1. The physics of inflation and dark energy 2.6 Acceleration in scalar field models Hubble “drag” Potential  V()V() Canonical scalar fields: If thekineticenergy => slow roll : energyis << than the potential With slow- roll,  works like a "time- dependent"  V  Equation of motion small

2. The physics of inflation and dark energy 2.7 Starting and stopping a slow-rolling scalar field  V()V() ! Hard to turn on at right time ! Inflation is easy to start : just prepare the scalar field somewhere, and "let it roll". If its energy   is not negligible, then it will quickly dominate, and the slow-rolling scalar field will cause spacetime to inflate.  -domination begins "here" when z~1,  0 = (2.3 x 10 - 3 eV) 4  V()V() begins ? reheating? not here ends But... not so good for dark energy: since   is nearly constant, there is usually a fine tuning problem with the the scalar field (as happens with  => it is very hard to adjust  and V(  ) such that   dominates the energy density at just the right time (z~1) !

3. The physics of inflation and dark energy 2.8 Dark energy - a theorist´s viewpoint Tracking dark energy w = w(t) w  = -1 Cosmological Constant w = - 1 w X = const. “Barotropic” dark energy w X = const. Fine tuning Coincidence Problem!!! Coincidence Problem!!! Self-adjusting (?) 1+z = a 0 /a radiation =>matter K-essence today

4. The physics of inflation and dark energy V()=m 4 exp[-]Wetterich 1988 V()=m 4+  - Ratra & Peebles 1988 V()=m 4 [cos(  )+1]Frieman et al 1995 V()=m 4 cosh[  ]Chimento & Jakubi 1996 V()=m 4+  - exp[  ]Binétruy 1998, Martin et al 1998 V()=m 4 exp[]Steinhardt et al 1998 V()=m 4 [ 1-(    exp[-]Albrecht & Skordis 1995 V()=whatevermany many authors   = (´) 2 /(1+a´+b´) (K-essence)Armendáriz-Picón et al 2001   = V()/[1-(d µ )2] (Tachyon)Sen 2002, Padmanabhan 2002 p  = -A/  (Chaplygin gas)Kamenshchick et al. 2001,... ??? (Branes)many many many many many Canonical ("honest-to-God") scalar fields: Other models: 2.9 The modelling battlefield Very common: attractor behavior for the background (Ferreira & Joyce 1998,Brax et al. 2000) and for the perturbations (Abramo & Finelli 2000) => results independent of the initial conditions!

5. The physics of inflation and dark energy Dark energy models - pro´s and con´s Scalar field, w = w (t) “Barotropic”, w X =const. (Ex: , w  = - 1) Pro: Easy to parametrize Cosmolog. quantities are simple expressions Observations so far only sensitive to w, not d w /dt Con: Fine tuning No good motivation (except for pure  ) Easy to model (Maybe) physically well- motivated Fair hope of explaining coincidence problem. Still, some fine tuning! Often not really well- motivated at all Can´t resolve time dependence of w Other models: K- essence, "rolling tachyon", Chaplygin gas, "vacuum metamorphosis",... ? ?????? ??????

6. The physics of inflation and dark energy 3. Inflation´s greatest achievement: particle creation and the origin of everything QUANTUM MECHANICS: "  E  t > h/2  " The vacuum is filled with virtual pairs of particles, which exist for very brief moments, before being annihilated back to nothingness.

7. The physics of inflation and dark energy 3.1 Virtual pairs and particle creation Right here, right now:

8. The physics of inflation and dark energy 3.2 Virtual pairs in an accelerating background Horizon H -1 Inflation (acceleration) converts virtual pairs into real pairs accelerated expansion rips pairs apart

9. The physics of inflation and dark energy 3.3 From quantum fluctuations to galaxies 10 -35 s 10 -33 s 3. 10 5 y 1.5 10 10 y quantum fluctuation classical fluctuation density and temperature perturbation Andromeda That´s us (Adapated from Lineweaver 1997)

10. The physics of inflation and dark energy Past light cone of an astronomer on Earth:

11. The physics of inflation and dark energy CMB sky CMB deconstructed

12. The physics of inflation and dark energy Conclusions There is an impressive amount of observational evidence suggesting that the Universe has suffered two phases of acceleration: one in the very early Universe (inflation, t~10 -35 s), and the other right about now (Dark energy age, t~10 10 years); The two phases appear to be completely unrelated: what works for one, does not seem to work for the other; Inflation will never be "confirmed": either the evidence will be consistent with it, or not. Must learn to live with that... The physics of inflation use, in a very deep way, both Quantum Mechanics and General Relativity - and the consequences are fully consistent with observations! The evidence for dark energy is persuasive and growing, but is still shaky. The case is not as solid as for inflation (yet); At present, nearly all dark energy models are contrived, fine-tuned and degenerate - in short, bad and ugly;