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Why can't we see light from beyond the observable universe?

I've done a lot of research on this and all I've found is unsatisfactory answers and straight up nonsense.

Some claim that the universe "expands faster than the speed of light" beyond the observable universe. Such a claim doesn't even make sense because the units of speed are m/s and for expansion are Hz. That's like saying "the area of this square is larger than the volume of this cube".

All that the expansion can do to light (as far as I know) is redshift it. And light doesn't have a minimum possible frequency or energy value. So even if the expansion of the universe is very rapid, why does the light of distant objects "never reach us". Surely it still would, just extremely redshifted. In this case it does still reach us, and yet the claim is that it cannot.

We often detect redshifted light, and that light has not been slowed down. When we detect it, it still goes at c, even though (in fact a better word than "even though" here is "because") it is redshifted. Light is always propagating at c no matter the reference frame.

More precisely: does the light really never reach us, or can we just not detect it?

If it never reaches us - why?

If we cannot detect it although it does reach us - why?

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The Hubble law is $v=Hd$. When you multiply H by the distance, you get a velocity (units distance/time). That is what the Hubble law is saying. Once you reach a distance (the cosmic horizon) in which the hubble law tell you that Hd=c, where c is the speed of light, that means that an object at such distance is moving away from you faster than light. This is not a mistake, the local speed of light cannot be larger than c, but in general relativity space expands, so even massive object can look to travel at more than c.

The light from a galaxy beyond the cosmic horizon may be sent in your direction, but in your reference frame that light ray will move away from you, because the space (and distance) created in between the light ray and you grows larger than the distance the light ray makes when trying to get "closer" to you. So basically, that light will never reach you. The light tries to get to you but too many space is being added in between, so it will never make it.

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  • $\begingroup$ Is this answer compatible with special relativity postulate that light travels in all reference frames at the SAME speed $c$ ? So if behind cosmic horizon somebody will shoot photon towards us, and space "gets backwards" faster,- in our reference frame photon still should be going towards us at speed $c$,- otherwise relativity postulate about light speed invariance makes no sense. No ? $\endgroup$
    – Agnius Vasiliauskas
    Commented Nov 30, 2023 at 9:01
  • $\begingroup$ I'm happy with the first paragraph. What you're saying in simple terms is that this recession speed isn't really a speed per se. These m/s are the number of "new metres created between us and the object per second". In this case light will travel thru them at c but there will be too many new metres to reach us. So as soon as the photon covers 3E8m, there will be at least 3E8 new metres in front of it. Is this correct? As for the second paragraph why would the light move away from us? We should still observe it as going at c through space, no? $\endgroup$
    – Krokodil
    Commented Nov 30, 2023 at 11:48
  • $\begingroup$ Just a recent thought. Maybe expanding universe with acceleration is not an inertial reference frame ? If so, then relativity $c$ speed invariance in this case does not apply and indeed photon can be "stopped" or turned back from us. But I'm not sure what scenario fits best, I'm not that strong in relativity. Hm... $\endgroup$
    – Agnius Vasiliauskas
    Commented Nov 30, 2023 at 15:25
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    $\begingroup$ @AgniusVasiliauskas "local" means "if you make the experiment in a lab that is small enough, then whatever effect you are calling non local, is below the error threshold for detection". $\endgroup$
    – Pato Galmarini
    Commented Dec 3, 2023 at 1:41
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    $\begingroup$ @AgniusVasiliauskas Sure, you are entitled to your own opinion. $\endgroup$
    – Pato Galmarini
    Commented Dec 4, 2023 at 0:27
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The observable universe gets bigger with each new advance in telescope technology. In this sense the "observable universe" is that part of it which is sending just enough photons our way to trigger a pixel inside our most sensitive telescope's detector system.

Ultimately, for any part of the universe to be visible to us requires that it be transparent, so any light given off by an object there can make it out and then travel to our telescopes. Because the earlier back in time you go the hotter and denser the universe becomes then means that there will be an early time when the universe was too hot and dense for light to travel freely through it. Anything that happened before that time (the epoch is called recombination) then is unobservable to us, no matter how sensitive our telescopes might be.

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    $\begingroup$ The observable universe includes things that could be observed with neutrinos or gravitational waves. $\endgroup$
    – ProfRob
    Commented Nov 30, 2023 at 5:03
  • $\begingroup$ @ProfRob but can we image neutrinos? $\endgroup$
    – niels nielsen
    Commented Nov 30, 2023 at 6:05
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    $\begingroup$ @nielsnielsen, yes: arxiv.org/abs/2309.10493. $\endgroup$
    – Martín-Blas Pérez Pinilla
    Commented Nov 30, 2023 at 10:35
  • $\begingroup$ So after enough time, you're saying light from those distant objects could in fact be detected by us because currently the universe is largely transparent? $\endgroup$
    – Krokodil
    Commented Nov 30, 2023 at 11:56
  • $\begingroup$ Yes, it is the transparency of the universe that lets photons from the recombination era travel unimpeded to us. In fact, the cosmic microwave background is exactly the relic radiation left over from recombination. $\endgroup$
    – niels nielsen
    Commented Nov 30, 2023 at 18:21

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