Big Bang Theory and physics view of universe., Slides of Physics

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Sean Carroll, Caltech

What We (Don’t) Know About

the Beginning of the Universe

1. What we know about the Big Ban
2. The spacetime viewpoint
3. The quantum viewpoint

What we know about the Big Bang:

1. Something Bang-like happened.

standard GR (CDM) today

allowed histories

[Planck] [Carroll & Kaplinghat]

cosmic background radiation (^) primordial nucleosynthesis

The universe 13.8 billion years ago was hot, dense, expanding very rapidly, and decelerating.

What we know about the Big Bang:

3. The early universe had extremely low entropy.

time

early universe S ~ S radiation ~ 1088

today S ~ S BH ~ 10103

future S ~ S dS ~ 10123

Of all the states that look macroscopically like our present universe, only a tiny fraction evolved from smooth states. Most were chaotic, Planckian, singular.

space of states

“macrostates” = sets of macroscopically indistinguishable microstates

Boltzmann, 1870s: entropy counts the number of states that look the same macroscopically.

Low initial entropy is an enormous fine-tuning Calls out for a robust explanation.

1. What it’s like to have a beginning.

The spacetime viewpoint on the beginning of th

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time

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2. Ways of avoiding a beginning
   • eternal universes. Bouncing

Hibernating Reproducing

Cyclic size

time

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What it’s like to have a beginning

Don’t ever say the universe “came into existence.” Sounds like a process within time, rather than the beginning of time itself.

Rather, there was an initial moment – a time before which there was no other time.

What “caused” the universe?

Wrong question. Rather: is it plausible that the laws of physics allow for a universe with a beginning?

(Yes.)

Bouncing cosmologies have an entropy puzzle:

• If entropy grows monotonically, requires infinite fine-tuning.
• If entropy has a minimum at the bounce, why?

size

time

entropy

?^?

Cyclic cosmologies

Repeat the bounce over and over.

[Turok, Steinhardt; Penrose]

Both cyclic and hibernating cosmologies have an entropy catastrophe:

• Entropy grows monotonically for all time. Requires infinite fine-tuning in the infinite past.

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entropy

[Farhi, Guth, Guven]

Reproducing cosmologies

Imagine a “parent” universe that is itself quiescent and high-entropy.

But through some mechanism it can give birth to new offspring universes, with initially low entropy.

E.g. spacetime quantum tunneling into disconnected “baby universes.”

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Result: a time-symmetric multiverse

• (^) New universes branch off from the parent universe both directions of time. Overall time-symmetric.
• (^) Easier to create new low-entropy universes than hig entropy ones.
• (^) This might explain why our Big Bang had low entrop

Reproducing cosmologies don’t have an entropy probl

• Entropy grows without bound toward past and future
• There is a middle point of lowest entropy, but it needn’t be “low” in any objective sense. (Indeed, it can be locally maximal.)

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entropy

[Carroll & Chen; see also Barbour, Koslowski & Mercati; Hartle & Hertog; Goldstein, Tomulka & Zanghi; Carroll &

time(?)

Derived/ emergent

space

particles^ fields causality

light cones

metric (^) collapse/ branching

wave functions

Hilbert space tensor products entanglement Hamiltonian information

entropy

pointer states

Fundamental

Emergence in QM

locality

Time evolution: the Quantum Eternity Theorem

• (^) Consider a universe described by a quantum state obeying Schrödinger’s equation

with nonzero energy, governed by laws of physics that are independent of time.

• (^) Then: the universe is eternal. (Time t runs from –∞ to +∞.)

[Carroll, 2008, arxiv:0811.3722]