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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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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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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33 views

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

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

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
58 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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109 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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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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Did Wheeler accept Carter's anthropic principle-based multiverse? [closed]

Physicist John Wheeler wrote an article called "Genesis and Observership" where he said near the end: "We have reviewed the evidence out of the big bang tat the universe did come into being and the ...
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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 (...
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1answer
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Expansion of the universe – impossible for future astronauts to make it back

The universe is rapidly expanding. If future astronauts built a spaceship that could travel close to the speed of light and took off, there is a certain point where they could not make it back again ...
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Does the age of the universe depend on the way it is defined?

I was reading some earlier posts on this question in stack exchange. These posts give me the impression that the age of the universe is defined in a certain way following prescriptions such as that: ...
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1answer
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Is the observable universe equivalent to 'our' light cone?

All the objects we can observe (stars, galaxies, ...) must be in our past light cone, since otherwise we couldn't see them. Presumably there are more objects located outside of our observable universe ...
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2answers
103 views

How do you distinguish between missing momentum from a neutrino and from dark matter?

I thought googling this would give me an answer quite quickly, but actually couldn't find much, so maybe it's a silly question. But I read that dark matter searches rely on measuring missing momentum ...
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35 views

Likelihood, posterior, prior interpretation and credibility/confidence_level with bayesian/frequentist approaches [closed]

I try to understand the following article : testing general relativity from curvature and energy contents at cosmological scale I don't understand the title of figure 1 : where it is indicated ...
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1answer
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If inflation is an exponential expansion, is the exponent known?

Inflation is often called exponential expansion. I can't seem to find what that exponent is. Is it 2, or 20, or completely unknown?
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Implications of an infinite and no-information lost universe [closed]

Assume, for the sake of the question, that the universe is infinite, and that no quantum information is ever lost (no-hiding theorem). Does this mean that every piece of information we generate today ...
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Cosmology and Fluid Dynamics

What will be the relation between the barotropic index (w) of fluid and the velocity (v) of the fluid ??? and please mention some citation or book.
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Big crunch scenario, would black holes form?

Imagine a big crunch scenario in which everything (or almost everything as for I have other doubts, eg about entropy...) rolls back. Shouldn't the contracting universe reach a point of density at ...
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'My question is about' …the size of the Universe after Inflation [duplicate]

Hitherto, I had assumed, on account of Inflation, "boom" the Universe was "out there" and it has taken light 13.6 BY to get here. However... Quoting the aerospace engineer: In my thinking if the ...
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1answer
113 views

The Zero Energy Hypothesis and its consequences for particle creation and dualist interactionism

Most attacks on the possibility of dualist interaction cite the conservation of energy as a definitive objection. I have attempted to investigate the validity of this objection, and have found a ...
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Accelerating Universe and dark energy or photon absorption or neergy decay?

I’m an RF/microwave engineer with physics background, and had a question regarding dark energy and experimental observations. FYI I don’t qualify as a professional physicist, hence asking a ...
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Why physics cannot affect cosmological modes whose wavelength is larger than the horizon?

I started to learn cosmology recently. Confused about this point in a book. The book is Modern Cosmology by Scott Dodelson. I am confused about this in Chapter 6 "The horizon grows as the scale ...
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Nature of plasma of the early intergalactic medium

Please I have a question to ask about the nature of plasma of the early intergalactic medium at large redshifts. We known that the most important unmagnetized cosmic plasma is the early intergalactic ...
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38 views

Do Holographic Screens eliminate the need of finding holographic dualities?

There are various models in physics based on the famous holographic principle (https://en.wikipedia.org/wiki/Holographic_principle) This does not always work since in these models we must find a ...
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Can one quantify the total luminosity of the cosmological horizon?

If I integrate all the power (radiation, matter particles, neutrinos) radiated from the cosmological horizon into the universe, what number do I get? Is it true that the integral power/luminosity is ...
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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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35 views

Neutron mass fraction evolution: approximation

In Mukhanov's "Physical foundations of cosmology" on page 102 the author considers an equation for the evolution of the neutron mass fraction $X_{n}\equiv n_{n}/(n_{p}+n_{n})$: $$ \tag 1 \dot{X}_{n}(t)...
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Flat yet finite Universe

I would like to ask you this question: Let's assume that the universe is perfectly flat and big bang happened as described by our theories some 14 billion years ago. Shouldn´t, therefore, the radius ...
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Forces between adjacent particles during big crunch

Ignoring the effects of gravity, consider two adjacent objects/particles in contact. If the universe was expanding, this would cause them to separate at $\vec{v}=H_0 \vec{r}$, where $H_0$ is the ...
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World line of a photon from a distant galaxy in the $\Lambda$CDM model

The title is pretty much explaining the question: I am used to the graphs of Einstein-De Sitter model horizons: But then I don't understand 1) Are we using this kind of diagrams also now to ...
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What proof do we have that dark matter is matter? [duplicate]

The Dark matter page on CERN's official website describes dark matter as if it were regular matter just with some mysterious properties. This was a surprise to me because every time someone mentions ...
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Is the “cosmological constant tension” the prime reason that we believe the expansion of the universe is accelerating?

Background: For a few years now there is significant difference in the calculation of the cosmological constant between standard candles methods and CMB methods. CMB measurement comes from a very ...
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Supertranslations & Superrotations and black holes

I am currently (attempting to) the black hole memory effect and I am really struggling to find introductory explanations which simply state “this is what is meant by a supertranslation on a black hole ...
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2answers
186 views

What causes the Sagnac effect?

Wikipedia explains the Sagnac effect as a result of the rotating disk, which moves the target so that one of the light beams has farther to travel and consequently, will arrive later than the other ...
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What is the entropy of the cosmological horizon?

The Bekenstein-Hawking entropy for a black hole horizon is $S_{BH}=\frac{c^3 A}{4Gℏ}$. Is there a similar expression for the cosmological horizon?
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106 views

What is the result that that differs by many orders of magnitude between QM and GR? [duplicate]

It is well known that QM and GR are deemed incompatible due to a discrepancy in some calculations which I have read can differ by large magnitudes. What are these calculations to which people are ...
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What is the Lagrangian for FRW universe?

A straightforward calculation to the Lagrangian for FRW as $\mathscr{L}=\sqrt{-g}R$ gets me the following result: $$\mathscr{L}\sim a^3\big(\frac{\ddot{a}}{a}+(\frac{\dot{a}}{a})^2+\frac{k}{a^2} \big)...
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1answer
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What is the relation between comoving particle horizon $d_{hor}^c(t)$ and comoving Hubble radius $1/(aH)$ for arbitrary expansion laws?

I was learning Inflation by reading Hannu's lecture. But on page 4 of this lecture (Chapter 7 Inflation, 7.2.1 Accelerated expansion), there are some things I don't understand. The content is shown in ...
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1answer
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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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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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61 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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1answer
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Scale factor for flat universe filled with radiation and cosmological constant [closed]

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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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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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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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 ...