How will the space-time continuum end?

The end of the future

“My topic can be summed up in three words: Time is ending.” This is how John A. Wheeler from the University of Texas at Austin began a lecture in 1981 in Munich. He meant the collapse of time in two ways: on the one hand locally, i.e. in many different places, and on the other hand globally, i.e. for the universe as a whole. Today, over three decades later, more and more physicists and cosmologists agree with Wheeler - and yet the time no longer appears to be the same as it did then. In addition, a few other "deaths" were discovered by her, while Wheeler's swan song may have been premature.

John Archibald Wheeler, who died in 2008, was one of the most important physicists of the 20th century. He researched nuclear fission with Niels Bohr, did pioneering work on relativity and quantum theory and coined basic terms such as “black hole” and “wormhole”. He called the big bang and the gravitational collapse “the two gates of time”: “The first gate means that there is a time before which there is no before. The second gate means that there is a time after which there is no after. ”Time, according to Wheeler,“ is neither music, nor metronome nor drumbeat ”of the world, but rather its slave, and it ends with it.

Wheeler identified a gravitational collapse as the future killer of the time. In the local case, time and space collapse in the center of black holes. These dark graves are formed from the cores of collapsed stars. Inside everything is crushed - matter, energy and even the dimensions. Although time continues outside of the gravity gullies, it should come to an end in them - at least within the framework of general relativity: Albert Einstein's field equations spit out infinite values ​​for curvature, density and temperature, while space and time become zero. Such a singularity not only marks the end of time, but also of classical physics in general.

The big crush

In the second case, which Wheeler mentioned, the whole universe is a kind of black hole. Space is currently expanding, but at some point the expansion could stall and then reverse. Then the galaxies would approach again, the cosmic temperature would rise until it finally surpassed that of the stars, whereupon these would dissolve, as would matter. After all, everything would have to implode in a brutal final bang, so that space would devour itself and disappear with time. Cosmologists call this furious finale "Big Crunch" - the great crushing. Whether this will actually happen is questionable - and now less likely than in Wheeler's time. For the universe to collapse, at least one of the following conditions must be met:

· The average density of matter in space is greater than a critical limit of around five protons per cubic meter. That doesn't sound like much. But it is at least three times what astronomers say is in space - not only in terms of luminous matter, especially gas and stars, but also of the ominous dark matter, which is probably made up of as yet unknown elementary particles. In short: the existing mass will not force the expansion of space to reverse.

· The energy density of the vacuum is negative. As a result, the room develops an inner “pull” and withdraws into itself. However, it was discovered in 1998 - and has just been crowned with a Nobel Prize - that the universe is not expanding more and more slowly, as is actually to be expected due to the slowing effect of the matter, but faster and faster. The most popular explanation for this is a positive energy density of the vacuum, which has an anti-gravitational effect. However, nobody knows what is hidden behind this mysterious dark energy. The simplest candidate is the cosmological constant introduced by Albert Einstein. According to speculative physical models, however, the vacuum energy could one day also become negative. Then the universe would at some point eradicate itself, albeit not for trillions of years, as Alex Vilenkin of Tufts University suspects.

· As yet unknown physical effects cause a collapse. This follows, for example, from some theories of a modified theory of relativity as well as from certain models for dark energy or from colliding universes. The astronomical evidence for this has so far been insufficient.

