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The Origin of the Universe

and the Arrow of Time

Sean Carroll, Caltech

How can we make sense of our unnatural-looking universe?

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The single most surprising thing aout the universe!

things change"

#ggs reak, ice cues melt, stars emit radiation,we record memories of the past" And it all happens in

a consistent $direction% &from the past to the future'

throughout the universe! the arrow of time"

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(h) does time have a direction?

*t+s not a feature of the microscopic laws of ph)sics"Those work euall) well forwards or ackwards in time

the) are $invariant under time reversal% &t-t, pi-p

i'"

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Toda)+s take-home messages!

." The origin of time+s arrow is cosmological"

/" (e don+t understand wh)"

0" (e might need to consider what happened

beforethe 1ig 1ang"

2" This is not 3ust recreationaltheolog)"

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#ver)one knows wh) there is an arrow of time! entrop)"

The /nd 4aw of Thermod)namics sa)s that entrop)tends to increase &in closed s)stems' as a function

of time" 5ot invariant under time reversal6

Alan 7uth+s office

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phase

space

sets of macroscopicall)

indistinguishale microstates

#ntrop) measures volumes in phase space"

1olt8mann! entrop)

increases ecause

there are more high-

entrop) states than

low-entrop) ones"

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#ntrop) increases ecause

there are more wa)s to e high-entrop)

than low-entrop)"

*f )ou start in a low-entrop) state,

evolving to higher entrop) is

the most natural thing in the world"

1ut wh) did the entrop) start out so low?

A uestion aout initial conditions! the earl) universe6

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The low entrop) near the 1ig 1ang is responsile

for ever)thing aout the arrow of time"

4ife and death

1iological evolution

9emor)

The $flow% of time

(ithout the arrow of time, we would e in thermaleuilirium -- ever)thing static, nothing ever changing"

The uestion is! wh) aren+t we in thermal euilirium?

:;enrose