# Particle horizon

Cosmological horizon

The cosmological horizon, (also known as the particle horizon) is the maximum distance from which particles could have traveled to the observer in the age of the universe. It represents the boundary between the observable and the unobservable regions of the universe. The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model being discussed.

The particle horizon differs from the event horizon in that the particle horizon represents the largest comoving distance from which light could have reached the observer by a specific time, while the event horizon is the largest comoving distance from which light emitted now can ever reach the observer.

— Wikipedia on Observable universe

2010.08.06 Friday ACHK

# 知己知彼

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「知己知彼，百戰不殆」的原因是：

— Me@2010.08.05

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2010.08.06 Friday $copyright ACHK$

# 無足夠資料 5.1

— Me@2010.08.06

[1] 唔識就飛 = 想不通的話，就立刻停止原本的題目，去做下一題（或者是同一題的另一部分）