If particle horizon is the maximum distance we can see presently in the universe, how come we are able to see CMB? CMB is radiation from surface of last scattering happened at t~380k years.

Suppose universe is expanding at a constant rate ( i.e. no acceleration), will we be able to see CMB again??


The particle horizon is the distance from which light emitted at the moment of the Big Bang will just now be reaching us.

The CMB was emitted 380,000 years after the Big Bang. So the CMB radiation we see has been travelling for less time than the light emitted at the Big Bang, and therefore the CMB radiation has travelled a shorter distance than the particle horizon.

In other words, the CMB horizon is always inside the particle horizon, so we will always be able to see it.

  • 2
    $\begingroup$ In fact, the CMB horizon is the optical horizon, which is always (strictly) smaller than the particle horizon. $\endgroup$ – Danu May 8 '15 at 10:54

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