# Cosmology in a Globally U(1) Symmetric Scalar-Tensor Gravity

## TL;DR

This paper proposes a scalar-tensor cosmological model where a complex scalar field drives cosmic evolution, avoiding traditional problems and generating primordial perturbations without tensor modes, challenging standard expansion-based cosmology.

## Contribution

It introduces a novel scalar-tensor cosmology with a complex scalar field in Minkowski spacetime, eliminating the need for space expansion and addressing key cosmological issues.

## Key findings

- Non-singular cosmological evolution with no horizon or flatness problems
- Generation of a flat spectrum of scalar perturbations without tensor modes
- Cosmology driven by mass growth rather than space expansion

## Abstract

A cosmological model is formulated in the context of a scalar-tensor theory of gravity in which the entire cosmic background evolution is due to a complex scalar field evolving in Minkowski spacetime, such that its (dimensional) modulus is conformally coupled, and the (dimensionless) phase is only minimally coupled to gravitation. The former regulates the dynamics of masses; cosmological redshift reflects the growth of particle masses over cosmological time scales, not space expansion. An interplay between the energy density of radiation and that of the kinetic energy associated with the phase (which are of opposite relative signs) results in a non-singular cosmological model that encompasses the observed redshifting phase preceded by a turnaround that follows a blushifting phase. The model is essentially free of any horizon, flatness or anisotropy problems. Quantum excitations of the phase during the matter dominated blueshifting era generate a flat spectrum of adiabatic gaussian scalar perturbations on cosmological scales. No detectable primordial tensor modes are generated in this scenario, and cold dark matter must be fermionic. Other consequences are also discussed.