Beyond six parameters: Extending Λ CDM

Eleonora Di Valentino (1) , Alessandro Melchiorri , Joseph Silk (1)

1 IAP - Institut d'Astrophysique de Paris

Cosmological constraints are usually derived under the assumption of a six-parameter Λ CDM theoretical framework or simple one-parameter extensions. In this paper we present, for the first time, cosmological constraints in a significantly extended scenario, varying up to 12 cosmological parameters simultaneously, including the sum of neutrino masses, the neutrino effective number, the dark energy equation of state, the gravitational wave background and the running of the spectral index of primordial perturbations. Using the latest Planck 2015 data release (with polarization), we found no significant indication for extensions to the standard Λ CDM scenario, with the notable exception of the angular power spectrum lensing amplitude, A_lens , which is larger than the expected value at more than 2 standard deviations, even when combining the Planck data with BAO and supernovae type Ia external data sets. In our extended cosmological framework, we find that a combined Planck+BAO analysis constrains the value of the rms density fluctuation parameter to σ₈=0.781_-0.063^+0.065 at 95 % C.L., helping to relieve the possible tensions with the CFHTlenS cosmic shear survey. We also find a lower value for the reionization optical depth τ =0.058_-0.043^+0.040 at 95 % C.L. with respect to the one derived under the assumption of Λ CDM . The scalar spectral index n_S is now compatible with a Harrison-Zeldovich spectrum to within 2.5 standard deviations. Combining the Planck data set with the Hubble Space Telescope prior on the Hubble constant provides a value for the equation of state w <-1 at more than 2 standard deviations, while the neutrino effective number is fully compatible with the expectations of the standard three neutrino framework.

Keywords: 98.80.-k; 95.85.Sz; 98.70.Vc; 98.80.Cq; Cosmology; Gravitational radiation magnetic fields and other observations; Background radiations; Particle-theory and field-theory models of the early Universe; Astrophysics - Cosmology and Nongalactic Astrophysics; General Relativity and Quantum Cosmology; High Energy Physics - Phenomenology

Domains: Sciences of the Universe [physics]

Eleonora Di Valentino, Alessandro Melchiorri, Joseph Silk. Beyond six parameters: Extending Λ CDM. Physical Review D, 2015, 92, ⟨10.1103/PhysRevD.92.121302⟩. ⟨insu-03644666⟩