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# de Sitter space acceleration horizon?

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1 hour ago, vlaiv said:

Not sure if this is the case. Current accepted theory does not put constraint on geometry of universe, and there are three distinct possibilities - K=-1, 0 and 1.

For simplest case of 6 parameter lambda CDM model, it is assumed that K is 0 and Plank data gives current measurement of ΩK, to be 0.000±0.005. This puts lower bound of curvature radius in cases of positive and negative curvature but it does not tell us if space is indeed positively, negatively or zero curved.

Nope ...

Observable universe is indeed of 13.8Bly radius if you describe distances by light-travel measure.

There are different ways to measure distance in expanding universe. One of those measures - light-travel measure is straight forward, and in that measure light indeed needed to travel for 13.8 billion years traveling at light speed to cross the distance needed to reach us.

Then there is co-moving distance - that one gives radius of about 46Bly. This one should be read as - if you had a long enough ruler and place it in universe now - point that emitted light that traveled 13.8By at speed of light and covered 13.8Bly would sit at 46Bly on that ruler.

This is precisely due to expanding universe. If we take a point in time where photon emitted is still traveling to us, and is about half way between origin and us - for remaining travel time both space "behind" the photon and "in front" of the photon will stretch as photon is "flying". This is why "physical" distance is 46Bly while photon only traveled 13.8Bly - other distance comes from stretching of space (actually some photon travel distance also comes from stretching - as space "in front" of photon is still stretching as photon "flies"

For different distance measures used in cosmology look here:

Thanks vlaiv. Your explanation is more eloquent than mine but that is what I was meaning, so it seems something is sinking in to the grey matter (that is perhaps another confusing term to throw into the mix ).

You clearly have deep understanding - I presume you are schooled, or work, in this area?

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8 minutes ago, Astrokev said:

Thanks vlaiv. Your explanation is more eloquent than mine but that is what I was meaning, so it seems something is sinking in to the grey matter (that is perhaps another confusing term to throw into the mix ).

You clearly have deep understanding - I presume you are schooled, or work, in this area?

Not really

This has been sort of hobby of mine longer than astronomy. Recently I just started a thread on apparent galaxy sizes - it's related to this, and after that I just revisited some video lectures on cosmology by Susskind - these are a bit technical, but I highly recommend them for anyone interested, so this topic is still "fresh" in my mind.

Actually there are excellent lectures on many different topics in physics from Susskind, and in my view they are all very worth watching (Classical Mechanics, Special Relativity, General Relativity, Statistical Mechanics, QM, QFT, Cosmology, ....).

At times I feel that he is moving at rather slow pace, but he is in fact teaching a lesson to people in audience and he often needs to clarify things for everyone to understand.

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