When black holes and other enormously massive, dense objects whirl around one another, they send out ripples in space and time called gravitational waves. These waves are one of the few ways we have to study the enigmatic cosmic giants that create them. Astronomers have observed the high-frequency “chirps” of colliding black holes, but the ultra-low-frequency rumble of supermassive black holes orbiting one another has proven harder to detect. For decades, we have been observing pulsars, a type of star that pulses like a lighthouse, in search of the faint rippling of these waves. In June 2023, pulsar research collaborations around the world – including ours, the Parkes Pulsar Timing Array – announced their strongest evidence yet for the existence of these waves.
One enigmatic feature of the interstellar plasma is the presence of dense, compact, and intensely-turbulent regions, akin to an interstellar tornado. These so-called extreme scattering events, or ESEs, are poorly understood because they are difficult to study. Our current understanding is so poor that scientists would expect such extreme objects to quickly destroy themselves. How they form and how they sustain themselves is a mystery. The solution to this puzzle likely involves the magnetic fields in our Galaxy but further study of ESEs is critical. Unfortunately, ESEs are so small by astronomical standards, that they are completely invisible to other areas of astronomy.
Pulsars—rapidly-spinning remnants of stars that flash like a lighthouse—occasionally show extreme variations in brightness. Scientists predict that these short bursts of brightness happen because dense regions of interstellar plasma (the hot gas between stars) scatter the radio waves emitted by the pulsar.