String theory, gravity and experiment

TL;DR

The lectures survey how string theory may intersect with gravity through the membrane paradigm for black holes, experimental tests of gravity, and string-inspired gravitational phenomenology. They connect BH dissipation and thermodynamics to horizon boundary dynamics and discuss Hawking radiation, while reviewing precision tests of GR—from solar-system to binary pulsar observations—and the implications for equivalence principles and fundamental constants. The text then explores long-range scalar-tensor modifications, cosmological attractor mechanisms, and brane-world scenarios, highlighting observable consequences such as tiny deviations in post-Newtonian parameters and time variation of constants. Finally, the work discusses cosmological signals from string theory, notably alternatives to slow-roll inflation and cosmic superstrings, including their gravitational-wave signatures and potential detectability with LIGO, LISA, and pulsar timing. Overall, it frames gravity phenomenology as a promising arena to connect string theory to observational physics and cosmology, with concrete predictions and upcoming experimental tests.

Abstract

The aim of these lectures is to give an introduction to several topics which lie at the intersection of string theory, gravity theory and gravity phenomenology. One successively reviews: (i) the "membrane" approach to the dissipative dynamics of classical black holes, (ii) the current experimental tests of gravity, and their theoretical interpretation, (iii) some aspects of the string-inspired phenomenology of the gravitational sector, and (iv) some possibilities for observing string-related signals in cosmology (including a discussion of gravitational wave signals from cosmic superstrings).

Figures (8)

• Figure 1: In this figure, we schematically illustrate the "Penrose process", i.e., the splitting of an ingoing particle into one that falls into the BH and another that exits at infinity.
• Figure 2: This figure depicts the classically allowed energy levels (shaded region) as a function of radius, for test particles in the neighborhood of a BH. There exist positive- and negative-energy solutions, corresponding (after second quantization) to particles and anti-particles. Classically (as in the Penrose process) one should consider only the "positive-square-root" energy levels, located in the upper shaded region. The white region is classically forbidden. Note the possibility of tunneling (this corresponds to particle creation via the "super-radiant", non-thermal mechanism briefly mentioned below).
• Figure 3: This figure is a schematic representation of the effective gravitational potential in the neighborhood of a BH. Note that as far as the particles are concerned, the spacetime is essentially flat both at infinity and near the horizon. The tidal-centrifugal barrier that separates the horizon from infinity gives rise to the grey body factor.
• Figure 4: Time-symmetric half-advanced half-retarded contributions to the gravitational interaction between particles A and B.
• Figure 5: The ends of open strings are attached to a brane, giving rise to SM particles, while closed strings are free to propagate in the bulk.