Did the universe generate dark energy
A look into the dark universe
No more than the head of foam on the waves of a deep ocean - that is the matter of which we humans and our world are made, viewed from the universe. Almost the entire cosmos is not formed by protons, neutrons and electrons, the components of "our world", but by something completely different that mysteriously calls itself "dark matter". Matthias Bartelmann from the Center for Astronomy communicates in an understandable and exciting way what dark matter is and which ingenious methods scientists have to use in order to gradually figure it out.
The type of matter we ourselves are made of and the world as it immediately surrounds us is essentially made up of protons, neutrons and electrons. We call it "baryonic". In the universe, however, it forms no more than the whitecaps on the waves of a deep ocean. We now know that almost all of the material that makes up the various cosmic structures is composed of something else that we call dark matter.
How do we know? Two kinds of cosmological observations in particular have brought about a real breakthrough in our knowledge of the universe in recent years. One type depicts the entire sky in the range of microwaves, i.e. at wavelengths that are fractions of a millimeter to about a centimeter. In this wavelength range it is possible to see the afterglow of the Big Bang, as shown in the early universe around 400,000 years after the Big Bang. This glow is remarkably intense: there are around 400 photons in every cubic centimeter of volume.
The remarkable thing is that this afterglow was imprinted with the first traces of the emerging structures that emerged in the later course of the development of the universe. These traces are tiny: they only reach about a millionth of the intensity of the entire microwave signal. Nevertheless, the American satellite "Wilkinson-MAP" recently succeeded in precisely measuring these tiny germ cells of today's structures.
The second type of observation that gave cosmology its breakthrough is a set of huge, wide-angle photographs of our cosmic neighborhood that were, and are, taken with five different color filters. When the experiment is completed, these wide-angle images will cover almost a quarter of the entire sky. The wealth of this data is immeasurable.
With the data recorded so far from this large, digital sky survey, the "Sloan Digital Sky Survey", it was possible to measure structures in the distribution of the galaxies up to considerable distances from us. So we now have a picture of the universe from its early youth and one from a more developed stage. It was the combination of the two that, for the first time in the history of modern cosmology, made it possible to determine the essential properties of the universe on a large scale.
We are primarily concerned with three of these properties: First, the universe is spatially flat. This cannot be taken for granted because it could just as easily be curved as a spherical or saddle surface. In order for it to become flat, the total energy density in the universe just has to assume a certain critical value. So we know from this how big the total energy density in the universe is. Second, the matter that makes up cosmic structures only contributes around 30 percent of this energy density. Even this matter is essentially alien to us because, as said above, only a small part of it is of the familiar baryonic form. Thirdly, the remaining 70 percent of the necessary energy density is completely puzzling because they are not only of an unknown form, but are also responsible for the seemingly paradoxical expansion behavior of the universe. The universe is expanding, but its rate of expansion does not decrease, it increases. We would expect gravity, as we are intuitively familiar with it, to slow down the expansion of the universe. Apparently it works in the opposite direction and accelerates it. Einstein's general theory of relativity, which we otherwise have great confidence in, only allows this if this form of energy has a negative pressure.
Without going into what this dark energy and dark matter might be, we both ask what observations we could make to learn more about their nature. In this respect an effect comes to our aid that follows directly from the innermost foundations of general relativity.
As a result of this effect, masses deflect light in such a way that light paths towards the masses are curved as if they were passing through glass converging lenses. One speaks of the gravitational lens effect. It is differential, that is, two rays of light traveling in only slightly different directions are generally deflected somewhat differently, thereby distorting sources that we see through such gravitational lenses. An optician would call gravitational lenses astigmatic. They are quite unsuitable as imaging optical systems, also because of their extremely long focal lengths.
However, it is precisely this astigmatism that we attach great importance to, because it gives away the gravitational lenses, which are otherwise largely dark because they consist predominantly of dark matter. In the distant universe, young galaxies are so dense that around 40 of them can be found in one minute of square arc. There are around 20,000 of them on the area of the full moon. These distant, innumerable galaxies form a kind of cosmic wallpaper, a tiny patterned background. Between this background and us lie the diverse cosmic structures that draw their astigmatic gravitational lensing effect on the cosmic wallpaper. As a result, we see a distorted image of the wallpaper and can deduce from the type and extent of the distortion how the dark matter is distributed, although it is otherwise almost completely invisible.
You can imagine it as if you were looking through a structured pane of glass into an evening living room. The lights in it form a diverse pattern of spots on the pane, which can be used to reconstruct how the thickness of the pane changes.
