- #106
sylas
Science Advisor
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- 9
russ_watters said:A non-accelerating expansion most certainly does have a horizon, as does a non-expanding universe! All that is required to have a horizon for the "observable" universe is for the universe to be larger in light years than it is old in years. Ie, a universe that is 1 year old and not expanding will have an observable size of 1 ly, a universe 14 billion years old will have an observable size of 14 billion light years.
For a constantly expanding universe that started at a big bang, that requires an expansion to always exceed the speed of light.
There are several horizons you can speak of. The one that was being discussed in [post=2291438]msg #16[/post] is often called the "event horizon". It partitions spacetime into (A) events we have "seen" or that we can "see" if we only wait long enough, and (B) events that we can never see, no matter how long we wait.
If someone speaks of a photon never being able to reach us, then that means they are proposing it is past the event horizon.
The event horizon only exists in an accelerating universe.
It doesn't make sense to speak of an expansion exceeding the speed of light. Expansion is not a velocity, in those units. The units of expansion are basically inverse time. Of it is given in km/sec/Mparsec. But you can also give it as sec-1, and it means the inverse of the time it would take the double in scale factor at the current expansion rate.
There are always regions in the universe that are receding faster that the speed of light. (Using proper time, proper distance co-ordinates.) It is perfectly possible to see a galaxy which is now and always has been receding with a recession velocity greater than c. As long as the expansion is not accelerating, a photon will be passing into new regions with smaller and smaller recession velocities, and eventually into regions where co-moving galaxies are receding at less than light speed and then finally to our own local region, allowing us to see that distant galaxy.
Cheers -- sylas