Letters
By Science News
Galactic collisions explained
Perhaps you can explain why Andromeda and the Milky Way are going to collide “Milky Way will be hit head-on,” (SN: 7/14/12, p. 10). Galaxies, as is always written, are rushing away from each other at ever-increasing speeds. How do things collide when there is never anything to collide with? Either galaxies are rushing outward in all directions with the same impetus from the point of the Big Bang, or they are not.
Bruce Smith, via e-mail
Galaxies do indeed collide with one another even though the universe is expanding at an ever-accelerating rate. In the case of the Milky Way and Andromeda, the two galaxies are massive enough and close enough that they are gravitationally attracted to one another. So they are headed for a collision even as both galaxies continue to be carried along on the flow of the expanding universe. You can think of it like two boats running into one another in the middle of a river; they collide even though both are also being carried downstream by the current. —Alexandra Witze
Clarifications
The high energy (100 billion billion electron volts) of a particle mentioned in “Chasing a Cosmic Engine” (SN: 7/14/12, p. 16) refers to the particle’s energy when it hit the atmosphere, not when it reached detectors on the ground. The article should have said that in 1962, the particle slammed into the atmosphere, creating a shower of particles that rained down on an array of detectors in Volcano Ranch, N.M.