What is Cosmological expansion: Definition and 36 Discussions

The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion whereby the scale of space itself changes. The universe does not expand "into" anything and does not require space to exist "outside" it. Technically, neither space nor objects in space move. Instead it is the metric governing the size and geometry of spacetime itself that changes in scale. As the spatial part of the universe's spacetime metric increases in scale, objects move apart from one another at ever-increasing speeds. To any observer in the universe, it appears that all of space is expanding while all but the nearest galaxies recede at speeds that are proportional to their distance from the observer – at great enough distances the speeds exceed even the speed of light.As an effect of general relativity, the expansion of the universe is different from the expansions and explosions seen in daily life. It is a property of the universe as a whole rather than a phenomenon that applies just to one part of the universe and, unlike other expansions and explosions, cannot be observed from "outside" of it.
Metric expansion is a key feature of Big Bang cosmology, is modeled mathematically with the Friedmann–Lemaître–Robertson–Walker metric and is a generic property of the universe we inhabit. However, the model is valid only on large scales (roughly the scale of galaxy clusters and above), because gravity binds matter together strongly enough that metric expansion cannot be observed on a smaller scale at this time. As such, the only galaxies receding from one another as a result of metric expansion are those separated by cosmologically relevant scales larger than the length scales associated with the gravitational collapse that are possible in the age of the universe given the matter density and average expansion rate. To paraphrase, the metric is forecasted to eventually begin to outpace the gravity that bodies require to remain bound together, meaning all but the most local bound groups will recede.
According to inflation theory, during the inflationary epoch about 10−32 of a second after the Big Bang, the universe suddenly expanded, and its volume increased by a factor of at least 1078 (an expansion of distance by a factor of at least 1026 in each of the three dimensions), equivalent to expanding an object 1 nanometer (10−9 m, about half the width of a molecule of DNA) in length to one approximately 10.6 light years (about 1017 m or 62 trillion miles) long. A much slower and gradual expansion of space continued after this, until at around 9.8 billion years after the Big Bang (4 billion years ago) it began to gradually expand more quickly, and is still doing so. Physicists have postulated the existence of dark energy, appearing as a cosmological constant in the simplest gravitational models, as a way to explain this late-time acceleration. According to the simplest extrapolation of the currently-favored cosmological model, the Lambda-CDM model, this acceleration becomes more dominant into the future. In June 2016, NASA and ESA scientists reported that the universe was found to be expanding 5% to 9% faster than thought earlier, based on studies using the Hubble Space Telescope.

View More On Wikipedia.org
  1. cianfa72

    I Meaning of the distance between galactic objects in GR

    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...
  2. Samama Fahim

    I Cosmological Scalar Field Density Dilution

    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}...
  3. S

    I More extended equilibrium configurations due to dark energy?

    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...
  4. S

    I Dark Energy contribution to plasma temperature 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 tempreature parameters of the gas and particles circulating between galaxies in clusters. In particular they...
  5. S

    I Entropy reversal in an infinite static universe?

    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...
  6. S

    I DeSitter cosmological horizon stability?

    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...
  7. elcaro

    I Cosmological expansion of space - question

    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...
  8. M

    I Expansion of the Universe and the cosmological principle

    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...
  9. S

    I Avoiding heat death in an accelerated expanding universe?

    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...
  10. S

    I Sachs-Wolfe effect in a Dark Energy universe?

    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...
  11. S

    I Can there be some kind of photon emission caused by space expansion?

    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?
  12. S

    I Is energy conserved during the formation of local systems?

    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...
  13. S

    I Mining the Universe's expansion?

    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...
  14. S

    I Receding movement of a stopped object in an accelerating universe?

    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...
  15. S

    I Information lost at the Big Crunch?

    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...
  16. J O Linton

    B What is the distance at which gravity and the cosmological force are equal?

    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...
  17. J

    Cosmological Redshift

    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}##...
  18. sergiokapone

    I Stress–energy pseudotensor of gravitation field for DE

    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...
  19. Buzz Bloom

    A A cosmological expansion problem too difficult for me

    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...
  20. R

    B Exactly how quantum tunneling leads to a Big Bang?

    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.
  21. S

    B Cosmological expansion's limit

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

    A Alpha Variation vs Cosmological Expansion: Basics Explained

    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...
  23. T S Bailey

    Cosmological expansion and uncertainty

    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...
  24. S

    Cosmological Expansion: Is There a Limit?

    is there any limit to cosmological expansion?
  25. K

    Cosmological Expansion: Estimating Present Horizon Length

    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...
  26. M

    Question about Cosmological expansion?

    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...
  27. bcrowell

    Effect of cosmological expansion on bound systems in realistic cosmologies

    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...
  28. N

    Cosmological expansion vs. stretching

    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)...
  29. N

    Cosmological expansion of earth orbit

    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"...
  30. C

    How do gravity and cosmological expansion strike an equilibrium?

    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...
  31. andrewkirk

    DIfferential equation for understanding cosmological expansion -

    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...
  32. I

    Cosmological expansion question

    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...
  33. D

    Do the cosmological expansion affect binding systems

    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...
  34. P

    Cosmological expansion effects

    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...
  35. turbo

    Virtual particles and cosmological expansion

    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...
  36. I

    Cosmological Expansion: Exploring Expansion Force Beyond Big Bang Theory

    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...