On the Effects due to a Decaying Cosmological Fluctuation

We present the initial conditions for a decaying cosmological perturbation and study its signatures in the CMB anisotropies and matter power spectra. An adiabatic decaying mode in presence of components which are not described as perfect fluids (such as collisionless matter) decays slower than in a perfect-fluid dominated universe and displays super-Hubble oscillations. By including a correlated decaying mode with a red or a scale invariant spectrum, the anisotropy pattern shows super-imposed oscillations before the first Dopplear peak, while with a blue spectrum the amplitude of the secondary peaks relative to the first one and the matter power spectrum can be altered. WMAP first year data constrain the decaying to growing ratio of scale invariant adiabatic fluctuations at the matter-radiation equality to less than 10%.