What is Space expansion: Definition and 39 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. H

    Solutions for nonrelativistic-matter perturbations

    I'm studying the nonrelativistic-matter perturbations if the expansion of the Universe is driven by a combination of components. I'm currently Following this document (The growth of density perturbations) from Caltech. However, the author doesn't explain how he has found the solutions for the...
  2. 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?
  3. J

    A Experimental test of shrinking matter theory?

    Prof Wetterich has proposed that atoms are shrinking rather than the Universe is expanding. Here is a 2013 Nature News article describing his theory: https://www.nature.com/news/cosmologist-claims-universe-may-not-be-expanding-1.13379 Here is his 2013 paper "A Universe without expansion"...
  4. ramim

    B Is it possible to see the history of space if it expands faster than light?

    I was wondering if it would be possible to see the history of space if one was to stand at the edge of the universe as it expanded faster than the speed of light. If the universe was to be expanding faster than the speed of light right now, and I was able to somehow go there and keep up with it...
  5. TheMercury79

    I Hubble relation to Scale Factor

    Imagine a Universe where the Hubble parameter is truly a constant, in both space and time. How much smaller would such a Universe be 14 billion years ago compared to today? Using the Hubble parameter in terms of scale factor: ##H(t) = \frac{\dot{a}}{a}## leads to the differential equation...
  6. J

    I Zero Active Mass in FRW cosmologies

    What do people think of Fulvio Melia's argument for the necessity of "zero active mass" in FRW cosmologies? (i.e. the overall equation of state must be ##\rho+3p=0## at all times) Here is a link to an interesting lecture video: Here is a recent paper: https://arxiv.org/abs/1807.07587
  7. M

    I Will Space Expansion Lead to the Formation of Black Holes and Neutron Stars?

    1) space is expanding at an increasing rate, therefore things are getting farther from each other and therefore increasing in velocity. 2) the faster an object moves relative to another, the more mass it has 3) supermassive objects can turn into neutron stars, black holes, etc. Therefore, will...
  8. l0st

    I Gravitational Waves Cause Uniform Space Expansion: Exploring MNRAS Papers

    I am looking at a couple of very interesting papers, published in MNRAS, that deduce, that the accelerated expansion of the Universe we observe can be attributed to gravitational waves, produced by a very distant merger of two or more universe-mass-scale black holes. The last one is on the...
  9. J

    I Does the Uncertainty Principle imply a linear cosmology?

    If the uncertainty in the age of the Universe is ##\Delta t## then the Uncertainty Principle implies that it has an uncertainty in its energy ##\Delta E## given by $$\Delta E \ \Delta t \sim h.\tag{1}$$ If this energy fluctuation excites the zero-point electromagnetic field of the vacuum then a...
  10. Stephanus

    B The speed of light and Hubble Flow

    Dear PF Forum, I've been wondering about how on Earth (and I do mean it, from earth :smile:) that we know there's a galaxy 20 billions light away. Considering that https://en.wikipedia.org/wiki/Age_of_the_universe is 13.8 billions years old. But before I'm asking about Supernova Ia and Hubble...
  11. U

    Gravity wave cancellation cause space expansion?

    Is it possible that the increased acceleration of the universe could be caused by the cancellation of gravity waves? If not, what would be the impact on space-time when gravity waves do cancel out?
  12. L

    Conservation of energy and space expansion

    This had me thinking for a while. Imagine a photon emitted by a very distant object at a redshift of z = 2.0 for example. As the photon travels through space, due to space expansion the photon's wavelength will shift towards red. With an increase in the wavelength there must come a decrease in...
  13. N

    Measuring space expansion without redshift

    Hi, Apart from the redshift of electromagnetic waves (which we do observe), is there any other evidence of space expansion that is not based on redshift? If not, is there a theoretical experiment that could let us observe and measure space expansion? Here I am assuming that space is not just...
  14. ErikZorkin

    I Space expansion and Universe as computation

    Good day. I do not know much about cosmology, rather computer science, but the following theoretical question bothers me a little. Some scientists, like Tegmark, Wolfram, Zuse or Fredkin, support the idea that the Universe might be just computation. Computable means that something can be...
  15. Glurth

    Describe the mechanism in GR causing *Space* expansion

    From what I have read, it sounds like GR predicts that under certain conditions, space itself will expand. If this is incorrect, please just let me know and ignore the below. While I don’t know the math involved in GR, I can understand explanations of how gravity distorts space-time, to alter...
  16. G

    Does space expansion affect the rate of time?

    Time can have 2 different connotations. One is inherent to an entity in spacetime by itself, say a lonely radioactive atom. It may decay or it may not decay, independently of what's going on around it. But another involves interactions between particles, for example "it takes 60 seconds for...
  17. G

    Isn't space expansion logically required?

    If spacetime is a single entity, shouldn't it be simply logical that space expands? We know that spacetime is one single entity, there is no space AND time, but just spacetime. As we move towards the future, more time gets 'created', the more we get into the future away from the Big Bang, the...
  18. Stephanus

    What is the cause of space expansion in the universe?

    Dear PF Forum, Thanks readers for giving me so many explanations about space expansion in my previous thread. However, I've forgot to ask 2 things that I'd like to know the answer. Space is expanding. According to Hubble Law? What cause space expands? 1. Dark Energy? 2. Does space its own have...
  19. Grinkle

    Plank length / space expansion question

    Do models of the expansion of space-time manifest that expansion by an increase in Plank-length, or by additional Plank-lengths appearing in between existing particles? I think it must be the latter because the Plank-length is not an empirically measured value and the constants it derives from...
  20. 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...
  21. A

    Why space expansion is limited to scales larger than galaxy clusters?

