Q: How can the universe expand faster than the speed of light?

Physicist: You’ll often hear that “the universe is expanding faster than the speed of light”.  However, this statement is akin to statements like “green is bigger than happy”.  It’s not even wrong.

There are two big things to remember about the expansion of the universe.  First, the universe doesn’t expand at a particular speed, it expands at a speed per distance.  Right now it’s about 70 kilometers per second per megaparsec.  That means that galaxies that are about 1 megaparsec (1 parsec = 3 lightyears and change) away are presently getting farther away at the rate of 70 km every second, on average.  Galaxies that are 2 megaparsecs away are presently getting father away at the rate of 140 km every second, on average.

Notice the awkward phrasing there: distant galaxies are “getting farther away”, but oddly enough they are not “moving away”.

Initially, the distance between Red and Yellow is 1, and the distance between Red and Green is 2.  After double the size of the "universe" the distances are 2 and 4, respectively.  Yellow receded by 1, but Green receded by 2.  Green would seem to be moving faster than Yellow, but in fact all of the dots are sitting still while the space they inhabit expands.

Initially, the distance between Red and Yellow is 1, and the distance between Red and Green is 2. After doubling the size of the “universe” the distances are 2 and 4, respectively. Yellow receded by 1, but Green receded by 2. Green would seem to be “moving” faster than Yellow, but in fact all of the dots are sitting still while the space they inhabit expands.

The easiest way to think about the expansion of the universe is to think about the expansion of something simpler, like a balloon.  If for some reason you have a balloon covered in ants, and you inflate it slowly, then the ants that are nose-to-nose (pardon, “antennae-to-antennae”) with each other will barely notice the expansion.  However, the farther two ants are apart, the more the expansion increases the distance between them.  If an ant on one side tries to run to one of her sisters on the far side of the balloon, she may find that the distance between the two of them is increasing faster than she can close that distance.

The distance at which this happens (where the rate at which the distance decreases because of the movement of the ant and the rate at which the distance increases due to the expansion of the balloon) is a kind of “ant horizon”.  Any pair of ants that are already farther apart than this distance can never meet, and any pair closer than this distance may (if they want).  In the picture above, if an ant can run a distance of 2 during the expansion time, then an ant starting at the yellow point could reach the red point, but an ant starting at the green point will always find itself maintaining the same distance from the red point.

The “ant horizon” is a decent enough analog for the edge of the visible universe.  The speed at which the ant runs is described with respect to the part of the balloon it’s presently standing on and the speed at which light travels is with respect to the space it travels through (technically with respect to objects that are “sitting still” in space).  The oldest photons we see are those that have come from just barely on the near side of the distance at which light can’t close the gap.  It’s not that things beyond that distance are moving away faster than light (almost all the galaxies and gas and whatnot are moving slowly with respect to “the balloon”), it’s that the light they emit just isn’t moving fast enough to overcome the expansion.  Light beyond that is still moving at light speed, and it may even be trying to move toward us, but the distance is simply expanding too fast.

Here the analogy breaks down and starts making our intuition incorrect.  When you inflate a balloon the sides are obviously moving apart.  You can use a rule (maybe a tape measure) and a stopwatch and you can say “dudes and dudettes of the physics world, the speed of expansion is ____”.  Even worse, when a balloon expands it expands into the space around it, which begs the question “what is the universe expanding into?“.  But keep in mind, all that physics really talks about is the relationship between things inside of the universe (on the surface of the balloon).  If you draw a picture on the surface of a balloon, then if the balloon is dented somewhere or even turned inside-out, the picture remains the same (all the distances, angles, densities, etc. remain the same).

Point of fact: it may be that the balloon is a completely false metaphor for the universe as a whole, since the best modern measurements indicate that the universe is flat.  That is, rather than being a closed sphere (hypersphere) it just goes forever in every direction.  This means that there is genuinely no way to describe the expansion of the universe in terms of a speed (there’s no “far side of the balloon” to reference).

