What is Cosmic expansion: Definition and 21 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. O

    A What is the maximal comoving distance that a space probe can reach?

    What is the maximal comoving distance that a probe can reach depending on its speed? If the probe travels at light speed, the maximal comoving distance that it can reach is 5 Mpc, which is called the cosmic event horizon. But what if it travels at some other speed?
  2. Filip Larsen

    B Dark energy might not be constant after all

    https://arstechnica.com/science/2024/04/dark-energy-might-not-be-constant-after-all/ https://www.desi.lbl.gov/2024/04/04/first-cosmology-results-from-desi-most-precise-measurement-of-the-expanding-universe/ Interesting preliminary indications from DESI (which I did not know about until now)...
  3. A

    B Cosmic expansion and shock waves

    Do galaxies that have surpassed the speed of light during the cosmic expansion emit radiation waves comparable to the sonic boom when an aeroplane breaches the sound barrier?
  4. S

    I Can gravitational waves gain energy in an expanding FRW spacetime?

    I was reading this paper (*Green's functions for gravitational waves in FRW spacetimes:* [https://arxiv.org/abs/gr-qc/9309025](https://arxiv.org/abs/gr-qc/9309025)) and I had a specific question about one statement in the paper that I would like to ask: At page 6, the author says that...
  5. wolram

    B Active galaxies point to new physics of cosmic expansion

    https://phys.org/news/2019-01-galaxies-physics-cosmic-expansion.html can dark energy vary over the history of the universe?
  6. S

    I Would complexity re-emerge if cosmic expansion reversed?

    Imagine that for some reason the current slow expansion of the universe is going to reverse, and the universe is going to collapse back to the pre-inflation state. [Let's make this an assumption without worrying about the mechnism] So we have a very dilute and cold distribution of energy, since...
  7. R

    B Cosmic Expansion: Benefits of Hubble Law over Stretching Space(-Time)

    Why should you prefer the view that cosmological expansion is due to distant galaxies moving apart according to the Hubble Law instead of cosmic space stretching, like stretching of space(-time) in the vicinity of black holes?
  8. wolram

    B Why Does Cosmic Expansion End?

    Why do we live in such a privileged time? arXiv:1810.10547 [pdf, ps, other] The End of Cosmic Growth Eric V. Linder, David Polarski Comments: 5 pages, 6 figures Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO) The growth of large scale structure is a battle between gravitational...
  9. G

    B Is the cosmic expansion uniform everywhere?

    I seek a little help in understanding why we can see so far into the past to be able to view events where the light from these events should have long ago overtaken wherever the masses ended up. I get it that there was a considerable time during which the limits of the space expansion exceeded...
  10. ohwilleke

    I Is suppressed gravitational pull a possible explanation for dark energy?

    One way to get the universe to expand is with dark energy that pulls at the matter of the galaxy separating it or equivalently for space-time to not be perfectly flat. An alternative, in principle, would be for the gravitational pull between objects like galaxies and galactic clusters to be...
  11. S

    B Matter-antimatter annihilation and inflation

    Is there any link between the energy released from the annihilation of matter-antimatter during baryogenesis and cosmic inflation or expansion/dark energy? This question came up when reading: https://www.nasa.gov/mission_pages/chandra/news/08-160.html
  12. G

    I Is time rate getting faster with cosmic expansion?

    Hi, layman here, but hopefully reasonably educated. I did not know whether this question might fit better under relativity or under cosmology so I'm posting it here. General Relativity tells us that time slows down in gravity wells. The cosmic expansion tells us that the energy density in space...
  13. P

    Cosmic expansion in quantum gravity

    Ive often heard that in quantum gravity space time is discrete. One analogy I've heard is to think of spacetime as being made up on space time atoms. In these models when the universe expands, is the number of space time atoms thought to increase?
  14. B

    Why can we still detect the CMB ?

    According to Wayne Hu here, http://background.uchicago.edu/~whu/intermediate/intermediate.html, the universe started hot and dense and then expanded and cooled. In the hot, dense conditions of the early universe, photons were tightly glued to matter. When the universe was about 300,000 years...
  15. V

    Expanding gravitational systems

    Suppose that everything in the Universe - the radius of the sun, the astronomical units, radius of atoms, and everything else were to expand proportionately during the same period of time. This would mean also that the ratio of the time period of the various planets around the sun, and all local...
  16. L

    Would the Big Rip make time stop?

    Would an accelerating expansion create any time dilation effects? And once the expansion velocity between any two adjacent points reached c would time stop altogether?
  17. P

    Velocity of receding galaxies. Hubble <-> scale factor

    Hi there, This is my first post but I've been a spectator for a long time now. So I've been working on some of the basics of cosmic expansion and there is one contradiction that I came upon that I can't seem to resolve. I've looked around some of the similar threads but I couldn't find anything...
  18. S

    Extracting Energy from Cosmic Expansion

    Sorry if this has been covered before, but I couldn't find anything satisfactory with a quick search. I realize there is another thread on here regarding conservation of energy with respect to expansion, but my question sort of takes a different direction; can you create energy from this effect...
  19. F

    Cosmic Expansion - Spectroscopy Experiment

    Astronomers tend to presume that between any given source and observer, all light travels the same distance at the same speed. But light bends in the presence of gravity. Observe a simple prism and you will note the red wavelength bends less than violet. Forget stars and planets, how many...
  20. B

    Understanding Energy Density and Cosmic Expansion

    I know the concept of the accelerating universe and expansion and all, but what exactly is energy density and how does it cause or explain the cosmic expansion of our universe? Thanks in advance.
  21. H

    Clarification on Cosmic Expansion

    I've read in books and seen on television programs that the universe is expanding, and not only is it expanding it is accelerating in its expansion. I was just reading "parallel worlds" by michio kaku, and I came across the Hubble constant, H. This number is the rate at which the universe is...