I understand that distant galaxies that are currently inside our particle horizon will, sometime in the future, not be observable by us anymore because they are receding faster than the particle horizon is expanding. I guess, put another way, if a distant galaxy, that we can see today (is currently inside of our particle horizon), has been outside of our event horizon since time $t$, then we will stop seeing that galaxy (it will be outside of our particle horizon) once all the light it emitted before $t$ arrives here.

Does that mean that if a galaxy in our universe has never been observable by us (i.e., none of the light that it emitted, at anytime in the past, has ever reached us), then none of the light that it emitted, at anytime in the past, will reach us in the future?

A similar question might be: if none of the light that it emitted, at anytime in the past, is reaching us now, then none of the light that it emitted, at anytime in the past, will reach us in the future?

I have not seen another question put quite this way, so I do not think that it is a duplicate, but if it is, I apologize. I did try to research this on this site, before posting.


1 Answer 1


If a distant galxy is currently inside our particle horizon, then it will be always visible, technically, even if it at some point it crosses the event horizon. It's just that its image that we are getting will be more and more delayed. As our time moves forward, we will get the images of galaxy closer and closer to the event horizon. Matematically, the image of th galaxy at the time when it crosses the event horizon will reach us at infinity. The image of the galaxy after it crosses the event horizon will never reach us. But, I repeat, technically we would always be able to see it, just more and more delayed, more and more dim, until the point it becomes to dim to detect.

Generally, it is possible for the particle horizon to expand so much to enclose a galaxy that wasn't previously visible, as long as that galaxy didn't start its existence outside the event horizon (which is possible in some, though not all, cosmological models). We would first seen its image from the moment it appeared, and then continue to observe its evolution.

Something from outside of the event horizon getting inside the particle horizon is mathematically impossible. If we see something at the present time, then any moment from its previous history would be visible to use somewhere in our past. Assuming of course it emitted any light at that time, and that light hadn't been stopped by the environment like hot plasma that filled the early universe. So, if something is outside of our event horizon, then it will always be outside of our event horizon. That's basically the definition of the event horizon: the outside of the horizon is the region that will never be visible to us, and since no massive object can travel faster than light, no material object will be able to escape it either.

  • $\begingroup$ Downvoted. Both paragraphs start with incorrect assertions. Once a galaxy is inside our particle horizon, it will never cross back out. It will eventually leave our event horizon, but this is a different beast. Since the oparticle horizon is aleways growing, even in comoving coordinates, galaxies may only enter it, not leave it. In fact, as I read second paragraph, its middle and ending are also wrong. $\endgroup$
    – Thriveth
    Commented Sep 24, 2019 at 12:29
  • $\begingroup$ @Thriveth You are right, I've got the particle horizon and the event horzion confused. I'll make the appropriate corrections. $\endgroup$ Commented Sep 24, 2019 at 15:57
  • $\begingroup$ So, if I understand: 1) once a galaxy is in our particle horizon, it is in (and we see light from it) forever. The only catch is that the light we see from something in our particle horizon, that has left our event horizon, can never have been emitted after the date that it left the event horizon and its light that we see, as time goes on, will get dimmer and dimmer. 2) some galaxies that were never in our particle horizon will enter it in the future and hence be visible to us for the first time. $\endgroup$
    – David
    Commented Sep 24, 2019 at 16:42
  • $\begingroup$ @David Exactly! $\endgroup$
    – Thriveth
    Commented Sep 24, 2019 at 17:37
  • $\begingroup$ @adam I removed my downvote now that your answer is edited. $\endgroup$
    – Thriveth
    Commented Sep 24, 2019 at 17:39

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