It’s a little subtle (“subtle” = “math heavy”), but there are a number of phenomena that allow astronomers to clearly tell the difference between things “moving away” and things “expanding away” from us.  For example, beyond a certain distance galaxies no longer get smaller (the way things that are moving away should), instead they get redder and stay about the same size independent of distance, due to a lensing effect of the expansion.  Isn’t that weird?

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78 Responses to Q: How can the universe expand faster than the speed of light?

  1. George says:

    In all the lab processes that I know, there is nothing that can speed up time or extend space. By relativity, only gravity can compress space and slow down time. Now you seem to say there is this great process that can expand space and speed up time in the process too which by the way sounds like antigravity to me. If we can see some of that in the lab, that would be great. Also I find it is hard to say and understand that the early universe is smaller. How is that exactly defined? Using current space and time as reference to gauge past space and time is confusing the matter further…

  2. John Campbell says:

    I have serious issues with the concept of universal expansion. At least, in the context of the Big Bang, universal symmetrical expansion.

    I see no reason whatsoever to believe in a Big Bang event having triggered universal inflation/expansion. What I see is expansion due to the release of energy from destroyed matter (in stars and galaxies). Expansion is relative to the amount of mass in the area currently undergoing nuclear interaction. in other words… the greater the concentration of stars and galaxies, the greater (faster) the observable expansion. Conversely, where stars and galaxies are less prevalent, expansion in less (slower).

    Expansion therefore, whilst universal, is neither consistent nor uniform, nor indeed does the universe increase in size overall. Instead what we are observing is “local” cosmological expansion whilst in other areas the cosmos is compressing.

    My best analogy would be a box (the cosmos) filled with millions of balloons representing galaxies and galaxial groups.

    Imagine the effects if these balloons were each filled with a different gas (representing differing concentrations/mixtures of matter). If we heat one corner of the box, the heat would penetrate into the balloons and the heat energy would make some balloons expand more than others, as the heat travels through the balloons and their contents, some balloons will shrink or compress as those around them expand and then contract again as the heat passes through the local group and into another group of balloons with different reactions to the heat on the balloons’ contents. These reactions represent the collective energy output of each galaxial group as the reactions within their stars pour energy, released in the breakdown of matter within them, out into space… this the fundamental driving engine behind cosmological expansion.

    In our cosmological box, the balloons would become dynamic, moving, expanding, contracting, deforming, contorting, as the pressure differences move through the cosmos. This results in energetic currents rippling through the cosmos, further effecting cosmological dynamism.

    This process could operate at a local level (5-1000 galaxies) and at a universal level (1000-1,000,000,000 galaxies) respectively (galaxial group balloons within larger universal balloons), all dependent upon differential energetic (mass) outputs.

    This, to my mind, explains what we observe as local and universal expansion.

  3. George says:

    I, for one, will certainly agree more with your description here. It sounds more physical and realistic and it is more in line with GR, in my opinion. Expansion should be more at the outside area or the edge of the universe or something like that. Don’t know what our current cosmologists are thinking…

  4. John Campbell says:

    Cosmologists, like most scientists, are restricted by what they can see, George.

    We live within and can see only an area within ONE of those cosmological balloons, so all we see is a generally uniformally-expanding observable universe (although I am aware that there was observed in recent times a part of the observable universe that was not behaving in-line with uniform expansion, lending weight to my opinion that expansion is not cosmological or uniform).

    Cosmologists therefore can only reasonably opine on what they are currently and historically observing. Only ourselves and theoretical physicists have the luxury of conjecture, as we are not restricted by the disciplines of science (observe and repeatably prove). My point of view is a theory; A seemingly sensible, rational theory, but a theory nonetheless!

    Add to this scientific restriction the political pressure of the religious…. there is a NEED for the ‘Big Bang’ in religious theism, because a creator “must” have created in a singular event… and evidence for that event is (apparently) all around us in the observable universal expansion, which “must” have started at a singular point and dominates all!

