Fat Gravitons, the Cosmological Constant and Sub-millimeter Tests
Raman Sundrum
TL;DR
The paper investigates whether the Cosmological Constant Problem can be addressed without new light fields by positing a finite-size, or 'fat', graviton that breaks the point-like gravity picture at sub-millimeter scales. It develops Soft Graviton Effective Theory (SGET), a framework that blends Weinberg's soft graviton couplings with heavy-particle EFT to describe soft gravity acting on hard SM matter without extrapolating GR to very short distances. The authors argue this fat-graviton scenario preserves macroscopic GR and the Equivalence Principle, while linking the vacuum energy naturalness to the graviton's size and predicting a qualitative modification to gravity emerging near $20\,\mu$m, testable in sub-millimeter experiments. By constructing SGET further, they show that heavy SM physics need not generate robust contributions to the gravitational effective action, thereby clarifying the fat-graviton loophole and offering a falsifiable EFT pathway for CCP resolutions.
Abstract
We revisit the proposal that the resolution of the Cosmological Constant Problem involves a sub-millimeter breakdown of the point-particle approximation for gravitons. No fundamental description of such a breakdown, which simultaneously preserves the point-particle nature of matter particles, is yet known. However, basic aspects of the self-consistency of the idea, such as preservation of the macroscopic Equivalence Principle while satisfying quantum naturalness of the cosmological constant, are addressed in this paper within a Soft Graviton Effective Theory. It builds on Weinberg's analysis of soft graviton couplings and standard heavy particle effective theory, and minimally encompasses the experimental regime of soft gravity coupled to hard matter. A qualitatively distinct signature for short-distance tests of gravity is discussed, bounded by naturalness to appear above approximately 20 microns.