How can we be seeing the beginning of the universe?

The Hubble Telescope has been viewing galaxies as they were near the
beginning of the universe, some 15 billion years ago. It has taken
that long for the light to reach us, so what we see is what these
galaxies looked like so long ago.

My question is how can the light emitted from these galaxies so long
ago only now be reaching us? If the universe has been expanding, then
way back then we were much closer than we are now. How did we end up
so far away? I thought light traveled faster than anything else, so
this light should have passed us a long, long time ago.

coastsider wrote:

The Hubble Telescope has been viewing galaxies as they
were near the beginning of the universe, some 15 billion
years ago. It has taken that long for the light to reach
us, so what we see is what these galaxies looked like so
long ago.

My question is how can the light emitted from these
galaxies so long ago only now be reaching us? If the
universe has been expanding, then way back then we were
much closer than we are now. How did we end up so far
away?

As you said, the Universe is expanding.

I thought light traveled faster than anything else, so
this light should have passed us a long, long time ago.

How long ago?

The speed of recession of distant galaxies increases
with distance. The farther away a galaxy is, the faster
we and that distant galaxy are moving apart. The farther
away a galaxy was when it emitted the light which reaches
us now, the longer the light had to travel to reach us.

Although the galaxies we now see as very distant were
closer to us billions of years ago, they still were very
far away. Light which has travelled toward us for ten
billion years (that is, light from a galaxy which now
appears to be ten billion light-years away) may have been
nine billion light-years away when the light was emitted.
The distance between us and them has increased considerably
over such a long period of time, so naturally the light
has to travel considerably farther to reach us, which
takes additional time.

If two swimmers in an ocean start out nine miles apart,
and swim away from each other at one mile per hour, a bird
which at the same moment starts flying from one swimmer to
the other at ten miles per hour will reach the second
swimmer after ten hours.

Expansion of space makes the situation more complex, but
the above essentially answers the questions you raised.
Times work out the way they do as a direct result of the
speeds and distances between galaxies.

Notice that the apparent distance of any object is equal
to the distance the light had to travel to reach us. That
is true whether the object is a hydrogen atom which emitted
a bit of light in the cosmic background radiation 13.7
billion years ago, or a phosphor on the computer monitor
screen two feet in front of you which emitted a bit of
light two nanoseconds ago.

-- Jeff, in Minneapolis

Your answer misses one of the basic points of my question. I'm not
asking how we can see galaxies from a long time ago. I'm asking how we
can see galaxies from the beginning of the universe.

... Light which has travelled toward us for ten
billion years (that is, light from a galaxy which now
appears to be ten billion light-years away) may have been
nine billion light-years away when the light was emitted.

The Hubble is observing galaxies that were less than a billion years
old at the time. And the light took about 14 billion years to reach
us. You can't account for that with the math in your swimmer example.
For instance, let's say we were 1 billion light years apart when the
light left that galaxy observed by Hubble. The light then travels for
14 billion years. Where would we be relative to the galaxy after the
14 billion years? Even if we were traveling away from eachother at 0.8
times the speed of light (which we aren't!) the light would still have
passed us several billion years ago.

Perhaps this question boils down to how fast are we traveling away from
galaxies so far away, i.e., galaxies from near the beginning of the
universe? If we can observe the light from those galaxies today, that
implies we are traveling away from the galaxy at something close to the
speed of light. I don't think galaxies are traveling apart from one
another at close to the speed of light. Or are they?

Jeff Root wrote:
coastsider wrote:

The Hubble Telescope has been viewing galaxies as they
were near the beginning of the universe, some 15 billion
years ago. It has taken that long for the light to reach
us, so what we see is what these galaxies looked like so
long ago.

My question is how can the light emitted from these
galaxies so long ago only now be reaching us? If the
universe has been expanding, then way back then we were
much closer than we are now. How did we end up so far
away?

As you said, the Universe is expanding.

I thought light traveled faster than anything else, so
this light should have passed us a long, long time ago.

How long ago?

The speed of recession of distant galaxies increases
with distance. The farther away a galaxy is, the faster
we and that distant galaxy are moving apart. The farther
away a galaxy was when it emitted the light which reaches
us now, the longer the light had to travel to reach us.

Although the galaxies we now see as very distant were
closer to us billions of years ago, they still were very
far away. Light which has travelled toward us for ten
billion years (that is, light from a galaxy which now
appears to be ten billion light-years away) may have been
nine billion light-years away when the light was emitted.
The distance between us and them has increased considerably
over such a long period of time, so naturally the light
has to travel considerably farther to reach us, which
takes additional time.

