THE OBSERVABLE UNIVERSE

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 it tells us how far away the source of these photons was at the time they were emitted. That does not tell us where that source is now or give any information about the size of the Universe.

Credit: Andrew Z. Colvin




What is defined as 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 currently 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.



Actually our maximum observable radius is even more restricted.

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. Of course it can only be light that was emitted after the Big. 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.


This means that there is a “cosmic event horizon” or Hubble Sphere beyond which any light source can never be visible. No photons that are emitted from beyond the Hubble Sphere will ever be able to reach us. These photons travel with the speed of light and we will always be beyond the reach of those photons. In physics this is referred to as that we have no “causal connection” to the Universe beyond the event horizon. The radius of this Hubble Sphere depends on the value of the Hubble constant and other cosmological parameters and is about 14.4 Gly in light travel time.

More here.


What is further out?

It is very likely that the Universe continues beyond the edge of the Hubble Sphere and even beyond the edge of the observable Universe. In cosmology there are theories about the Universe beyond our observable Universe (even about multiple Universes), but we can never test those theories with observations.

Read more here.