CMB anisotropy at degree angular scales and the thermal history of the Universe

We study the anisotropy of the cosmic microwave background (CMB) in cold and mixed dark matter (CDM and MDM) models, with non scale-invariant primordial power spectra (i.e. $n\mathrm{\ne }1$) and a late, sudden reionization of the intergalactic medium at redshift $z_{rh}$. We test these models against recent detections of CMB anisotropy at large and intermediate angular scales. We find that current CMB anisotropy measurements cannot discriminate between CDM and MDM models. Our likelihood analysis indicates that models with blue power spectra ($n\simeq 1.2$) and a reionization at $z_{rh}\sim 20$ are most consistent with the anisotropy data considered here. Without reionization our analysis gives $1.0\le n\le 1.26$ (95 % C.L.) for ${\mathrm{\Omega }}_b=0.05$.