If two swimmers in an ocean start out nine miles apart,
and swim away from each other at one mile per hour, a bird
which at the same moment starts flying from one swimmer to
the other at ten miles per hour will reach the second
swimmer after ten hours.

Expansion of space makes the situation more complex, but
the above essentially answers the questions you raised.
Times work out the way they do as a direct result of the
speeds and distances between galaxies.

Notice that the apparent distance of any object is equal
to the distance the light had to travel to reach us. That
is true whether the object is a hydrogen atom which emitted
a bit of light in the cosmic background radiation 13.7
billion years ago, or a phosphor on the computer monitor
screen two feet in front of you which emitted a bit of
light two nanoseconds ago.

-- Jeff, in Minneapolis
Jeff Root wrote:
coastsider wrote:

The Hubble Telescope has been viewing galaxies as they
were near the beginning of the universe, some 15 billion
years ago. It has taken that long for the light to reach
us, so what we see is what these galaxies looked like so
long ago.

My question is how can the light emitted from these
galaxies so long ago only now be reaching us? If the
universe has been expanding, then way back then we were
much closer than we are now. How did we end up so far
away?

As you said, the Universe is expanding.

I thought light traveled faster than anything else, so
this light should have passed us a long, long time ago.

How long ago?

The speed of recession of distant galaxies increases
with distance. The farther away a galaxy is, the faster
we and that distant galaxy are moving apart. The farther
away a galaxy was when it emitted the light which reaches
us now, the longer the light had to travel to reach us.

Although the galaxies we now see as very distant were
closer to us billions of years ago, they still were very
far away. Light which has travelled toward us for ten
billion years (that is, light from a galaxy which now
appears to be ten billion light-years away) may have been
nine billion light-years away when the light was emitted.
The distance between us and them has increased considerably
over such a long period of time, so naturally the light
has to travel considerably farther to reach us, which
takes additional time.

If two swimmers in an ocean start out nine miles apart,
and swim away from each other at one mile per hour, a bird
which at the same moment starts flying from one swimmer to
the other at ten miles per hour will reach the second
swimmer after ten hours.

Expansion of space makes the situation more complex, but
the above essentially answers the questions you raised.
Times work out the way they do as a direct result of the
speeds and distances between galaxies.

Notice that the apparent distance of any object is equal
to the distance the light had to travel to reach us. That
is true whether the object is a hydrogen atom which emitted
a bit of light in the cosmic background radiation 13.7
billion years ago, or a phosphor on the computer monitor
screen two feet in front of you which emitted a bit of
light two nanoseconds ago.

-- Jeff, in Minneapolis

"coastside1000" wrote in message
oups.com...

Perhaps this question boils down to how fast are we traveling away from
galaxies so far away, i.e., galaxies from near the beginning of the
universe? If we can observe the light from those galaxies today, that
implies we are traveling away from the galaxy at something close to the
speed of light. I don't think galaxies are traveling apart from one
another at close to the speed of light. Or are they?

The initial expansion of the universe during what is
known as the "inflation era" took place at speeds
far in excess of the speed of light (General Relativity
places no speed limit on how fast regions of space
may expand relative to one another. It only constrains
the speed of objects embedded within space to less
than the speed of light).

So, light from what were relatively close-by objects
which are now (thanks to inflation and expansion)
very distant, is only now reaching us.

coastsider wrote:
The Hubble Telescope has been viewing galaxies as they were near the
beginning of the universe, some 15 billion years ago. [...]
My question is how can the light emitted from these galaxies so long
ago only now be reaching us?

This is a very frequently asked question. The answer is usually that the
questioner has an incorrect mental picture of the big bang. Take a look at
Ned Wright's cosmology tutorial:

http://www.astro.ucla.edu/~wright/cosmolog.htm

and see if helps.

-- Ben

Greg Neill wrote:
(General Relativity
places no speed limit on how fast regions of space
may expand relative to one another. It only constrains
the speed of objects embedded within space to less
than the speed of light).
I've never heard about that bit. Are there any books,
papers or other ressources on it?
Until now I just considered hyperinflation a point where
we something happened we assume to be impossible but
arbitrarily decided not to send a speeding ticket to
the universe.

Lots of Greetings!
Volker

The Hubble Telescope has been viewing galaxies as they were near the
beginning of the universe, some 15 billion years ago. It has taken
that long for the light to reach us, so what we see is what these
galaxies looked like so long ago.

