Understanding Cosmic Distances and Time: A Question on Stellar Measurements

[thetexan]

I have a question about stellar distances and time.

If I look up and see a star that I know is 10 light years away (determined by some current and accepted method of determining a star's distance) isn't it true that what I am actually saying is that 10 years ago (when the light left the star) that star was 10 light years distant from us (give or take the small relative change of position of the star and the Earth during that time)?

 

[Drakkith]

You are correct.

 

[thetexan]

Is it also true that if I look and find a star in the opposite direction, also 10 light years away, that I can say that 10 years ago when the light left both stars that each of those two stars, being 10 light years away from us (give or take the small relative change of the position of the stars and the earth), were, at that time, being in opposite directions, 20 light years from each other?

 

[Drakkith]

Yep.

 

[thetexan]

Then if I look and find a galaxy 13 billion light years away in one direction and another 13 billion light years in the opposite direction, then can I say that 13 billion years ago, nearly back to the beginning of the universe, that it's THEN observable diameter was at least 26 billion light years?

Stated another way, the farther out I observe (the farther back in time and closer to the beginning) it seems the bigger the universe was at that time. Is this the correct deduction?

 

[Jack21222]

Then if I look and find a galaxy 13 billion light years away in one direction and another 13 billion light years in the opposite direction, then can I say that 13 billion years ago, nearly back to the beginning of the universe, that it's THEN observable diameter was at least 26 billion light years?

Stated another way, the farther out I observe (the farther back in time and closer to the beginning) it seems the bigger the universe was at that time. Is this the correct deduction?

The opposite... the further you look back, the smaller the universe was at the time the light was emitted.

 

[thetexan]

The opposite... the further you look back, the smaller the universe was at the time the light was emitted.

Not according to the previous responder. And, if the logic holds as is laid out in the first few posts of this thread, then I am really confused by your statement.

I know that's what everyone says but can someone please explain the paradox. If I see two galaxies, each 13 billion light years away in opposite directions, why doesn't that indicate that their distance from each other 13 billion years ago was 26 billion light years from each other?
And...

If I were to then spot two galaxies, each 13.5 billion light years away in opposite directions, that would indicate that 13.5 billion years ago (even farther back in time) their distance from each other was 27 billion light years...a even larger distance!

What you say may be true, but I would like someone to explain the contradiction please.

 

[Arch2008]

Right after the Big Bang, the tiny universe began inflating. Dark energy causes the space between galaxies to continue to inflate. When the light left those galaxies it traveled through 13.5 billion light years of space to get here, because the original space in between was expanding during those 13.5 billion years. The space in between was much smaller to begin with, 13.5 billion years ago, and then expanded.

 

[Nabeshin]

Not according to the previous responder. And, if the logic holds as is laid out in the first few posts of this thread, then I am really confused by your statement.

I know that's what everyone says but can someone please explain the paradox. If I see two galaxies, each 13 billion light years away in opposite directions, why doesn't that indicate that their distance from each other 13 billion years ago was 26 billion light years from each other?
And...

If I were to then spot two galaxies, each 13.5 billion light years away in opposite directions, that would indicate that 13.5 billion years ago (even farther back in time) their distance from each other was 27 billion light years...a even larger distance!

What you say may be true, but I would like someone to explain the contradiction please.

The universe expands.

 

[thetexan]

Well, I am trying to grasp how the inflation would have anything to do with this.

Anyway you slice it, in my example, I am looking at photons that have traveled 13.5 billion light years thru something...either gradually expanding or rapidly inflating space...but whatever it is, there was 13.5 billion light years of it...IN BOTH DIRECTIONS, no less! That means the photons from those two galaxies began from points 27 billion light years from each other...13.5 billion years ago...doesnt it?

So does it matter what the condition of space was? Expanding slowly or inflating rapidly. 27 billion light years is 27 billion light years. I am looking at photons that have been traveling for 13.5 billions years that are from a galaxy that was IN FACT 13.5 billion light years away.

Unless there is some other factor that modified the speed of light in relation to distance I am in a quandry.

 

[russ_watters]

Not according to the previous responder.

What you say may be true, but I would like someone to explain the contradiction please.

The two scenarios are completely different, so there is no contradiction. There is no expansion detectable at a distance of 10 LY, so it doesn't have any impact on that scenario, whereas there is expansion detectable on the 13 LY scale, so it must be considered in the calculation of a past distance.

That means the photons from those two galaxies began from points 27 billion light years from each other...13.5 billion years ago...doesnt it?

No, not if the space in between is expanding.

 

[thetexan]

Help me think this out.

Lets say that 13.5 billion years ago the universe was, 10 light years in diameter and moving out fast as inflation.

At that moment the two galaxies (or photon producing light emitters) send out their respective photons, presumably at the speed of light. Due to inflation, the two emitters are expanding away from each other at hyper speeds...maybe even faster than light. Meanwhile their respective photons are racing towards each other at the speed of light. Their progress towards each other is retarded because the space within which they are traveling is expanding, so even though they are moving, they are moving against the stream so to speak...traveling at light speed in a space that is expanding at or faster than light speed.

So in other words, even though the photons have been moving for 13.5 billion years the emitters from which they were emitted were inflating outward as well as the space within which the photons are traveling. Is that right?

 

[Drakkith]

So in other words, even though the photons have been moving for 13.5 billion years the emitters from which they were emitted were inflating outward as well as the space within which the photons are traveling. Is that right?

The emitters AND the photons themselves are inflating outwards since the space that both occupied expanded, yes.