Composition of the Universe

One more constant needs to be mentioned: the cosmological constant, \Lambda , which has units of (mass)^4 . As we will see later, it turns out that \Lambda^{1/4} has roughly the same scale as M_{\text{neutrino}} ; again a quantity that is very small and needs to be explained. We will discuss that later in this chapter. \Lambda is related to what is called the “dark energy,” which is the energy of empty space and is related to the cosmological constant. This is because dark energy leads to an accelerated expansion of the cosmos. It was an amazingly important discovery in physics a couple of decades ago that the composition of energy in the universe is dominated by this mysterious form of energy by measuring this accelerated expansion of the universe. Figuring out where this dark energy comes from constitutes one of the biggest puzzles in all of physics today. The composition of energy in the universe today is as follows: