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 ever 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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21 Responses to Q: How can the universe expand faster than the speed of light?

  1. Doug says:

    Saying that the light has more to transverse as the universe expands so that there is a horizon beyond which light cannot reach us suggests that the expansion creates more void for the light to transverse. That void is something. It seems to imply the expanded universe has more void, more “ether”stuff the light must travel across.

  2. LarryD says:

    We also have to remind ourselves that we are talking about those parts of the universe where light has been able to reach us. Another idea might be that the ‘void’ may not be a void at all but contains other similar domains but so far from ours that light has not yet, and may never, reach us. Yesteryear they said the Earth was at the center and now we are saying what we see is everything. Imagine what we will be saying in another 500 years or so.

  3. Karthick says:

    How long does the balloon keep expanding?

  4. RichF says:

    “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 ever direction.”

    Or the hypersphere is so large that what we can observe appears flat.

  5. squi says:

    Shouldn’t it be dudes and “dudettes”? “Dudets” looks like it should be pronounced “doo-dayz” – following the same rule as “ballet” or “bidet”.

  6. The Physicist The Physicist says:

    Fixed! If I could spell or write I wouldn’t be in physics.

  7. Nikki Ty says:

    Supposing one thinks of a large ball of yeast dough containing raisins. As the dough sits there it begins to expand … all the raisins are stationary but the dough around them is expanding. Now transpose yeast dough to the Universe and the raisins to planets/stars/celestial bodies. This makes the expansion of the Universe much easier to understand than the balloon … which accepts the theory that the Universe has limits and forces us only to consider those ants on the surface of the balloon, not those within it.

    Now why should it have limits? Shall we say that nothing moves faster than the speed of light … the Universe is expanding at an increasing speed with the farthest reaches expanding proportionately faster. Red shift and all that stuff. Okay, so far so good. Now theorists can assume when we no longer can see light from the furthest celestial bodies that they aren’t there. The Universe ends.

    But how do we KNOW nothing can move faster than light. Suppose the Universe at its furthest reaches eventually accelerates to a speed faster than the speed of light. Then as it expands, the light from the bodies moving faster than light would never reach us. But an expanding Universe may still be there. Even though we’ll never be able to see light from those distant bodies from the point the expansion of the Universe exceeds that 186,000 miles per second. At that point the visible Universe is no longer accessible to us. But it might still be there.

  8. Sean says:

    Based on the rate of expansion, should we be able to determine how large the universe “should be”? Also, as the rate of expansion is increasing does that mean our observable limit is actually decreasing because “stuff” is moving away from the horizon faster and faster every second?

  9. Carefree Mathematician says:

    Wow this is all so interesting! I have a couple questions, first, if the universe is a hyper sphere, then does that mean that things are moving away from one point in space to inevitably crash together in the opposite point, or is it instead possible that our area of the universe is expanding while another part is contracting? Though I may have missed the point… Everything’s expanding equally, theoretically, right? Second question, this essentially just means that distances are getting “bigger,” but might that also equate to saying that the speed of light is slowing down in certain areas? I’m sure the math can work out weirdly like that; just slow down light by some ammount in certain areas and you get the same effect, just done differently, sort of like the difference between making object A bigger and making object B smaller, there’s nothing different unless there’s an unchanging object C to compare them to.

    Thanks for considering these thoughts!!!
    :)

  10. Andres says:

    What would happen if I “connect” two bodies that are one megaparsec away with a pole (a pretty long pole!). Would they immediately stop making contact because of space dilation? Does it means that to maintain “contact” the pole needs to increase its length 70 km/s?

  11. Nicholas Lee says:

    If we conclude the universe is expanding because the distances between separate lumps of matter are getting smaller, then couldn’t we equally assume that the distances are staying the same whilst the lumps of matter (and our measuring sticks) are shrinking?

    Is there any empirical way to tell which scenario is happening?

    What would the implications be for our world view if it was the size of the matter particles and the constant ‘C’ that was shrinking rather than the universe expanding?

  12. Josh says:

    There are many “flat” shapes that do not extend forever in every direction, e.g. the flat 3-torus.

  13. brandon says:

    If everything is moving away from everything else, that must include the atoms in my body. Am I expanding? Why does that expansion not affect the density of all things? If the expansion is accelerating, will it reach a speed that is fast enough to bring the density of black holes down to a point where they cease to be black? If there is a photon right on the “ant horizon” moving directly perpendicular to the horizon does that mean it is effectively stationary?

  14. The Physicist The Physicist says:

    @brandon
    There’s a post here that tries to talk about that.

  15. RichF says:

    At non-cosmic distances gravity is sufficiently strong to overcome the expansion of space. That’s why you don’t notice it. Even clusters of galaxies stick together because of gravity.

  16. Thomas Aikenhead says:

    Nikki Ty, maybe that is what we are looking at with Dark Matter?

  17. ben hadj ali med says:

    Find your scientific researches and explanation very important and really convince me well.
    in a question on the possibility for human contact with extra terrestrial , i noticed non
    And human cannot control him self only in the instant where he is now , and the God created the time , the matter and the place and he is only can control the universe and these equation of three and he is the only can be in the 4th dimension .

    A question on the sciences in the Bible and Koran of all creations, water , iron human, space, thee invisible black holes, the stars renew,- astro nova , universe permanent expansion, evens creation, pulsars, the water osmo inverse the atom and all , about 1500 explanations , some scientific takes that like handicap of researches other consider like an illumination and can help for the best way of all researches .
    this question makes me in live .
    can you please help me the right answer

  18. Rick Curtis says:

    I we;; seriously consider the Expansion of a balloon, seems much ;ike the Economy. But I am trying to go to a planet 1000 light years away and the speed of light is too slow. I would be over 700 years old by the time I reached there. And if the speed of light is equal to the speed of gravity, why did it take 13.5 billion years for the light of the Big Bang to reach Earth and what the hell was gravity doing here when it arrived, it appears it had to travel faster than light. If it made a direct bee-line like light. Why wasn’t it here? The Big Bang is said to be nuclear high and low, gravity, and electro-magnetism. So why is gravity travelling so slow? Gravity equals the speed of light seems very questionable.

  19. james newton says:

    I accept the universe is expanding at a greater and greater rate, but i hear that a term called now a “black hole” pulls matter and energy into it and nothing escapes! Could the reverse or counterbalance to an ever expanding universe be the so called black holes that physicist John Wheeler coin the term of. could a black hole producing and controlling technology be beyond warp drive to an ultimate cosmic drive system. Using the new NASA warp drive with a tuning guidance(tuner element 131) and energy reserve systems like on the science fiction Star trek with its dilithium (fictional) for tuning and direction(navigation) now resembling a modified microwave radar sensor ( like impulse drive when ions or light is used), and dark matter and antimatter (real not fictional) for energy storage! I also could see a relation between communicating faster than light throw a subspace corridor that does not violate E=MC^2. I use Science fiction as a creative model to view science from. Kind of like others now are viewing the movie Gravity in this same perspective! The proposed warp ship with warp drive technology is suppose to be have been already been tested!

  20. iAmNobody says:

    Imagine everything is light…a void of light. You are that light so really nothing has ever moved. Only the place your conscious awareness of yourself, light, is focused at the time. You don’t move. The universe moves around you.

  21. Philipp says:

    Great analogy for a layman like me :) Thx

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