Observational Implications of Axionic Isocurvature Fluctuations

The axion is the most attractive candidate to solve the strong CP problem inQCD. If it exists, the inflationary universe produces axion fluctuations whichare mixtures of isocurvature and adiabatic fluctuations in general. Weinvestigate how large isocurvature fluctuations are allowed or favored in orderto explain observations of the large scale structure of the present universe.Generic flat universe models with mixed (isocurvature + adiabatic) densityfluctuations are studied. It is found that the observations are consistent withthe mixed fluctuation model if the ratio \alpha of the power spectrum ofisocurvature fluctuations to that of adiabatic fluctuations is less than \sim0.1. In particular, the mixed fluctuation model with \alpha \sim 0.05, totalmatter density \Omega_{0} =0.4, and Hubble parameter H_0=70 km/s/Mpc gives avery good fit to the observational data. Since the height of the acoustic peakin the angular power spectrum of the cosmic microwave background (CMB)radiation decreases significantly when the isocurvature fluctuations arepresent, the mixed fluctuation model can be tested in future satelliteexperiments. Ratios of the amplitude at the peak location to that at the COBEnormalization scale for various models are given. Furthermore, we also obtainthe amplitude of isocurvature fluctuations as a function of axion parametersand the Hubble parameter during the inflation. We discuss the axionfluctuations in some realistic inflation models and find that a significantamount of the isocurvature fluctuations are naturally produced.Comment: 31 pages (RevTeX file), the final version to appear in Prog. Theor. Phy