Gravitational waves offer a potential window into the first moments of cosmic history. If the Universe underwent and abruptly exited an early matter-dominated phase, as predicted by many particle-physics models involving heavy particles, primordial black holes, or oscillating scalar fields forming $Q$-balls, the resulting gravitational-wave signal could be amplified via the recently discovered “poltergeist mechanism” discussed in this review. The authors explain the physical origin of this amplification, analyze the mechanism across several cosmological scenarios, and discuss prospects for detecting the resulting signals with current and future gravitational-wave observatories.

Although most treatments of fluid mechanics consider the fluid viscosity to be constant or uniform, many systems have a viscosity that varies in time or space. Examples arise in microbial habitats, in the ocean, and in Earth’s mantle. In this review, the authors explain how viscosity gradients occur in real systems, examine how these gradients lead to new phenomena at both low and high Reynolds number, and highlight open research questions.