We study the Cosmic Microwave Background (CMB) anisotropies produced by cosine-type quintessence models. In our analysis, effects of the adiabatic and isocurvature fluctuations are both taken into account. For purely adiabatic fluctuations with scale invariant spectrum, we obtain a stringent constraint on the model parameters using the CMB data from COBE, BOOMERanG and MAXIMA. Furthermore, it is shown that isocurvature fluctuations have significant effects on the CMB angular power spectrum at low multipoles in some parameter space, which may be detectable in future satellite experiments. Such a signal may be used to distinguish It is widely believed that the present cosmological observations are consistent with low density Cold Dark Matter (CDM) models with scale invariant adiabatic density fluctuations. The required energy density of non-relativistic matter is about 30 − 40 % of the critical density (i.e., Ωm ≃ 0.3 − 0.4). On the other hand, the recent BOOMERanG [1
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