What are some interesting unanswered questions

Unsolved mysteries of science

What is dark matter? How does the universe end? Are there naked singularities? Science still knows many puzzles - a selection of the most exciting open questions

"I know that I know nothing". In the search for knowledge one could consider this winged word ascribed to the philosopher Socrates as the only established fact. This is an imprecise, even falsifying translation from the Greek. "I know that I don't know" calls for a separation of assumptions, bogus knowledge, and proven facts - if researchers want to create knowledge, it only works if one simultaneously makes it clear which questions are still unanswered. Often enough, that's even the more exciting part of science, as our little compilation is supposed to show.

It is certain that the universe as we know it emerged from a singularity - even if the exact mechanism cannot be clarified with today's means of physics. The question of what is "before" is just as nonsensical in the narrower sense as the question of what is "behind" the universe, because space and time only emerged in the Big Bang. In this sense, there is no before. However, cosmology still allows the question of previous universes. Depending on the theory, she comes to different answers.

What was before the big bang?

String theory, for example, builds the cosmos out of one-dimensional strings and multi-dimensional surfaces (branes), the surplus dimensions of which are wound up in certain ways in space-time. There are very different ways of winding the membranes and strings, and depending on which one you choose, a different universe is created. There are a total of 10100 different universes possible.

However, a large part of these universes may have consisted or consist of the basic structures, the membranes, without elementary particles ever being formed. Only when two three-dimensional worlds came too close when moving across an additional eleventh dimension, they could have collided - which is how our universe was born in the Big Bang.

In terms of the world before the world, the competitor of string theory, loop quantum gravity, is even more productive. According to her, the universe is only apparently continuous. In fact, everything, really everything, is quantized, that is, divided into small bits, including gravity. Space is no longer the container for the universe, but a part of it that is also dismembered and takes the form of a network of lines and nodes.

The elementary particles then correspond to different node types, between the lines and nodes there is: nothing. No empty space, nothing at all. The loop quantum gravity theory leads to some strange conclusions, but describes some interesting phenomena better than other theories.

The German physicist Martin Bojowald simulated for the first time in 2004 what results from it for the Big Bang. First of all, one avoids the concept of singularity, because the loop quantum universe has a certain minimum structure size, which it cannot fall below. If you count yourself closer and closer to the Big Bang, it no longer appears as an insurmountable barrier.

Instead, with a "plop" you reach minus time - a new, different or even predecessor universe in which all directions (including those of time) are reversed. This is not a big problem for physicists because the laws of nature are practically symmetrical. This pre-universe universe is contracting in the direction of the Big Bang.

What are dark matter and dark energy?

Most of the universe eludes our observation: only four percent of its energy or mass content consists of matter in the form we know. In addition, there is 23 percent dark matter and 73 percent dark energy.

The existence of both results from the observation of visible structures. On the one hand, distant areas of the cosmos move faster than can be explained by the gravity of visible objects. It is believed that this is due to dark matter. On the other hand, the expansion of the universe is accelerating, while it should actually be slower. The mysterious dark energy is to blame for this.

What both consist of is not yet clear. After all, there are candidates. The neutrinos have long been considered favorites, but they probably don't have enough mass to explain the astronomers' observations. Therefore one assumes the existence of so called "WIMPs" (Weakly Interacting Massive Particles), which like the neutrinos are only subject to the weak interaction, but are much heavier. They move much more slowly than neutrinos and therefore belong to the so-called cold dark matter.

Even less is known about the nature of dark energy. Perhaps it is simply one of the fundamental properties of the universe. In quantum field theory, it could be represented as the energy of the vacuum. In some models, dark energy occurs as the effect of a hypothetical scalar field called "quintessence".

Is the fine structure constant constant?

The fine structure constant describes the strength of the electromagnetic interaction. Among other things, it can be determined from the frequency with which an electron emits a photon or how strongly charged particles interact with one another.

It has not yet been proven whether the fine structure constant has changed since the Big Bang. If it were not constant over time, it would have considerable effects on all cosmological models, because it is around 1036 times as large as the equivalent coupling constant of gravity.

How does the universe end?

The theories about the death of space are as varied as the knowledge of researchers about them is limited. These scenarios are seriously discussed - the first is most likely:

Death by freezing to death: According to this theory, also called "Big Freeze", the universe is expanding more and more. As a result, the temperature drops steadily until at some point there is no more free, usable energy available, as required by physical and chemical processes.

Death by tearing apart: If the expansion of the universe accelerated further, at some point everything, including the smallest building blocks of matter, would be torn apart in a "big rip".

Death by crushing: The "Big Crunch" theory assumes that the expansion of the universe will stop at some point (it doesn't look like that at the moment) and that the universe will then contract again.

Death by radiation: It is possible that our universe is not the only one. If it ever meets a sibling made of antimatter, the result would be a complete annihilation of the universe, without warning, from one moment to the next.

Death by falling apart: We may also be sitting on a gigantic trick by nature. What we consider to be space in the ground state, the vacuum, could also be in a quantum-physically higher state. If so, one day the vacuum, nothing to me, nothing to you, could move into an energetically lower state. In the worst case, the entire universe disintegrates.

Freeze death: The amount of dark energy may be increasing faster than the universe is expanding. The density of the universe grows until it becomes viscous and at some point remains in a solid state, as if frozen.

Are there naked singularities?

When a star that is heavy enough collapses, the gravity of its mass pulls all available matter together into such a small space that the known laws of physics no longer apply. Researchers speak of a singularity. However, such a hole in space is always hidden from curious observers - an event horizon forms around it, behind the curtain of which we cannot see.

That is why these singularities are also called black holes. But does that really always happen? That is an unanswered question. The hypothesis of the cosmic censor put forward by Roger Penrose assumes this, but has not been proven. In any case, the general theory of relativity does not result in a ban on naked singularities (The Fate of the Collapsing Star). Indeed, under certain initial conditions, it seems possible that the event horizon will not emerge quickly enough. A completely different (and also unanswered) question is how an observer would see a naked singularity.

What is the shape of the universe?

One of the fate-determining properties of the universe is its geometric shape. Since it is spanned by four-dimensional space-time, one must think away one dimension for easier understanding. For example, a line appears straight to the resident of a curved sheet of paper, although the sheet has a curvature to the resident from the outside.

The shape of the universe is mainly determined by its density. Since we cannot look at the universe from the outside, it is very difficult to determine the actual shape of the universe. To do this, we have to limit ourselves to the geometry: If the universe is curved, the internal angle sum of a triangle (at great distances) can no longer be exactly 180 degrees.

Recent measurements with the WMAP satellite have shown that the universe has an Euclidean geometry, so it is most likely flat. That applies at least to the area of ​​space that is a maximum of 24 gigaparsecs away from us. But what happens behind it - nobody knows. Some researchers have brought up the shape of a soccer ball - we would then live on one of the surfaces of the soccer ball that are almost flat. Some of its properties would go well with a funnel or donut shape too.

In the next part, you are guaranteed not to find an answer to the following six questions:

  1. Is the Standard Model of Physics correct?
  2. What does the world formula look like?
  3. Do quarks have a structure?
  4. How does the Kaye effect work?
  5. How does sonoluminescence work?
  6. What is reality

The author's eBook "The New Biography of the Universe", which describes the development and structure of our universe in detail, is available on Amazon (Mobi, DRM-free), iTunes (for iPad) and Beam eBooks (ePub, PDF, DRM- free) available.

(Matthias Matting)

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