Why is the universe so predictable?
Inflation and Cycles in the Multiverse
Research Report 2011 - Max Planck Institute for Gravitational Physics
The limits of the big bang theory
The most important cosmological discovery of the last century was undoubtedly the discovery of the expansion of the universe by Edwin Hubble in 1929. No other breakthrough has fundamentally changed our view of the world. Previously, most scientists were convinced that the universe would remain roughly unchanged and static. The expansion of space gave the universe a story for the first time, so to speak.
This expansion has dramatic consequences: if one traces the universe back in time, it becomes clear that the universe must have been smaller and smaller and therefore denser and hotter. Georges Lemaître  was the first to take this idea to extremes by proposing the Big Bang hypothesis. Accordingly, the universe is said to have started at one point with a huge explosion, and this small, hot universe has expanded and cooled since then.
Shortly after the Big Bang, the energy density was so great that there weren't even atoms, just elementary particles that constantly collided. Even photons (light particles) could only fly straight ahead over short distances until they collided with electrons again. As a result, the universe was completely opaque at that time. About 380,000 years after the Big Bang, it was cold enough for the first atoms to form. Since photons interact much weaker with atoms than with free-flying electrons, for example, the universe suddenly became transparent and the light, which at this point could fly freely for the first time, still penetrates our cosmos. This "cosmic background radiation" in the microwave range was measured for the first time in 1965 (Fig. 1). It gives us, so to speak, an infant image of the universe and shows that our universe was almost the same temperature everywhere. Slight temperature fluctuations are shown by the different colors. These small temperature differences later determined the distribution of galaxies. In colder regions the matter was a little denser, and due to the influence of gravity, the matter gradually "clumped" in these areas and formed stars and galaxies. In contrast to this, the slightly warmer areas were also a bit emptier, and have become increasingly emptier over time, because nearby, denser regions have drawn the matter out even further. In this way the great empty spaces of our universe were created. The cosmic background radiation therefore offers us convincing evidence that the universe (almost 14 billion years ago) was dense and hot.
The discovery of background radiation led to widespread acceptance of the big bang hypothesis. However, the big bang theory leaves a number of questions unanswered. One such question is known as the "horizon problem". It is still unknown how big our universe is, but it extends for at least 14 billion light years in each direction, because that's how far we can currently see. If you take this expansion and trace it back to shortly after the Big Bang with the help of the equations of the theory of relativity, you can see how the universe does not contract to one point, but to a large area. So the Big Bang did not take place at one point, but on an extensive area! But this area consists of several regions that could not have any contact with each other up to this point, as there should have been nothing before the Big Bang. Nevertheless, the Big Bang is said to have taken place in all of these places at the same time! This hypothesis is not justifiable if one believes in cause and effect, because how are you supposed to synchronize countless big bangs when there is no time? It therefore makes much more sense to assume that the Big Bang was not the beginning, but an event in the history of our universe. But what was before? What could trigger the big bang?
Inflation or cycle?
There are currently only two good theories about the pre-Big Bang era. The best known is the inflation theory , according to which there was a short phase of extremely rapid expansion before the Big Bang. Such a phase has the effect that even tiny regions are extremely stretched and are homogeneous and isotropic over large areas within a short time (any "folds" in space are tightened by the rapid expansion). When this phase comes to an end, the energy of expansion is partially converted into radiation and matter - this corresponds to the Big Bang. This then also happens over large areas at the same time, since the same conditions prevail everywhere there.
A second theory is that of a cyclical universe . According to this theory, there was a phase of contraction before the Big Bang. It turns out that a phase of slow contraction can also cause the universe to become homogeneous and isotropic. Therefore, according to this theory, the Big Bang is also synchronous over large areas. Here the Big Bang corresponds to the change from the contraction phase to the expansion phase, which in turn generates radiation and matter. This turnaround is currently the least explored aspect of the theory. However, it seems plausible that such a downturn can take place. In this case, the phases of contraction and expansion can alternate as often as desired, so that the universe behaves cyclically over large periods of time.
Which of these two theories applies to our universe? This may turn out to be as early as next year. Both theories can explain the origin of the temperature fluctuations in the infant image of our universe equally well. However, as to the details of these temperature differences, the two theories make different predictions. For example, inflation theory cannot currently explain why temperature fluctuations are of the magnitude measured. In the theory of the cyclical universe, on the other hand, there are arguments that show that the fluctuations are just as strong as they can be maximally .
The biggest difference in the predictions of the two theories concerns the distribution of the fluctuations in the cosmic background radiation, i.e. the pattern in Figure 1. An inflation phase is driven by (still hypothetical) particles, which Inflatone and which have very little interactions with one another (this means that two such particles have almost no effect on one another). These particles are practically "free" and accordingly the statistical distribution of temperature fluctuations is also described by a Gaussian curve . The cyclic model is also based on the dynamics of similar particles, but this time they have strong interactions with one another. This leads to significant deviations from a Gaussian curve in terms of the distribution of temperature differences .
Unfortunately, the current measurements of cosmic background radiation are still too rough. To decide which of the theories is more plausible, we need an even more detailed picture of the background radiation. But just such an image is currently being taken by the European PLANCK satellite, and the first results are eagerly awaited in 2012!
However, both theories still leave many questions unanswered that can only be answered with the help of a quantum theory of gravity. Although the string theory is not yet fully developed, it still represents the best approach to such a unified theory. It is therefore worthwhile to find out which statements (in its current, unfinished state) it already provides about the universe. It turns out that, according to the string theory, there can be both inflationary and cyclical universes. In addition, the string theory also allows countless other universes, some of which are very similar to our universe, but most of them are fundamentally different. In addition, these different universes are continuously interconnected (through the variation of a number of parameters). In this so-called landscape a new universe can arise within an already existing universe . This new universe is usually different from the older one. In this way, an infinite number of universes arise and so all theoretically possible universes also become real!
If this "multiverse" (Fig. 2) corresponds to reality, it becomes the task of the cosmologist to also describe it mathematically. This is a difficult challenge, mainly because of the infinities that exist in the multiverse: on the one hand there is an infinite number of universes, on the other hand many of these universes are themselves infinitely large! It is therefore not surprising that the various studies of the multiverse have so far led to different results. Many mathematical descriptions of the multiverse lead to paradoxes and "insane" predictions. Only a few such descriptions are currently known that seem more meaningful. So far, cyclical universes have also been largely ignored in these investigations because they were assessed as unimportant. Recently, however, calculations  have shown that it is rather the other way round: If cosmic cycles can be repeated any number of times, it is actually much more likely that we live in a cyclical universe and not in an inflationary one. (Otherwise, if there can only be a limited number of cycles, the probability depends on as yet unknown details of the string theory.) This makes it even more exciting to wait for the results of PLANCK. If the results indicate a cyclical universe, we also get an indication of the description of the multiverse. However, if PLANCK paints the picture of an inflationary universe (which most cosmologists expect), there will be a new paradox to solve.
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