What is Expansion of space: Definition and 71 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. E

    I Spacetime expansion - time dimension expansion

    I am trying to find some sources about time dimension expansion during spacetime expansion. Why do We assume that only spacial dimensions expand? I only found a lot of weird arguments. Btw, temporal dimension expansion does not mean that second was shorter in the past. Similarly to spacial...
  3. Dogya

    Hubble Constant: Force Acting on Hydrogen Atom Due to Expansion of Universe

    TL;DR Summary: Express through the Hubble constant the force that acts on the hydrogen atom due to the expansion of the universe We have a hydrogen atom, in a gravitationally bound system nothing interesting happens to it. Let's put it in an empty world with only an electron and a proton...
  4. 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...
  5. S

    I Conservation of energy in quasar outflows?

    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...
  6. S

    I Particle production in an expanding universe?

    I was reading this interesting article [1] which talks about particle production in an expanding universe. Usually this process is proposed to have occurred in the early universe, when the expansion was in the inflationary phase and it was so powerful that matter was created in particle...
  7. T

    B Expansion of Space Itself, rather than due to an Explosion like the Big Bang?

    Please explain how we know that it is space itself that is expanding rather than an expansion due to an explosion such as the big bang. In either case wouldn’t every object appear to be moving away from us? How do we determine that space/ time is expanding? Tex
  8. M

    I Expansion of Space: Questions on How the Universe Works

    I first apologize if this has been posted more than once; sorry. There is a lack of clarity at my end as to whether I am logged on here? Say we have two galaxies, the Milky Way (MW) and galaxy A separated along a straight line 20 x the radius of the Universe visible to us. If I understand...
  9. M

    I Expansion of Space: Exploring the Rate Between Galaxies

    Hi, may I ask a question, please? Say we have two galaxies, the Milky Way (MW) and galaxy A separated along a straight line 20 x the radius of the Universe visible to us. If I understand correctly then the rate of the physical expansion of space between MW and A could be at the rate of say...
  10. S

    B Expansion of space affecting photon's energy

    On a long trip the photon goes, but it occupies a wavelength of space at any particular time. If the space between start and finish (inspection) is expanding all the way all the journey time, then most of the expansion has no effect on the photon. Like eg second tenth is section currently passed...
  11. K

    I Questions about the expansion of space on galactic scales

    How would the expansion on a scale of 10 kpc be measured, by a red or blue shift? How can expansion of space be differentiated from the velocity of stars? It seems that the expansion of space weakens the effects of gravity?
  12. A

    I Expansion of space & expansion of matter within space

    Space evidently is expanding, so we say that the very fabric of space time is expanding uniformly in all directions, so two questions 1)originally this expansion was driven by the energy content within the universe aka the dense hot matter , versus nowdays the acceleration is picking up speed...
  13. H

    I Can an object fall into a black hole at faster that the speed of light?

    I am wondering if an object can fall into a black hole at faster that the speed of light. I have heard that the expansion of the universe can make distant galaxies appear to recede from one another at velocities faster than the speed of light.

 Intuitively, this makes sense to me. I am...
  14. Euthan

    I Energy stored in space-time or space?

    I was talking to a graduate physics student about the issue of energy conservation in an expanding universe. I paraphrased the argument against energy conservation as follows - Suppose we have a photon in outer space that is very far from earth. The universe is expanding (by this I meant that in...
  15. benorin

    B The geometry of the expansion of space

    I've been trying to wrap my head around the geometry of the expansion of space, from Science Channel shows I vaguely understand the "every point in space is moving away from every other point in space" and iirc this was uniformly so. Is that correct? If not ignore the rest of this post I suppose...
  16. K

    I Observational Evidence for Metric Expansion of Space?

    Imagine that the CMB did not exist. What observational evidence exists to support the theory of the metric expansion of spacetime, as opposed to having a static spacetime and it's the matter distribution that is expanding - as it would in an explosion?
  17. Mayed Al-Tunaiji

    I Time & Space Expansion: High Schooler Qs

    Before I get to a question that's been on my mind for a while, I would like to make it clear that I am a high school student with no understanding of physics beyond my school's curriculum and some books that I've read, so excuse me if this sounds rather absurd. I know that the expansion of...
  18. R

    B Understanding the Expansion of Space: Galaxies Moving Away and Proving Expansion

    How does knowing that galaxies are all moving away from each other and the farthest ones are moving more quickly prove that space is itself expanding and not just that the galaxies are moving through it?
  19. Addez123

    B Does red-shifted light cause expansion of space?

