We show that the Cosmic Microwave Background ( CMB ) polarization data gathered by the BOOMERanG 2003 flight and WMAP provide an opportunity to investigate 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 } .