[thedoc (OP)]

Jean-Louis Dessalles  asked the Naked Scientists:
    Hi Chris,
 
I would like to ask this question to the Naked Scientist.
 
 
Since all galaxies far far away are old, and since the universe at that time was small, these ancient galaxies should appear to us as clustered in a specific corner of the sky, as they were necessarily close to each other. Yet, I suspect they can be spotted all around us. Can you explain this better than my son did to me by invoking paradoxical distance inversion?
 
 
 
Thanks,
 
Best wishes,
 
 jean-louis
 
 --
 Jean-Louis Dessalles - Telecom ParisTech jean-louis Dessalles
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What do you think?

[evan_au]

The Big Bang theory states that the universe started as hot, concentrated matter, that filled the entire volume of space.

The universe expanded, taking this matter with it, so that matter was still filling all of space - but less dense, and cooler.

Once the matter cooled sufficiently, it was able to form stars and galaxies. Because the matter filled all of the universe, stars and galaxies formed throughout the universe. Because the matter was expanding with the universe, the stars and galaxies carried with them the velocity at which the universe was expanding.

Even today, as the universe continues to expand, the stars and galaxies fill the universe, and carry the velocity that allows them to fill the expanding universe (but the density of matter is getting lower over time, as the universe expands).

[Bill S]

If, for example, I look at two galaxies that are now 13 billion ly from us, and 14 bly apart; I am looking at them as they were 13by ago. I must also be looking at them where they were 13by ago, so why are they not close together. 

That puzzled me for a long time until I thought about what we mean by “where”.  I reasoned that these galaxies still occupied the same locations, with respect to the Universe, as they had done 13by ago; thus they could be said to be in the same places.  Is my reasoning right?

This still left me with a problem.  If I am seeing them as being 14bly apart, they must have been that distance apart 13by ago, so how big was the Universe 13by ago?

[jeffreyH]

If, for example, I look at two galaxies that are now 13 billion ly from us, and 14 bly apart; I am looking at them as they were 13by ago. I must also be looking at them where they were 13by ago, so why are they not close together. 

That puzzled me for a long time until I thought about what we mean by “where”.  I reasoned that these galaxies still occupied the same locations, with respect to the Universe, as they had done 13by ago; thus they could be said to be in the same places.  Is my reasoning right?

This still left me with a problem.  If I am seeing them as being 14bly apart, they must have been that distance apart 13by ago, so how big was the Universe 13by ago?

That is an excellent question. Let me have a think.

[Bill S]

Let me have a think.

Should that have been "drink"?  It's the sort of question that a single malt helps with. []

[evan_au]

If I am seeing them as being 14bly apart, they must have been that distance apart 13by ago, so how big was the Universe 13by ago?

What you observe is two quasars that you measure as being 13 bly away from us, based on their red shift.
You observe them at widely-separated places on the sky - if you drew this as a Euclidean triangle, you might calculate the distance between the quasars as 14 bly.

However, on large timescales, the universe is not Euclidean. The photons emitted by these quasars are carried along with the expansion of the universe, so they follow curved paths, not straight lines.

According to current theories, quasars which are 13 bly from us would have been within 1-2 bly of each other, at the time the light was emitted.

[jeffreyH]

Yeah. What Evan said. Now where is my drink.

[syhprum]

In the strange 4 dimensional universe in which we exist is not every one at the centre