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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Time in the Standard Model of Cosmology
Beyond a formal preference for background independence, what is stopping us from setting cosmological time as a de facto universal timeline, analogous to newtonian absolute time? General relativity ...
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Difference between new inflation and chaotic inflation
I'm trying to understand the difference between new inflation and Linde's chaotic inflation. From what I understand, according to the old inflation, during inflation empty space remains empty, so its ...
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Mystery of cold spot in the CMB [closed]
The cold spot in the CMB is a statistical anomaly there is only a 2% chance there isnt on purpose. Can the cold spot of the CMB be due to an alien Type II or even Type III civilization and not new ...
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Cosmological horizon and Hawking/Unruh radiation? [closed]
I have two questions about cosmological horizons and their emission of radiation
The first one is:
There are some authors that propose that dark energy or the accelerated expansion of the universe ...
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How big was the surface of the cosmic background radiation?
The cosmic microwave background radiation is the furthest and oldest visible light in the universe. But the universe has expanded considerably since that light was emitted. At the time that that light ...
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Is the universe closed or flat?
Apparently there is a tension in the measuring of the curvature of the universe (https://arxiv.org/abs/2307.07475) as apparently in 2018 the Planck collaboration got a series of results consistent ...
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Fourier coefficients of fluid velocity in Cosmology
I start with the following equation, that can be obtained by taking the curl of the continuity equation, and in which $\omega=\vec{\nabla}\times\vec{v}$ is the curl of the velocity field of the ...
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What is the intuition behind the decrease in comoving Hubble radius during inflation and then its increase later on?
We know that the comoving Hubble radius decreases during inflation when the universe is exponentially expanding and then increases during matter and radius domination. What is the reason behind this? ...
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Visibility of our Universe by an Observer outside [closed]
In my own theory/understanding of our universe I believe that EM radiation does not leave the universe because the method /medium for light travel was created during the expansion thus only existing ...
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Is differential number density the same as flux density?
I'm trying to convert the differential number density of photons to luminosity (using slide 17 of https://www.astro.rug.nl/~sctrager/teaching/OA/Photons.pdf) and ...
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Are the mass, diameter and age of the Universe frame dependent?
Mass of the observable Universe is known to be $1.50×10^{53}$ kg. Age is approximately known to be 13.7 billion years.The observable Universe is a sphere with diameter of roughly $8.8\times10^{26}$ m.
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Is there an "escape velocity" for closed dimensions? [closed]
Assuming a closed universe, the shape of the universe is often considered, or at least presented in pop science, to be a glome (4-sphere), and popularly depicted as behaving analogously to a 3-sphere, ...
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One-Way Speed of Light and the Big Bang
Variations on this question have been asked a few times (e.g. here, here, and a few YouTube videos here and here). The claim seems to be that because we can only measure the round-trip speed of light, ...
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Does inflation theory assume a finite universe?
Inflation theory has it that the early universe was causally connected, and could “mix”, hence explaining relative homogeneity of the CMB. The universe then rapidly expanded and became causally ...
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Comoving Space as a Manifold: How Does It Work? [closed]
One of the open questions in cosmology is:
"What is the 3-manifold of comoving space, i.e. of a comoving spatial section of the universe, informally called the "shape" of the universe?&...
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Does the top-down cosmology hold for anything? [closed]
According to Top-Down cosmology, galaxies might have multiple histories that coexist and are equally real. I think this has to do with the time symmetry of most physical laws.
Can this theory also ...
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Can a receding body due to the accelerated spacetime expansion be "rescued"?
Once a body "crosses" the limit where dark energy wins over gravitational forces (Is there a distance from a gravitational source where the influence of gravity and dark energy are balanced ...
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Is there a Doppler effect on the surface of an expanding balloon?
Imagine a stationary transmitter which vibrates the surface of the balloon and a stationary receiver half way around the balloon that can pick up these waves. Let the balloon expand. Will the ...
