Faster than Light: Understanding the Expansion of the Universe

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Discussion Overview

The discussion revolves around the relationship between the speed of light and the expansion of the universe, particularly focusing on whether the universe's expansion can exceed the speed of light and the implications of this for observable phenomena in cosmology.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that at certain redshifts (z>1), the universe is indeed expanding faster than the speed of light, leading to photons emitted beyond the observable universe that will never reach us.
  • One participant suggests that stars at the edge of the observable universe may fade and disappear over time due to this expansion.
  • Another participant argues that comparing the rate of expansion of the universe to the speed of light is problematic due to differing units, noting that expansion is a speed per distance rather than a straightforward speed.
  • It is mentioned that while many objects are receding faster than light, photons can still reach us from regions that are receding faster than light under certain conditions, as they can enter areas with lower recession velocities.
  • A later reply emphasizes the distinction between the event horizon and the observable horizon, clarifying that only in cases of accelerating expansion do we encounter regions from which no photons can ever reach us.
  • One participant provides an example involving cosmic background radiation, explaining how photons emitted from a rapidly expanding universe took billions of years to reach us despite the initial proper distance being relatively small.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the universe's expansion relative to the speed of light, with some agreeing on the existence of photons that will never reach us, while others challenge the comparison of expansion rates to speeds and highlight the complexities involved.

Contextual Notes

Participants note ambiguities in the concept of distance on cosmological scales, particularly regarding the use of "proper distance" coordinates in their discussions.

DARTZ
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What is moving faster

The speed of light
or
The expansion of the universe

If the universe is expanding faster then the speed of light, then light be traveling away from us?
 
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It is true that at certain redshifts (z>1) the universe is expanding away from us faster than c, so yes it is correct to say that there are photons that are emitted beyond this horizon (the observable universe) that will never reach us because space is expanding faster than they can travel.
 
In which case I would suppose that there are stars, et al that we can see now at the edge of the observable universe that will appear to fade and then disappear due to this expansion over time?
 
1. It is not actually possible to compare a rate of expansion to a speed. It's a unit problem: the expansion rate has units of speed per distance. You can't compare a speed per distance to a speed. The comparison makes no sense.

2. That said, there are many objects in the universe that are receding at faster than the speed of light. That is to say, if we consider a photon traveling from some far away galaxy, if that galaxy is far enough away, then more space will have been created between us and the photon than the photon is able to travel. So yes, there are photons out there that are traveling in our direction but are nevertheless getting further away.
 
FunkyDwarf said:
It is true that at certain redshifts (z>1) the universe is expanding away from us faster than c, so yes it is correct to say that there are photons that are emitted beyond this horizon (the observable universe) that will never reach us because space is expanding faster than they can travel.

As noted, expansion of the universe is not a velocity; the units are different. But it does imply that distant parts of the universe are receding at speeds proportional to distance, and these can be arbitrarily large.

However, you need to be careful. It is only in a case of accelerating expansion that there is an "event horizon" marking regions from which no photon can ever reach us no matter how long we wait. This is not the same as the observable horizon, which marks the regions from which photons can reach us up until now.

The other tricky point is that it is quite possible for a photon to reach us, even if coming from regions that are receding faster than the speed of light. Basically, a photon passes into regions with smaller and smaller recession velocities, and can eventually pass into regions where the recession velocity is less than light speed again. In this case, the "proper distance" between us an an approaching photon actually increases at first; but then eventually this reverses and starts to reduce again.

An extreme example of this is the cosmic background radiation, which was emitted at cosmologically small distances in a very rapidly expanding universe. Those photons were intially at a "proper distance" of something like 50 million light years or so; but were being carried away from us (or from the matter from which we were eventually formed) as the universe expands and disperses. It has taken 13.7 billion years for those photons to reach us.

Technical caution: distance is actually ambiguous on these scales; I am using what is called the "proper distance" co-ordinate in this post.

Cheers -- sylas
 

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