The space can be stretched indefinitely

Infinity, part 2: Universe without end?

It is a primal human experience: that mixture of fascination and shudder under the night vault covered with twinkling stars. From star to star, the view gropes its way out into space, further, further and further, apparently without end.

Is this space really infinite? Or does the universe have a limit somewhere? What's beyond the threshold? The nothing? God? And what shape does the cosmos have? Is it an endless plane, a sphere, a four- or even more-dimensional, curved structure?

Astronomers are currently being carried away by the spirit of optimism in their guild, electrified by a series of recent discoveries. In principle, these confirm the model of the Big Bang, according to which everything we see once emerged in a huge explosion from a point with an unimaginably high density. But at the same time, the new findings force radical corrections to be made. The situation is reminiscent of the gutting of a building in which new ceilings and walls have to be put in behind the old facade and on the proven foundation.

In the beginning there was a bang

The 20th century brought the discovery that the galaxies are moving away from us, that the universe is expanding like a rising yeast dough in which the celestial objects are like raisins. According to this, the visible cosmos must once have been concentrated in one point before it flew apart as a colossal "ball of fire" in the Big Bang, creating space and time.

So one thing was clear: our universe did not exist for an indefinite period of time. According to the latest knowledge, it began around 14 billion years ago. The question remained after the end: will the cosmos ever cease to be? Will the world shrink again in an unimaginably distant era and finally collapse? Or will the universe expand into eternity, the matter diluting and cooling down until all structures have disappeared and deadly monotony prevails?

The year 2001 brought the realization: The universe is condemned to infinite expansion. The decisive factor for this conclusion was the investigation of the cosmic background radiation, which is still perceptible today as a faint reverberation of the Big Bang. In the "fireball" that flared up at the time, there was a thick soup of atomic building blocks and light particles that constantly collided with each other - which made it impossible for the photons to escape. After a few hundred thousand years, electrons, protons and neutrons combined to form atoms, free space was created between them, and the light particles were able to travel long distances for the first time - the universe became transparent.

The earth is surrounded by a "wall of fire"

According to the calculations of the astrophysicists, the glowing ball had a diameter of 300,000 light years and was 2,700 degrees Celsius. From the earth we look today in all directions on the inside of this "wall of fire", the radiation of which, however, has cooled down to 2.7 degrees above absolute zero due to the expansion of the cosmos. Just like the space between the galaxies, the distance between the wave crests of the electromagnetic rays has expanded on the way to us. They have become longer-wave and are now in the microwave range.

This rumbling of the Big Bang is surprisingly even. It shows only slight fluctuations, which go back to the fact that the cosmic primordial sphere was shaken by vibrations: These shifted the frequency of the emitted electromagnetic waves by one nuance - depending on whether they originated "in the valley" or "on the mountain" of an oscillation . Physicists have calculated that this primal vibration should be reflected in the sky as a pattern of warmer and cooler spots with a maximum of twice the size of the full moon. When three research teams confirmed the prediction on the basis of measurements in April 2001, the astronomers went crazy. "This means that there is no doubt that the universe is flat," says Matthias Bartelmann from the Max Planck Institute for Astrophysics in Garching.

The doctrine of the ball

What does flat mean in this context? And how is the flatness of space related to its fate? In his general theory of relativity, Albert Einstein broke the classic notion of space and postulated that it can in principle assume different geometries, depending on the density of the matter contained and each with a different future. In a "dense" world, space curves positively or convexly. A universe of this structure would one day contract again.

To imagine a three-dimensionally curved space is beyond our imagination; the principle becomes tangible, however, if we forego one dimension and consider the two-dimensional analogue: the surface of a sphere. The two-dimensional beings living in this world feel as little of its curvature as three-dimensional creatures of that of their 3-D space. The spatial geometry is only visible to those who "stand one dimension higher" and look down from there.

In a "thin" cosmos the space bends negatively or concavely, in the 2-D simplification to a saddle surface. This universe would have been condemned to dissolve into gloomy infinity. Flat or Euclidean space, the analog of which is a plane, is a kind of compromise. The matter in it is just enough to prevent the collapse. The fact that of the three possible geometries the universe has adopted the flat, hence the simplest, simplifies our view of the world in a certain way. Because Euclidean geometry is what we learn in math class and with which we work in everyday life when we draw a triangle and expect the angles to add up to 180 degrees.