    But universal expansion is not necessarily cosmological expansion. And nor does it necessarily rewind to a single ‘Big Bang’ event. As pointed out previously, it could just be a natural consequence of the tendency of matter to destroy itself in the furnaces of stars and release their energetic contents into space, resulting in expansion which (conceivably) could be measured against the concentration of stars in different areas. My bet being that areas with high populations of stars and galaxies expand faster than those with less, and that areas virtually devoid of stars and galaxies are shrinking (compressing).

    As a subnote to this compression of these seemingly empty areas of cosmological space, as pressure increases within them, due to the activity surrounding them, matter, in the form of exotic particles (electrons, muons, mesons, bosons, etc) become atoms, which will then propagate into gases. Clouds of gases will compress, under their own gravity until the pressure within becomes so great that a star is born… and the process starts all over again!

  5. Nic Harvard says:

    Interesting, as i was discussing exactly this last night with a non-mathematical, non-cosmologist, non-physics person, who nonetheless is highly intelligent.

    Balloons aside, i pointed out two things which perplex me, and which my research has not yet come up with a good answer to:

    1) Should the current theory regarding a big bang be correct, the energy constants and temperatures do not add up…. so, then either:
    a) My back-of napkin maths is wrong. That’s entirely possible – i’m good at math, but it is not my field.
    b) The universe seems to have expanded in early stages at rates greater than C (in a vacuum)
    c) I’m ignoring the fact that C is variable, as it is the speed of light *IN A MEDIUM*

    A corollary to this: Shortly after, while watching an interesting documentary on lensing effects, the history of optics, and prisms and spectrum’s, i pointed out that the bending of different frequencies of light (photon energy levels basically) can only be explained by the fact that different energy levels for a photon result in different values of C for that particle/wave/probability effect as it passes from one medium to another.

    The argument is long and complex, and i have no space to write it in the margins of this post, so i shall not go into it, but nothing else works as elegantly.

    Third thought: If this is so, then how, should a stream of coherent light pass through a non-bending (perfectly parallel, say) intervening medium, does it stay coherent?

    So, my question is this:

    Does anyone have good pointers to solid research on varying values of C in different mediums, or how the value of C might differ for certain values of energy in the EM spectrum?

    I need to fill certain gaps in my knowledge here….

    With anticipatory thanks, Nic

  6. “Faster than the speed of light” is observed only at the greatest distances. But if we transfer ourselves instantly at that “place” and look back, it will be our galaxy that will be “faster than the speed of light”.

    Wich means that, that “speed” is an illusion created by the distance. On the other hand going “instantly” at that place would be going back in time. Which indicates that time and distance are two faces of the same volume we call “space-time”.

  7. Observer says:

    I don’t understand the balloon example that you gave. Let’s consider the red and green dots (call them ants) in your figure. They are at a distance of (pi/2)*r. Let’s say the balloon expands at a rate of ‘v’ along the radius. So the rate at which distance between the red and green dots increases will be (pi/2)*(r + vt) – (pi/2)*r = (pi/2)*vt. This is independent of r. Only if the ants travel slower than ‘v’ they will not be able to meet.

    Therefore only if the universe is expanding faster than c, will light not be able to travel from point A to B. It is independent of how far A and B are from a reference point.

    I know it is a very simplistic calculation, but at least for the balloon analogy to work, this calculation should work out.

  8. Alan C. Donoho says:

    I have a question on the speed of light travel. If in the future man succeeds to be able to travel at the speed of light, going that fast wouldn’t you probably hit some space debris out there ending the journey in tragedy. Because at that speed you wouldn’t have time to avoid any collision. If that is the case, wouldn’t you say traveling at the speed of light is not practical even if it becomes possible. Thank You. Alan

  9. Marvin says:

    I do agree with you, in this calculus it really doesnt depend on the distance to a reference point.
    It is even worse! As to my calculations, each ant will reach another ant at some point, independently of its speed.
    Lets say we have 2 ants: Ant A and B. Lets also say they are separated by 1 radian. Their separation by 1 rad doesnt change as long as it doesnt move (the fact that its 1 rad doesnt change, the actual distance does change as the value of 1 rad changes with a change of radius!!!)
    So, indeed, their increase of separation distance would be:
    d(AB) = (R_0 +vdt) – R_0 = vdt

    So one would be inclined to say: If the ant moves with u < v, it will never reach B. However, there is a thing: If A moves towards B, even with u 0(R + vdt)-0(R) = 0.
    It reaches its target.

