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Light year distance question



 
 
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  #1  
Old February 22nd 04, 12:18 PM
Tony Sims
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Default Light year distance question

I've recently read that a distant galaxy 11 billion light years away
has been spotted.

Does this mean that in actuality it was 11 billion light years away 11
billion Years ago or that it is that far away now?

If it was 11 billion light year away 11 billion years ago then how can
this be the case assuming the universe is 14 billion years old. Each
galaxy would have had to travel 5.5 billion light years in just three
billion years i.e. moving faster than the speed of light? This is
assuming they originated in the same place as per the big bang theory.

Please let me know if I'm way off the mark here, just interested to
know.

Tony Sims
  #2  
Old February 23rd 04, 09:30 AM
T
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Default Light year distance question

Tony Sims wrote:

I've recently read that a distant galaxy 11 billion light years away
has been spotted.

Does this mean that in actuality it was 11 billion light years away 11
billion Years ago or that it is that far away now?

If it was 11 billion light year away 11 billion years ago then how can
this be the case assuming the universe is 14 billion years old. Each
galaxy would have had to travel 5.5 billion light years in just three
billion years i.e. moving faster than the speed of light? This is
assuming they originated in the same place as per the big bang theory.

Please let me know if I'm way off the mark here, just interested to
know.

Tony Sims



Hmmm, this has gotten me the thinking.

Givens:

Age of Universe, since Big Bang; 14 Billion Years.

Distance from Earth of Observed Galaxy; 11 Billion Light Years.


OK, the light we observe from this Galaxy (I'll call it G1) has been
traveling for 11 Billion years. "Eleven Builliyon Years" to quote Carl
Sagan.

At the time the light we now observer was generated G1 was 11B Light
Years away. In the time it took to get here G1 was continued to move.

So, how far away _IS_ G1?


TBerk











  #3  
Old February 23rd 04, 02:29 PM
Mark Weinreb
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Default Light year distance question

It means that light from that galaxy has taken 11 billion years to reach us.
Every second light travels approximately 186,000 miles, which makes the
galaxy 11,000,000,000 * 186,000 * 60 * 60 * 24 * 365 miles away. I tried to
work it out on my calculator, but it screamed and hid behind the sofa!
Since the universe is expanding, it used to be closer. How close? well, one
as close as these two dots .. if all the theories are correct.

"Tony Sims" wrote in message
om...
I've recently read that a distant galaxy 11 billion light years away
has been spotted.

Does this mean that in actuality it was 11 billion light years away 11
billion Years ago or that it is that far away now?

If it was 11 billion light year away 11 billion years ago then how can
this be the case assuming the universe is 14 billion years old. Each
galaxy would have had to travel 5.5 billion light years in just three
billion years i.e. moving faster than the speed of light? This is
assuming they originated in the same place as per the big bang theory.

Please let me know if I'm way off the mark here, just interested to
know.

Tony Sims



  #4  
Old February 27th 04, 02:42 AM
Greg
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Default Light year distance question

T wrote in message .com...
Tony Sims wrote:

So, how far away _IS_ G1?



Well we don't measure how long it takes the light to travel. Its based
on the observed data (ie red shift, and the current Hubble constant or
type 1a supernova?), from this we estimate how far away it is more or
less now. IIRC..
  #5  
Old February 28th 04, 01:21 PM
william mook
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Default Light year distance question

(Tony Sims) wrote in message . com...
I've recently read that a distant galaxy 11 billion light years away
has been spotted.

Does this mean that in actuality it was 11 billion light years away 11
billion Years ago or that it is that far away now?

If it was 11 billion light year away 11 billion years ago then how can
this be the case assuming the universe is 14 billion years old. Each
galaxy would have had to travel 5.5 billion light years in just three
billion years i.e. moving faster than the speed of light? This is
assuming they originated in the same place as per the big bang theory.

Please let me know if I'm way off the mark here, just interested to
know.

Tony Sims


Well, you've asked a number of questions. The first, is how do
astronomers know how far away the object is. The answer is, they just
measured the Z of the object - its relativistic doppler shift. That's
all.

This is described elegantly at the following URL

http://hyperphysics.phy-astr.gsu.edu...v/reldop2.html

And you can check out the high-red-shift quasar here;

http://antwrp.gsfc.nasa.gov/apod/ap981211.html

These are some 15 billion light years from Earth. The Z's are on the
order of 5. Taking our equation we can see the relation between Z and
speed of light;


Z = SQRT((c+v)/(c-v)) - 1

And distance is given by the Hubble constant which astronomers believe
is between 16 km/sec and 25 km/sec per million light years. Let's
take the average of 20.5 km/sec per million light years. That's equal
to 0.006833% per million light years.

So, to produce a Z=5 requires a v=283,800 km/sec or v=94.6% the speed
of light. Dividing by 20.5 km/sec per million light years or
0.006833% obtains a distance of 13,843,902,000 light years.

That's pretty far away.

How big is the universe today?

