Double Inflation and the Low CMB Quadrupole

Recent released WMAP data show a low value of quadrupole in the CMB temperature fluctuations, which confirms the early observations by COBE. In this paper we consider a model of two inflatons with different masses, $V(\phi_1,\phi_2)={1\over 2}m_1^2\phi_1^2 + {1 \over 2} m_2^2 \phi_2^2$, $m_1>m_2$ and study its effects on CMB of suppressing the primordial power spectrum $P(k)$ at small $k$. Inflation is driven in this model firstly by the heavier inflaton $\phi_1$, then the lighter field $\phi_2$. But there is no interruption in between. We numerically calculate the scalar and tensor power spectra with mode by mode integrations, then fit the model to WMAP temperature correlations TT and the TE temperature-polarization spectra. Our results show that with $m_1\sim 10^{14}$ GeV and $m_2\sim 10^{13}$ GeV, this model solves the problems of flatness {\it etc.} and the CMB quadrupole predicted can be much lower than the standard power-law $\Lambda$CDM model.