# Gravitational Atoms

## TL;DR

This paper explores the formation and decay of gravitational atoms in a dark sector, analyzing their potential gravitational wave signals and implications for fundamental physics and cosmology.

## Contribution

It introduces the concept of gravitational atoms in a dark sector, examines their gravitational wave signatures, and discusses implications for gravity theories and early universe cosmology.

## Key findings

- Gravitational wave signals from decaying gravitational atoms are typically above 10^{13} Hz in standard cosmology.
- A non-standard early universe with matter domination can produce signals in a detectable frequency range.
- Gravitational atoms naturally arise in the minimal PIDM dark matter scenario.

## Abstract

Particles in a yet unexplored dark sector with sufficiently large mass and small gauge coupling may form purely gravitational atoms (quantum gravitational bound states) with a rich phenomenology. In particular, we investigate the possibility of having an observable signal of gravitational waves or ultra high energy cosmic rays from the decay of gravitational atoms. We show that if ordinary Einstein gravity holds up to the Planck scale, then, within the $\Lambda \text{CDM}$ model, the frequency of the gravitational wave signal produced by the decays is always higher than $10^{13} \, \text{Hz}$. An observable signal of gravitational waves with smaller frequency from such decays, in addition to probing near Planckian dark physics, would also imply a departure from Einstein gravity near the Planck scale or an early epoch of non-standard cosmology. As an example, we consider an early universe cosmology with a matter-dominated phase, violating our assumption that the universe is radiation dominated after reheating, which gives a signal in an interesting frequency range for near Planckian bound states. We also show how gravitational atoms arise in the minimal PIDM scenario and compute their gravitational wave signature.