TY - JOUR

T1 - Gravitational instability of de Sitter compactifications

AU - Contaldi, Carlo

AU - Kofman, Lev

AU - Peloso, Marco

PY - 2004/8

Y1 - 2004/8

N2 - We consider warped compactifications in (4 + d)-dimensional theories, with four-dimensional (4d) de Sitter dS4 vacua (with Hubble parameter H) and with a compact internal space. After introducing a gauge-invariant formalism for the generic metric perturbations of these backgrounds, we focus on modes which are scalar with respect to dS4. The physical eigenmasses of these modes acquire a large universal tachyonic contribution -12d/(d + 2)H 2, independently of the stabilization mechanism for the compact space, in addition to the usual KK masses, which instead encode the effects of the stabilization. General arguments, as well as specific examples, lead us to conjecture that, for sufficiently large dS curvature, the compactified geometry becomes gravitationally unstable due to the tachyonic growth of the scalar perturbations. This means that for any stabilization mechanism the curvature of the dS geometry cannot exceed some critical value. We relate this effect to the anisotropy of the bulk geometry and suggest the end points of the instability. Of relevance for inflationary cosmology, the perturbations of the bulk metric inevitably induce a new modulus field, which describes the conformal fluctuations of the 4d metric. If this mode is light during inflation, the induced conformal fluctuations will be amplified with a scale free spectrum and with an amplitude which is disentangled from the standard result of slow-roll inflation. The conformal 4d metric fluctuations give rise to a, very generic realization of the mechanism of modulated cosmological fluctuations, related to spatial variation of couplings during (p)reheating after inflation.

AB - We consider warped compactifications in (4 + d)-dimensional theories, with four-dimensional (4d) de Sitter dS4 vacua (with Hubble parameter H) and with a compact internal space. After introducing a gauge-invariant formalism for the generic metric perturbations of these backgrounds, we focus on modes which are scalar with respect to dS4. The physical eigenmasses of these modes acquire a large universal tachyonic contribution -12d/(d + 2)H 2, independently of the stabilization mechanism for the compact space, in addition to the usual KK masses, which instead encode the effects of the stabilization. General arguments, as well as specific examples, lead us to conjecture that, for sufficiently large dS curvature, the compactified geometry becomes gravitationally unstable due to the tachyonic growth of the scalar perturbations. This means that for any stabilization mechanism the curvature of the dS geometry cannot exceed some critical value. We relate this effect to the anisotropy of the bulk geometry and suggest the end points of the instability. Of relevance for inflationary cosmology, the perturbations of the bulk metric inevitably induce a new modulus field, which describes the conformal fluctuations of the 4d metric. If this mode is light during inflation, the induced conformal fluctuations will be amplified with a scale free spectrum and with an amplitude which is disentangled from the standard result of slow-roll inflation. The conformal 4d metric fluctuations give rise to a, very generic realization of the mechanism of modulated cosmological fluctuations, related to spatial variation of couplings during (p)reheating after inflation.

KW - Cosmological perturbation theory

KW - Cosmology with extra dimensions

UR - http://www.scopus.com/inward/record.url?scp=23244446878&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=23244446878&partnerID=8YFLogxK

U2 - 10.1088/1475-7516/2004/08/007

DO - 10.1088/1475-7516/2004/08/007

M3 - Article

AN - SCOPUS:23244446878

SN - 1475-7516

SP - 149

EP - 168

JO - Journal of Cosmology and Astroparticle Physics

JF - Journal of Cosmology and Astroparticle Physics

IS - 8

ER -