What is dark energy and why is it so important?

If the universe only contained ordinary matter and dark matter, we would expect the gravitational pull of all the mass in the universe to be slowing down the universe’s expansion in the same way that if you throw a ball up, Earth’s gravity pulls it back down.

However, in 1998, astronomers measuring the distance to far-away supernovae discovered that the expansion had started to get faster instead of slower.

To explain this, scientists invoked dark energy, an unknown something that, unlike matter, gravitationally repels instead of attracts, pushing the universe apart almost like “anti-gravity.” The simplest version of dark energy is Einstein’s original idea for a cosmological constant, as a way to balance the action of gravity in his theory of general relativity.

The mysterious dark energy constitutes nearly 70% of the universe today. And while we can’t see dark energy directly, it determines how our universe is expanding and its eventual fate.

Previously, the gold standard for cosmic microwave background measurements was provided by Planck satellite data, taken a decade ago.

The improved measurements from the South Pole Telescope, when combined with the DESI experiment and other CMB datasets, reduce the likelihood of a cosmological constant and increase the preference for time-evolving dark energy models.