Astrophysical and cosmological implications of large volume string compactifications

We study the spectrum, couplings and cosmological and astrophysical implications of the moduli fields for the class of Calabi-Yau IIB string compactifications for which moduli stabilisation leads to an exponentially large volume ν ∼ 10¹⁵1_S⁶ and an intermediate string scale m_s∼10¹¹ GeV, with TeV-scale observable supersymmetry breaking. All Kähler moduli except for the overall volume are heavier than the susy (supersymmetry) breaking scale, with m∼ln(M _P/m_3/2)m_3/2∼(ln(M_P/m _3/2))²m_susy∼500 TeV and, contrary to standard expectations, have matter couplings suppressed only by the string scale rather than the Planck scale. These decay to matter early in the history of the universe, with a reheat temperature T∼10⁷ GeV, and are free from the cosmological moduli problem (CMP). The heavy moduli have a branching ratio to gravitino pairs of 10^-30 and do not suffer from the gravitino overproduction problem. The overall volume modulus is a distinctive feature of these models and is an M_planck-coupled scalar of mass m∼1 MeV and subject to the CMP. A period of thermal inflation may help relax this problem. This field has a lifetime τ∼10²⁴ s and can contribute to dark matter. It may be detected through its decays to γγ or e ⁺e^-. If accessible the e⁺e^- decay mode dominates, with Br(χ → γγ) suppressed by a factor (ln(M_P/m_3/2))². We consider the potential for detection of this field through different astrophysical sources: the Milky Way halo, the diffuse cosmic background and nearby galaxy clusters and find that the observed gamma ray background constrains Ωχ≲10^-4. The decays of this field may generate the 511 keV emission line from the galactic centre observed by INTEGRAL/SPI.