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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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1answer
70 views
+50

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....
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2answers
113 views

The Big Bang and universal elemental abundance. Predictions seems bad and unconvincing?

Allow me to preface this question by stating that I am not a Big Bang denier. I am reading the book A Universe from Nothing by Lawrence M. Krauss. In his book, he presents the following card as one ...
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1answer
282 views

Discrepancy problem in lithium?

Why is there a discrepancy between the amount of lithium-7 predicted to be produced in Big Bang nucleosynthesis and the amount observed in very old stars?
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0answers
25 views

Galactic Birkeland currents

Does Milky Way or any other galaxy produce Birkeland currents such as seen on Earth in auroras? If yes, how strong would they be? According to this questions answer, galaxies do have magnetic fields. ...
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0answers
31 views

Hubble Tension is really a tension

I find a recent article that claims, "The results are shown in Figure 1 which makes it evident that the derived value of $H_0$ varies by several $km s^{−1}Mpc^{−1}$ depending on which public SNe Ia ...
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1answer
48 views

In cosmology, what means the 'Fourier modes'?

I am reading papers of cosmology for my thesis and always I read 'Fourier modes' but I don't understand why is necessary that we work in the Fourier space and what means 'Fourier modes'? Maybe it is a ...
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1answer
19 views

What means $h$ which appears following cosmological measured parameters?

For example, Big Bang Nucleossynthesis says that baryonic content in the Universe is around $0.018 \leq \Omega_{b}h^{2} \leq 0.024$. I know it is something related to measurement error but I do not ...
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1answer
92 views

Does the speed of universal expansion lead to time going backwards? [closed]

After reading that time stops at the speed of light and at event horizons I wondered if when the speed of light is exceeded does time loop backwards ... The thought struck me too that the term space-...
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2answers
282 views

How to Change Coordinate Systems in General Relativity

Let me preface by stating that I have no experience with General Relativity. I am working on a project for school that requires a little knowledge of it, so I am hoping to find some help. I do have ...
3
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1answer
41 views

What was above and below the QCD chiral symmetry breaking temperature?

Above a critical temperature in the Universe, there was probably a phase of unbroken approximate QCD chiral symmetry. Mathematically, the symmetry breaking is triggered when the operators $\bar{\psi}{\...
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1answer
145 views

What happens here when the Second Law of Thermodynamics doesn't hold true?

In the 1920s, theoretical physicists, most notably Albert Einstein, considered the possibility of a cyclic model for the universe as an (everlasting) alternative to the model of an expanding universe. ...
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1answer
126 views

Spacetime diagram of LCDM

I have a question concerning the LCDM spacetime diagram https://i.stack.imgur.com/Uzjtg.png published on the Physics Forum Stack Exchange website Can space expand with unlimited speed? How are the set ...
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0answers
44 views

Lower bound on the size of the universe, if closed [duplicate]

As the measurements show the flatness of our universe is approximately 1 but the measurements are with a slight error. If we assume that the flatness is greater than 1 but still within the error of ...
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0answers
47 views

Question about quantum fields, elementary particles, and quantum numbers, and fundamental forces

As I understand it the allowable spin states of elementary particles is tied to the number of space and time dimensions, so for any two universes with the same number of space and time dimensions the ...
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2answers
1k views

Where does the energy go when light is redshifted? [duplicate]

Imagine a galaxy millions of lightyears away and, obeying Hubble's law, moving very quickly away from us. Now imagine the same galaxy emitted a green photon in our direction (a photon with a ...
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0answers
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Doesnt the law of conservation of energy conflict with the theory that eventually the universe will die out into nothing?

Many cosmologists theorize that the universe will eventually die out some day, but I could never figure out how this will happen if the law of conservation of energy, one of the most fundamental laws ...
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0answers
19 views

Comoving sound horizon at last scattering

I tried to calculate the comoving sound horizon at last scattering using the integral $$r_s=\int_{0}^{380,000}{c_s\cdot (1+z)\ dt}.$$ According to most of the measurements it is approximately $150$ ...
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1answer
31 views

If you accelerate every particle in the whole universe, would it slow down universe expansion?

