What is the strength of the dark energy

Dark energy is one of the greatest mysteries of modern cosmology. It makes up over 70 percent of the universe - but so far nobody knows what it is made of. New measurements on exploding stars, so-called supernovae, now show that dark energy was just as strong nine billion years ago as it is today.

Baltimore (USA) - At least those explanations are out of the running, in which the dark energy should change over this period of time.

Supernovae are used by astronomers as "standard candles" to measure the universe. From their brightness and the course of the star explosions, the researchers can determine both the distances of the supernovae and the speed at which the universe was expanding at the time of the explosions. By examining many supernovae at different distances - and thus at different cosmic epochs - the expansion history of the universe can be reconstructed.

In the late 1990s, Adam Riess from Johns Hopkins University in Baltimore and his colleagues caused a sensation. Until then, astronomers had taken it for granted that the expansion of the universe in the course of cosmic history would be slowed down by the gravitational pull of matter. But the supernovae examined by Riess and his team showed the opposite: the expansion is accelerating. The researchers named the mysterious driving force of this acceleration "dark energy".

But what is this dark energy? Is it a kind of tension in the vacuum? Or energetic fields that weaken over time? A large number of different hypotheses are circulating in specialist circles. With their new observations, Riess and his colleagues now provide an improved basis for the discussions. Riess and his team measured a total of 23 supernovae at a distance of up to nine billion light years with the Hubble space telescope. The finding: since nine billion years the strength of dark energy has not changed. But not all cosmologists are completely convinced. Riess also admits that the error bars in the measurements are still so large that minor changes in the dark energy cannot be completely ruled out. In addition, the dark energy can of course also have changed in earlier epochs of the cosmos. Together with his colleagues, Riess therefore wants to continue on the hunt for distant star explosions.