# What We (Don't) Know About the Beginning of the Universe

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- Sean Carroll, Caltech What We (Donât) Know About the Beginning of the Universe What we know about the Big Bang The spacetime viewpoint The quantum viewpoint
- What we know about the Big Bang: 1. Something Bang-like happened. [Carroll & Kaplinghat] [Planck] 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: 2. Classical GR suggests singularities are generic. Highly symmetric universes tend to have an initial singularity (LemaÃ®tre). More strongly, Hawkingâs theorem: compact expanding universes obeying the Strong Energy Condition (gravity attracts) always have singularities. But the Strong Energy Condition can be violated. And theorists are happy to consider modifying GR.
- What we know about the Big Bang: 3. The early universe had extremely low entropy. time early universe S ~ Sradiation ~ 1088 today S ~ SBH ~ 10103 future S ~ SdS ~ 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 âmacrostatesâ = 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.
- Inflation doesnât explain why entropy was initially low. Inflation: if a patch of space starts in a false vacuum, it naturally accelerates, creates energy, smooths out, and reheats into matter and radiation. But that initial proto-inflationary state is even lower-entropy than the conventional hot big Bang (1 < Sinflation < 1015). You donât explain low entropy by positing even lower entropy.
- What itâs like to have a beginning. The spacetime viewpoint on the beginning of the universe 2. Ways of avoiding a beginning â eternal universes. Bouncing Reproducing Hibernating Cyclic
- What itâs like to have a beginning Donât ever say the universe âcame into existence.â Sounds like a process within time, rather than the beginning of time itself. Rather, there was an initial moment â a time before which there was no other time. What âcausedâ the universe? Wrong question. Rather: is it plausible that the laws of physics allow for a universe with a beginning? (Yes.)
- Bouncing cosmologies Smooth out the singularity, either through new degrees of freedom (fields, branes, dimensions), or through intrinsically quantum effects. Stringy Bounce [Veneziano] Quantum Cosmology [Bojowald, Ashtekar, Page, Hartle, Hawking, Hertog] de Sitter Bounce [Aguirre, Gratton] Ekyprotic Bounce [Khoury, Ovrut, Steinhardt, Turok]
- Bouncing cosmologies have an entropy puzzle: If entropy grows monotonically, requires infinite fine-tuning. If entropy has a minimum at the bounce, why?
- Cyclic cosmologies Repeat the bounce over and over. [Turok, Steinhardt; Penrose]
- Hibernating cosmologies Universe is quiescent and quasi-stationary into the eternal past; at some point undergoes a phase transition and begins to expand. [Brandenberger, Vafa] [Greene, Hinterbichler, Judes, Parikh] String gas cosmology Primordial degravitation
- Both cyclic and hibernating cosmologies have an entropy catastrophe: Entropy grows monotonically for all time. Requires infinite fine-tuning in the infinite past.
- [Farhi, Guth, Guven] Reproducing cosmologies Imagine a âparentâ 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 âbaby universes.â
- 2 large dimensions Alternatively: spontaneous compactification Six-dimensional de Sitter space w/electromagnetic fields will spontaneously nucleate four-dim de Sitter universes. [Carroll, Johnson, & Randall]
- Result: a time-symmetric multiverse New universes branch off from the parent universe in both directions of time. Overall time-symmetric. Easier to create new low-entropy universes than high- entropy ones. This might explain why our Big Bang had low entropy.
- Reproducing cosmologies donât have an entropy problem! Entropy grows without bound toward past and future. There is a middle point of lowest entropy, but it neednât be âlowâ in any objective sense. (Indeed, it can be locally maximal.) entropy [Carroll & Chen; see also Barbour, Koslowski & Mercati; Hartle & Hertog; Goldstein, Tomulka & Zanghi; Carroll & Guth]
- The quantum viewpoint on the beginning of the universe Quantum theory describes the evolution of quantum states living in a Hilbert space H, obeying SchrÃ¶dingerâs equation . We often start with a classical system and âquantizeâ it, yielding a quantum theory âofâ that system. But thatâs human convention, not Nature. Honest quantum questions are about what happens to vectors in Hilbert space, evolving under the SchrÃ¶dinger equation.
- time(?) Derived/ emergent space fields particles 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 [Carroll, 2008, arxiv:0811.3722]
- In quantum mechanics, if time is fundamental, it never ends. Expand the state in energy eigenstates: Each phase just rotates in a circle; the set of all of them move in a straight line through a torus. No singularities, barriers, etc. A generic quantum universe lasts forever, without a beginning or an end.
- Recurrence theorem: if Hilbert space H is finite-dimensional, states return to their starting points infinitely often. Problems with an eternal quantum universe: recurrences, fluctuations, Boltzmann brains. Entropy is usually maximal (equilibrium). Downward fluctuations are suppressed: Almost all observers are minimal fluctuations: âBoltzmann brains.â
- Possible solution: Hilbert space is infinite-dimensional. There is no recurrence theorem in an infinite-dimensional Hilbert space. Quantum equivalent of an unbounded phase space. The quantum state has infinite room to grow and change. This is the kind of quantum theory that might ultimately have as an emergent classical spacetime a bouncing or reproductive cosmologies. Entropy growing without bound in both directions of time.
- Alternative: time is emergent, not fundamental Loophole for Quantum Eternity Theorem: we live in a single energy eigenstate. E.g. . Seems non-generic, but is exactly what we get by quantizing general relativity: the Wheeler-DeWitt equation for a wave function of spatial three-metrics. Where does time come from? [e.g. Hartle, Hawking]
- Time can emerge in quantum mechanics because we can superpose different states [Page & Wootters 1983]
- If Hilbert space is infinite-dimensional, emergent âtimeâ can run forever. No need for a beginning â but there could be one. But if Hilbert space is finite-dimensional, there are only a finite number of possible clock states. Therefore, time will have a beginning.
- universe had a beginning universe may or may not have had a beginning universe is eternal, with a finite recurrence time (& Boltzmann brains) universe is eternal, and need never experience recurrence Was the Big Bang the beginning of the universe? time is emergent time is fundamental

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