Dark Radiation in LARGE Volume Models (1208.3562v2)

Abstract: We consider reheating driven by volume modulus decays in the LARGE Volume Scenario. Such reheating always generates non-zero dark radiation through the decays to the axion partner, while the only competitive visible sector decays are Higgs pairs via the Giudice-Masiero term. In the framework of sequestered models where the cosmological moduli problem is absent, the simplest model with a shift-symmetric Higgs sector generates 1.56 < N_{eff} - N_{eff,SM} < 1.74. For more general cases, the known experimental bounds on N_{eff} strongly constrain the parameters and matter content of the models.

Dark Radiation in LARGE Volume Models: A Comprehensive Overview

The paper "Dark Radiation in LARGE Volume Models" by Michele Cicoli, Joseph P. Conlon, and Fernando Quevedo, presents a detailed analysis of dark radiation production in the cosmological context of the LARGE Volume Scenario (LVS), a framework stemming from string theory compactifications. The authors investigate the reheating process driven by volume modulus decays, which inherently generates non-zero dark radiation, primarily through decays into the axionic partner. The paper is set within a framework where the cosmological moduli problem (CMP) is resolved.

Key Findings and Methodology

The research explores the reheating period following inflation, which is critical in converting vacuum energy into Standard Model (SM) thermal relativistic degrees of freedom. This energy transfer is quantified through the effective number of neutrino species, $N_{\text{eff}}$. Observed deviations in $\Delta N_{\text{eff}}$, relative to the SM expectations, hint at the presence of additional dark radiation components.

The paper presents a computation of decay channels and rates for the volume modulus in the LVS. A critical decay mode discussed is into the axionic partner, contributing to dark radiation. In models where the Higgs sector is shift-symmetric, the authors derive a $\Delta N_{\text{eff}}$ ranging between 1.56 and 1.74. This range fits within experimental observations, although more general scenarios face significant constraints due to current bounds on $\Delta N_{\text{eff}}$.

The research is predicated upon a specific geometric form of the compactified volume, with the LARGE Volume Scenario stabilizing this at exponentially large values. This stabilization establishes a hierarchy among moduli masses, positioning the volume modulus as the lightest, yet long-lived due to its reliance on decaying into gravitationally coupled modes. The decay dynamics primarily hinge on interactions with visible sector fields, primarily through Higgs pairs via the Giudice-Masiero term, and axionic partners.

Theoretical and Practical Implications

From a theoretical perspective, the unavoidable generation of dark radiation within this framework has significant implications for model-building efforts addressing cosmological observations. Particularly, the constraints on $\Delta N_{\text{eff}}$ act as a stringent sieve for various string-derived phenomenological models, limiting the presence of additional moduli or axions beyond specific bounds, conventionally described as the "axiverse."

Practically, these results highlight the need for precise cosmological measurements, such as those anticipated from the PLANCK satellite, to distinguish the detailed features of dark radiation produced in the early universe. The paper underscores the utility of cosmological probes as constraints on high-energy theories, bridging observational astrophysics with string theoretic constructions.

Conclusion and Future Direction

The paper offers a comprehensive account of how dark radiation manifests within the LVS framework. While presenting a plausible reconciliation with observational data, it underscores potential challenges in extending the model to accommodate a multitude of axion-like particles—reflecting the constraints imposed by $\Delta N_{\text{eff}}$.

Future research suggested by the authors could involve refining the predictions with more complete datasets, exploring the ramifications of hidden sector dynamics, and aligning theoretical infrastructure with progressively detailed cosmic background measurements. Such efforts will be vital for the further understanding and verification of string theoretic predictions in cosmological contexts, putting them to empirical test.