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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Spherical harmonics with 2 cosmological probes : considering $a_{\ell m, photo}$ as a constant and $\hat{a}_{\ell m, spectro}$ as an estimator

I am in cosmological context where the survey on which I am working has 2 probes : a photometric galaxy clustering ($GC_{ph}$) probe and a spectroscopic galaxy clustering ($GC_{sp}$ probe). We use an ...
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Comoving distance vs physical distance

In a recent post in Medium.com Ethan Siegel wrote the following: "... as you look farther away, objects appear smaller until a critical point: a minimum size that objects will appear in our ...
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What from our current understanding in physics prohibits vacuum space to be possible a superluminous medium? [closed]

Thus, an exotic form of energy-matter at a higher energy state than our normal matter and luminous $c$ bounded energy of our spacetime. A different space-time domain out of phase from our spacetime ...
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Problems with negative pressure, imaginary speed or super-luminous speed?

Currently, dark energy is described as a being that exerts a negative pressure while having a positive energy density. However, there is a serious problem when considering a physical existence with ...
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Conformal Cyclic Cosmology

In this conformal cyclic cosmology, where did the very first eon come from? How did the very first eon start with possible lowest entropy?
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What is the Ricci curvature tensor in a region of sparse matter?

In empty space, where the energy-momentum tensor $T_{\mu\nu} = 0$, any solution to the Einstein field equation has Ricci tensor $R_{\mu\nu} = 0$. So what does a solution look like in a region of space ...
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Oscillating solutions to Friedmann equations

Does homogeneous and isotropic Friedmann cosmologies allow for periodic (simple harmonic solutions) Universes? Can they (universal oscillators) solve the issue of the initial (or future) singularities?...
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Conformal cyclic cosmology, black hole and Weyl curvature

Roger penrose in his theory "conformal cyclic cosmology" states that after a google there would be a giant black hole. Does black hole have Weyl curvature? If it does, then we know that ...
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Is there a consensus among cosmologists about the big freeze or any “fate of the universe” scenario?

Any insight into the matter would be helpful.
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Rydberg energy and Hubble constant

Although they are of different dimensions, the value of the Rydberg energy is very close to that of the Hubble constant. Rydberg energy (R): $2.179 \times 10^{-18}$ [Joule] = 13.6 [eV] Hubble ...
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How to test for possible negative mass of dark matter?

What is the phenomenology of how to test if dark matter has possibly a negative mass (WP negative mass) in particle physics experiments, cosmology or astrophysics? I lately came across this ...
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What implications to cosmology would it have if Webb Telescope probing the far end of our observable Universe finds out a large number of galaxies?

Since it is predicted that Cosmos is no less than 250 times that of the observable Universe (13.8 Blyrs) in size: MIT Technology Review I wondering how you can calculate any age of matter creation (...
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What would happen if the two mouths of a wormhole are receding away from each other faster than light?

If wormholes could form naturally, and point A and point B were in galaxies receding from each other faster than light, what would happen to something going from one end to the other? Physicist Kip ...
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Cosmological expansion around ropes between galaxies

The FLRW cosmology is a fair approximation of cosmological expansion for space between galaxies and clusters, however within galaxies themselves, is known to not hold, and in fact observational ...
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How does it make sense for the universe to have started from a big bang?

It has been said that the Big Bang started from a singularity. Think about a balloon radially growing over time. Fix a time $t_0, t_1 > 0$, and let $M_0, M_1$ be two balloons at time $t_0, t_1$ ...
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What is the exact ratio of dark energy to mass?

I need to know the exact ratio between mass and dark energy (total dark energy in the universe / total mass in the universe). I could only find it to 2 decimal points (0.68). I need this to make this ...
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What does $C^{XY}_{\ell}$ mean when we weasure $a_{\ell m}$ in the sky?

In cosmology context, we have the general formula for the angular power spectrum $C_{\ell}$ : $$C_{\ell}=\left\langle a_{l m}^{2}\right\rangle=\frac{1}{2 \ell+1} \sum_{m=-\ell}^{\ell} a_{\ell m}^{2}=\...
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Could dark matter be just a gravitational effect of dark energy?

