Cosmological properties of a class ofΛdecaying cosmologies

We investigate some properties of flat cosmological models with a term that decreases with time as a^-m (a is the scale factor and m is a parameter 0<~m<3). The models are equivalent to standard cosmology with matter and radiation plus an exotic fluid with the equation of state pₗ= (m/3-1) ₗ. We study the effect of the decaying term on the cosmic microwave background anisotropy and by using a seminumeric method we compute the angular power spectrum (up to l=20) for different values of m and ₌₀. We also investigate the constraints imposed on the models by the magnitude-redshift test in which high-redshift type Ia supernovae (SNe Ia) are used as standard candles. We obtain the 95. 4%, 90%, and 68% confidence levels on the parameters m and ₌₀ and compare them with those arising from lensing statistics. Our analysis reveals that the SNe Ia constraints are stronger for low values of m and ₌₀, while those from lensing statistics are more important for m1. Models with ₌₀0. 2 and m1. 6 are in good agreement with the data.