We don’t know, all previous history has been wiped out Assume radiation dominated era

1. The Going Gets Weird What came before inflation? • We don’t know, all previous history has been wiped out • Assume radiation dominated era • We have unified three of the forces: Strong, Electromagnetic,and Weak • At about 1018GeV, gravity becomes as strong as other forces: • We need a quantum theory of gravity • Weird stuff can start happening • The “Planck Era” • Some complicated possibilities: • Entire universe might quantum tunnel from nothing • Time might become meaningless • Whole universe might be inflating, with only little pockets escaping to make universe

2. History of the Universe EventkBT or TTime Planck Era 1018GeV 10-43 s Inflation begins 31015GeV 10-38 s Inflation ends, baryons created, Grand Un. 31015GeV 10-36 s Supersymmetry Scale, Dark matter created 500 GeV 10-12 s Electroweak Scale 50 GeV 10-10 s Quark Confinement 150 MeV 1.410-5 s Neutrinos Decouple 1 MeV 0.4 s Neutron/Proton freezeout 0.7 MeV 1.5 s Electron/Positron Annihilation 170 keV 30 s Primordial Nucleosynthesis 80 keV 200 s Matter/Radiation Equality 0.76 eV 57 kyr Recombination 0.26 eV 380 kyr First structure formation 30 K 500 Myr Now 2.725 K 13.75 Gyr

3. Chaotic Inflation • Universe may have began in inflationary era, everywhere • Everything is expanding, very fast, everywhere • A small pocket manages to escape and start forming a universe • We’ll have to get slow roll, or something, to make it continue inflating enough • This pocket grows to make observable universe • Different bubbles will not collide – the universe is expanding too fast • It is quite possible that there is more than one way to escape from inflation • Different “universes” would have different fundamental constants

4. The String Landscape: • Most popular theory combining gravity and quantum mechanics is string theory • Many people (including me) criticize it since it has yet to produce a verifiable prediction • String theory is incredibly complex • No one understands it • The potentials (like the graphs we have) are effectively infinite dimensional • The number of minima – potential universes – is very, very large • The “String Landscape” • There may be many, many minima • Many possible universes with apparentlydifferent laws of physics

5. What Remains to be Explained? • Things we’ve resolved: • Why the universe has the fraction of hydrogen/helium, etc. we see • The nature of the cosmological background radiation • How all the structure in the universe formed from initial perturbations • Things we’ve got good guesses on: • Where the dark matter came from • Why  = 1 • Why the universe is nearly uniform • The likely causes of initial perturbations • Why there’s more matter than anti-matter in the universe • Things we don’t really know: • Where the universe came from • Why the vacuum energy density is so low • Why the various particle physics parameters are what they are

6. The Smallness of the Vacuum Energy Density • The vacuum energy density of the universe is just now becoming important • Naïve calculation says it should be infinite • Less naïve: It should be 10120 times bigger than it is • What if it hadn’t been so small? • If it had been 1000 times bigger, it would have dominated the universe at z = 14 • Exponential growth begins at this time, all structure formation stops • No globular clusters would have formed • If it had been 100 times bigger, it would have dominated the universe at z = 6 • No galaxies would have formed • If it had been 2-3 orders of magnitude bigger, we wouldn’t be here • No life would exist • Other similar coincidences seem to favor the existence of life • For example, resonance helps make carbon in stars

7. The Anthropic Principle • On the boundaries of respectable science • If string landscape is right (or similar ideas) there may be many “local minima” of whatever the potential is • Many “bubble universes” might be formed with different physical constants, vacuum energy densities, etc. • If there are enough of them, some will inevitably contain life that asks, “why is the universe the way it is?” • The anthropic principle: • Only universes where complex life is possibleshould be considered • Smallness of vacuum energy density may besimply because we need it to exist • Some other physical constants may similarly beinevitably “fine tuned” for life

8. The Final Exam • Not yet written – but I’m working on it • Makeup of exam – Current guess • 300 points total (30% of your final total grade) • 60 points: multiple choice • 40 points new material • 20 points old material • 120 points: short essays • 80 points new material • 40 points old material • 120 points: computation • All new material

9. History – what should I know? EventkBT or TTime Planck Era 1018GeV 10-43 s Inflation begins 31015GeV 10-38 s Inflation ends, baryons created, Grand Un. 31015GeV 10-36 s Supersymmetry Scale, Dark matter created 500 GeV 10-12 s Electroweak Scale 50 GeV 10-10 s Quark Confinement 150 MeV 1.410-5 s Neutrinos Decouple 1 MeV 0.4 s Neutron/Proton freezeout 0.7 MeV 1.5 s Electron/Positron Annihilation 170 keV 30 s Primordial Nucleosynthesis 80 keV 200 s Matter/Radiation Equality 0.76 eV 57 kyr Recombination 0.26 eV 380 kyr First structure formation 30 K 500 Myr Now 2.725 K 13.75 Gyr Events and Order Just Below 1 MeV Just Below 1 eV

10. Equations • Hubble’s Law: • Expansion of everything: • Scaling of density: • Radiation: • Friedmann Equation: • Omega: • Time-Temperature Relations Crude approximation: Include when mc2 < 3kBT Exclude when mc2 > 3kBT • Dens.Curv.Name • < 1 k = -1 Open • = 1 k = 0 Flat • > 1 k = +1 Closed • Collision rate:

11. Some Other Things to Know • Universe is always close to flat (=1) • When can you use the time formulas accurately? • Actual age of universe is about 13.7 Gyr. • Current temperature around 2.7 K • Which elements created during nucleosynthesis? • (1H), 4He, 2H, 3He, 6Li, 7Li • 3H and 7Be created but decayed • What problems does inflation seem to solve? • Flatness, Horizon, Primordial fluctuations • What is the relative size of each of the following components of the universe now? • Ordinary matter, dark matter, dark energy, radiation • Class Evaluations: • Please Fill them Out • Hand in to Volunteer • Check off your name Never forget the wonder and majesty of the Universe!