Perhaps an odd question, but (Proposition): say I want to slow down everything that is happening in the universe, I believe this could be done by accelerating everything to a velocity close to the ...
4
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1answer
152 views

How are the dark matter (DM) thermalization rates derived in singlet DM model?

This well-cited paper talks about a minimal renormalizable extension to the Standard Model (SM) to incorporate particle dark matter (DM) into it by adding a real scalar field $S$ which (unlike the ...
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1answer
78 views
+50

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 ...
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2answers
211 views

What is the interpretation of a wave function of the Universe in Hawking's no boundary proposal?

In the path integral formalism we have an in state $\Psi_{in}[\phi]$ and and out state and we find the amplitude for going from one to the other: $$\Delta[\Psi_{in},\Psi_{out}] = \int \Psi_{in}[\phi]...
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1answer
48 views

Are most atoms (or nuclei) unchanged since they were created?

Most atoms we experience (on this planet, for simplicity) were created very long time ago in supernova explosions and the like. The light elements (eg helium made in the Sun) and some heavy ones (...
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0answers
45 views

What's the farthest away from us a photon could theoretically reach?

Assuming the expansion of the universe will be dominated by dark matter, is there a limit to how far a photon could travel in terms of co-moving distance, as the Universe will be expanding faster than ...
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0answers
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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 ...
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0answers
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Cosmology / Interpretation between credibility/confidence_level with bayesian/frequentist approaches

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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0answers
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Wheeler-deWitt equation in words?

Looking at the Wheeler-deWitt equation. My attempt to describe it in words is this: "For each 3d-manifold given by metric tensor field $\gamma$, associate a complex number $\Psi$. The Wheeler-de-Witt ...
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1answer
124 views

Changes to a Length of Physical Ruler Caused by Gravity vs Caused by Cosmological Expansion of Space

I read here (Feynmann Lectures, Lecture 42) that "Just as time scales change from place to place in a gravitational field, so do also the length scales. Rulers change lengths as you move around." (...
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3answers
57 views

What is meant by “Big Bang” in expressions like “$10^{-36}$ seconds after the Big Bang”?

Sometimes I find expressions like "The inflationary epoch lasted from $10^{−36}$ seconds after the conjectured Big Bang singularity [...]". What is meant by "the Big Bang" in this context? I ...
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1answer
36 views

How to measure the distance of an object using standard candles?

Standard Candle techniques can only be used to determine the exact distance from Earth to another standard candle, right? I mean, imagine I want to know the distance to a star A in galaxy $\alpha$. ...
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0answers
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What is the unit of the Angular Power Spectrum $C_\ell$ of Galaxy Clustering and Weak Lensing?

If the matter power spectrum $P_m(k,z)$ is measured in $h/Mpc^3$ and $k$ is measured in $h/Mpc$, what about its Fourier Transform $C_\ell$ and the conjugate vector to $k$, $\ell$? Thank you so much
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0answers
56 views
+150

Cross-sections at zero temperature and high temperature for a process and its reverse

If the Feynman amplitude for a $2-2$ forward scattering $ab\to cd$ is denoted by $\mathcal{M}_{ab\to cd}$ and that of the reverse scattering process, $cd\to ab$, is denoted by $\mathcal{M}_{cd\to ab}$...
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2answers
73 views

Hubble constant: a constant or might be a function?

When I read or watch youtube videos about Hubble constant everybody agree it is no more considered as a constant but why nobody talk about acceleration difference between closer and distant objects ...
3
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2answers
135 views

Is the fluctuation pattern of the Cosmic Microwave Background (CMB) identical in every location of the universe?

I know that the Cosmic Microwave Background (CMB) is the leftover radiation from the "surface of last scattering". Let's say an alien civilisation lives on andromeda or further away. Would they see ...
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1answer
33 views

What are gravitational waves from the early Universe/how could they be produced?

