Primordial fluctuations without scalar fields

Abstract

We revisit the question of whether fluctuations in hydrodynamical, adiabatical matter could explain the observed structures in our Universe. We consider matter with variable equation of state $w=p_0/\ep_0$ and a concomitant (under the adiabatic assumption) density dependent speed of sound, $c_s$. We find a limited range of possibilities for a set up when modes start inside the Hubble radius, then leaving it and freezing out. For expanding Universes, power-law $w(\ep_0)$ models are ruled out (except when $c_s^2\propto w\ll 1$, requiring post-stretching the seeded fluctuations); but sharper profiles in $c_s$ do solve the horizon problem. Among these, a phase transition in $c_s$ is notable for leading to scale-invariant fluctuations if the initial conditions are thermal. For contracting Universes all power-law $w(\ep_0)$ solve the horizon problem, but only one leads to scale-invariance: $w\propto \ep_0^2$ and $c_s\propto \ep_0$. This model bypasses a number of problems with single scalar field cyclic models (for which $w$ is large but constant).

References in corpus (8)

• New Ekpyrotic Cosmology
• Generating Ekpyrotic Curvature Perturbations Before the Big Bang
• Intermediate inflation in light of the three-year WMAP observations
• Speedy sound and cosmic structure
• Curvature perturbations from ekpyrotic collapse with multiple fields
• Near Scale Invariance with Modified Dispersion Relations
• Holography and the scale-invariance of density fluctuations
• Observing the temperature of the Big Bang through large scale structure

Cited by in corpus (10)

• The Pseudo-Conformal Universe: Scale Invariance from Spontaneous Breaking of Conformal Symmetry
• Generating Scale-Invariant Perturbations from Rapidly-Evolving Equation of State
• Towards a Cosmological Dual to Inflation
• Non-Gaussianity in single field models without slow-roll
• General conditions for scale-invariant perturbations in an expanding universe
• Scale Invariance via a Phase of Slow Expansion
• Strong Coupling Problem with Time-Varying Sound Speed
• Horizon-preserving dualities and perturbations in non-canonical scalar field cosmologies
• Primordial perturbations in a rainbow universe with running Newton constant
• Non-adiabatic primordial fluctuations