    Light can get red-shifted due to expansion of space, this leads to a loss of energy. Would it be possible that this energy is what's causing the universe to expand in the first place?
  20. V

    B Exploring the Expansion of Space & Time

    We are told that space is expanding and that the rate of expansion is speeding up. However, as relativity shows that space and time are inextricably linked, does this mean that time is expanding too?
  21. K

    I Dark energy and expansion of space

    universe is expanding since the bigbang so therefore space is expanding, but apparently dark energy remains constant, it apparently isn't getting diluted as the universe expands is energy being created, specifically the energy in vacuum?
  22. 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...
  23. Buckethead

    B Expansion of space and momentums of galaxies

    It is my understanding from reading a few threads that remote clusters don't have any "speed" relative to us, they are simply moving away. Does this mean they have no momentum? And if that is true, does that imply that they have no mass (in this context, not in a local context). And if that is...
  24. Yash1993

    Speed of light/electromagnetic waves is constant?

    Since Edwin Hubble proposed distant galaxies move apart from each other as spectra of each galaxy is shifted towards the red end. this created the law that universe is expanding exponentially since big bang. and these galaxies spectra are due to stretching of wavelength of light incident from...
  25. A

    I Expanding Universe: Big Bang & Beyond

    Hi all, After the 'big bang' it is said that space is expanding. Atom is almost 99% space then why are they not expanding, same can go with all the stars and galaxies and all stuffs. Hope this is not sounding crazy. If so pls forgive me. Cheers.
  26. A

    Does mass slow the expansion of space?

    I read that the mass of the universe slows the expansion of the universe. I also heard that if the mass of the universe was above a critical value the universe would contract. I believe this is called the big crunch scenario. And that it is currently not considered a probable scenario. Is this...
  27. J

    Relationship Between Spatial Expansion and Gravity's Force?

    Fellow Nerds, I'm looking for a quantitative relationship between the gravitational strength of a point on a field and the speed of expansion of space at that point. Given a cosmological constant and a metric, is it possible to pinpoint a certain point of space and ask how quickly that space is...
  28. yazanhomsi

    The Inflationary and The Big Bang Theory

    Hello everyone, My question is: The big bang model proposes that the rate at which the universe is expanding is now decelerating. The inflationary theory proposes that the speed of expansion and separation started out slow and now it is accelerating. But as we know, recently it has been found...
  29. D

    Does Space Expansion Add Potential Energy?

    Space is expanding, which separates objects from each other in space. Won't this add potential energy? Doesn't this violate the law of conservation of energy? But if it does not, how can space be added without adding potential energy?
  30. Strilanc

    Does the Hubble expansion of space have a preferred frame?

    I have a paradox that I don't know the answer to. Special relativity has no preferred frame, but it seems like adding an exponential expansion to space introduces such a frame. The Setup Suppose we're in a universe with a much faster expansion rate, where the space between any two objects...
  31. V

    A layman's guide to the accelerating expansion of space

    I am writing this thread because I have too often had to explain this and I basically wanted to have a resource available to direct folks to, and, frankly, the Wikipedia topics on this are pretty vague on the explanation. Any questions, comments, or corrections are greatly appreciated. So, here...
  32. CassiopeiaA

    Does Inflation Refer to the Expansion of Space or the Entire Universe?

    A very fundamental doubt. Was inflation expansion of just space or the universe with whatever the state of matter it had?
  33. virgil1612

    Expansion of space in our neighborhood

    I know that expansion of space operates farther away from our Local Group of galaxies. But the reason for not considering expansion in our galactic neighborhood is: A. The space expands everywhere, the expansion is just too small close to us so we don't need to talk about it. or B. The space...
  34. AlexDB9

    FTL Expansion of Space: Rapidity & Infinity

    If the expansion of space in the macrolevel is faster than the speed of light then it should have "more" than infinite rapidity. How does that work out? Thanks.
  35. Ghostcrown

    Could the expansion of space tear us apart?

