Primordial fluctuations without scalar fields

J. Magueijo, J. Noller

Research output: Contribution to journalArticlepeer-review


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=p00 and a concomitant (under the adiabatic assumption) density dependent speed of sound,
cs. We find a limited range of possibilities for a setup when modes start inside the Hubble radius, then leaving it and freezing out. For expanding universes, power-law w(0) models are ruled out (except when cs2∝ w≪1, requiring post-stretching the seeded fluctuations); but sharper profiles in cs do solve the horizon problem. Among these, a phase transition in cs is notable for leading to scale-invariant fluctuations if the initial conditions are thermal. For contracting universes all power-law w(ε0) solve the horizon problem, but only one leads to scale-invariance: w∝ε02 and cs∝ε0. This model bypasses a number of problems with single scalar field cyclic models (for which w is large but constant).

Original languageEnglish
Article number043509
JournalPhysical Review D
Publication statusPublished - 2010


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