Well if you put a v=c in the Z calculation, you'll find that Z becomes
infinite, but distance does not. With the hubble constant given the
universe is 14.634 billion light years in diameter. The Hubble
constant gives the size of the universe. If its smaller (16 km/sec
per million light years) the universe is 18.75 billion years across
and years old. If its larger (25 km/sec per million light years) the
universe is 12.00 billion light years across and years old.

The second part of your question deals with things in motion relative
to one another. In this regard its important to note that the speed
of light is constant for all observers and that light does not pass
for the photon. In fact the light ray that connects an observer to an
object is known to physicists and astronomers as the null-time path.
All light rays leaving an object in all directions at a particular
instant create what is known as a lightcone surrounding that instant
in their minkowski, or 4D space time, or hyperspace representation
(these all mean the same thing).

In this context its important to note that the assumption here (which
is in the process of changing given even more detailed analysis of new
data from Hubble and COBE and elsewhere) is that if something is
moving at 94.6% the speed of light now, it was moving at 94.6% the
speed of light 14.634 billion years ago. That's the way everything
comes back to a precise point at the beginning of time.

Now, you may recall that one part of relativity is the twin paradox.
That's where a twin moving rapidly in space relative to a twin living
at home ages more slowly than the stay at home twin. The interesting
thing is that BOTH twins observing one another with a telescope
observe the OTHER TWIN aging more slowly.

Your question revolves around your trouble with this paradox.

Which will require another ton of explanation... if you are unaware
of it, let me know, I'll take it offline with you!
  #6  
Old April 14th 05, 04:14 AM
Physicist1 Physicist1 is offline
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First recorded activity by SpaceBanter: Apr 2005
Posts: 5
Default

Quote:
I've recently read that a distant galaxy 11 billion light years away
has been spotted.

Does this mean that in actuality it was 11 billion light years away 11
billion Years ago or that it is that far away now?
Your questions are hopefully answered in the March Scientific American on page 36. 11 billion years ago it was closer, I believe. And now it is farther than 11 billion light years.

Quote:
If it was 11 billion light year away 11 billion years ago then how can
this be the case assuming the universe is 14 billion years old. Each
galaxy would have had to travel 5.5 billion light years in just three
billion years i.e. moving faster than the speed of light?
On page 42 I sumarize: An object 14 billion light years away now is receeding faster than light. But as the Hubble distance grows the photons will find themselves in a region of space that is receeding slower than light. Then they can reach us.
The current distance to the most distant object we can see is 46 billion light years. But we are seeing it as it was a long time ago and I doubt if we will ever see the light it is sending now. Better read it yourself.
  #7  
Old April 19th 05, 11:01 PM
Steve
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Default

I've recently read that a distant galaxy 11 billion light years
away
has been spotted.

Does this mean that in actuality it was 11 billion light years away

11
billion Years ago or that it is that far away now?


This is a little complicated, which probably explains why there have
been so many incomplete or wrong answers. Part of the problem is that
there are several different definitions of "distance" that are
important in cosmology.

Probably the best resource for questions of this type is Ned Wright's
Javascript "cosmology calculator" at
http://www.astro.ucla.edu/~wright/CosmoCalc.html
The calculator has links to explanations of the concepts involved. By
the way, the Doppler formula does not give correct answers although it
gives qualitatively the correct behavior that distance is finite at
infinite redshift.

If you put in the current favorite cosmology parameters H_0 = 71,
Omega_M=0.27 and a guess for redshift z=2.5 and push 'Flat', you find a
light travel time of 11 Gyr. This corresponds to a current comoving
radial distance of 19.3 billion light years. That is to say, if the
light left the galaxy 11 billion years ago and if you could measure the
distance away it is "now," you would find 19.3 billion light years. Of
course it will take 19.3 billion years for the light that galaxy is
emitting "now" to reach us. (I put "now" in quotes because time is
complicated, but you can think of "now" as meaning the epoch when the
microwave background temperature is 2.73 K.) It's hard to tell from
something as vague as a press release, but this was probably the
intended meaning.

If instead you put in z=1.02, you find a comoving radial distance of 11
billion light years and a light travel time of 7.8 billion years. This
meaning is also a possible interpretation.

  #8  
Old April 29th 05, 04:41 PM
spacejunkie spacejunkie is offline
Junior Member
 
First recorded activity by SpaceBanter: Apr 2005
Posts: 9
Default

Quote:
Originally Posted by william mook
And you can check out the high-red-shift quasar here;

http://antwrp.gsfc.nasa.gov/apod/ap981211.html

These are some 15 billion light years from Earth. The Z's are on the
order of 5.

…Sniped…

How big is the universe today?

Well if you put a v=c in the Z calculation, you'll find that Z becomes
infinite, but distance does not. With the hubble constant given the
universe is 14.634 billion light years in diameter. The Hubble
constant gives the size of the universe. If its smaller (16 km/sec
per million light years) the universe is 18.75 billion years across
and years old. If its larger (25 km/sec per million light years) the
universe is 12.00 billion light years across and years old.
I'm trying to understand this stuff with only a small amount of physics training so please forgive my ignorance. How is it possible for the universe to be "12.00 billion light years across" if we can observe objects that are 15 billion light years away?
 




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