    Is it possible that the forces behind the expansion of space could at some point overcome the fundamental forces holding our molecules together?
  36. Z

    Understanding Expansion of Space

    So what I don't get about the expansion of space, and what I'm assuming one of you can explain to me, is that it seems like if space were expanding, how would we have any way of noticing it? It seems like as the distances between particles expands, so must the size of the particles themselves...
  37. C

    Expansion of Space vs Velocity in Space

    As far as I know, the main argument for the statement that the universe is expanding is that the velocities of galaxies increase proportional to the distance. This together with the cosmological principle indicates that every point in the universe observes the same thing, something that is...
  38. I

    Expansion of space vs stuff just moving away

    NOTE: I am not a cosmologist, so if any of my statements are not correct please tell me. When we observe distance galaxies we can measure how fast they move away using the red-shifting of their light. So how do we know space itself is expanding vs the galaxies are just moving away relative to...
  39. A

    Could dark energy and expansion of space be just a topological effect?

    Sigh. My first post. I wrote rather long message here and as I tried posting it, "you need to login" - and it vanished. :( Anyways. I have no high physics/math education but still I consider myself enthusiasts. So be gentle! :) Consider the following "my way of filling sudokus", just thinking...
  40. M

    Expansion of space: Earth - Moon

    Hi, I made a (rough) calculation of how far the Moon would be moving away from Earth according to Hubble's law = 72.6 (km/s)/Mpc -- 1 megaParsec = 3.08 × 1022 m ≈ 4 × 1022 m Distance between Moon and Earth ≈ 4 x 108 m Expansion rate ≈ 7.2 104 (m/s)/4 × 1022 m = 7.2 (m/s)/4 × 1018...
  41. W

    Question about the expansion of space.

    If space is expanding and I put a ruler inside this expanding space, the ruler should expand with the space. Therefore, inside this expanding space two particles would always measure, to an observer with the ruler inside the same space, the same distance away, even as that space expands since...
  42. K

    Expansion of space - Red shift

    I have always been puzzled by how we see an object from the past and can tell what it is doing now if it is billions of light years away. Given that; 1. the shift in the measurement of the light spectrum indicates the speed at which an object is retreating or getting closer. 2. that...
  43. W

    Does the expansion of space only occur in unbound systems?

    I'm coming with a good background in Big Bang expansion as the following sci-am article shows (which I've mastered): http://space.mit.edu/~kcooksey/teaching/AY5/MisconceptionsabouttheBigBang_ScientificAmerican.pdf What I'd like to understand is this. Expansion can only be felt in unbound...
  44. S

    Universe Expansion: Why Isn't Space Between Atoms Expanding?

    I realize that because of gravity and the relatively high density of our place in the universe compared to the voids, cosmologists generally agree expansion isn't really significant, but I don't understand why this is so. If the universe is expanding, then why isn't the space between every atom...
  45. 2

    Question about the expansion of space.

    Hi. So if the space in the Universe is expanding, does that mean that the space existing between me and my monitor, or the space between object A and object B is also expanding in some sense?
  46. K

    Understanding Accelerating Expansion: Exploring Space Interjection and Density

    I'd like to understand accelerating expansion better. It seems that space is interjected proportionately where there is low density (intercalated is the term in biology). Can this interjection of space be sent back where it came from when that area of space becomes more densely populated?
  47. C

    Max. rate of expansion of space?

    Since I see the Big Bang was the beginning of space; if space is infinite, does that mean that space can expand at an infinite rate? (Thanks in advance from this layman; I've started Brian Greene's "Hidden Reality" and despite laymen being it's target audience, I'm stuck on this question.)
  48. S

    Expansion of space and dark matter

    question 1. i need some help understanding this. it is proven that the universe is infinitely expanding correct? what is the exact reason for that? question 2. i have a couple questions on dark matter. does dark matter give off its own gravity bending space-time, I am assuming ?
  49. E

    Expansion of space (conceptual question)

    From what I understand the beginning of the universe was period of rapid expansion. I've been told that this expansion is not to be thought of as the universe expanding into an empty space and filling things up but space expanding instead? Is there something off with this and if not how...