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The Observable Universe

Location of Earth small


The age of the Universe is estimated at 13.8 Gyr which is the same as the light travel time to the edge of the Universe. It is not very useful to say that the size of the Universe therefore is 13.8 Gly. We saw before that light travel time means the time that the photons we receive now have been travelling through an expanding universe. If we multiply that with the speed of light what does that tell us more about the size of the Universe?


The observable Universe is that part of the Universe that is within our present light travel time range. This means that the oldest light we can see is that which left the source at “first light” of the Universe. This is a physical limit caused by the finite speed of light and is independent of the technology we use for observations. We will never be able to look any further.


There is a small difference between “Edge of the Observable Universe” located at the Big Bang and “Edge of the Visible Universe” which is from the time of “recombination”  , the time that the Universe became transparent for EM-radiation.


A proper distance indicator for the size of the Universe is the comoving distance we discussed above.
The comoving distance to the edge of the Observable Universe is 46.5 Gly and to the edge of the Visible Universe is 45.7 Gly.


An interesting fact is that we on Earth are at the centre of the observable Universe. We should say “Our” observable Universe. An alien who looks around from a distant galaxy will see a different observable Universe that is centred around that galaxy. If we can see that galaxy there is an overlap between the two observable Universes. If not, we can have no meaningful discussion about it.


Every year that passes the Universe will have expanded a bit further. So the edge will be further away from us over time. This also means that we will be able to see new light that finally has a chance to reach us as regards the age of the Universe. Of course it can only be light that was emitted after the Big Bang (more precise after the epoch of recombination). But that won’t help very much, because at those extreme distances the Universe is already expanding faster than the speed of light. This is not a violation of Special Relativity as it is space itself that is expanding; it is not a movement of mass. So this means that there is a “cosmic event horizon” or Hubble Sphere beyond which we have no “causal connection” to the Universe. In cosmology there are theories about the Universe beyond our observable Universe, but we can never test those theories with observations.


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Image: Earth (top) at the centre of our Observable Universe.
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