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

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Discussion Overview

The discussion revolves around the concepts of cosmological horizon and particle horizon within the context of the Milne universe. Participants explore the definitions, implications, and interpretations of these horizons in cosmology, particularly focusing on their differences and the nature of the observable universe.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants inquire about the difference between cosmological horizon and particle horizon, suggesting confusion in terminology.
  • One participant references external resources to clarify the concepts of cosmological and particle horizons.
  • Another participant proposes that the term "cosmological horizon" often refers to the "cosmological event horizon," indicating a common interpretation.
  • There is a discussion about the conditions under which particle horizons occur, specifically mentioning the Milne universe as an example where particle horizons do not exist due to its characteristics.
  • A participant questions whether the boundary of the observable universe in the Milne universe can be referred to as a particle horizon, indicating uncertainty about the terminology.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and implications of cosmological and particle horizons, and there is no consensus on whether the boundary of the observable universe in the Milne universe can be classified as a particle horizon.

Contextual Notes

Participants reference specific conditions and assumptions related to the expansion of the universe and the nature of horizons, which may not be fully resolved in the discussion.

micomaco86572
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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?
 
Last edited:
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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