Cosmological Perturbations without Inflation

TL;DR

This paper proposes an alternative to inflation where a scalar field with zero active mass generates scale-free fluctuations, potentially linking CMB features to trans-Planckian physics without requiring inflation.

Contribution

It introduces a novel mechanism for primordial fluctuations using a zero active mass scalar field, bypassing the need for inflation and addressing the horizon problem.

Findings

Zero active mass scalar field can produce scale-free fluctuations.

CMB fluctuations correspond to Planck length at Planck time.

Supports the idea that CMB probes trans-Planckian physics.

Abstract

A particularly attractive feature of inflation is that quantum fluctuations in the inflaton field may have seeded inhomogeneities in the cosmic microwave background (CMB) and the formation of large-scale structure. In this paper, we demonstrate that a scalar field with zero active mass, i.e., with an equation of state rho+3p=0, where rho and p are its energy density and pressure, respectively, could also have produced an essentially scale-free fluctuation spectrum, though without inflation. This alternative mechanism is based on the Hollands-Wald concept of a minimum wavelength for the emergence of quantum fluctuations into the semi-classical universe. A cosmology with zero active mass does not have a horizon problem, so it does not need inflation to solve this particular (non) issue. In this picture, the 1-10 degree fluctuations in the CMB correspond almost exactly to the Planck length at the Planck time, firmly supporting the view that CMB observations may already be probing trans-Planckian physics.