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Since space is expanding, and accelerating, with distant locations accelerating at such a high rate that they are receding at greater than the speed of light, how can the Webb detect these distant objects.

I have read that the size of the “visible universe” is decreasing with time because of the expansion. Infrared energy propagates at the speed of light in vacuum.

So I'd like to ask how the JWST telescope can detect matter “shortly” after the big bang?

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    $\begingroup$ We easily see galaxies receding faster than the speed of light, but it is a common misunderstanding that we don't. See this answer. $\endgroup$
    – pela
    Jan 4, 2022 at 20:36
  • $\begingroup$ "the Webb" what? You may know what you're asking about, but future readers of this post might not. Could you please clarify what Webb item you are asking about. $\endgroup$
    – Fred
    Jan 4, 2022 at 23:21

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It is true that space is expanding[citation needed] and due to this expansion, over time, distant objects will become unobservable. Eventually distant galaxies will be beyond our Particle Horizon (the maximum distance a particle of light could be and still be observed by us). In the far flung future, if we're still around, the observable universe will essentially be the closest galaxies to us.

However, that effect is still very, very minimal in our young universe. Galaxies in our universe did not form immediately. Nor has our universe been expanding at the same rate for its entire existence. Those two concepts combined essentially mean that even the earliest galaxies are still within our particle horizon.

As a specific example, Wikpedia's List of the most distant astronomical objects show's that the most distant objects we have found are at redshifts in the range of 6 to 11. This represents an age of a few hundreds of millions of years after the Big Bang.

A relevant source to this discussion is Loeb (2001) which states:

objects with the current redshift z from 5 to 10 will remain observable for no more than 4–6 billion years

While the early galaxies are far off and on the way out of our particle horizon, we still have a long time to go (on a human scale) before we won't be able to see these objects "shortly" after the Big Bang. Just to give you a sense of the time scales involved in this process, Loeb (2001) states

Within $\lesssim 10^{11}$ years, we will be able to see only those galaxies that are gravitationally bound to the Local Group of galaxies, including the Virgo cluster and possibly some parts of the local supercluster

This means it'll be a few billion years before the most distant objects are no longer observable and a hundred billion years before almost everything is no longer visible.

What's more, even once they're outside our event horizon (meaning no newly emitted light will reach us) it still takes billions of years for the already emitted light to finally reach us meaning we'll still see the objects long after they're "unobservable" Loeb (2001) shows this concept in some of the equations and plots.

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  • $\begingroup$ And yet, the further and further we look into space, we continue coming across fully formed galaxies - even at distances they shouldn't exist. $\endgroup$
    – White Prime
    Jan 6, 2022 at 17:38
  • $\begingroup$ @WhitePrime Do you have a citation to back that statement up? I don't know of any instance of a "fully formed galaxy at a distance it shouldn't exist". If we look back at the early universe, we don't see fully formed galaxies, we see proto-galaxies without much structure and incredibly high star formation rates. $\endgroup$
    – zephyr
    Jan 6, 2022 at 18:08
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Light is emitted or scattered by objects and travels forward through time at the speed $c$ until it reaches a camera, but you can turn that around and imagine that a beam is emitted from the camera and travels backward in time at $c$ until it reaches the object being photographed.

So, imagine a beam leaving the JWST and traveling back to "shortly after the big bang", whatever that means. (It obviously has to be a late enough time that galaxies have formed.) The JWST will see galaxies from that time in that direction iff there are galaxies at the location of the light beam. In the standard cosmological model, there are galaxies there, because there are galaxies everywhere. The universe is homogeneous, so it doesn't matter how far the beam traveled: every location is the same.

The model may be wrong. But we know that the universe is homogeneous out to at least the distance of the beam from the previous paragraph, because of the cosmic microwave background. The CMB was emitted before any galaxies formed (and therefore from a greater distance than the galaxies that the JWST should see), and is almost perfectly homogeneous.

Distant galaxies recede faster than $c$ in a certain technical sense. They don't recede faster than light, since light also moves faster than $c$ in that same technical sense. These speeds are not measured relative to anything in particular. It's better not to worry about faster-than-$c$ recession as it doesn't have any straightforward physical interpretation.

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  • $\begingroup$ I don't feel that this actually answers the question. All you've done is state the universe is homogeneous and distant objects are receding faster than the "speed of light". How does that explain why early galaxies haven't yet moved out of our particle horizon? $\endgroup$
    – zephyr
    Jan 10, 2022 at 14:14
  • $\begingroup$ @zephyr It doesn't matter where the galaxies go after they emit the light, only where the light goes. I've seen conflicting definitions of "particle horizon", but the one in this diagram expands with time and galaxies can only enter it, not leave it. $\endgroup$
    – benrg
    Jan 11, 2022 at 0:55

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