String Theory Signatures in Cosmological Observables: Swampland Bounds, Cosmic Superstrings, and Axiverse Phenomenology - A Synthesis and Original Analysis

This paper presents a synthesis of current results bearing on String Theory’s observational footprint in the era of precision multi-messenger cosmology, augmented byoriginal superradiance calculations constraining the string axiverse. We distinguishbetween mathematically derived framework theorems — such as the derivation of General Relativity from worldsheet consistency — and model-dependent conjectures thatconnect string theory to phenomenology.Our analysis proceeds along three observational channels. First, we review howfiltering effective field theories through the Swampland Distance Conjecture yields afalsifiable bound on the tensor-to-scalar ratio r < 10−3, a target accessible to theLiteBIRD mission, and discuss the assumptions underlying this bound. Second, weassess the cosmic superstring interpretation of the NANOGrav 15-year pulsar timingsignal, surveying current parameter constraints on string tension Gµ and reconnectionprobability P. Third — and as the primary original contribution of this work — wepresent a first-principles superradiance calculation constraining ultralight axions usingfour well-measured stellar-mass black hole systems. We derive exclusion bounds onaxion masses in the range 10−12.2 eV ≲ ma ≲ 10−8 eV from Cygnus X-1 alone, withcomplementary bounds from GRS1915+105, GX339−4, and MAXIJ1820+070.We conclude by identifying the remaining original analyses — a Bayesian fit topulsar timing data and fuzzball echo template construction — that would complete the observational program.