A Constraint on Planck-scale Modifications to Electrodynamics with CMB
  polarization data
  We show that the Cosmic Microwave Background (CMB) polarization data gathered
by the BOOMERanG 2003 flight and WMAP provide an opportunity to investigate
{\it in-vacuo} birefringence, of a type expected in some quantum pictures of
space-time, with a sensitivity that extends even beyond the desired
Planck-scale energy. In order to render this constraint more transparent we
rely on a well studied phenomenological model of quantum-gravity-induced
birefringence, in which one easily establishes that effects introduced at the
Planck scale would amount to values of a dimensionless parameter, denoted by
$\xi$, with respect to the Planck energy which are roughly of order 1. By
combining BOOMERanG and WMAP data we estimate $\xi \simeq -0.110 \pm 0.076$ at
the 68% c.l. Moreover, we forecast on the sensitivity to $\xi$ achievable by
future CMB polarization experiments (PLANCK, Spider, EPIC), which, in the
absence of systematics, will be at the 1-$\sigma$ confidence of $8.5 \times
10^{-4}$ (PLANCK), $6.1 \times 10^{-3}$ (Spider), and $1.0 \times 10^{-5}$
(EPIC) respectively. The cosmic variance-limited sensitivity from CMB is
$6.1\times 10^{-6}$.
