Cosmology Definition and 951 Threads

Please select up to 3 members who were most impactful in the Astronomy and Cosmology forums in 2023. This is a popular vote. Polls were created by weighing activity and measure of helpfulness. Everyone nominated should feel honored. Many more could be added to this poll, we can never...

Dark Energy puts a constrain on the size of overdensities (like clusters and superclusters of galaxies) and their growth. A higher Dark Energy density would reduce the radius of the zone where matter would be gravitationally bound, because more Dark Energy density would mean that objects would...

I have a table of densities of galaxies : Expected number density of galaxies for photometric survey per unit area and redshift intervals, ##\mathrm{d} N / \mathrm{d} \Omega \mathrm{d} z\left[\mathrm{sr}^{-1}\right]## and the corresponding density of galaxies per ##\operatorname{arcmin}^2## for...

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 tempreature parameters of the gas and particles circulating between galaxies in clusters. In particular they...

Hello, I have asked a similar question before, but this time I want to ask it a bit differently to be understood better. I am a physics student at a university and this semester I am going to make a term project about Dark Matter Density Distribution by using rotation curves. However, I am a...

Is there a neat way to "not" run the internal Boltzmann solver (for perturbations) in CLASS code and rather just solve for the background quantities? This way I can save the time otherwise spent in evaluating the perturbations and transfer functions. I am only interested in the time evolution of...

I recently found a question in a physics discussion site [1] about whether there was a "distance" between two galaxies where both the gravitational force and the influence of dark energy would be balanced. The answers and comments seem to indicate that there is indeed such a "radius" around a...

Hi. I'm not sure if this should be in Cosmology or Astronomy - a moderator can feel free to move it if required. Also the prefix (study level) may be an I or an A, you can change that, just thought I'd get a bigger audience with an I. I'm trying to read this paper: "The Inner Structure...

I have found some papers (like this one: https://www.aanda.org/articles/aa/full_html/2009/45/aa12762-09/aa12762-09.html) which say that dark energy increases the potential energy in a system of a quasi-stationary gravitationally bound many-body system. It also says that because of this, the...

1) In a cosmology context, when I add a centered Poisson noise on ##a_{\ell m}## and I take the definition of a ##C_{\ell}## this way : ##C_{\ell}=\dfrac{1}{2\ell+1} \sum_{m=-\ell}^{+\ell} \left(a_{\ell m}+\bar{a}_{\ell m}^{p}\right)\left(a_{\ell m}+\bar{a}_{\ell m}^{p}\right)^* ## Is Poisson...

Suppose the universe were to eventually collapse in a Big Crunch [1]. How closely could the universe's final moments resemble those at the beginning of the universe? Could the universe return to its original state exactly in some kind of "Big Crunch" or "Big Bounce" model? [1]...

Hi, I am new here so apologies if i am not using the right subforum. I don't have a physics background so i am not very technical but i do have a little bit of understanding. I was reading this paper by hawking/hertog and came across something that ended up confusing me. Here is it: "Pre-big...

I cannot find an answer online. Help please.

More of a biology person but always had an interest in cosmology as well.

If the universe keeps expanding at an accelerated rate (given by the cosmological constant) then the universe would approach a DeSitter spacetime where there would be a cosmological horizon that would radiate just as the event horizon of a black hole radiates Hawking radiation I thought that...

In the far future there will be most likely a point where a maximal state of entropy will be reached in the universe and after the last black hole evaporates there could be no more structures and no more work could be done. According to the Poincaré recurrence theorem for a closed universe...

Václav Vavryčuk has written two articles on replacing the standard FLRW metric in cosmology with what he calls a "conformal FLRW metric", which he claims explains astrophysical and cosmological phenomena traditionally attributed to dark energy and dark matter/MOND, such as the dimming of type 1a...

The article https://phys.org/news/2023-07-age-universe-billion-years-previously.html#:~:text=42-,New%20research%20puts%20age%20of%20universe%20at%2026.7%20billion%20years,as%20old%20as%20previously%20believed&text=Our%20universe%20could%20be%20twice,%22impossible%20early%20galaxy%20problem.%22...

new article by Pengfei Li: Distance Duality Test: The Evolution of Radio Sources Mimics a Nonexpanding Universe

I never stop learning physics, especially astrohysics and cosmology.

Hi, I've recently developed an interest for the history of the development of cosmology and find it very interesting. The key events I have been reading up on are: 1915 - Einstein's theory of General Relativity was published. 1923 - Hubble discovered a Cepheid variable in the Andromeda...

Physicist Grigory Volovik has put forward some ideas about the universe undergoing a topological phase transition (especially in the early stages of the universe). He published a book called "*The Universe in a Helium Droplet*" where he explained his ideas. You can find a brief discussion here...

What is the value of M_{Pl} used in the Planck (CMB) collaboration's observation papers, such as the one referenced in this link: https://arxiv.org/pdf/1807.06211.pdf. Specifically, I am wondering if it refers to the Planck mass or the reduced Planck mass?

I found some interesting discussions in this site (e.g: https://www.physicsforums.com/threads/smolin-lessons-from-einsteins-discovery.849464/; https://www.physicsforums.com/threads/relatismo-to-the-max.83885/) which are related to Lee Smolin's ideas that laws are not immutable and can therefore...

I apologize in advance if this is a stupid question but... According to some scenarios about the beginning of the universe (namely cosmological inflation), in layman's terms, everything was born out of a quantum fluctuation which caused a violent expansion. In this case, since an expanding...