"We live in a grotesque universe"

"On the other hand, the world has now become more complicated in terms of physics," points out Gerhard Börner, theoretician at the Max Planck Institute in Garching. His colleague Michael Turner from the University of Chicago puts it more drastically: "We live in a grotesque universe." Because this must have a certain mass that "pulls" the room flat with its force of gravity, which prevents it from curving into a saddle. And this mass is missing in the balance sheet of the astronomers back and forth.

The critical density is five protons or neutrons per cubic meter. But all galaxies, stars and gas clouds "dissolved and evenly distributed over space - bring only 0.2 particles per cubic meter, around four percent of the total mass. The substance that suns, planets, people and microbes are made of is therefore only a drop in the cosmic ocean.

But there seems to be something that increases the density in space: astronomers believe that they have spotted considerable masses in the vicinity of galaxies. These star islands would inevitably fly apart if only there was the matter that we see glow. Astrophysicists have calculated that a gravitational pull of ten times more mass is required to hold the swirling multitudes together. "There is probably no getting around the realization that most of the matter in the universe is dark," complains Mario Livio of the Space Telescope Science Institute in Baltimore. The situation reminds him, he says, of a circus act in which two white hands seem to act freely - because whoever moves them remains invisible against the black background.

Researchers are now really in the dark about the nature of this mysterious matter. After all, they can put their share in the cosmic inventory at just under 30 percent. But to get the room flat, two thirds of the critical density is still missing. In order to close the annoying gap in coverage, astrophysicists devised a "dark energy", a field that appears almost uniform everywhere, the origin of which they do not know and the properties of which they figure together. "No cosmologist is happy with it," admits Gerhard Börner, "but no one knows any other solution."

Lights in the depths of space

To take refuge in this curious concept, the sky explorers were forced by sensational discoveries of truly cosmic monsters - the supernovae of type Ia. When a burned-out sun collapses and explodes under its own weight, it shines so incredibly bright that we can see it from billions of light-years away. Since the thermonuclear eruption always proceeds according to the same pattern and the brightness is apparently always the same, "we have a standard candle," says Bruno Leibundgut, supernova specialist from the European Southern Observatory in Garching. "If you know you have a 60-watt bulb in front of you, you just have to measure the brightness and use it to determine the distance."

According to this principle, two research groups - the High-z Supernova Search Team and the scientists from the Supernova Cosmology Project - have been searching the depths of space for several years. In 1998, members of the high-z team reported what they initially thought was absurd: the two to five billion year old supernovae were about 20 percent further away than theoretically expected. "They must have been pushed away," concludes Bruno Leibundgut, "so there is a force there that accelerates the expansion of the universe." A universe that is not only expanding infinitely, but also expanding faster and faster - it's just like throwing a stone into the air and it no longer returns, but moves away from us at an increasing pace.

The authors of the publication were afraid of making a fool of themselves. But a little later they got the backing of the second supernova search team: That had come to the same result independently of them. Behind this drifting power is said to be that "dark energy" which, bizarrely, acts as anti-gravity.

What does "dark energy" look like?

From the pieces of the puzzle, cosmologists have constructed the following scenario: In the young universe with high mass density, gravity predominated. It slowed the momentum of the Big Bang. At some point the "dark energy" took over the regiment and drove the star islands apart faster and faster. Adam Riess from the Space Telescope Science Institute compares the behavior with that of a car driver "who starts to brake at a red light and then really accelerates when it turns green".

In fact, astronomers believe they have discovered a first indication of this. In 2000, Riess and colleagues came across the 1997ff supernova in old images of the Hubble telescope, which is at a record distance of 11.3 billion light years - and thus a little further away than the unaccelerated expanding universe would have placed it; but closer than if the acceleration had been effective from the start. For the supernova researchers, this is proof of the original deceleration in space. In Gerhard Börner's opinion, however, this is a bold interpretation. Because it is based only on an object that is also at the limit of observation.

For some, the question of infinity is decided in advance. Because the idea of ​​infinite space is simply unbearable for him. In fact, the part of the universe that we can survey is finite - even unlimited powerful telescopes do not change that. Because we can only see as far as light has been able to travel since the creation of the world - as many light years in 14 billion years, or the equivalent of 130 billion trillion kilometers. "We don't know what it looks like behind this horizon," says Gerhard Börner, "it can get totally crazy. But we assume that what we observe here is typical and continues out there."