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The covariance matrix $\mathbf{C}$: Why does $\ln \text{det} \mathbf{C}=\text{Tr}\ln \mathbf{C}$ hold? [migrated]
Prof. Max Tegmark first introduced the Fisher information matrix into cosmology in his paper titled Karhunen-Loeve eigenvalue problems in cosmology: How should we tackle large data sets?
As I read the ...
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How fast would a hypothetical microscopic quantum black hole evaporate with an effective mass of two protons?
According to the analysis shown in this research here, see link the evaporation time can be calculated in seconds using this equation:
$$
\begin{array}{l}
t_{\text {evap }}=\left(\frac{5120 \pi G^2}{\...
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How to derive deceleration parameter in terms of redshift?
How do I get q in terms of $z$ specifically of this form:
I want to derive the second equation from the first but I'm getting an additional $ \frac{a^2}{H^2}\ $ in the second term
$$ q= -\frac{\ddot{a}...
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Three-Point correlation function in cosmology
I have been studying this review article on Non-Gaussianity from inflation. It was mentioned that $n$-point correlation function can be obtained by the expression
\begin{equation}\label{eq:1}
\langle\...
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Understanding the additional field in shape dynamics calculation of big bang
I read this fascinating paper (RG).
The total collision is a well-known singularity in Newton mechanics: the distances become zero and, therefore, the potential becomes infinite.
In a paper before ...
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Is the size/age of the universe dependent on your velocity? [duplicate]
As Photons do not experience time or space, then according to my thought experiment, all photons must occupy some kind of singularity as well as what WE observe from earth.
I was also thinking that ...
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Interaction between energy and spacetime
I hope it's not too stupid but if we say that what we call matter or energy is creating or consuming the spacetime, would not that make the big bang more logical?
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Are dark energy and vacuum space actually one and the same?
It is my understanding and correct me if I'm wrong, that what we call as vacuum space is the zero-point energy ZPE and its quantum fluctuations. However, because ZPE is constantly increasing with ...
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How to intuitively understand the Boltzmann equations that come up in Cosmology?
This may be a silly question, but I have been reading through Dodelson's Modern Cosmology textbook and n Chapter 4 we derive the Boltzmann equation for different constituents of matter and radiation. ...
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Is the universe really a closed physical system? [closed]
It is assumed in most physics discussions that the universe is a closed physical system. But is it really? I am defining the universe as the entirety of everything physical, not merely the observable ...
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Andromeda Galaxy and Hubble
When Hubble determined the distance to Andromeda he based his estimation on Cepheids. However, the result was less than half the current value. What was the cause of this error and could another ...
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Trying to find the derivative of a function that depends on the comoving momentum modulus
How to show that for a function $f=f(P^{2})$ and for $P^{2}=\delta_{ij}P^{i}P^{j}$, we have
$$ P_{i} \frac{\partial f}{\partial P_{i}} = P \frac{\partial f}{\partial P}$$
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Does Dark Energy contribute to increase the isothermal temperature of plasma in galaxy clusters?
I have a question about this work called "Dark energy and key physical parameters of clusters of galaxies"
There, towards the end, the authors talk about the isothermal velocities and ...
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What is the connection between matter in the universe and the baryon number not being conserved?
Towards the end of "Quarks, the Stuff of Matter", the author discusses the implications of the proton is not stable and ultimately decays. He states, that if the proton decays, then the ...
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Galactic Rotation Curves
While researching rotation curves, I've noticed a variety of velocity behaviors in different galaxies. In some, the velocity decreases, in others, it remains relatively constant, and in some cases, it ...
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The equation of motion for the scalar field
Consider a scalar field ϕ with the following Lagrangian density in the FRW scale factor:
$L= (-\frac{1}{2} \partial_{\mu} \phi \partial^{\mu} \phi -V(\phi)) \sqrt{-g}$,
the motion for ϕ in this ...
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Size and boundary of the Milky Way Galactic disk
I have been pondering a question that arose while I was reading a research paper that mentions galactic disk stars have been found up to distances as far as 25 kpc from the galactic center and ...
