Astrophysical and Cosmological Implications of Large Volume String Compactifications

TL;DR

Large-volume IIB string compactifications stabilise the Calabi–Yau volume at $\mathcal{V}\sim 10^{15}$, yielding a gravitino mass $m_{3/2}\sim M_P/\mathcal{V}$ and a string scale $m_s\sim M_P/\sqrt{\mathcal{V}}$, with TeV-scale soft terms $m_{soft}\sim m_{3/2}/\ln(M_P/m_{3/2})$. The moduli spectrum splits into a heavy small-cycle modulus $\Phi$ with $m_\Phi \sim 2 m_{3/2}\ln(M_P/m_{3/2})$ and a very light volume modulus $\chi$ with $m_\chi \sim m_{3/2}(m_{3/2}/M_P)^{1/2}$; heavy moduli couple to matter at the string scale while the volume modulus couples at $M_P$, and $\mathrm{Br}(\Phi\to 2\psi_{3/2})\sim 10^{-30}$ suppresses gravitino production. The heavy moduli decay early, reheat the universe to $T_{RH}\sim 10^7$ GeV, while $\chi$ faces the cosmological moduli problem and may require thermal inflation for dilution; $\chi$ can contribute to dark matter and produce observable signals via $\chi\to \gamma\gamma$ or $\chi\to e^+e^-$, with gamma-background and 511 keV constraints providing testable bounds. Overall, the model predicts high reheating temperatures and distinctive late-time signatures, including potential gamma-ray lines and a marginal link to the galactic center 511 keV emission, motivating further renormalisation studies and geometry-dependent predictions.

Abstract

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 V ~ 10^{15} l_s^6 and an intermediate string scale m_s ~ 10^{11}GeV, with TeV-scale observable supersymmetry breaking. All Kähler moduli except for the overall volume are heavier than the susy breaking scale, with m ~ ln(M_P/m_{3/2}) m_{3/2} ~ (\ln(M_P/m_{3/2}))^2 m_{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 ~ 10^7 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 can help relax this problem. This field has a lifetime ~ 10^{24}s and can contribute to dark matter. It may be detected through its decays to 2γor e^+e^-. If accessible the e^+e^- decay mode dominates, with Br(χ\to 2 γ) suppressed by a factor (ln(M_P/m_{3/2}))^2. We consider the potential for detection of this field through different astrophysical sources and find that the observed gamma-ray background constrains Ω_χ <~ 10^{-4}. The decays of this field may generate the 511 keV emission line from the galactic centre observed by INTEGRAL/SPI.