Does the cosmic microwave background recede at the speed of light? Is it possible it recedes even faster because of the stretching of space?


1 Answer 1


Well obviously the CMBR arrives (towards us, not receding) at us at the speed of light as it is a form of electromagnetic radiation and local velocities are not affected by expansion.

What I guess you may mean though is the surface of last scattering, which is the surface from which the CMBR that we receive at a given instant was emitted in the early Universe. This surface of course is no longer emitting CMBR in the present cosmological time and it is not a fixed (comoving) surface either as which surface we receive the CMBR from changes with time, however we can give a recession velocity to the comoving set of points which make up the surface of last scattering at any given instant.

In the standard model of cosmology, the Lambda-CDM model, the instantaneous surface of last scattering is receding from us at approximately 3.2c (i.e. a recession velocity 3.2 times the speed of light).

  • $\begingroup$ Yes, this is what I meant. Thank you for the clarification, and the answer! $\endgroup$
    – Jonathan
    Feb 4, 2015 at 13:15

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