Re: Two questions


"Bjoern Feuerbacher" <feuerbac@xxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:d5l4fi$fos$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Peter Webb wrote:

<snip>

>>>>and the edge recedes at the speed of light.
>>>
>>>What edge?
>>
>> The horizon that defines the visible Universe.
>
> O.k.

That would imply that some galaxy at the limit
of visibility at some time in the past was also
still at the limit of visibility.

>> Using the Hubble constant, you can calculate the radius of a visible
>> Universe as being that distance at which the recession is equal to c.
>
> Are you sure that this gives the horizon? Looks strange to me.

It gives the limit visibility because anything
nearer has high but finite redshift but light
from anything beyond that distance cannot reach
us. It creates an 'event horizon' though not in
the sense of a black hole. The same thing applies
in SR for a constantly accelerating observer.

However, if the expansion is accelerating, I don't
see that the second definition produces a horizon
that "recedes at the speed of light", surely it
would depend on the acceleration parameter.

George


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