Quintessence Dark Energy & Acceleration of the Universe B URIN G UMJUDPAI The Tah Poe Academia Institute for Theoretical Physics & Cosmology Department of Physics - Naresuan University - Phitsanulok Invited seminar Department of Physics, Chiang Mai University, Chiang Mai, Thailand September 6, 2006 Fundamental Physics & Cosmology Research Unit

B.S. (Physics), Chiang Mai University 1996 Now, back home!

Outlines 1.Observation of acceleration 2.What causes acceleration ? 3.Quintessence dark energy exponential potential scaling solution 4.Fate of the universe 5.Comments and conclusion Join the DARK SIDE !

1. Observation of acceleration 1.1 type IA supernovae (discovered in 1998) A. G. Riess et al. [Supernova Search Team Collaboration], Astrophys. J. 607, 665 (2004)

1.2 Cosmic Microwave Background anisotropies (WMAP) combined with Hubble Space Telescope J. L. Sievers et al., Astrophys. J. 591, 599 (2003) 1.3 WMAP combined with large scale galaxy surveys N. A. Bahcall, J. P. Ostriker, S. Perlmutter and P. J. Steinhardt, Science 284, 1481 (1999)

COBE 1990 WMAP 2003 the surface of last scattering

2. What causes acceleration ? 2.1 Dark energy (scalar field & cosmological constant) For review see E. J. Copeland, M. Sami and S. Tsujikawa, hep-th/ Cosmological constant might be coming back! Favored by recent CMB result! 2.2 Modification of gravity (including braneworld models) many authors 2.3 Backreaction of cosmological perturbations E. W. Kolb, S. Matarrese, A. Notari and A. Riotto, hep-th/

3. Quintessence dark energy There are (too) many models of dark energy Cosmological constant ( w = -1 ) Scalar fields: –Quintessence –K - essence –Tachyon field –Phantom field –Dilaton field –Chaplygin gas and many more...

Fune-tunning problem Astrophysics measures Particle physics predicts 121 order of magnitude difference!

3.1 Motivation -S-Solution of fine-tunning problem 3.2 Fundamental physics supports Supergravity inspired models Pseudo-Nambu-Goldstone models 3.3 The Quintessence model Quintessence scalar field action Variation of action gives Klein-Gordon field equation

The Eq. can also be obtained from using Acceleration eq. Friedmann eq.: energy+matter BALANCE with geometry : How it affect expansion With scalar field domination

Eq. of state From fluid eq., solution is Value within a range However, to attain acceleration, we need Then where

Exponential potential Expressed in tern of Hubble parameter so that Dark Matter can be taken into account.

Scaling solution Energy density of the scalar field mimics the background fluid energy density. When scaling solution is the attractor, any initial conditions, the field sooner or later enter the scaling regime Possibility of solving fine tuning problem of dark energy.

Single exponential suffering from the density exit i.e. it does have scaling behavior but cannot dominate the universe. There are many newer and more complicated models of quintessence potentials. There are many more other models of dark energy.

4. Fate of the Universe The universe will end up with some types of future singularities e.g. Big Rip at finite time, size & p & energy density infinity, Sudden at finite time, size is finite, p is finite but energy density infinity and others...

Dark energy is real Scaling solution can help solving the fine-tunning problem Exponential model is the earliest, simplest and give scaling solution but unviable due to being unable to dominate matter. There are many other models The universe will end up with more and more acceleration Attempting for avoidance of future singularities is one of our current work. Doing this, we are working on Loop Quantum Cosmology background with a dark energy dubbed “phantom field” 5. Conclusion & comments

Acknowledgement R. Maartens & D. Wands (Ph.D. supervisors) M. Sami & S. Tsujikawa (collaborators) F.P.C. Research Unit of the T PTP The Thailand Research Fund Home institute: Naresuan University The host: Prof. Thirapat Vilaithong & Dept. of Phys. Chiang Mai University Gratitude to my beloved parents