PDA

View Full Version : Age & distance of the edge of the visible universe

flipper1792
2008-Dec-17, 05:04 AM
13.7 billion years. I've read a lot, searched a lot, & wondered a lot about this. If the universe is said to be 13.7 or so billion years old, and what we can see is 13.7 billion light years away, took light 13.7 billion years to travel that expanse, then we must be seeing it AS IT WAS 13.7 billion years ago. If the universe has been expanding at an accelerated pace for the last 13.7 billion years (and much longer than that if what we are seeing is 13.7 billion years ago - and it was a singularity at the time of the Big Bang 13.7 billion years before that), would that not be 13.7 billion years less than where the edge of the universe is right now? Wouldn't the actual edge of the universe actually be much farther away - like at least twice that - than 13.7 billion light years? And that is assuming that the universe has been expanding at THE SPEED OF LIGHT!!

Just a thought...... but it is bugging me to no end......... I can't get my head wrapped around this one.....

Jeff Root
2008-Dec-17, 06:14 AM
Hello, flipper1792! So you are the 1792nd generation of the original
dolphin I saw on TV as a kid? I feel really old, now.

Much of what you say is correct. Only some bits are wrong.

If the universe is said to be 13.7 or so billion years old, and what we
can see is 13.7 billion light years away, took light 13.7 billion years to
travel that expanse, then we must be seeing it AS IT WAS 13.7 billion
years ago.
The oldest thing we can see is the cosmic microwave background
Big Bang. It was emitted by the hot hydrogen gas that was forming
from separate protons and electrons. Before that time, the plasma
of protons and electrons was opaque, so we can't see farther back.
After that time, the entire Universe was dark for millions of years, so
there is nothing to see during that era. The first stars and galaxies
formed tens or hundreds of millions of years after the Big Bang.

The matter which emitted the CMBR was only 42 million light-years
away from where we are now when it emitted the light. Because of
the expansion of space, the light traveled a distance of 13.7 billion
light-years to reach us. The matter is now much farther away,
estimated to be 46 billion light-years distant.

We can see the CMBR in all directions despite the enormous distance
the light had to travel because it was emitted everywhere, and there
was so much of it. The earliest stars and galaxies are harder to see
even though light from them hasn't had to travel as far, because they
were scattered much more sparsely through space. Stars so far away
require very long observing times. The Hubble Space Telescope is
probably just able to make out a few of the earliest galaxies in the
very longest exposures it has made.

In general, the farther away the light source we see now, the farther
back in time the light was emitted. The oldest, most distant galaxies
we can see are less than 13 billion years old.

If the universe has been expanding at an accelerated pace for the last
13.7 billion years (and much longer than that if what we are seeing is
13.7 billion years ago - and it was a singularity at the time of the Big
Bang 13.7 billion years before that), would that not be 13.7 billion years
less than where the edge of the universe is right now? Wouldn't the
actual edge of the universe actually be much farther away - like at least
twice that - than 13.7 billion light years? And that is assuming that the
universe has been expanding at THE SPEED OF LIGHT!!
The actual size of the Universe is completely unknown, except that
it appears to be considerably larger than the part of it we can see.
Recent analysis of the CMBR showed no indication that we can see
the same parts of the CMBR in different directions, implying that the
Universe is too big for light to have traveled in opposite directions to
reach us from a single location. I personally think that the Universe
is finite, but it is theoretically possible that it is infinite in size.

Because we don't know whether there is any "edge" to the Universe,
we don't know how fast it is moving away from us, if it exists. If an
edge does exist, it most likely is moving away from us faster than the
speed of light. Speeds greater than the speed of light are not ruled
out by relativity theory when observed and observer are far apart.

The conventional wisdom is that the expansion slowed for the first
several billion years after the Big Bang, due to gravitational attraction,
and only began to speed up again a few billion years ago, due to dark
energy-- whatever it may be. I'm not familiar enough with the
observational data to say whether or not that interpretation is
justified. Your description of the accelerating expansion could be
correct: Maybe it has been accelerating from the beginning, not
just for the last few billion years.

Extrapolating backward in time from what we can see shows that
real tight. If you carry the extrapolation all the way back to time
zero, you get a singularity at which the Universe was infinitely dense
and infinitely hot. Current physics has no theory to describe how
the Universe got started, so we don't know what was actually going
on at the very beginnng. Undoubtedly the Universe was not infinitely
dense or infinitely hot, but we don't know what physics to put in
place of the mathematical singularity at time zero.

-- Jeff, in Minneapolis

01101001
2008-Dec-17, 06:32 AM
Wouldn't the actual edge of the universe actually be much farther away - like at least twice that - than 13.7 billion light years?

Wikipedia: Observable universe :: Size (http://en.wikipedia.org/wiki/Observable_universe#Size)

The comoving distance from Earth to the edge of the visible universe (also called particle horizon) is about 14 billion parsecs (46.5 billion light-years) in any direction.

Note, that is the comoving distance. If you want other methods of measurement, see Wikipedia: Distance measures (cosmology) (http://en.wikipedia.org/wiki/Distance_measures_%28cosmology%29).