What is Expansion of the universe: Definition and 22 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.

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  1. mister i

    B Is the Accelerating Expansion of the Universe Wasting Energy?

    This is my first (non-professional) post: It seems accepted that space is "created" (otherwise the expansion of the universe would exceed the speed of light). We are talking about "creation" (!!). The question would be: does current physics tell us if this represents a waste of energy for the...
  2. S

    I Avoiding structure collapse due to dark energy?

    I had a question about this paper (https://arxiv.org/abs/1401.3742) There, the authors indicate that dark energy competes against gravity in oversdensities and can slow down or even prevent their collapse. I have a simple question about this: Galaxies will in principle evaporate their outer...
  3. Sarg

    B The expansion of the Universe and time travel (By a non-scientist)

    Hello I have a question about space. I am no physicist nor student of any kind of science but I'm curious and I've heard some things about the speeding up of the expansion of the universe which got me thinking... If the expansion is accelerating without end and believed to even surpass the...
  4. S

    I Hubble flow kinetic energy into other types of energy?

    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...
  5. S

    I Energy from quantum systems in an expanding universe?

    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?
  6. 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...
  7. S

    I Cosmic strings increasing internal energy?

    I was reading an article by Edward Harrison, which tackles the problems of conservation of energy at cosmological scales. At some point (point 2.4) he cites several article, including one by Rees and Gott, which he says indicates that the internal energy of a comoving volume (e.g. a cosmic...
  8. T

    I How do we know that red-shift is caused by expansion?

    Hi, I am not a cosmologist, but this question has been bugging me for some time. I am an engineer, and I own an aquarium, at least I owned an aquarium with an awesome LED light. Under certain conditions, I found that when the blue night light was on, organics in the water would fluoresce, green...
  9. C

    I Does the expansion of the Universe affect orbiting bodies?

    Does the expansion of the universe affect orbits? Would the orbits of the Magellanic Clouds, for example, be different if the universe were not expanding? If orbits are affected, at what scale do we first detect the effects?
  10. T

    I Why do particles around us have finite momenta?

    Since there is no privileged inertial frame, I would have expected the first particles in the universe to have no particular bias in their momenta. Relative to an observer I would expect the distribution to be uniform and unbounded. The mean momentum of the initial particles relative to an...
  11. A

    B Photon Decoupling: Effects on Universe Expansion

    Why did the expansion of the universe cause an increase in the wavelength of the photons that existed during the time of Photon decoupling ? Does this mean that expansion of universe stretches everything and stars that were present when the universe considerably expanded also got stretched?
  12. nomadreid

    I A question about a popular example of expansion

    From the outset let me assert that I am not putting into question the expansion of space. Redshifts and all that. But there is an example which one often reads in popular astronomy articles which appears at first glance to be faulty, although I suspect it is my reasoning that is faulty, and I...
  13. P

    I How is the universe expanding if the speed of light is finite?

    The universe seems to be expanding since the farther away an object is, the faster it is moving. However, because of the finite speed of light, the farther away we look in distance, the further back in time we look. Does that mean that galaxies were moving faster in the past and are now slowing...
  14. S

    I Gravitational Waves vs Universe Expansion Rate

    Is there any relationship between the Speed of gravitational waves and the Universe's "local" expansion rate? Speed of gravitational waves is supposed to be equal to the speed of light. Gravitational waves don't travel faster than light. But we can observe far galaxies moving away from us with...
  15. Ranku

    I Cosmological constant and rate of expansion of the universe

    Does the presence of the cosmological constant modify the rate of expansion of the universe even during the earlier deceleratingly expanding phase of the universe?
  16. nomadreid

    I Predicted sizes of (observable) universe

    I have read the advice of Nugatory and Jorrie in order to get me started on understanding cosmology; I have played around with the cited calculators (except http://www.einsteins-theory-of-relativity-4engineers.com/CosmoLean_A20.html doesn't open for me), without mastering them, and read for a...
  17. M

    I Object moving out of the Observable Universe due to Expansion

    With the LDCM, cosmological constant, model I understand that the scale factor of the Universe grows more rapidly than the Horizon. I believe the correct horizon I need to be considering is the Hubble Horizon and the point when objects recessional velocity hits the speed of light they disappear...
  18. S

    B Exploring Intergalactic Space Expansion Using Light Propulsion

    Hello all, Is electromagnetic radiation considered a driving factor of intergalactic space expansion similar to directed energy propulsion? I assume every point in space has countless photons passing through it from every direction at all times.
  19. P

    I How do inflatons and gravity interact?

    Is the theory that inflatons become dominate when gravity is strong (as in right after the big band) and when gravity is weak (as in driving the current expansion of the observable universe)?
  20. G

    I Accelerated expansion and Hubble plot

    Hello! I have a question regarding the effect of the accelerated expansion of the universe on the Hubble plot (redshift over luminosity (or distance). I understand that for relatively nearby galaxies, this appears to be a linear relationship but that because of the accelerated expansion of the...
  21. P

    I Who really found out the Universe's acceleration?

    Prof. Brian Schmidt got the Nobel Prize for his work in expansion of the universe which he did in 1998. His team was able to figure out that earlier in the past, the universe's expansion rate was slower than it is right now and it's actually increasing day by day. However, wasn't the same thing...
  22. J

    A The equation of state of non-relativistic energy

    I just have a quick question about a concept I think I haven't fully grasped from my cosmology course. Why does non-relativistic energy have an equation of state with w=0? Also, is the concept of pressure different in general relativity than in thermodynamics or statistical mechanics?
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