    So, at some point the ant A WILL reach ant B. It may take a while, but it will.

    So concluding: This example doesnt seem to work so well for a constant increase in radius (R = R_0 +vt). However, I think it works just fine if -for example- the Radius doubles its size in each dt, meaning an accelerated expansion. This should solve the radius-independence and reaching problem)

  10. Ruvian says:

    Mr. Physicist
    Didn’t get when you said the universe is not spherical. What is it, then?
    You said flat, so it should mean 2 dimensions, which means a circle. Is universe circular?
    I can’t imagine this, because we live in a 3 dimensional space…
    Only if the big-bang made things expand in a circular way. Is that it?

    Another question: How is the radius of the expansion behaving? Is it increasing in a constant way or it it speeding up?
    You know I need this to understand if the radius of the current universe is a function of the time or not.

  11. Jay Vaghela says:

    Yes Mr. Ruvian cent percent Right….

    Univerce is expanding in 3 Dimensions.

    Its not circular or Elliptical or any of the shape as well it does not follow the radius rules [radius of univerce since bigbang] because univerce have infinite space to be expanded into…..

    One more example consider the big bang as fire ball we lightn up and this fire ball anyhow anyway explodes so the fire flames have infinite space tobe expand into…

    If univerce in which it is expanded is infinite than univerce speed never be decreased….

    Plzz email me on [email protected] for this dicussion….

  12. Joey says:

    What I don’t see any information on anywhere is what ‘size’ of space is expanding at 70km/sec per megaparsec? A cubic cm? Some quantum measurement? There’s no frame of reference.

  13. The Physicist The Physicist says:

    All of space is expanding at that same rate. If you’re looking at two points that are one megaparsec apart, then the distance between them will be increasing at 70 km/s. If they’re only sitting half that distance apart (500 kiloparsecs), then the distance between them will be increasing half as fast (35 km/s).

  14. Joey says:

    Okay, that makes sense. That leads me to my follow up question (which I forgot to ask): Why don’t we experience the expansion of space on the local level? That is, if the space between atoms or even planets were expanding, then we’d observe the effects. But we only notice the effect it seems over interstellar distances. Are the nuclear forces and gravity overcoming the expansion of space regionally? If so, how far into space and away from mass does one need to be for space itself to expand? Thanks.

  15. The Physicist The Physicist says:

    If you’re interested, there’s an old post that talks about that specifically.

  16. Antonio Carlos motta says:

    For the relativity theory the expansion of the universe if gives geometrically,not are the energies that Iran speeds greater than the speed of light.because nothing by the General theory of relativity could travel faster than the speed of light. I believe that the idea of that if the spacetime was discrete in some occasion as in the Inflation,some particles could run with speed greater than the speed of light,so the photons would have variable speeds.then a speed of light would be constant light speed into of spectrum of energy major,and others photons with major energy,in the blue frequency would have speed of light greater than the speed of light known.all is in the vacuum

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  18. Sal DeMarco says:

    To a laymans way of thought,the universe must have traveled much,much faster than the speed of light at the big bang, if the farther away we can see,the younger things look! Otherwise things would all look the same age if this weren’t true. Does this make any sense??

  19. Richard Edward Peters, Jr says:

    The initial pressure wave, created during the “big bang” event is the cause & effect that allows for the faster than light speed expansion of our universe ! The initial explosive event was so powerful that it created such a positive pressure wave that exited the event center well beyond the normal speed of light ! This initial pressure wave exited the event center before the mass ejection of matter itself !
    This immense pressure wave, traveling well beyond the speed of light allows for the “unimpeded” faster than light speed, expansive travel of our universe !!!

