Cosmic crash

The Milky Way and Andromeda galaxies will merge in about 10 billion years, and the supermassive black holes at their centers will collide less than 17 million years later, Sid Perkins reported in “Crash will follow ‘Milkomeda’ debut” (SN: 3/27/21, p. 9).

Perkins wrote that any civilization within about 3.25 million light-years of Earth that has gravitational wave–sensing technology similar to ours would be able to detect the collision. Some readers questioned why the crash would be detectable only to 3.25 million light-years when the gravitational wave observatories LIGO and Virgo can detect black hole mergers billions of light-years away (SN: 1/30/21, p. 30).

The reason has to do with the mass of the black holes, the story’s editor Chris Crockett says. The black hole mergers discovered to date involved relatively lightweight black holes, and the smashups emitted gravitational waves that have frequencies within LIGO and Virgo’s detection range of 10 to 10,000 hertz. But when supermassive black holes collide, they emit gravitational waves at much lower frequencies.

To observe such a crash today, researchers need pulsar timing arrays. This technology can detect low-frequency gravitational waves using variations in the steady radio blips of highly magnetized stars called pulsars. Pulsar timing arrays have yet to detect a supermassive black hole collision, but they are currently listening for the “background hum” of such events throughout the universe. The arrays could single out a smashup like the Andromeda–Milky Way out to about 3.25 million light-years away, Crockett says.

Bug off

Catnip wards off mosquitoes by triggering a chemical receptor that, in other animals, senses pain or itch, Erin Garcia de Jesús reported in “How catnip repels pesky mosquitoes” (SN: 3/27/21, p. 9).

Reader Rick Gillespie wondered if catnip’s active component, nepetalactone, also deters other insects such as fleas.

It would depend on the version of the receptor those species have, Garcia de Jesús says. TRPA1, the receptor that catnip triggers in mosquitoes, is common in many animals and typically responds to irritants such as cold, heat, wasabi and tear gas. But catnip doesn’t spark the same reaction in all insects. “Some aphids, for instance, use nepetalactone as a pheromone, and green lacewings are attracted to it,” she says. More studies are needed to learn how catnip affects other insects.

Leave it to beaver

Building simple structures made of sticks and stones in streams can entice beavers to build their own dams and keep water where it’s needed to fight drought and wildfires, Brianna Randall reported in “Reviving riverscapes” (SN: 3/27/21, p. 22).

Such beaver restoration tactics would not be as effective in streams that flow intermittently, Randall reported. Reader Pam Nelson asked why.

“Intermittent streams usually don’t support beavers, since the rodents need ponded habitat year-round to survive, both for food sources and escape from predators,” Randall says.

Beaver dam analogs could potentially work in such settings, but the stick structures usually work best when beavers are around to build on, repair and rearrange them as part of the natural process in healthy streams, she says.