(The fourth installment in the series “Alphabet of Cosmology“, in which I introduce a concept or project in cosmology.)
If you don’t know Calvin & Hobbes, we are no longer friends.
Let’s face it: “dark energy” is one of the dumbest names for an important concept in physics. First, it sounds like something from an action movie or superhero comic (or superhero action movie). Second, the term makes it sound like dark energy is somehow related to dark matter, which was a deliberate choice by cosmologist Michael Turner, who coined it. However, while dark matter (which as I’ve pointed out before would be better called “invisible matter”) does appear to be matter—it clusters and attracts ordinary baryonic matter gravitationally—dark energy has very different properties.
What do we mean by “dark energy”?
Since the 1920s, we’ve had strong evidence that the Universe is expanding. If the Universe contained only matter (both ordinary and dark matter) and light, then its expansion would gradually slow down, but it doesn’t. In 1998, two groups of astronomers discovered that the rate of expansion is increasing, a discovery for which three of the project leaders—Saul Permutter, Adam Riess, and Brian Schmidt—received the 2011 Nobel Prize in ...