What is the boundary of the observable universe in the Milne universe?

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The discussion centers on the distinctions between cosmological horizon and particle horizon in cosmology, particularly within the context of the Milne universe. Participants express confusion over whether the commonly referenced cosmological horizon is synonymous with the cosmological event horizon. The conversation highlights that for particle horizons to exist, the universe's expansion rate must be infinite at time zero, a condition not met in the Milne universe. Despite this, the Milne universe still has a boundary of the observable universe, raising questions about its classification as a particle horizon. Overall, the thread emphasizes the complexities of cosmological horizons and their interpretations.
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What is the difference of cosmological horizon and particle horizon in cosmology?
 
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Nobody knows the answer?
 
chronon said:
Take a look at http://www.chronon.org/Articles/cosmichorzns.html and http://www.chronon.org/articles/Cosmological_Event_Horizon.html on my website

Thx Chronon! I benefited a lot from your website, which is really cool! lol!

I have two more question: the cosmological horizon people often talk about actually means the the cosmological event horizon, doesn't it?
You said:"for particle horizons to occur the rate of expansion of the universe (the derivative of the scale factor with time) must be infinite at time zero.", why?
 
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micomaco86572 said:
the cosmological horizon people often talk about actually means the the cosmological event horizon, doesn't it?
I think that 'people' are sometimes confused about the issue, but that would be the most reasonable interpretation of cosmological horizon.
micomaco86572 said:
You said:"for particle horizons to occur the rate of expansion of the universe (the derivative of the scale factor with time) must be infinite at time zero.", why?
Well a freely coasting a universe without large scale gravity or cosmological constant - the (0,0) universe, also known as the Milne universe - doesn't have particle horizons. If you transform to conformal coordinates (See http://www.astro.ucla.edu/~wright/cosmo_03.htm#MSTD) then the conformal time goes back to minus inifinity.
 
But in the Milne universe we still have the boundary of the observable universe, do we? Can we refer to it as a particle horizon? thx
 

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