I am puzzled about an apparent contradiction. In a universe with a Hubble law, you see that, the farther an object is, the faster it is moving away from you. On the other hand, you also have a cosmological horizon (when this speed reaches $c$), that looks like pretty much a black hole horizon look like from far away, that is, objects seem to move slower as they reach the horizon and get packed there.

So, does an object far away near the horizon seem to move away from us or freeze from us?

  • $\begingroup$ Why do you think cosmological Hubble horizon looks like a black hole horizon? It is very different - with the Hubble horizon, we are inside, and also there is no strong gravity keeping things inside or outside. $\endgroup$ Oct 25, 2023 at 23:57
  • $\begingroup$ @JánLalinský That is what L. Susskind said in one of his talks. I understand that they are not exactly the same, for the reasons you said and a couple of others, but there are strong analogies too, assuming what he said is correct. $\endgroup$ Oct 26, 2023 at 0:09
  • $\begingroup$ Which talk is that? Btw. talks and books by science communicators are full of fancy misleading analogies to boost their popularity and sales. Take science popularizers and online physics personas with a truckload of salt. $\endgroup$ Oct 26, 2023 at 13:18
  • $\begingroup$ @JánLalinský youtube.com/watch?v=n7eW-xPEvoQ&t=3407s&ab_channel=mrtp $\endgroup$ Oct 26, 2023 at 17:58

1 Answer 1


For objects "moving" away from us faster than c due to the expansion of space between us and the object, the light from that object will never reach us and the object will vanish.

By analogy, if a marble is rolling down an infinitely strechable ramp and you stretch the ramp fast enough that the net movement of the marble does not go DOWN the ramp, but moves the marble higher (due to the stretching between the marble and the bottom of the ramp) the marble will not reach the bottom of the ramp.

  • $\begingroup$ Thanks, I do understand the redshift would render them invisible in practice, but let us assume we could measure any wavelength. Also, I am asking about the objects that are between us and the horizon, not those past the horizon. So, if I understand the analogy correctly, would that mean that the apparent slowing down is an artifact due to the finite speed of light (and that the distance is increasing but we just cannot see it)? $\endgroup$ Oct 25, 2023 at 4:30
  • $\begingroup$ Objects near the horizon will be moving away from us, and at those further distances do not follow Hubble's Law, so they would be accelerating away from us faster. -- as they cross the horizon they would no longer be visible to us at any wavelength (no causal connection, "c"). $\endgroup$
    – Mason Berg
    Oct 25, 2023 at 14:18
  • $\begingroup$ I am not sure what do you mean by "do not follow hubble law", I assumed in the question that I was talking of a universe that does follow a Hubble law (which does not seem to have a cutoff, as far as I remember). Or is this mathematically impossible? $\endgroup$ Oct 25, 2023 at 16:49

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