QUESTION:  Is it now possible to map the edges of the universe (or at least
a portion that we are relatively close to) by estimating the distance of
the most distant objects we can see with Hubble. These objects should all
be at the edge of the universe and all the same age, shouldn't they?


ANSWER from Meg Urry on May 10, 1996:
If I understand you properly, you want to know if Hubble can see out to,
or even beyond, the edge of the Universe?

It is true that Hubble can see very distant objects --- galaxies,
basically --- that existed when the Universe was very young, perhaps
only 1 or 2 billion years old. Astronomers use the term "redshift" to
denote distance and therefore age; redshift is defined as the relative
shift in wavelength of light emitted by the body due to its motion away
from us. Galaxies with redshifts of 3, 4, and almost 5(!) have been
observed with HST and ground-based telescopes. These are very distant,
many billions of light years away, and hence considerably younger than
the everyday stars and galaxies in our local universe. And Hubble sees
individual galaxies in greater detail than any other optical telescope.
But the COBE satellite, which measured very accurately the 3-degree
microwave background radiation, saw much farther still, to redshifts
of a thousand or so, when the Universe became transparent to such
radiation. So in that sense, Hubble is not seeing so far back in time
at all!

As for where the edge of the Universe is, whew! that's a complicated
cosmological question, and one that I can't pretend to answer fully
or even well. Basically, it's difficult to define an edge. If the Universe
is "open" --- i.e., it does not have enough mass to slow its expansion ---
then its topology is different than if its closed. And if it is closed,
then in some sense it turns back on itself and is edgeless. Alternatively,
if you think of the "edge" as being the place at which the recession
velocity due to the global expansion of the Universe becomes the speed
of light, if there were such an edge, then it is well beyond the redshift
of recombination, that is, of the redshift at which the Universe became
transparent, so we certainly couldn't see it anyway.

So, to get to the bottom line, the best "map" of the farthest reaches
of the Universe is the one NASA's COBE mission made, showing the bright
glow of infrared light --- a remnant from the Big Bang explosion --- and
the tiny fluctuations that eventually will coalesce due to gravity into the
lumpy Universe of galaxies we see today.