Astronomers have found stars dating from a long-ago collision between the Milky Way and another galaxy. The crash helps to explain why the Milky Way looks the way it does.
Two ways of measuring the universe’s expansion rate yield two conflicting answers. Many point to the possibility of new physics at work, but a new analysis argues that unseen errors could be to blame.
The newly developed theory of emergent gravity, proposed as an alternative to dark matter, struggles in one of its first trials.