  20. Andre Lefebvre says:

    I doubt that any “pressure wave” was created at the Big bang. Simply because there wasn’t anything existing at the time to make a wave with (no space, no time). The Big bang was the “apparition” of space-time; before that moment there wasn’t any “volume” of whatsoever.

  21. Richard Edward Peters, Jr says:

    Do you really believe that “nothing” existed before the “Big Bang” ? Remember that “energy” can not be created, nor destroyed ! I believe in an “oscillating” universe ! It makes sense “logically” & it makes sense through “sequentially logical processes” ! I have been utilizing both analytical tools since a child !!!

  22. Andre Lefebvre says:

    Tge Big bang is the “explosion” of the universe wich is a volume. This volume cannot appear before Plancks time and cannot have less than Plancks lenght in diameter; so the Big bang happenned at 10^-43 sec after time zero. Énergy was created during Plancks era between time zero and 10^-43 sec.

    The Big bang was not at the “beginning”.

  23. Konstantinos Kourmousis says:

    I am not a physicist nor a mathematician. I have a question:

    If the universe it is expanding, therefore all the objects in it are moving, constantly, away from the center of the universe in which the Big Bang is supposed to have taken place.

    If that is correct, then according to Einsteins Theory of Relativity, isn’t the entire’s universe time slowed down because of the speed in which it moves? I mean, if our planets moves fast enough away from the center of the universe, isn’t our planet’s time affected and slowed because of our movement?

    If that is true, then the 14 billion years we have estimated that the universe exists for are possibly greatly fewer to our perception. We may even be among the youngest beings in the universe end we don’t even know it. Is that possible?

    Please forgive my ignorance, and my bad English.

  24. Gary Youree says:

    These discussions are all in my area of interest – as a layman, however until we come to a limited consensus on block time, determinism, and/or block time infinity it is difficult or impossible to understand or conjecture on the physics of the universe(s), expansion beyond the speed of light without there being infinite mass creating what we perceive as the beginning – just prior to the big bang. “prior” being used loosely – from our perspective.

  25. carmen says:

    i’m not a physicist, nor a scientist of any kind! but i was curious as to why propulsion through supernova isn’t being accounted for… instead of a balloon, in which there are 3 ants, why not think of space like that of a graveled beach complete with little sandy patches? all the little pebbles represent a star. now, picture a random idiot coming along with a leaf blower pointed towards the ground. every once in a while the idiot turns on the leaf blower and jettisons some little pebbles out of the way. given enough time, enough mini blasts of the leaf blower you will see your little pebbled beach expand. time being what it is is the same, but the distance between all the pebbles have slowly started to scurry away.

  26. Alan Feldman says:

    As Lawrence Krauss once said:

    “Space can do what it wants.”

  27. Floyd says:

    I am not a scientist but I am curios about this concept.
    If the universe is expanding faster than the speed of light, and time stops at the speed of light, What exists in between ?

  28. RobRoy says:

    Why continue guessing? I’ve found this topic puzzling for a long time: Most explanations seem to be by folks who in fact don’t understand the concepts. This acticle covers it all very well: https://en.m.wikipedia.org/wiki/Metric_expansion_of_space. One note, the space/time cones need to have either the top or bottom view flipped L to R, as they are confusing (only in that regard). Otherwise exceptional explanation as how we can “see” 13B light years back to things that we were previously nearer (big bang singularity).

    Short version: Yes, objects that are very distant ARE becoming more distant at a rate greater than the speed of light, as “space itself” is stretching. The balloon analogy is very on point. And over time, regions of the universe will ultimately red shift, then disappear. And… It explains why these 13B light year distant objects may not appear as vanishingly small as you might think; their image size was larger relative to our field of view considering how “far” (how long their light has been traveling) as they weren’t THAT far when their light set out.

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