cosmological-constant

In 1998, two teams studying distant Type Ia supernovae found that cosmic expansion is speeding up. The cause, dark energy, makes up about 68% of the universe's energy budget, and nobody knows what it is.

The universe's physical constants appear finely tuned for life: change them slightly and stars, atoms, or chemistry might not exist. The anthropic principle argues this is a selection effect, since we can only observe a universe that permits observers. Whether that explains fine-tuning or merely restates it is one of the deepest debates in modern cosmology.