Galaxies farther away than let's say z=2 are beyond our reach. If we today receive a message from such a galaxy, our response message will never reach back to them. So goes the current knowledge. This should also mean that sometime in the future, long before our response message should be due to reach that faraway galaxy, observers there should see our galaxy disappear beyond their horizon. Reciprocally this means that also we in the future will se that galaxy "suddenly" disappear out of our sight. But it can't of course pass through the CMBR. The CMBR must have been gone long before. So my question is: At what date in the future will the CMBR "suddenly" disappear beyond our - and for that matter anybody's - horizon?

I have no problem whatsoever with the CMB being everywhere, that is not even my point - the question is instead whether the CMB is forever, in the time dimension, not in the space dimension.

Again, I am not talking about "somewhere", it is about the time dimension!

So if we were observing galaxies approaching and passing through our horizon, they first have to transverse the CMB ("back to the womb")? Go backwards in time to "380 000 Anno BB"? The original CMB never gets older than that.

  • 4
    $\begingroup$ Rob Jeffries answers your question perfectly, but you might also be interested in this question/answer on astronomy.SE which addresses exactly your problem with "galaxies leaving the horizon". $\endgroup$ – pela Oct 30 '18 at 22:27

The CMB is everywhere and was emitted from everywhere and (almost) isotropically fills the universe.

At any given epoch in the past or in the future (subject to there being sufficient photons to measure), there will be photons that were emitted somewhere at the epoch of recombination and are only just reaching the Earth at the epoch of observation.

The future of the CMB is that it will continue to be further redshifted in proportion to the increasing scale factor of the universe and it's energy density will inversely scale as the fourth power of the scale factor. It will always be observable until it dilutes or redshifts below any sensitivity threshold.

As a postscript, in standard cosmologies, it is not true that galaxies will disappear in the future. They will become fainter and more redshifted, but there is no sharp horizon that is ever crossed. The observable universe, defined by the particle horizon, continues to expand - that is, the co-moving distance of the most distant point in the universe from which we can have received light (emitted in the past) continues to increase.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.