[seqfan] Re: Silly thought

Jonathan Post jvospost3 at gmail.com
Wed Dec 3 15:54:19 CET 2008

Speaking for a moment as an ex Adjunct Professor of Astronomy, that's
not silly at all.

I quote:

"Just one clarification to make. The cosmological event horizon is
also receeding, and receeding faster than any object inside of it.
Distant galaxies will eventually fade from view as they become too
distant to detect, but will not cross an event horizon and suddenly
disappear. Isn't it true that if the rate of expansion is
accelerating, then galaxies can cross the cosmological event horizon
and disappear forever? That seems to be what's being said in the last
section of the article Misconceptions About the Big Bang
What does mark the edge of observable space? Here again there has been
confusion. If space were not expanding, the most distant object we
could see would now be about 14 billion light-years away from us, the
distance light could have traveled in the 14 billion years since the
big bang. But because the universe is expanding, the space traversed
by a photon expands behind it during the voyage. Consequently, the
current distance to the most distant object we can see is about three
times farther, or 46 billion light-years."

"The recent discovery that the rate of cosmic expansion is
accelerating makes things even more interesting. Previously,
cosmologists thought that we lived in a decelerating universe and that
ever more galaxies would come into view. In an accelerating universe,
however, we are surrounded by a boundary beyond which occur events we
will never see--a cosmic event horizon. If light from galaxies
receding faster than light is to reach us, the Hubble distance has to
increase, but in an accelerating universe, it stops increasing.
Distant events may send out light beams aimed in our direction, but
this light is trapped beyond the Hubble distance by the acceleration
of the expansion."

"An accelerating universe, then, resembles a black hole in that it has
an event horizon, an edge beyond which we cannot see. The current
distance to our cosmic event horizon is 16 billion light-years, well
within our observable range. Light emitted from galaxies that are now
beyond the event horizon will never be able to reach us; the distance
that currently corresponds to 16 billion light-years will expand too
quickly. We will still be able to see events that took place in those
galaxies before they crossed the horizon, but subsequent events will
be forever beyond our view."


G. F. R. Ellis and T. Rothman, "Lost horizons," Am. J. Phys. 61 (1993) 883.
Here is the abstract:
Cosmological horizons play an essential role in determining the causal
structure of spacetime and are of central importance in the
inflationary universe scenario. We review the topic of horizons in
simple language, pointing out a number of widespread misconceptions.
The use of spacetime diagrams plotted in terms of proper time and
proper distance coordinates helps sort out some of these difficulties.
They complement the widely used conformal diagrams, which show causal
relations clearly but severely distort proper distances.

Comments on ``Lost Horizons'' by G. F. R. Ellis and T. Rothman [Am. J.
Phys. 61 (10), 883–893 (1993)] by Michael Rauch, Am. J. Phys. 63 87

Past light cone shape and refocusing in cosmology, A Response to
Michael Rauch's ``Comments on `Lost Horizons' '' [Am. J. Phys. 63, 87
(1995)] by G. F. R. Ellis et al., Am. J. Phys. 63 88 (1995)

Start here: http://www.aapt.org/ajp/

On Wed, Dec 3, 2008 at 6:31 AM, David Wilson <dwilson at gambitcomm.com> wrote:
> I had a probably silly idea...
> What if the universe were slightly elliptical, and near our opposite
> pole were a massive black hole. From our standpoint, its gravitational
> effects on increasingly distant objects would appear as an expanding
> universe, its event horizon as "the edge of the universe", and the dying
> screams of consumed galaxies as background radiation.
> But seriously, can the edge of the universe be likened in any way to the
> event horizon of a black hole?
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