The challenge of ruling out inflation via the primordial graviton background

TL;DR

Detecting the primordial graviton background could definitively rule out inflation, as inflationary models would dilute this background below observable levels, making its detection a crucial test for early universe theories.

Contribution

This paper proposes that measuring the cosmic graviton background can distinguish inflation from alternative models, highlighting its potential as a model-independent test.

Findings

CGB contributes $ riangle N_{eff,g} oughly 0.054$

Next-generation probes could detect the CGB

Inflationary models dilute the CGB below detectability

Abstract

Recent debates around the testability of the inflationary paradigm raise the question of how to model-independently discriminate it from competing scenarios. We argue that a detection of the Cosmic Graviton Background (CGB), the relic radiation from gravitons decoupling around the Planck time, would rule out the inflationary paradigm, as realistic inflationary models would dilute the CGB to an unobservable level. The CGB contribution to the effective number of relativistic species, $\Delta N_{{\rm eff},g} \approx 0.054$, is well within the reach of next-generation cosmological probes. We argue that detecting the high-frequency stochastic gravitational wave background associated to the CGB will be challenging but potentially feasible. We briefly discuss expectations within alternatives to inflation, focusing on bouncing cosmologies and emergent scenarios.