Noelia Sanchez Gonzalez

We study the decoupling of branes sourcing AdS flux vacua from the asymptotic bulk using scalar-wave absorption probabilities. In a one-modulus truncation, the result depends only on the dimension of the AdS vacuum and on the asymptotic steepness of the scalar potential. We find that for any steepness and for any dimension greater than two the absorption probability vanishes in the low-energy limit, showing that the branes decouple from the bulk. We illustrate the general analysis in several scale-separated AdS vacua. For DGKT we find P_abs∼ω^(27/7) and an effective transverse dimension d_eff = 20/7. Interestingly, the scalar potential along the brane-induced moduli trajectory agrees with that obtained by compactifying a hypothetical (4+13/7)-dimensional gravity theory on a 13/7-dimensional sphere threaded by flux. We also apply the analysis to scale-separated AdS_3 examples and to the simplest one-modulus KKLT model.

The growth of adiabatic density perturbations during an era of early matter domination induces O(1) fluctuations in pre-inflationary QCD axion dark matter across a broad, string-theory-motivated parameter space. Remarkably, at ΛCDM matter-radiation equality the scale of these perturbations coincides with the quantum Jeans scale, so they collapse to solitonic ''axion stars''. These axion stars have densities up to 10^4 eV^4, and, including their surrounding halos, they contain as much as 50% of dark matter. Direct searches for a smooth axion background can be suppressed, but transient enhancements or indirect astrophysical signals at axion masses m_a ≲ 10^(−5) eV would point to a non-standard cosmological history.

When moduli roll in the early universe, all physical scales - including string tensions - simultaneously evolve. The dynamics of cosmic string loops with time-varying tension can produce cosmic string loop trackers in which most of the energy density of the universe lies in the form of string loops. This solution can exist as an attractor until the rolling modulus reaches its minimum, when the loops ultimately decay through gravitational wave emission. We explore the spectrum of gravitational waves produced by such string loop trackers. The resulting spectrum is high-frequency and peaks in the GHz regime today. The amplitude of the signal is diluted by any subsequent matter-dominated epochs, and thus the potential observability of the signal crucially depends on the duration of the moduli-dominated epoch that follows once the moduli settle down and oscillate about their minimum.