Quasi-matter Bounce Cosmology in Light of Planck

Abstract

We study quasi-matter bounce cosmology in light of Planck cosmic microwave background (CMB) angular anisotropy measurements along with the BICEP2/Keck Array data. We propose a new primordial scalar power spectrum by considering a linear approximation of the equation of state $w\cong w_0+\kappa (\eta -{\eta }_0)$ for quasi-matter field in the contracting phase of the universe. Using this new primordial scalar power spectrum, we constrain the zeroth-order approximation of the equation of state $w_0=-0.00340\pm 0.00044$ and first-order correction $10^4\zeta =-1.67_{-0.83}^{+1.50}$ at the $1\sigma$ confidence level by Planck temperature and polarization in combination with the BICEP2/Keck Array data in which $\zeta =12\kappa /k_{\ast }$ with pivot scale $k_{\ast }$. The spectral index of scalar perturbations is determined to be $n_{\mathrm{Bs}}=0.9623\pm 0.0055$ which lies 7$\sigma$ away from the scale-invariant primordial spectrum. We find scale dependency for $n_{\mathrm{s}}$ at the $1\sigma$ confidence level and a tighter constraint on the running of the spectral index compared to $\Lambda$CDM+${\alpha }_s$ cosmology. The running of the spectral index in quasi-matter bounce cosmology is ${\alpha }_{\mathrm{Bs}}=\pi \zeta /2c_{\mathrm{s}}=-0.0021\pm 0.0016$ which is nonzero at the $1.3\sigma$ level, whereas in $\Lambda$CDM+${\alpha }_{\mathrm{s}}$ it is nonzero at the $0.8\sigma$ level for Planck temperature and polarization data. The sound speed of density fluctuations of the quasi-matter field at the crossing time is $c_{\mathrm{s}}=0.097_{-0.023}^{+0.037}$, which is not a very small value in the contracting phase.

13 pages, 6 figures

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