I'm wondering if we just looking at the two sides of the same coin and if there is actually a correlation of DM with DE? Is it possible that DM just to be a gravitational effect (or an effect that ...
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Could one, in principle, make any predictions using the wavefunction of the universe? [closed]

Do physicists talk about the wavefunction of the universe? What does that wavefunction even mean? Usually, wavefunctions describe probabilities of measurements of a system. But in this case, every ...
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What happens when the universe runs out of fuel?

After some X billion years, one would think the stars in the entire universe will run out of hydrogen. What would happen next? Is there any way to get hydrogen out of heavy metals (extreme fission)? ...
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How is the Big Bang compatible with the preservation of quantum information?

As detailed at https://en.wikipedia.org/wiki/No-hiding_theorem , in the quantum world, information cannot be created or destroyed. The Bekenstein bound limits the amount of information that can be ...
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The space of the universe is expanding. Is there any corresponding universal phenomenon for time (contraction/dilation)? [duplicate]

Just like how electric and magnetic fields are complimentary, space and time are complimentary as they share the same "fabric". We observe that space in the universe is expanding at about a ...
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Is redshift an accurate method for measuring distances of distant galaxies and quasars?

I'm currently in 11th grade working on a science project about measuring distances using redshifts. I understand how the wavelength of a light wave increases when an object moves away from the point ...
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Calculate the luminosity distance in terms of redshift and the scale factor

Suppose you are in a spatially flat universe with a single parameter of the equation of state, $w$. Now for $w\neq -\frac{1}{3}$ calculate the luminosity distance in terms of redshift ($z$) and $w$. ...
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An exercise on the adiabatic initial condition from Daniel Baumann's new cosmology book

Additional info for those not familiar with cosmology: The $\delta$ s are the density contrast whose subscripts indicate species. It is defined as: $\delta_{a}=\frac{\delta\rho_{a}}{\bar{\rho}_{a}}$ ....
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4 votes
4 answers
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With how many Newtons of force is the universe expanding?

The universe is expanding at an accelerating rate. They key word here is "accelerating". Meaning that there is an equivalent "force" (in Newtons) that would cause that same ...
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How do you define present from physics point of view [closed]

So how would you exactly define present, aside from fact that time is an illusion; is it the age of the universe in this sector that we experience subjectively as present, does it have to do anything ...
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17 votes
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Wouldn't the cosmic background radiation (CMB) produce drag and thus create a preferential inertial frame?

Because the CMB is everywhere and is isotropic, if an object would have a certain velocity, it could have a pressure differential produced by the CMB which would produce drag till it would stop with ...
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1 vote
1 answer
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Understanding primordial Statistical Fluctuations in the Universe

I am trying to answer the question, Whether gravitational collapse began from Statistical fluctuations in the early Universe? In order to answer this I must find out the total number of particles in ...
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2 votes
1 answer
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In an expanding universe, can two people communicating to each other about their cosmological horizons get around their horizon limit?

I want to pose a preamble question that I will answer first to build up to the main scenario. Then I will pose the main question. The main question concerns the special case of an expanding universe ...
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2 answers
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What is there at a point the universe hasn't expanded past yet? [duplicate]

(Please don't mark as a duplicate) If the universe is constantly expanding that means that there is a point the univese hasn't expanded past, with that what would be past that point? This isn't about ...
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What is at the edge of space? [duplicate]

if the universe is constantly expanding that means that there is an end where the universe hasn't expanded yet if you could be at that before it was to expand more, or go past/into that edge, what ...
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16 votes
3 answers
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How can the universe be hot or dense in the first moments after the big bang when it has no matter?