There is, however, a disturbing possibility: the dormant danger of a phase transition of the vacuum. (Physicists speak of a “vacuum”, even if the universe is not an absolutely empty space.) If the physical state of our universe is “metastable”, it can “disintegrate” or tunnel into a lower-energy “ground state” (see graphic “The big one Decay "). That wouldn't necessarily be the end of everything, but it would be the end of time as we know it. However, the collapse of the vacuum could just as easily lead to a big crunch - under certain conditions even in a few dozen million years. Raphael Bousso of the University of California at Berkeley and his colleagues recently made a name for themselves with this prediction, which is as shocking as it is surprising. If it is correct, the end would be really close from a cosmological perspective - even our sun could still shine. If everything leads to a big crunch, it doesn't necessarily mean the end of time. Because like the black holes, the final bang is an extreme physical situation, but not necessarily a space-time singularity. This is initially only a mathematical limit of the theory of relativity - that is, strictly speaking, not an object of physics and not an object of nature, but the artifact of a theory that has been strained beyond its scope. Nature does not have to abolish itself here just because the theory of relativity fails - or better: indicates its limit of validity.

All that is needed is a better theory. Such a theory of quantum gravity would have to combine the theory of relativity with the other cornerstone of modern physics: quantum theory. It describes the realm of the very smallest and is not compatible with Einstein's field equations, because they are continuous, but quantum processes take place in leaps and bounds, so to speak portioned. This applies not only to radiation, particles and fields, but presumably also to space and time. Because due to the uncertainty principle discovered by Werner Heisenberg, nothing can be precisely determined and determined in nature. On the Planck scale, which Max Planck, the founder of quantum theory, described as early as 1899, space and time should therefore become “grainy”. This scale is of course extremely tiny: it is on the order of 10–35 meters and 10–43 seconds. If the extrapolations are correct, then time and space are not arbitrarily divisible - just like radiation and matter - but they are fuzzy, fold, waver and jerk, as it were. Wheeler called this "space-time foam".

Strictly speaking, quantized spacetime also marks an end of time - or at least our idea of ​​it. It is just not a fundamental continuum, but rather has the smallest rhythm in which it ticks. And like space, it seems to be constituted by something more fundamental. Wheeler coined the term “pre- or embossing geometry” for this, and he was convinced: “Time as such cannot be the last concept in describing nature. Time is neither original nor accurate. It's an estimate. It is a secondary term. At some point it will move into the second tier in terms of its importance. "

In the meantime, physicists and mathematicians have developed several theories for such an embossing geometry, which combine relativity and quantum theory. The leading are the loop quantum gravity and the M or string theory (Bild der Wissenschaft 4/2004, "Strings against loops"). The latter describes the substrate of the world with all its forces from one-dimensional vibrating strings, the strings, and multi-dimensional "branes", which perhaps also constitute space-time itself. According to the M-Theory, in addition to the three large spatial dimensions that are familiar to us, there are also seven more, but these are "rolled up" and so tiny that they may only be noticeable on the Planck scale.

Knots come and go

Loop quantum gravity is even more radical: According to this theory, space is not a three-dimensional continuum, but a network of one-dimensional structures. It is difficult to get an idea of ​​this fabric of the world, and its mathematical description is extremely demanding. It can best be compared with a photo, for example in Bild der Wissenschaft: From a distance it appears homogeneous, but on closer inspection it is composed of individual pixels, so it is “quantized”. Similarly, the room is also made up of one-dimensional components. This tissue is constantly changing, individual knots loosen and new ones arise. That brings out time. But from this quantum gravity theory, according to the physicists Abhay Ashtekar, Carlo Rovelli and Claus Kiefer, an end of time also follows - not time per se, but time as a fundamental quantity that cannot be traced back to anything else. Other physicists, such as Vesselin Petkov and H. Dieter Zeh, have argued similarly in the context of the theory of relativity: Since space and time do not exist separately, but only in the unit of space-time, the "old" time before Einstein's revolution is long at the end. It doesn't really exist.

