How much is the universe accelerating

Mysterious force in space - dark energy

The mutual attraction of matter should actually slow down the expansion of the cosmos, but instead it expands faster and faster. So far, nobody knows what could be responsible for it.

Simulation of the universe structure

It was sometime in the spring of 1998 - an international team of researchers led by Brian Schmidt, an Australian astronomer at Mount Stromlo Observatories, evaluated systematic observations of distant stellar explosions for the first time. Astronomers are still fascinated and confused by the data that literally move the world, recalls Bruno Leibundgut, Swiss astronomer at the European Southern Observatory ESO in Garching near Munich, who is part of Brian Schmidt's team: “The measurements pointed this out that seventy percent of the universe is in a form that we did not know before. This part of the universe is often referred to as dark energy. In the meantime, many new models for cosmology have been developed - and physics is also trying to explain this dark energy. Even for dark matter, which was postulated earlier, physics still has no explanation. Dark energy and dark matter together make up about 95 percent of the universe. "

Bruno Leibundgut and his colleagues are on the lookout for certain stellar explosions, so-called type Ia supernovae. Here white dwarfs play a crucial role - these are compact stars that have about as much mass as the sun, but are only as big as the earth. If such a white dwarf circles closely around a giant star, it can pull matter away from its companion. At some point the white dwarf has picked up so much matter that it becomes unstable and explodes as a supernova. The limit at which a white dwarf becomes unstable is relatively precisely known: the so-called Chandrasekhar mass is 1.44 solar masses. Because the physics of all Type Ia supernovae is very similar, the explosions are always very similar. These supernovae always shine with the same brightness on site.

Star explosions as a range finder

In the light of the supernovae, the information about how fast the universe was expanding at the time of the explosion is also frozen. The cosmos has been expanding since the Big Bang and the mutual attraction of matter should actually slow down the expansion slowly. But stellar explosions six or eight billion light years away - that is, six or eight billion years ago - show that the cosmos expanded more slowly then than it does today.

Supernova six billion light years away

The result of the group around Brian Schmidt and Bruno Leibundgut was quickly confirmed by another group. So far, all observations support the unimaginable finding in the mid-1990s: Something is driving the cosmos apart ever faster - and no one knows what that could be. Nobody knows what is physically behind it. One idea is the so-called vacuum energy, an additional component of the universe with a repulsive force. How strong normal attractive matter works depends on how densely it is packed in the cosmos. In contrast, the repulsive vacuum energy in the cosmos is always the same - and that has amazing consequences, explains Robert Kirshner from the Harvard-Smithsonian Center for Astropyhsics in the USA and also a member of Brian Schmidt's team: “If you go back in time, then If you look deeper and deeper into space, then the matter is much more densely packed than it is today. At that time, matter dominated with its gravitational pull - the universe was expanding, but slowing down. At some point the matter was so thinly distributed in the expanding universe that its attraction was smaller than the repulsive force of the vacuum energy. The universe has been accelerating since then. At some point, so to speak, the universe switched - from braking to accelerating! "

The dark energy puzzle

Chandra Deep Field

A fascinating idea: above a certain distance, the supernovae would have to show traces of the deceleration from the youth of the cosmos instead of the acceleration. So far, specimens some ten billion light years away have hardly been observed. But soon, with the help of the supernovae, astronomers could literally see how the repulsive vacuum energy in the cosmos gained the upper hand and since then has been drifting our universe apart ever faster. Bruno Leibundgut and his team are planning a large new observation program: “We want to decide whether the acceleration of the universe comes from the cosmological constant or from the so-called quintessential fields. The point in time at which the acceleration gains over braking depends on what is causing the acceleration and how strong this additional component is. If it is very strong, the acceleration starts earlier. If it is weaker, this transition is later in the universe or, from our point of view, at a shorter distance. We need extremely precise measurements for this. We think we need over two hundred supernovae in the crucial area to really do that. It will take a long time. We now have to convince the rest of the world that they give us the observation time so that we can do that. "

To do this, astronomers have to find supernovae at a distance of four to around nine billion light years and observe them closely. Not an easy undertaking, but feasible from the ground with top telescopes. Bruno Leibundgut is looking forward to the new big Omega project, which should clarify whether cosmological constant, quintessence or maybe something completely different is driving the cosmos apart at an accelerated rate. “The beauty of cosmology these days is that we are really dominated by data compared to thirty or forty years ago. Thirty or forty years ago, essentially everything was based on theory. At that time, the theories could in principle not be checked with the data. That has now been reversed. "

Data and models address the very fundamental questions of our world. What is the universe made of? What is Dark Energy? Will the cosmos expand forever? The ancient light of long-extinguished star explosions provides the answers - even if researchers have so far not understood all the answers and data from supernovae and background radiation in one big picture. The only way out is through further observations - and good ideas! “We have 95 percent of the universe that we just don't understand: 25 percent is dark matter, which we still have no idea what it is. Then we have things like dark energy, cosmological constant or quintessence. These make up maybe seventy percent of the energy density of the universe and we have no idea at all. This is actually an interesting situation. Maybe in ten years we will have a theory that explains all of this - but we need a breakthrough now. "