Why is the horizon not expanding during inflation?*

Why is the horizon not expanding during inflation?*
Because the texture of spacetime expands in-between, regions which are very distant from us seem to recede from us at a velocity larger than the speed of light, and thus cannot send us information. The horizon separates these regions from the ones causally connected to us. This means that a photon emitted outside the horizon but towards us is actually receding from us because space is being created in the region in-between the photon and us (see figure below (*))


In the case of standard expansion, such a photon emitted just outside the horizon towards us will overcome the expansion and, after a while, move towards us. This means that the horizon will have overtaken it and moved away from us: it has expanded.

But in the case of inflation, the expansion is as fast as it can be (we say “exponential”): the photon emitted towards us just outside the horizon will never overcome the expansion: the horizon does not move.

(*) Note that anyone measuring the velocity of the photon would obtain the standard velocity of light c; but, because of expansion, both the galaxy and the photon it emits recede from us, although this is not observable because they are beyond the horizon


  • Anna Skornyakova

    Doesn’t it mean that the Big Bang area (or period) is now beyond our horizon and we cannot study it? Still not sure I understood this notion (horizon) correctly…

  • dan

    How come the photon already emitted is receding? Shouldnt light have the same speed measured from outside no matter the space? What happens with it’s redshift or wavelenght?

  • Hank

    This is another “Just So” story to give an explanation of the phenomenon.
    What is the ‘texture’ of space-time?
    Maxwell thought it was aether.

  • Piero Martorana

    I do not know if this is the right place for questions, perhaps iI misunderstood. I have a problem with the concept of “horizon” in cosmology. Is there somewhere any information easy to digest (videos etc. a sort of “the cosmological horizon for dummies) ?

    Thank you


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