From the CERN website: In the first moments after the Big Bang, the universe was extremely hot and dense. I've always heard this about the big bang but I've never thought about it before now. If &...
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Friedmann equation solution and age of the universe in terms of $H_0$ and $Ω_{(r,0)}$ for an open/closed universe [duplicate]

I cam currently doing a problem involving Friedmann equation for $a(t)$ and solving for $t$ when $a(t)=a_0$ in order to get the age of the universe. Since the integrals are quite nasty, I want to make ...
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Covariance of transformed random variable

So i have the covariance $cov (x_0 , x_1)$ and i know $m_b = -2.5 log_{10} (x_0) + constant$, then how do i calculate $cov (m_b , x_1)$ i found this but i guess it isnt useful here since it is ...
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3 votes
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Deriving the age of the universe

I am trying to work out the solution to exercise 8.4 from An Introduction to Modern Cosmology by Andrew Liddle. I could derive the Friedmann equation as below, $$\dot{a}^2 = H_0^2 \left[\Omega_0a^{-1} ...
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1 answer
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Based on structure formation and the lifetime of the universe why is there an upper bound on the cosmological constant?

I understand that significantly greater values than the cosmological constant would result in difficulty for the formation of large gravitationally bound structures within the lifetime of the universe....
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Inverting an equation in the derivation of the comoving distance

It's given that $$z \approx H_0(t_0-t_e)+(1+q_0/2)H_0^2(t_0-t_e)^2~~(1)$$ and later they invert this equation and write $$t_0-t_e \approx H_0^{-1}[z-(1+q_0/2)z^2]~~(2)$$ I have tried to derive this ...
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Harmonic (coherent or not) waves from inflation?

I am just wondering if inflation could produce harmonic (coherent or not) waves and what are they strength (in the limit of frictionless media) or amplitude now for each mode, and if we are searching ...
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Is the set of reachable arrangements of an indeterministic universe (of a given material substrate) sensitive to its initial arrangement?

Suppose two equally massive universes have an identical material substrate of the same fundamental particles. Suppose then that these two universes initially have different arrangements of these same ...
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1 vote
1 answer
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Comoving distance in $\Lambda$CDM - Understanding an approximation

I am trying to find the comoving distance, $$\chi = c\int_0^z \frac{dz}{H(z)}$$ for the $\Lambda$CDM model (spatially flat universe, containing only matter and $\Lambda$). $$H^2 = H_0^2[\Omega_{m,0}(1+...
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8 votes
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Why did the quark epoch occur earlier than the lepton epoch?

I don't understand the reason and hypothesis behind why the quarks appeared first—as per the big bang cosmology—shortly after the strong and electroweak force separated. We don't know what the quarks ...
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1 answer
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Is the set of reachable states of an indeterministic universe sensitive to its initial conditions?

Suppose two universes with the same amount of mass-energy and evolving according to the same natural laws, but having different initial conditions. Is the set of states that are reachable by the ...
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Dark matter and un-smoothness in spacetime [duplicate]

Since dark matter currently is only observable with its gravitational effects and nothing else can we theorize that dark matter is only non-smoothness in spacetime that has been there from the Big ...
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0 votes
1 answer
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If I moved in a straight line forever, would I hit something?

Say you could move infinitely fast/gravity doesn't affect you. You move in a straight line. Do you hit anything, like stars or black holes? Not talking about cosmic dust.
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2 votes
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Could the universe be a 4-ball?

I recently thought of the idea that the universe could be an infinite 4-ball. The Big Bang would be its centre, and time would be outward from its centre (one layer would be one point in time). I ...
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2 votes
1 answer
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About a hypothetical universe

Imagine a universe dominated by matter, but it is balanced with a cosmological constant $\Lambda=4\pi G\rho$ so the universe is static ($H=0$). However, what would happen if some of that matter turns ...
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Calculation of positive energy of the universe [duplicate]

Being new to this site, I find it very difficult to navigate. I just had a question deleted for reasons I don't understand. So, I will try again. Two part question--1. How is the positive energy of ...
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2 votes
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Programs/Scripts for plotting penrose diagrams

What programs do people generally use to plot penrose diagrams? I need to plot some simple ones for the dS and AdS and Schwarzschild metric and my hand drawn ones dont look very good.
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How are these equations derived?

This is from Wayne Hu's lecture slides on cosmology Pg.20 (http://background.uchicago.edu/~whu/Presentations/crete1.pdf) He starts out with a diffusion/conservation equation for comoving photon number ...
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