Read out Triumph for Einstein's "Donkey". The universe is probably infinitely large, and it will expand forever - even faster, as new measurements say. This is a surprising rebirth of Albert Einstein's mysterious Cosmological Constant, in which two thirds of the energy of the universe should be. Of two numbers, the fate of the universe depends on the Hubble constant, which indicates how fast the universe expands, and the braking parameter that describes the slowdown of this expansion over time. Will the expansion of the universe last for all eternity, or will it be stopped by the gravity of matter one day, and vice versa, so that the universe collapses and disappears in a final bang?

"Cosmology: the search for two numbers, " was the title of a scientific article from 1970. The author is Allan Sandage from the observatories of the Carnegie Institution in Washington in Pasadena, California and successor to Edwin Hubble, who discovered in 1929 that the Universe is expanding. Sandage has devoted much of his research life to these two numbers.

After decades of persistent work with the largest telescopes, the development of ever more sophisticated observation methods and a constant increase in accuracy and sensitivity, the answer is now emerging: The Universe will continue to expand forever - and it is probably infinitely large. The sensation, however, is that expansion does not slow down, as would be expected from the braking effect of matter, but it actually accelerates! An unknown force that acts like an antigravity drives the universe apart.

The key role in the new measurements is a certain kind of starburst in distant galaxies. They are called Type Ia supernovae and are oversized hydrogen bombs. They ignite when the mass of a white dwarf star grows above a critical value because it pulls matter from a neighboring star. Since the absolute - ie distance-independent - luminosity of the observable explosion debris hardly fluctuates and can be measured very accurately, the supernovae can be used as a cosmic scale. The astronomers speak of "standard candles". "Standard bombs would be the more appropriate term, " says Sandage, who had first begun measuring space with the stellar explosions. If one knows their absolute luminosity, the change in the rate of expansion over the course of the aeons can be determined on the basis of their observable brightness. And that in turn allows conclusions to be drawn about the density of matter in the universe. display

Meanwhile, the astronomers have enough evidence for far-reaching conclusions. "We are in strong agreement, " jokes Perlmutter with a view to the competition. Because both research teams came to the same conclusions independently of each other and with different supernovae:

- The braking parameter is not enough to stop and reverse the expansion of the universe. The universe does not contain enough mass to collapse at some point. It will therefore extend to all eternity. - The "borderline" model preferred by many cosmologists for theoretical reasons is refuted. It assumed that the mean matter density of the universe is as high as the critical density at the dividing line between collapse and eternal expansion. - Moreover, the new measurements suggest that the universe is infinitely large - and not just limitless, but spatially finite, such as the surface of a sphere that has a finite surface area but no boundaries because it runs back in itself. - But the results of the cosmic investigation reveal even more: distant star explosions are fainter than they are likely to be even at a constant rate of expansion. This means that they are farther away than expected. It follows that the expansion of space has not slowed down for several billion years, but has become faster.

Charles Lineweaver from the University of New South Wales, Sidney, uses a down-to-earth metaphor to attempt a cosmic survey: "If you compare the universe to a cappuccino, then coffee is the weird vacuum energy. The equally mysterious dark matter is the milk. And the planets, stars and galaxies are the chocolate powder on the foam. "

=== Rüdiger Vaas

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