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# Are we at the centre of the Universe?

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Yes, I can imagine that but surely the two points (one on each line) which are at zero distance are coincident?

Steve

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yes, if we define "coincident" to mean zero distance apart. It does not make them necessarily the SAME point however.

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Just to throw a spanner in here, we have to be careful in interpreting this 13.7 billion light years. A source whose light has been in flight for 13.7 billion years is not, alas 13.7 billion lightyears away. The expansion of the universe works like compound interest. In its first year of the light's flight a small universe expanded at a certain rate per unit volume. In its last year of flight a much, much larger universe expanded at the same rate per unit volume- so since there was much more volume there was much more expansion, so the source was pushed back, so to speak, by a much greater extent.

(I know the Hubble Constant seems not to have been constant but for the sake of the point I'm making I'm treating it as if it were.)

Olly.

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I have to confess that I don't get that. So if the light has been travelling for 13.7 billion years, how far away was the object which sent it, when it started out, 13.7 billion years ago?

Steve

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I have to confess that I don't get that. So if the light has been travelling for 13.7 billion years, how far away was the object which sent it, when it started out, 13.7 billion years ago?

Steve

It WAS 13.7 billion light years away, but that was 13.7 billion years ago. It just might have moved even further away by now, based on the speed that it was travelling.

Kaptain Klevtsov

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Yes, that's it. The point the light set out from is not where it used to be. The expansion of the universe has pushed it back during the light's flight. It is no longer where it was at the start of the flight so it won't any longer be just 13.7 billion LY away.

However we would all do ourselves a key favour if we were more careful with the word 'movement.' A cosmologist would not describe two remote galaxies as getting further apart because they were 'moving.' Their mutual recession is not driven by their 'movement' but by the space between them mysteriously expanding. The usual analogy is this; stick coins onto a half inflated balloon with superglue so they can't move. These are galaxies and the rubber surface is space. Blow into the balloon. The glued coins can't move in their own local space but they can and do get further apart.

If they were not glued and could slide on the balloon any movement they performed would be called their Proper Motion and have nothing to do with the Hubble Flow (the expanding of the rubber surface.)

Some good cosmological research compared the proper motion of many galaxies with the rate of the Hubble Flow (expansion of universe) in their area and discovered that many are vainly beating against the current of expansion, attracted to some mysterious unknown christened 'The Great Attractor.' Love it! Love this stuff!!

Olly.

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I've looked but can't find an answer to this. I've read from lots of sources that 'we' believe that the edge of the Universe is 13.7 billion light years away. But in which direction? I can't see that mentioned anywhere.

Steve

It's wiered, yet simple, actually.

It's called the cosmological principle and states that no matter where, in the universe, you stand, everything always looks the same. So, every place is the centre, and every place is the edge. Also, this theory was a sort of a pivotal point that helped Hubble formulate the big bang theory*.

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(Please dont comment on threads that have been idle for many many months)

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(Please dont comment on threads that have been idle for many many months)

Sorry!

Stuff seemed interesting, that's all.

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