


Dark Energy and the CC problem
Particle physics theories typically predict the presence of a vacuum energy, at worst of the
order of the Planck mass M_{Pl} ~ 10^{72}GeV^{4}, which would cause the Universe to accelerate with a Hubble parameter H ~ 10^{61}km/s Mpc, or at best of the order of the supersymmetry breaking
scale M_{susy} ~ TeV^{4}. Since this is clearly incompatible with observations, physicists have for
a long time assumed that some symmetry sets this vacuum energy to zero. However, in 1998,
observations of type Ia supernovae measured for the first time the acceleration of the Universe
with precision, corresponding to a Hubble parameter of order H ~ 70km/s Mpc, i.e. consistent
with a cosmological constant of order 10^{48}GeV^{4}, that is 120 orders of magnitude smaller than
predicted from particle physics. This measurement was soon confirmed by many other sources,
such as the CMB, matter density and gravitational lensing and is to date arguably the most
embarrassing puzzle of modern cosmology.