My question is how can the light emitted from these galaxies so long
ago only now be reaching us? If the universe has been expanding, then
way back then we were much closer than we are now. How did we end up
so far away? I thought light traveled faster than anything else, so
this light should have passed us a long, long time ago.

Well, the Universe is something like this:

http://news.yahoo.com/photo/061018/p...BHN lYwN0bXA-

After two galaxies collided, the whole thing is a mess, and from that
mess stars born, and a new galaxy forms.

The big bang may VERY LIKELY be a big collision in space.

We don't see the beginning because the visible Universe was evolving
after a big bang, and for some stupid reason we can't see before
the big bang.

Once the Universe clears from a giant mess caused by a massive
collision in the past, it takes an organized form (a necessity of
gravity).

Usually the simpler explanations tend to prove correct, and a flat
Earth is refuted by scientists on all scales. Since we don't know,
we perceive the Universe as flat and square.

Greg Neill wrote:
[...] So, light from what were relatively close-by objects
which are now (thanks to inflation and expansion)
very distant, is only now reaching us.

Inflation had nothing to do with it. The visible universe was still very
small at the end of the inflationary epoch ("the size of a grapefruit"), and
that's assuming that inflation actually happened at all, which is far from
certain. The fact that we can see ancient galaxies is fully explained by the
FRW cosmology, which is much less speculative and better confirmed than
inflation.

-- Ben

Volker Hetzer wrote:
I've never heard about that bit. Are there any books,
papers or other ressources on it?
Until now I just considered hyperinflation a point where
we something happened we assume to be impossible but
arbitrarily decided not to send a speeding ticket to
the universe.

No, certainly not. Inflation is just another solution of the field equations
of general relativity. The spacetime manifold defines what it means for
objects "in" the manifold to be going at a certain relative speed, but the
manifold itself doesn't follow any speed constraints. It's not that it
violates any speed constraints either, it's just that there's no way to
define the constraints, because the manifold pretty much by definition can't
move or have a speed. According to the usual definition of expansion speed,
the visible universe expanded faster than light in the inflationary epoch,
and it's *still* expanding faster than light (around 3.4 times the speed of
light at present). But that's not a physically meaningful quantity; I think
it was invented just because people expect it to exist. Astronomers normally
talk about expansion in terms of the Hubble constant, which doesn't have
units of speed.

I'm not sure what reading to recommend. There are tons of books and papers
on general relativity and cosmology, of course. You should certainly read
Ned Wright's cosmology tutorial, which I linked elsewhere in this thread. He
talk about inflation in part 4, but not in much detail.

-- Ben

coastside1000 wrote:
Your answer misses one of the basic points of my question. I'm not
asking how we can see galaxies from a long time ago. I'm asking how we
can see galaxies from the beginning of the universe.

... Light which has travelled toward us for ten
billion years (that is, light from a galaxy which now
appears to be ten billion light-years away) may have been
nine billion light-years away when the light was emitted.

The Hubble is observing galaxies that were less than a billion years
old at the time. And the light took about 14 billion years to reach
us. You can't account for that with the math in your swimmer example.
For instance, let's say we were 1 billion light years apart when the
light left that galaxy observed by Hubble. The light then travels for
14 billion years. Where would we be relative to the galaxy after the
14 billion years? Even if we were traveling away from eachother at 0.8
times the speed of light (which we aren't!) the light would still have
passed us several billion years ago.

Suppose the light was emitted at a cosmic age
of 1Ga (1 billion years). If the galaxy formed at
say 600 Ma then it would appear 400 Ma old in
that light. Time now is 13.7 Ga so the light has
been travelling for 12.7 Ga. We must be 12.7 Ga
away from it now so if we started just under
1 billion light years from the emitter then we have
moved 11.7 billion light years in that time. That is
an apparent speed of 0.92c.

Perhaps this question boils down to how fast are we traveling away from
galaxies so far away, i.e., galaxies from near the beginning of the
universe? If we can observe the light from those galaxies today, that
implies we are traveling away from the galaxy at something close to the
speed of light. I don't think galaxies are traveling apart from one
another at close to the speed of light. Or are they?

In that sense, yes, hence the extreme Doppler shifts.
In fact GR says that galaxies even farther away are
"moving" faster than the speed of light. What you need
to realise is that it isn't motion through space in the
sense of SR. The space between the source and us
is expanding and there is no limit on how high the
resulting apparent speed can be.

George