I had a question regarding cosmic event horizon. Let's say that a far away solar system has just crossed the cosmic event horizon due to the expansion of the universe. In that solar system let us say there is a planet orbiting the solar system's star, for half of the planet's orbit, it will be traveling towards us, and the other half it travels away from our planet. So, if the planet crosses the cosmic event horizon while the orbit is going away from us, what happens when the planet travels back towards us so that its relative velocity is less than light? Does the planet re-appear crossing back over the event horizon? And what happens to the entropy of the planet as it crosses back on our side of the horizon? Does entropy of the planet decrease in this case? Or, because the relative velocity is so high, does the planet from our perspective not orbit because time slows down to the point that the planet appears stationary?

  • $\begingroup$ What should the speed of a particle be to get out of the cosmic event horizon? $\endgroup$
    – Ali
    Aug 31, 2013 at 2:33

1 Answer 1


The radius of our cosmological horizon is expanding at a speed that is on the order of the speed of light. As measured by an observer at our horizon, the horizon sweeps past at exactly the speed of light. The planet is not goint at any speed anywhere near the speed of light. So once our horizon sweeps past it, the planet is not going to recross the horizon.


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