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Confusion about Hubble law vs. objects freezing at the cosmological horizon
I am puzzled about an apparent contradiction. In a universe with a Hubble law, you see that, the farther an object is, the faster it is moving away from you. On the other hand, you also have a ...
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“Negative energy” definition of “gravitationally bound” in expanding cosmology
First off, I realize that there is already a number of questions relating to gravitational binding in cosmology:
Gravitationally bound systems in an expanding universe
Gravitationally bound systems ...
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Effects of dark energy in the kinetic energy of a body?
If I launch a ball into the sky it would reach a distance after which it would return into the ground transforming the potential energy into kinetic energy as it hits the ground
This is similar to ...
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Does General Relativity, without Cosmic Inflation, predict a perfect blackbody for the CMB radiation?
My understanding is that the universe did not have enough time to thermalize before the epoch of recombination, so many patches of the sky were not in causal contact with each other, which means they ...
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Clocks at different redshift: does calculating luminosity distance shift the frame of reference?
Luminosity is energy per time.
When we calculate a luminosity distance, using angular diameter distance times $(1+z)^2$, I think we are correcting the energy (from redshifted photons) by $(1+z)$ and ...
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Is it possible to take the time derivative of sound speed of an ideal fluid when the background $w=0$?
Context: To determine the evolution for a particular unified dark matter model we use the following equation:
\begin{equation}
\delta''+3(s\delta)'+(\delta'+3s\delta)\left(2+\frac{H'}{H}\right)-\...
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How to obtain the continuity and Euler equations by taking moments of the Vlasov equation in Cosmology?
In a set of notes about cosmology, I have found the following claim:
The 0th moment of the Vlasov equation yields the continuity equation. For that, upon integrating over the momentum, we have to ...
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In a homogeneous cosmological fluid, which position vector is constant, $\vec{r}$ or $\vec{x}$?
This question occurred to me while trying to prove an equation in the context of Cosmology. When I decompose the Newtonian gravitational potential $\Phi$ as the sum of a potential $\phi_b$ due to the ...
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Cosmological understanding from Penrose diagram of de Sitter spacetime
The conformal diagram of de Sitter spacetime looks like this
I think I understand the causal properties of this diagram. Someone who is static in the south pole can send messages only to the upper ...
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Question about cosmological constant and radius of curvature of the universe
Dimensional analysis suggests that $\Lambda R^2 \sim O(1)$, where $\Lambda$ is the cosmological constant and $R$ is the radius of the universe. $\Lambda$ is measured to be around $10^{-52}$ m$^{-2}$, ...
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Theoretically, is it feasible for the dark matter density to be constant and homogeneous, as dark energy is, and the two to be related?
I know that currently dark matter and dark energy are separate things, not related and one not deriving from the other. But if both are included in a generalized gravitation theory, the picture can ...
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Why does the light intensity of stars distribute almost uniformly across the night sky, if earth is not special?
According to special relativity, the direction of light should change when a reference frame is moving at near-light speed. There was a question on stack exchange about this topic before: Light in ...
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Why is it true that $\mathcal{H}'\vec{x}=-\vec{\nabla}\phi_b$?
A professor at university made the following claim during a Cosmology lecture:
In the background we have $\mathcal{H}'\vec{x}=-\vec{\nabla}\phi_b$ with $\nabla^2\phi_b=4\pi G\bar{\rho}$.
I am having ...
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Cosmology, MOND and TeVeS
I have read the Wiki page about TeVeS. It says that MOND (or rather, AQUAL) is its nonrelativistic limit.
I wrote a thesis on cosmology eons ago (no, seriously, more than 50 years ago!) and though I ...
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Constraints on the CMB temperature redshift dependence at matter-radiation equality?
As pointed out in the 2011 paper Constraints on the CMB temperature redshift dependence... here the relation between redshift and the CMB temperature, $T_{CMB}(z)=T_0(1+z)^{1-\beta}$ (with $\beta=0$ ...
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