Dark Energy: the threefold way
The apparent acceleration of the universe (``dark energy”) is among the most amazing discoveries - and challenging problems - of contemporary physics. One of the most puzzling aspects of this phenomenon is the extremely tiny energy scale to which it is associated. A relevant part of my research has been devoted to tackle this problem, from three directions:
Top-Down
Bottom-Up
Inside-Out
Take your favorite unifying/supersymmetric/quantum gravity theory and see whether it has something to say about dark energy. It is hard to make the right energy scale come out naturally, but you can have new mechanisms and degrees of freedom (fields) to explain the acceleration. Try to look for complementary phenomenology (e.g. coupling variations, E.P. violations, growth of structures) that can constrain the model independently.
Bibliography:
M. Gasperini, F. Piazza and G. Veneziano, ``Quintessence as a run-away dilaton”, Phys. Rev. D65 023508 (2002), gr-qc/0108016
T. Damour, F. Piazza and G. Veneziano, ``Runaway dilaton and equivalence principle violations”,
T. Damour, F. Piazza and G. Veneziano, ``Violations of the equivalence principle in a dilaton runaway scenario”, Phys. Rev. D66 046007 (2002) hep-th/0205111
F. Piazza and S. Tzujikawa, ``Dilatonic ghost condensate as dark energy”, JCAP 0407:004 (2004)
Forget about your favorite unifying/supersymmetric/quantum gravity theory and try to describe the behavior of dark energy effectively. Use general hints from the theory to put sensible priors in your parameter space.
Bibliography:
L. Amendola, M. Gasperini and F. Piazza, ``Fitting Type Ia supernovae with coupled dark energy”
JCAP 0409:014 (2004) astro-ph/0407573
L. Amendola, M. Gasperini and F. Piazza, ``SNLS data are consistent with acceleration at z=3''
R. Crittenden, E. Majerotto and F. Piazza, ``Measuring deviations from a cosmological constant: a field-space parameterization'', Phys. Rev. Lett. 98, 251301 (2007) astro-ph/0702003
S. Nesseris, F. Piazza and S. Tsujikawa, ``The universe is accelerating. Do we need a new mass scale?'' 0910.3949
The appearent acceleration of the universe might be calling for a major reassessment of our current understanding of gravity at low energy. Have you been daring enough?
Bibliography:
F. Piazza, ``Modifying Gravity in the Infra-Red by imposing an 'Ultra-Strong' Equivalence Principle''
To appear in the December 2009 Special Issue of Int. J. Mod. Phys. D, 0907.4299
(Honorable mention at the Gravity Research Foundation Essay Competition 2009)
F. Piazza, ``The IR-Completion of Gravity: What happens at Hubble Scales?'', New J. Phys. 11 (2009) 113050 0907.0765
S. Nesseris, F. Piazza and S. Tsujikawa, ``The universe is accelerating. Do we need a new mass scale?'' 0910.3949
F. Piazza, ``Some new views on gravity at low energy'' 0910.4677
Invited Talks:
``The accelerating Universe: new physics at small energy”, APC, Paris, Feb 5, 2009
The Search for Variations of Fundamental Couplings and Mass Scales.
Perimeter Institute of Theoretical Physics, Waterloo, July 14 - 18, 2008
``Measuring deviations from a cosmological constant: a slow-roll paradigm for dark energy"
Facts and Fictions in Cosmology, Sils Maria (Switzerland), March 26-April 2, 2006
``Dark Energy and alpha variations"
Dark Energy Day, Milano-Bicocca University, November 22, 2002
``Cosmology and gravity of the dilaton at strong coupling"
String/Brane Cosmology, IHES and Universite de Paris-Sud, Orsay (France) September 23-27, 2002