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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54 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!

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