Primordial cosmic fluctuations for variable gravity

TL;DR

This paper shows that primordial cosmic fluctuations can be observed without an expanding universe, using a conformally invariant formulation that allows for scale-invariant spectra even in flat spacetime with variable Planck mass.

Contribution

It introduces a conformally invariant approach to cosmic fluctuations, demonstrating that observable primordial spectra do not require an expanding universe or a geometric horizon.

Findings

Primordial fluctuations can be observed without a geometric horizon.

Scale-invariant spectra can arise in flat Minkowski space with variable Planck mass.

Conformal invariance preserves primordial information against thermal washout.

Abstract

The observability of primordial cosmic fluctuations does not require a geometric horizon $H^{-1}$, which is exceeded temporarily by the wavelength of fluctuations. The primordial information can be protected against later thermal washout even if all relevant wavelengths remain smaller than $H^{-1}$. This is demonstrated by formulating the equations governing the cosmic fluctuations in a form that is manifestly invariant under conformal field transformations of the metric. Beyond the field equations this holds for the defining equation for the correlation function, as expressed by the inverse of the second functional derivative of the quantum effective action. An observable almost scale invariant spectrum does not need an expanding geometry. For a variable Planck mass it can even arise in flat Minkowski space.