After expanding to first order in ##\epsilon## and subtracting off the unperturbed equation, I get\begin{align*} \frac{\partial \delta \rho}{\partial t} + 3H \delta \rho + \frac{\bar{\rho}}{a} \nabla \cdot \delta \mathbf{v}=0 \end{align*}I'm not sure how to deal with the ##3H \delta \rho## term...

Does the polarization spectrum TE measured by the Planck and WMAP satellites show evidence for superhorizon fluctuations at low multipoles and are these evidence for pre-bigbang inflation?

Is it possible to estimate the number of hot and cold spots of average diameter of 1° in a full-sky map of the CMB?

I was reading a discussion where some physicists participated* where the topic of Lorentz invariance violations occurring in cosmology is mentioned. There, they mention that we can imagine a Lorentz-violating solution to the cosmological equations. What do they mean by that? Can anyone specify...

I have heard that some types of inhomogeneties and topological defects (like cosmic strings) in cosmology have been proposed to be able to break fundamental symmetries of nature such as the Poincaré, Lorentz, diffeomorphism CPT, spatial/time translational...etc symmetries... However, I have not...

Assuming dark energy is fairly, uniformly distributed through out the cosmos, how strong is it, or how much energy is associate with it, out in the deepest, emptiest voids in space? I'm specificlaly refering to the great voids in between the great walls of galaxy clusters. I'm making the...

Perhaps this is a stupid question but, if Lorentz symmetry and time translational symmetry are not global in an expanding universe, wouldn't that mean that is possible that other Hubble spheres outside our observable universe could have other symmetries or an absence of the Lorentz symmetry? I...

Wikipedia states the following in their article about the expansion of the universe: If the cosmological principle was discovered to be false in our universe, i.e. our universe was discovered to be inhomogeneous or anisotropic or both on very large scales and the FLRW metric does not hold for...

Observational evidence for cosmological coupling of black holes and its implications for an astrophysical source of dark energy Comments?

I'm currently in my first year of a combined major in Physics and Astronomy. I'm hoping to study theoretical cosmology, and get a PhD in physics. I've been told it would be wise to get a double major in Physics and Math if I wish to study theory, but would it also be wise to get a combined major...

When arriving at the standard model of cosmology, i.e. the exapnding universe, we assume based on experirmental data that the cosmos is homogenous on large enough scales. But when we go back in time, when the galaxies are beginning to form, we note that because of the growth of density...

tldr summary

A recent thread asked about Penrose's proposal on cyclic cosmology. It was closed due to lack of any remotely acceptable sourcing, even after prompting. Much of the original professional publication on this is not available on arxiv. However the following includes a summary of conformal cyclic...

Spacetime expands at an accelerated rate and the particles with movement associated to this expansion are coupled to the Hubble flow. In many papers that I've read, objects coupled to the Hubble flow are treated as if they have some velocity and kinetic energy associated with it.However, can...

I would like to arrive at the following expression for the quantity ##o_{\ell}## ( with "DM" for Dark Matter ): ##o_{\ell}=b_{s p}^2 C_{\ell}^{D M}+B_{s p}## with Poisson noise ##B_{s p}=\frac{1}{\bar{n}}(\bar{n}## being the average number of galaxies observed). the index "sp" is for spectro...

TL;DR Summary: I dont enjoy my lab subject. I am currently in grad school (not US) and in the theoretical Cosmology laboratory, however I feel like I ended up in the wrong side. I did undergrad research project in astrophysical data simulation and thought that this is what I want to do in...

Wondering if there are any online “book clubs” that people read cosmology textbooks together?

I found this article* about the behavior of quasar outflows in cosmology and how they can create a magnetic field. In section 2.1.4., the authors say that when a quasar produces a "wave" or an outflow, the material will be emitted with energy coming from both the quasar itself and the Hubble...

https://en.m.wikipedia.org/wiki/Heat_death_of_the_universe Is the heat death of the universe completely unavoidable in an universe with an accelerated expansion dominated by dark energy like ours? Or can there be any way to avoid it according to current knowledge, observations and experiments...

In describing the spacetime around a massive, spherical object, one would use the Schwarzschild Metric. What metric would instead be used to describe the spacetime around multiple massive bodies? Say, for example, you want to calculate the Gravitational Time Dilation experienced by a rocket ship...

If energy is "not conserved" in General Relativity (or at least, it is difficult to define it) in the context of an expanding accelerating spacetime (like it happens in our Universe), are there any observations of deviations from the strict conservation laws in the evolution and formation of...

How do we know that cold neutrinos do not make up 100% or a large percentage of the dark matter content in the universe? In my mind, the only way to prove that dark matter is not simply cold neutrinos would be to measure the density of cold neutrinos in the universe and then calculate the...

I found a paper (https://arxiv.org/pdf/astro-ph/0411299.pdf) which talks about quantum systems emitting energy due to spacetime expansion. Is this true or only a hypothesis?

Hello, I'm an independent researcher in cosmology and working on topics like cosmological redshift, galaxy formation and galaxy cluster dynamics. Came across the forum searching for details about the solar system orientation in the galaxy rotation disk. Very inspired and enthusiastic about the...

Before anything, I would like to clarify that I am aware that this is speculative physics more than established mainstream physics (and Smoot is not claiming that his ideas are true). However I think that it is interesting to discuss these models even if they are not yet proven to be right. I...