Re: Seeing old galaxies




<lodtop@xxxxxxxxxxx> wrote in message
news:1152279636.140499.78810@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Just been listenin to "In Our Time"


If Hubble captures the light of galaxies (not just background
radiation) that are 13 bn years old and the universe is approximately
14 bn years old, how come we can see it?

Wouldn't the universe be a maximum of 1 billion light years across when
this light was created and it therefore would have gone past us 12 bn
years ago? I thought that the speed of light is an absolute limit, so
to repeat the obvious it would have been produced when the universe was
1bn years old, but 13 bn light years across (mimimum).

Please answer ASAP as I need to sleep.

Thanks.

PS No maths please.

PPS This probably a really stupid question and the 1bn light years bit
is more to illustrate the question rather that being a real figure.

It would be easier with a diagram but here goes.
The speed of light isn't an absolute limit to the rate of expansion of
the universe. 13 Bn ly is the
"lookback time" of Hubble, it can see galaxies as far back/away as 13Bn
years ago. There may be more
to see further back, or space may have expanded enough to shift really
old/distant light out of the
visible.
===============================================================================
http://zebu.uoregon.edu/~js/ast123/lectures/lec14.html

Expanding Universe:

A common question in cosmology is "why are all the galaxies receding
from each other?" In other
words, the cosmological principle requires that we not be at a special
place in the Universe. Since
all the galaxies are moving away from us, then they must all be moving
away from each other. This is
explained if the Universe, as a whole, is expanding.

In a real sense, Hubble's law, the recession velocity of galaxies, is
an illusion. The galaxies are
not moving, the space between them is literally expanded.

Lookback Time:
The large size of the Universe, combined with the finite speed for
light, produces the phenomenon
known as lookback time. Lookback time means that the farther away an
object is from the Earth, the
longer it takes for its light to reach us. Thus, we are looking back in
time as we look farther
away.


The galaxies we see at large distances are younger than the galaxies we
see nearby. This allows us
to study galaxies as they evolve. Note that we don't see the
individuals evolve, but we can compare
spirals nearby with spirals far away to see how the typical spiral has
changed with time.

http://www.citebase.org/cgi-bin/citations?id=oai:arXiv.org:astro-ph/9306002

We present explicit expressions for the calculation of cosmological look back time, for zero
cosmological constant and arbitrary density parameter ?, which, in the limit as redshift becomes
infinite, give the age of the universe. The case for non-zero cosmological constant is most
easily solved via numerical integration. The most distant objects presently known (approaching
redshift z = 5) have implied ages of ? 1-2 Gyr after the the Big Bang. The range of such age is
narrow, in spite of a variety of cosmological models one might choose. We give a graphical
representation of a variety of cosmological models and show that a wide range of Hubble constants
and values of the age and density of the universe compatible with modern studies are consistent
with adoption of a positive cosmological constant.<<

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