The Stochastic Gravitational Wave Background from Primordial Gravitational Atoms

We propose a scenario of primordial gravitational atoms (PGAs), which may exist in the current and past universe due to spinning primordial black holes (PBHs) and very light bosonic fields. In a monochromatic mass scenario with a sizable dimensionless spin, which may arise in a short matter dominated (MD) era, we analyze the resulting stochastic gravitational wave background (SGWB) signal. Its spectrum is approximately characterized by a rising ∝ f³ followed by a falling ∝ f⁻¹ where f is the frequency. Then, we investigate the constraints and prospects of such a SGWB, and find that PGAs with a core mass MBH~ O(10)~M_ and a cloud of light scalar with mass μ ~ O (10⁻¹³) eV could yield constraints even stronger than those from bare PBHs. Future detectors such as LISA, Taiji and TianQin are able to explore PGAs over a narrow and elongated strap in the (μ,MBH) plane, spanning over 10 orders of magnitude for the maximum spin, 10⁻⁸~M_ MBH 10⁴~M_, 10⁻¹⁶~ eV μ 10⁻³~ eV. If the PGA is dressed with a vector cloud, the SGWB signal has a much better opportunity to be probed.