Probing the primordial Universe from the low-multipole CMB data

TL;DR

This study uses low-multipole CMB data to constrain cosmological parameters, revealing a preference for a blue tilt in scalar perturbations and compatibility with a red tilt in gravitational waves.

Contribution

It provides the first constraints on primordial cosmological parameters using only low-multipole CMB data, focusing on tensor and scalar perturbations.

Findings

Blue tilted scalar spectrum is preferred at 2σ confidence level.

Red tilted gravitational wave spectrum is compatible with the data.

Low-multipole CMB data can effectively probe primordial Universe physics.

Abstract

Since the temperature fluctuations in cosmic microwave background (CMB) on large-angular scales probe length scales that were super-horizon sized at photon decoupling and hence insensitive to microphysical processes, the low-multipole CMB data are supposed to be a good probe to the physics of the primordial Universe. In this letter we will constrain the cosmological parameters in the base $\Lambda$CDM model with tensor perturbations by only using low-multipole CMB data, including Background Imaging of Cosmic Extragalactic Polarization (B2), Planck released in 2013 (P13) and Wilkinson Microwaves Anisotropy Probe 9-year data (W9). We find that a red tilted power spectrum of relic gravitational waves is compatible with the data, but a blue tilted power spectrum of scalar perturbations on the large scales is preferred at around $2\sigma$ confidence level.