The magic window of physics

Even if the current theories of quantum gravity are by no means secured or even only experimentally confirmed, they make it clear how correct and almost prophetic John Wheeler's considerations on embossing geometry were. He aptly described physics as “a magic window”: “It shows us the appearance that lies behind reality - and the reality that lies behind the appearance. Physics goes much further than was previously imagined. We are no longer satisfied with just understanding elementary particles, force fields or even space and time. Today we demand insights into the being of things themselves from physics. So we can say that we will only understand how simple the universe is as soon as we realize how strange it is. "

After all: The assumption of a quantized space-time promises a resolution of the singularities in the theory of relativity - i.e. a healing of that painful stab in the tissue of space-time. No gravitational collapse tears apart space-time, as Wheeler and Einstein feared, but rather it springs back, as it were. This is what both string theory and loop quantum gravity suggest. What exactly happens is still unclear, however. But Wheeler's gates of time do not seem to lead nowhere, that is, to the end of reality. Either the door swings straight back or it opens into a new world. Another universe could begin “before” the Big Bang and “behind” a black hole - or the compressed space-time bounces back like a squash ball hit against the wall. In any case, time would continue. And a new Big Bang could rise from the final bang - like a phoenix from the ashes. So the big crunch would really be a big bounce: a big rebound and swing.

However, time does not necessarily have to disappear spectacularly, as in Big Crunch, or jump back wildly, as in Big Bounce. It could also creep away miserably - by gradually fading or fading. Cosmologists speak of the Big Whimper. Then everything would not have disappeared, as in the classic final bang, but something would still be left that would not change, however. Such a scenario can be called a pseudo end, analogous to the pseudo beginning of time (Bild der Wissenschaft 10/2004, “The Big Bang from Almost Nothing”). In both cases there is a micro time, but no macro time. Characteristic of the macro time is an arrow of time, i.e. irreversible processes as well as clear changes, and thus a differentiation between the past and the present. In micro-time, on the other hand, there are no larger structures or directions of development, only random quantum processes - such as the conversion of radiation into particles and vice versa. So in micro-time everything remains in a dreary equilibrium: One moment is almost like the other.

Barren space without time

According to the cosmological scenarios with a pseudo beginning, the Big Bang was the beginning of the direction of time, but not the beginning of everything. For example, it could have occurred as a random fluctuation in a quantum vacuum or as the breakdown of a time loop. A pseudo beginning thus marks the origin of the macro time, but not an emergence from nothing. Analogous to this is the pseudo-end: here not everything disappears completely, but the macro time stops. Then there is no time direction and no more developments. Either the universe freezes, or it becomes an eventless and structureless, almost empty space in which only here and there photons meet or random quantum fluctuations come and go. The end of time would then be the end of macro time.

Most hypotheses about dark energy - including those of a positive cosmological constant - predict such a bleak future. Accordingly, space will expand ever further and faster - for all eternity. In many trillion years the stars will go out, the raw materials for new suns will be used up, the protons and thus atoms will decay and finally even the black holes will have evaporated through quantum processes. When nothing happens in the almost empty space and nothing more can function as a clock, not even theoretically, then time is drawing to a close - according to Albert Einstein's bon mot "Time is what the clock shows". Roger Penrose from Oxford University recently worked this out in his new model of conformal cyclical cosmology - in which he also speculated about a new beginning of time or macro-time (bild der Wissenschaft 12/2010, “The Eternal Return of Time”). A self-emptying and ultimately time-consuming universe may either seem depressing or a kind of nirvana. But due to quantum processes, it also harbors the possibility of a rebirth of time. This follows from several cosmological models, not just Penrose's.

But dark energy could kill time much more vehemently, much earlier and forever. In extreme cases, it tears apart all structures - stars, molecules, atomic nuclei and finally space-time itself. Such a big rip is not particularly likely. But it cannot be excluded as a temporal exitus by astronomical measurements, if the so-called phantom energy is behind the dark energy, which makes the expansion infinitely fast.

