A recent study proposes that the primordial gravitational-wave background (GWB) can be a key tool for detecting primordial black holes (PBHs) with masses below 10⁹ grams. These PBHs are difficult to observe directly because they evaporated before Big Bang Nucleosynthesis (BBN). However, their existence could have generated a period of early matter domination in the early universe, leaving a characteristic imprint on the GWB spectrum. This imprint would manifest as specific frequencies associated with the onset and end of PBH domination.

The researchers have developed numerical relations that directly link these characteristic frequencies to the mass and initial abundance of the PBHs. This allows for the reconstruction of PBH parameters from GWB observations. The sensitivity of future gravitational wave experiments spans a wide frequency range, enabling probing of PBH masses from the BBN bound (approximately 10⁹ g) down to as low as 10 g.

A notable finding of the study is that the nanohertz signal reported by the NANOGrav collaboration, if primordial in origin, is already probing PBHs with masses between 4 and 90 megagrams (Mg). This demonstrates the potential of gravitational wave observations to access a vast region of the PBH parameter space that would otherwise be beyond the reach of current experimental techniques.