What’s most of the universe made of? The answer isn’t stars, galaxies, or even dark matter. Instead, over two-thirds of the universe is energy — a mysterious form of it that is pushing the universe apart faster with every passing second.
This astonishing fact was discovered in 1998 by two teams of astronomers observing distant Type Ia supernovae. These are white dwarf stars that explode in a runaway thermonuclear reaction when their mass exceeds roughly 1.4 times that of the Sun. This consistent threshold means that Type Ia supernovae are all about the same brightness. So, when one goes off in a faraway galaxy, we can determine its distance based on how bright it appears to us. The fainter it looks, the farther it must be. These observations revealed that distant galaxies were receding much faster than predicted, and that the universe’s expansion was not slowing down or staying steady, but accelerating over time.
This source of this acceleration is what we now call dark energy — and what exactly it is remains a mystery. The simplest explanation can be represented mathematically by what theorists call a cosmological constant, written in equations by the Greek latter lambda (Λ). One interpretation of the cosmological constant is that it is a force that is part of space itself — and so as the universe expands, the amount of dark energy grows. But simply being able to capture this in an equation doesn’t tell us much about its nature.
The leaders of the supernovae survey teams who discovered the accelerating universe shared the Nobel Prize in physics in 2011. If you can figure out what dark energy is, you’re sure to snag one yourself.
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