    Hello everyone, I have a question about metric expansion of space. According to Wikipedia (ok probably is not the best source, but I have only a qualitative understanding of physics) the expansion occurs only at scales larger than galaxy clusters...
  22. E

    Space expansion, gravity waves, warp

    If gravity waves are supposed to propagate at c, how could the theoretical warp drive wave propagate at greater than c values? I have heard and red and understand that space itself can expand (and warp? ) faster than c and, in fact, that will happen in the future when space in between...

    Can Space Expand Faster Than the Speed of Light?

    As I understand, every point in the the universe may be considered for that observer to be the center. Everything seems to be expanding away from that point. From our perspective on earth, the further away we observe, the faster that space is expanding, up to, and faster than the speed of light...
  24. M

    Exploring the Nature of Space Expansion: Self-Expansion vs Dark Energy

    A few years ago I came to this forum to ask about the issue of space expansion...and the answer I got then was that the expansion is the nature of the space (or space-time) according to relativity or some other more advanced theory...no one used dark energy to explain the issue at that...
  25. W

    Is Space Expansion 100% proven?

    In a counterpart of General Relatlvity called Field Theory of Gravitation where spacetime is really flat and spin-2 fields cause gravity. See: http://arxiv.org/pdf/gr-qc/9912003v1.pdf Space is already there as the following description state: "Cosmology is another field of application...
  26. S

    A thought experiment regarding the nature of space expansion

    I devised this using a setup that I found easiest to imagine, while trying to be clear on how modern physics considers space to expand. We have a nifty device that can cause spatial distortion (no, you cannot has one!). Before we use it, we lay down two meter rulers, end to end. They both...
  27. M

    Space expansion vs. time speeding up

    Hi! I've been reading these forums off and on for a year or so, and finally registered! There have been other threads here on space expansion vs. time speeding up, but I have some questions that weren't addressed. Background: I was told by a notable astrophysicist, Neil DeGrasse Tyson...
  28. C

    Redshift from space expansion, & conservation of energy?

    Greetings, When a photon travels through expanding space, it's frequency decreases and it has less energy. The energy of photons of the cosmic background radiation have decreased dramatically. Where does the energy go? Is it lost? Thanks
  29. C

    All forces neuralized at big bang, & space expansion

    What caused and is causing space to increase or expand? Greetings, The answer to a major question has raised another problem...then solution...then questions. Why didn't gravity keep everything together at the big bang like a black hole? Because *all* of space was filled with...
  30. G

    Space expansion on smaller scales

    Space is expanding. This is evident from the redshift of distant objects. My question is what effect does it have on smaller scales? Since the expansion of space carries objects along with it (necessary for the objects to be farther) wouldn't the expansion within galaxies slowly pull them...
  31. P

    Space expansion in our perspective?

    So if space is expanding everywhere, does this mean the space in my room is expanding? I'm assuming that its expanding so slow because of the small scale, but is there still a measurable amount of space expansion occurring in my room in any time interval? If i were to wait a few billion...
  32. H

    Space Expansion and Light question

    As a steady stream of light particles (or any other form of measurable energy) from some distant star, say a billion light years away, makes its way toward us, shouldn’t space expansion cause consistent and measurable gaps to form between each light particle, causing a blinking effect? The...
  33. G

    Q: Space expansion and black holes

    It is my understanding that with the passing of time, space actually expands in size. In about 14G years, space will be twice its current size. What will this do to black holes (BH)? If a BH is in intergalactic space and has nothing to consume, if space actually expands, the density of the BH...
  34. A

    Confused about space expansion? Me too.

    Confused about space expansion?? Me too. Space expansion is occurring from what we can observe. In another thread about gravity i read that gravity is space time essentially, as if you introduced an object into Earth's atmosphere it would instantly be subjected to Earth's gravity as it...
  35. A

    Does space expansion theory obey relativity laws?

    Does space expansion theory obey relativity laws?? I've been wondering about the idea of space expansion that's been used to explain red shift effects in most of our universe, which forgive me if i am wrong is nothing(space) propelling matter. Please could someone tell me if i am...
  36. T

    How fast does the universe expand and what are the theories behind it?

    Just a fast question, Ill be glad if you answer it. I was wondering how fast does the space expand, but I wasn't able to find answer anywhere around the internet, the only thing I was able to get is the rate of expansion - 72 km/sec/Megaparsec, but I don't know if I can derive the speed of...
  37. B

    Exploring Space Expansion: Questions & Answers

    I don't understand what you mean with: universe is expanding. Is space that -as an elastic membrane- is expanding or galaxies have a proper velocity? And however, shouldn't all become bigger during the expansion? for example, in a sphere-universe of diameter 100 metres there is a...
  38. G

    Isn't space expansion expectable?

    [SOLVED] Isn't space expansion expectable? Since Einstein showed that space and time are in fact entwined as a single entity spacetime, it makes me wonder if it's not just natural that we see space expanding with time. As we examine spacetime forward in time (looking at regions where "the...
  39. Y

    Space Expansion from Dark Matter

    General Relativity GR, and Loop Quantum Gravity LQG as well, says that space is derived from the gravitational field of matter. But the total matter in the universe is constant; whereas the space of and the amount of Dark Energy in the universe is growing. It is supposed that most of this...