Cosmological expansion Definition and 37 Threads

I would like to ask some questions about an interesting paper that was written back in the late 90's (https://arxiv.org/abs/astro-ph/9701131) There, the authors propose how the universe may evolve from the near future to extremely far time scales Near the end of it (Section VI, D.), they...

Hi, I was thinking about the claim that for instance Sagittarius A* (Sgr A*) black hole is a at 26996±29 light years from the Earth from a GR point of view. Assuming a FLRW model for the Universe, maybe the above meaning is that at a given cosmological time ##t## (the "present" time) the proper...

This is from Modern Cosmology, Scott Dodelson, Chapter 6. For the part "Show that its energy density dilutes as ##a^{−3}##", following is my attempt: In the equation ##\frac{\partial \rho}{\partial t} = -3H(P+\rho)##, put ##P = \frac{1}{2} \dot{\phi}^2-V(\phi)## and ##\rho=\frac{1}{2}...

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 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...

As far as I know, entropy could be reversed by the Poincaré recurrence theorem if it had a finite horizon given by some amount of vacuum energy causing an accelerating expansion. However, I found this lecture by Leonard Susskind () where he tells a way through which the vacuum could decay into...

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...

I the lambda-CDM model, is the expansion of spacetime uniform around all of spacetime, is there a smooth transition between expanding parts of spacetime (the voids) and non-expanding parts of spacetime, or is there a sharp distinction between expanding and non-expanding parts of spacetime. Is...

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...

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...

The integrated Sachs-Wolfe effect occurs when a photon goes through a gravitational potential that changes due to spacetime expansion (presumably caused by dark energy). For that reason, a photon going through a gravitational well would gain energy (blueshift) when entering and it would lose...

Are there any kind of observed and experimentally verified processes or mechanisms where photon emission occurs and which are directly cause by spacetime expansion in some way?

I found an old article (https://journals.aps.org/pr/abstract/10.1103/PhysRev.137.B1379) which talks about conservation of energy in an expanding space. Apparently, the author found that energy is conserved at local scales (like the motion of planets in our solar system) as one would expect, but...

There is an article written by astrophysicist Edward Harrison [1] which defends that energy could be extracted from attaching an imaginary cosmologically long string to a receding object from us in an expanding universe. He says that the energy extracted is potentially limited (in decelerating...

Imagine we attach an imaginary cosmological scale rope to an object that is very far away from us. Before attaching the string, the object would be receding from us due to spacetime expansion. After attaching it, tension would form in the string and we would eventually stop the object. After...

Let's imagine for a moment that the universe stopped expanding somehow (even though the evidence we have suggests this is not going to happen) and gravity made it contract until reaching a Big Crunch state. According to our actual understanding of physics and our current working models, Is it...

In considering the excellent answers given to my previous query about a photon in a box, I was led to consider what force was needed to hold the walls of the box stationary (w.r.t. an inertial observer inside the box.). If you place a mass m on the end of a very long string of inextensible...

Homework Statement Consider a point in the intergalactic medium at some cosmic time ## t_{obs}##, the time of arrival of a photon of wavelength ##λ_{obs}## as seen by a hydrogen atom at that location. The source of this photon a comoving distance ##r## away emitted it at wavelength ##λ_{em}##...

Suppose we have Einstein equation for *Universe free of matter* in form \begin{equation} G_{ik} = \chi T_{ik}, \end{equation} where the cosmological constant $\Lambda$ is transferred to the RHS of equation and written in the form of stress–energy tensor of Dark Energy...

I began thinking about the problem I describe below from trying to understand the discussions in another thread. https://www.physicsforums.com/threads/concordance-cosmology-paper.909603 This problem is about the expansion behavior of a “simple” universe model that might demonstrate a distinction...

In wikipedia says that in a big amount of time quantum tunneling can create a new Big Bang. EXACTLY, how this happens. https://en.wikipedia.org/wiki/Future_of_an_expanding_universe ("Beyond" part) *note: I'm in high school, but feel free to explain this with complicate process.

is there any limit to the expansion of the universe? (Cosmological Expansion) Can anyone refer to some research papers on this topic?

hello, My question will be quite naive for experts and reflects the fact that I'm new to the subject of varying the fine structure constant alpha and mainly need an introductory reference ... so if someone has a good one to advice ...thanks a lot i just don't understand the basics: how people...

Firstly, I assume that I'm correct in assuming that since expansion is accelerating it will increase to any arbitrarily large value at some point in the future. If this is true, there must be some point at which particle/antiparticle pairs (due to uncertainty) are carried away from one another...

is there any limit to cosmological expansion?

Homework Statement If light traveled a distance L = H_{eq}^{-1} at M-R equality, how large does this distance expand to at present? (in Mpc) Homework Equations z_{eq} = 3500 \Omega_m = 0.32 at present \rho_c = 3.64 \times 10^{-47} GeV^4 present critical density The Attempt at a...

I've long wondered about an assumption that we have today and I've never found a direct answer to my question. Presently we can observe that there is a direct proportionality between an object distance and the factor by which its light is redshifted. We deduce that this observation implies...

This paper dates to 1998: Cooperstock, Faraoni, and Vollick, "The influence of the cosmological expansion on local systems," http://arxiv.org/abs/astro-ph/9803097v1 They show that systems such as the solar system, galaxies, and clusters of galaxies experience nonzero effects from cosmological...

Cosmological expansion vs. "stretching" From a relativistic point of view, how can we tell the difference between a universe that’s expanding (outward) from a singularity versus one that is being sucked INTO a singularity? In the case of the latter, it seems that those objects closer (than us)...

I came across the following at Ned Wrights website and wondered [a] What does the boldface statement mean? [b] What is thought about the statement that "Cosmological expansion of the Earth orbit around the sun is negligible but not zero" instead of "its not been determined"...

I was just reading on wikipedia about the expansion of space. ( http://en.wikipedia.org/wiki/Metric_expansion_of_space ) It explains that the expansion of space can be treated as if it were a repulsive force between particles proportional to the distance between them, ie the further apart...

I am trying to understand cosmological expansion and how it is possible to see objects that are receding from us faster than the speed of light. This is explained in words at http://www.mso.anu.edu.au/~charley/p...DavisSciAm.pdf" and I have tried to describe a simple mathematical model to...

I am working off the premise that: If Cosmological expansion is really occurring a redshift (as we currently observe) can only be obtained if (stars, planets, atoms) do not expand. See Misner, Thorne and Wheeler comment: "Only later does he realize that the atom does not expand, the...

It is known that in result of the cosmological expansion the distances between galaxies increase according Hubble law v=H*r. Distant SNeIa show that the dark energy accelerates the expansion. I heard in a scientific popular transmission that in result of the acceleration of expansion, every...

In another thread, someone was talking about cosmological expansion effects on planetary orbits. (Actually it was about lunar orbits, but I think planetary orbits are more to the point). Through a somewhat round-about path, I eventually got to thinking about the following question. Suppose we...

I re-read parts of "Genius" by James Gleick earlier today after visiting a web-site that discussed steady-state cosmology. Apparently, Feinman felt that the creation and obliteration of virtual particles (diagramed by equivalent waves moving BOTH forward and backward in time) supported the idea...

At the risk of further muddying up the epicycling, let me offer a wild speculation concerning the expansion force. At the close of the infaltion epoch assumed by its advocates to have been at 10^-35 (superscript negative 35) seconds after time zero of the big bang, the universe is believed...