Cracks in the fabric of space-time

Even more brutal is a possibility that Max Tegmark from the Massachusetts Institute of Technology came up with just a few months ago and which he called Big Snap: Not only are all structures shredded down to the subatomic level, but it even happens to the Space itself. If, as Wheeler speculated, it has a “grainy” or “foam-like” structure, it should not be able to expand indefinitely, otherwise new “space atoms” would have to form out of nothing. Tegmark illustrates this with an example from everyday life: "Since a rubber band consists of a finite number of atoms, it will tear if you stretch it far enough." In the Big Snap model, the dark energy would not only do this with a rubber band, but also do with the room itself. "But in contrast to rubber bands, it is unclear what that means exactly," admits Tegmark. He even speculates that the space only dissolves between the galaxy clusters when there is no phantom energy, so that the galaxies held together by gravity would be spared - they would be isolated islands in "nothing". Then the Big Snap could even survive. There would be no escape, however, if the dark energy fulfills a very specific form of its equation of state: Then the universe would come to a screeching halt and shatter internally. With such a big brake or big freeze, everything would come to an abrupt standstill (Bild der Wissenschaft 4/2010, “Universe in shock”). Micro and macro time would be cut off abruptly. No clock could tick anymore, no heart could beat.Even the vibrations of the atoms and the vibrations of the strings would be frozen in shock forever. In the apocalyptic words of the writer Theodor Fontane: “I see it fall, end, / and how everything collapses. / I can't turn the day around, / but I don't want to live it! ”■

by Rüdiger Vaas

Six ways of the end of the world

The future of the universe depends on whether space continues to expand as it has before. If instead the universe contracts in the distant future, it will collapse in a final bang that even destroys time (1). However, it could also lead to a new Big Bang if the collapse "swings back" (2). If the expansion goes on forever, even protons and black holes will decay at some point (3). At present the expansion is even getting faster, which means that the universe is becoming more and more “emptier” (4). Perhaps it accelerates so much that everything is even torn apart (5). Under special conditions, however, the expansion could come to an abrupt end (6) - then everything would be crushed or freeze, and nothing would change.

When time becomes space

A crazy-sounding finale of time is its transformation into a fourth dimension of space. In fact, in the theory of relativity, time only differs formally from the three dimensions of depth, width and height by an opposite sign. And that could jump from minus to plus.

Marc Mars from the University of Salamanca, José Senovilla and Raül Vera from the University del País Vasco in Bilbao worked this out in a string cosmology model. The Spanish physicists assume that our four-dimensional universe, a 4-brane, moves through a higher-dimensional space (“bulk”). If this happens faster and faster, up to the speed of light, then this is accompanied by a slowdown in movements or time within our universe. In a sense, we resemble aphids on a leaf, the 4-bran universe, and if it flutters too quickly, we can no longer crawl around, but have to cling to and finally remain motionless. And not just us, but everything.

"This would be a dramatic event, although the bulk and the bran remained regular everywhere," says Marc Mars. In our universe, time would end in the shock of a big freeze, even though that would not be a singularity from a higher dimensional perspective. Within our universe, the structure of cause and effect would come to an end, while outside, in bulk, everything would remain normal, including time itself. And the dark energy would be a sham effect that the Bran movement leads us to believe.

"If you assume that time began with the Big Bang, then it can also disappear again - that's just the opposite effect," comments cosmologist Gary Gibbons, a close associate of Stephen Hawking at Cambridge University. Hawking had already proposed a reverse signature change in 1981: According to his model, time in the Big Bang emerged from a fourth spatial dimension. It would not only have an end, but also a beginning.


· The fate of the universe depends on the values ​​of a few cosmological parameters, which are measured more and more accurately.

· Depending on the density of matter, the type of ominous dark energy and the modification of the relativity theory, time ends slowly or brutally: it fades away, collapses, is torn apart or has to literally freeze.

The great disintegration

Physicists describe the vacuum state of our universe in a simplified way with the potential of a field and its variable sizes. If the state is metastable, it can, as it were, decay. If the universe goes through such a phase transition to a state of lower potential energy, nothing is as before. This would be the end of the world we know - even the natural constants would change and earthly life would be impossible.

February 21, 2012