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 and a concomitant (under the adiabatic assumption) density dependent speed of sound, . 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 models are ruled out (except when , requiring post-stretching the seeded fluctuations); but sharper profiles in do solve the horizon problem. Among these, a phase transition in is notable for leading to scale-invariant fluctuations if the initial conditions are thermal. For contracting universes all power-law solve the horizon problem, but only one leads to scale-invariance: and . This model bypasses a number of problems with single scalar field cyclic models (for which is large but constant).
- Received 10 July 2009
DOI:https://doi.org/10.1103/PhysRevD.81.043509
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