It seems that Erwin Schrödinger was one of the first proponents of a multiverse concept. In Dublin in 1952, Erwin Schrödinger gave a lecture in which he said that when his equations seemed to ...
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27 views

Energy equivalence of the Universe? [duplicate]

Let us have the following constants: speed of light $c$, gravitational constant $G$, mass of the Universe $M$ and the Hubble's radius $R$. Firstly, is it just a silly coincidence that we have the ...
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1answer
34 views

Confusion on co-moving distance and the Big Bang

I'm at sea. From the wiki. entry https://en.wikipedia.org/wiki/Comoving_and_proper_distances : “..Most textbooks and research papers define the comoving distance between comoving observers to be a ...
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1answer
81 views

Is the cosmological horizon of an accelerated observer spherical?

I guess the question says it all. If you move across the universe (I mean: across the cosmic background radiation) with accelerated motion, does your cosmological horizon (I mean: the one you see) ...
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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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1answer
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Is there any “realistic” metric of type $\mathrm{(A)dS}_2 \times \mathcal{T}_2$ in General Relativity?

After studying the Bertotti-Robinson metric, which describes a $\mathrm{AdS}_2 \times \mathcal{S}_2$ universe, I was wondering about other kind of closed topologies with holes, like $\mathrm{(A)dS}_2 \...
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2answers
113 views

Are these Stephen Hawking's statements legitimate? [closed]

I am reading 'Brief Answers to the Big Questions' by Stephen Hawking and it seems to me that some of his statements are just hypotheses, but they are written in such a way that they 'sound' like they ...
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1answer
57 views

“Cosmological constant problem” and dark energy

Simply stated: is the "cosmological constant problem" (the discrepancy of about 120 orders of magnitude between measured values of vacuum energy and the ones predicted by Quantum Field Theory) in any ...
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19 views

Primordial dark matter synthesis

In standard cosmology, there is a general picture of: Primordial nucleosynthesis, i.e., when and how, nuclei or elements were formed. It is not quite tested, but it is more or less known where it ...
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2answers
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What are the pros and cons of Einstein-Cartan Theory?

As an alternative to General Relativity, i hear that it can avoid the initial big bang singularity as well as the singularities in black holes, so why does it appear to be talked about so little? If ...
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2answers
133 views

Any theory/author using the white hole model to explain the birth of the universe?

The maximally extended Schwarzschild solution (Kruskal diagram) envisages a white hole, which is the time-reverse of a black hole, i.e. a part of spacetime from which things can escape to us, while we ...
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2answers
72 views

How to get the Bertotti-Robinson metric into its conformally flat version?

The Bertotti-Robinson metric is a very nice exact solution to the Einstein-Maxwell field equations in vacuum. It is usually stated into these equivalent versions (where $r = \sqrt{x^2 + y^2}$ and $\...
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Quintessential models for dark energy

Following Sean Carroll here There are good reasons to consider dynamical dark energy as an alternative to an honest cosmological constant. First, a dynamical energy density can be evolving slowly ...
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Relativistic constant acceleration in the framework of general relativity

Regarding the constant acceleration of a spacecraft towards relativistic velocities, the general equations are found at the following link: Baez However, if the distance covered is in excess of 1 ...
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1answer
65 views

Would Wheeler's oscillatory universe model create absolutely different universes in each cycle? [closed]

According to physicist John Wheeler, the universe is part of a sequence of cycles of Big Bangs and Big Crunches, called cyclic or oscillatory universe model: https://en.m.wikipedia.org/wiki/Multiverse ...
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1answer
61 views

What am I missing here? (How do we know the universe has a cause?) [closed]

I apologise if this has been asked before or is otherwise an ill-formed question. Consider the following predicates: $B(x)$: "$x$ began to exist". $C(x)$: "$x$ has a cause". Let $U$ be the ...
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4answers
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What is a layman explanation for why the wavelength of light increases when space itself expands?

I understand that the universe is generally agreed to be expanding based on observations. What I have read is that space itself is expanding. My question is why does this expansion of space affect the ...
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2answers
64 views

If light speed determines the rate of time, could the speed of light be varying over time and we be unaware of it?

Is this a valid implication/alternative explanation of universal red-shift? I thought I'd ask this when I read this question about light clocks. I had speculated this way: If light speed determines ...
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2answers
116 views

Is Young Earth universe distinguishable from non-creationist one? [closed]

While talking today about some religious beliefs I was struck by the next question: So what if some super-natural being forks our universe 6,000 years ago, i.e. creates the exact same copy of ...
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Factors of the Tolman Surface Brightness and the rate of photons

From the Wikipedia: First, the rate at which photons are received is reduced because each photon has to travel a little farther than the one before. Second, the energy of each photon observed is ...
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35 views

Interpretation problem with Melvin and Bertotti-Robinson magnetic universes

I'm having an interpretation problem with two models of magnetic universes in General Relativity. Firstly, consider the Melvin universe described by the following static metric: \begin{equation}\tag{...
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Big-Bang theory / Lambda CDM model [duplicate]

Is there any evidence that the 'big-bang' was indeed the 'beginning'? or, is it basically an assumption, based on the fact that we have 'discovered' nothing else that fits the bill?
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The limit of the Universe that we'll ever be able to see

After watching Neil Turok's talk at 13:00 - 13:20 he says that the Universe expands and there is a limit beyond which we will never be able to see anything. And that's due to the Dark Matter which (...