Speaker
Description
Blue-tilted Gravitational Waves (BGWs) have emerged as a promising candidate for explaining the cosmic gravitational wave signals observed by Pulsar Timing Arrays (PTA). Within the standard cosmological model, the frequency range of BGWs is restricted by the Big Bang Nucleosynthesis (BBN) limit on gravitational wave amplitude, which prevents their detection at interferometer scales. However, the inclusion of an early matter-dominated phase can dilute BGWs at higher frequencies, ensuring consistency with both BBN and LIGO constraints on stochastic gravitational waves. This process enables BGWs to match PTA data while generating a unique and testable gravitational wave signature across a wide frequency range. Ultralight Primordial Black Holes (PBHs) could serve as the source of the necessary early matter-dominated phase to facilitate this mechanism. When interpreted through the framework of BGWs, the PTA results provide a means to constrain the parameter space of a novel scenario involving modified Hawking radiation, referred to as the "memory burden" effect, linked to ultralight PBHs. This scenario can be further investigated using high-frequency gravitational wave detectors. In particular, we show that PBHs with masses as light as $10^{2-3}~{\rm g}$ can leave observable imprints on BGWs at higher frequencies while remaining compatible with PTA observations.