Gravitational waves can be produced by the merger of two black holes or two neutron stars and those have been detected. Now people also thinking about GWs produced in the early universe. By what ...
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0answers
18 views

How can the Milne model of the Universe have curvature? [duplicate]

The title says it all. The Milne model of the universe is a model where the universe is empty, there is no matter at all, yet the spatial curvature is different from 0. I'd expect that a spacetime ...
4
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1answer
1k views

What is the curvature of an empty universe?

My calculations tell me an empty universe has hyperbolic curvature. Is this correct? If it is, can anyone help me understand why this is intuitively?
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1answer
133 views

Are the possible universes in brane cosmology embedded in string theory landscape?

String theory has a landscape of possible universes. Brane cosmology is related to string theory because it needs extra dimensions, but are the possible different universes of brane cosmology ...
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1answer
94 views

The inflaton field exists in a superposition of energies. How does that affect the rate of cosmological expansion?

The inflaton field is a scalar field that each field value corresponds to different rates of expansion. The field exists in a superposition of energies, that is, the wave function hasn’t collapsed. ...
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0answers
48 views

New metrics for Time? [closed]

We say that the Universe originated some 14-15 billions years ago with an event called the Big Bang. However, it does not make any sense, according to nowadays physics, to ask what did exist, let's ...
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0answers
20 views

Expanding space outside our galaxy

Some other Galaxy would have its own Hubble sphere. It is possible then that we could be on the edge of someone else’s Hubble sphere. Why is it then we are not experiencing this accelerated expansion ...
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2answers
200 views

What is the volume of spacetime we can survey?

How large is the fraction of spacetime volume we can observe within a sphere with the radius of the present particle horizon distance?
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1answer
56 views

Multiple levels of expansion of space? [closed]

It can easily be seen from the plot that there are at least 3 gaps between objects in the large, there is the gap between the planets and the moons, and between the planets and stars, and between ...
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1answer
123 views

De Sitter spacetime affine parameter

I am reading Chapter 8 in Carroll's "Spacetime and geometry " textbook and I was lead to exercise 8.2, given as: Consider de Sitter space in coordinates where the metric takes the form $$ds^{2} =...
4
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1answer
59 views

Are there any CMB-independant probes of the curvature of the Universe?

There is a preprint today (think it also appeared in Nature Astronomy on Nov 4) which argues that a $\Lambda{\rm CDM}+\Omega_k$ model with negative $\Omega_k$ fits the Planck Legacy 2018 CMB data ...
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1answer
25 views

How the dark matter density parameter appears in Big Bang Nucleosynthesis (BBN)?

I know that it is possible to obtain the light element abundances ($^{4}\mathrm{He}, \mathrm D, ^{7}\mathrm{Li}$ etc.) and to determine the baryon-to-photon ratio $\eta$ from the total baryon ...
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1answer
72 views

A change to Friedmann's 2nd equation

$$\tag{1}\frac{\ddot a}{a}=-\frac{4\pi G}{3}\left(\rho + \frac{3p}{c^2}\right)+\frac{\Lambda c^2}{3}$$ $$\tag{2}\frac{\ddot a}{a}=+\frac{4\pi G}{3}\left(\rho + \frac{3p}{c^2}\right)+\frac{\Lambda c^2}...
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1answer
206 views

Heat death in eternal inflation

Heat death is the predicted fate of the universe. Eternal inflation theory states that new hydrogen atoms are created in each pocket universe so that means new hydrogen atoms are still being created ...
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0answers
41 views

Could the increasing expansion of the universe be proof of the multiverse? [closed]

Einstein included a cosmological constant in his equations, this leads to the search for dark matter as I understand. Currently we fail to detect this dark matter. So what if it isn't there, but ...
7
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2answers
107 views

Hawking radiation and Unruh effect

I recently read about the Hawking effect and I noticed that formally, in its general lines, the derivation is very similar to that of the Unruh eff. In the latter case we write the Fock states in a ...
3
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1answer
50 views

Measuring mass density of electrons in the universe

Pardon me if the question is too naive, but only baryon and dark matter masses are considered while making measurements of the mass density in the universe. What I am interested to know is how small ...