The gravitational lensing effect of large cosmic structures is very weak. It only changes the images of distant galaxies by a few percent. But since there are so many of them, we can still reliably measure this weak effect. The first breakthrough came in 2000, when initially four groups shortly after each other reported that they had succeeded in measuring the weak gravitational lensing effect of cosmic structures. The measurements were taken with different telescopes and different cameras under different conditions and evaluated with different methods, but they gave a uniform picture. Without question, the first measurements of the lens effect were successful in very large structures made of dark matter. Since then, such observations have become almost routine. This area is developing at an impressive rate.
The strong variant of the gravitational lensing effect leads to rarer, but much more spectacular observations. In roughly every fourth cluster of galaxies there are strongly distorted, arched structures, so-called arcs, which are images of distant galaxies that are pulled apart in such an extreme manner by the deflection of light in the gravitational field of the galaxy cluster. There are two types of such arcs; those that are tangent to the galaxy cluster and others that point radially away from its center. While tangential arcs allow the total mass of a galaxy cluster to be determined, the radial arcs, which are much closer to the cluster center, allow us to measure how steeply the density of dark matter drops outward from the center of the galaxy cluster. In this way, it is possible to use the gravitational lensing effect not only to detect structures made of dark matter, but also to measure the properties of the mass distribution in them.
That’s the principle. Reality is much more complicated and requires much more subtle research. But why do you focus on galaxy clusters? They are the largest structures in the universe that are still held together by gravity. They get their name from the typically a few hundred to a thousand galaxies that are gathered in them. In addition, they contain a plasma that is so hot that it glows in the X-ray range, but they are essentially accumulations of dark matter, because in them too the proportion of the "normal", familiar baryonic material is very small.
As the largest cosmic structures bound by their gravity, galaxy clusters emerge last in the evolution of the universe. So they are young entities, much younger than the galaxies they are made of. That is why they are still in the process of developing, at times violently. Their matter is usually not distributed in any spherical symmetry, but they are often structured and irregular, precisely because they are still subject to stormy changes.
This turns out to be decisive for the strong gravitational lensing effect. With the same mass, but with an irregular shape, a gravitational lens can cause considerably stronger distortions. On the one hand, it becomes much more complicated to determine how the dark matter is distributed in galaxy clusters, but on the other hand, it opens up the possibility of investigating the state of evolution of the galaxy clusters. As we shall see, this creates a direct bridge to dark energy.
What is the basis of our interest in how exactly dark matter is distributed in galaxy clusters? Theoretical studies with the help of large computers predict that the density of dark matter in galaxy clusters should drop outwards in a very specific way: flat on the inside, steep on the outside. If we succeeded in testing this prediction and either rejecting it or confirming it, we would have come a great deal closer to the elucidation of the nature of dark matter. A considerable part of the research program at the Institute for Theoretical Astrophysics is dedicated to this goal.
From here the arc to dark energy stretches. Their negative pressure, which mocks all experience, leads to the fact that they have a major influence on the structural growth in the universe. We know how pronounced the structures are in our cosmic neighborhood. The older it is, the faster the dark energy drifts apart. Structure formation must fight against this accelerated expansion. The dark matter, which eventually ends up in galaxies or galaxy clusters, for example, is attracted to the resulting structures, but is torn away from it by the general expansion of the universe. The more the dark energy determines the expansion in the later development phases of the universe, the earlier the structure formation had to start in order to arrive at the structures as we see them today. Depending on how the dark energy has behaved in the course of cosmic development, structures must have arisen sooner or later.
When we look out into medium or long distances, we simultaneously look back into the past, because because of its finite speed, the longer it takes for light to reach us, the further away it went on its journey. A look into the distant universe is also a look into the past. If we can determine how strongly developed the cosmic structures were at this time, we can infer how the expansion behavior of the universe has developed in the course of cosmic history. This then makes it possible to draw conclusions about the dark energy again without having to perceive it directly in any way.
Galaxy clusters presumably went through their most violent development at the very time when dark energy took over the direction of the cosmic expansion. This is precisely what makes them such preferred objects for us: On the one hand, with the strong gravitational lensing effect, we have a possibility at hand to diagnose the state of development of the galaxy clusters and the distribution of dark matter in them. On the other hand, the phase of the most violent development of the galaxy clusters shifts towards earlier or later cosmic epochs, depending on the type of dark energy. Our expectation is based on this that with the help of the strong gravitational lensing effect it could be possible to fathom both dark matter and dark energy more closely. The weak gravitational lensing effect will also play an essential role, because it also allows us to measure the way in which cosmic structures have grown in the recent cosmic past.
We are not there yet. Very detailed studies, most of which require large-scale computer simulations, are necessary to test these ideas first and then to turn them into action and ultimately to cognition. But there are a number of astonishing observations that inspire us. For example, there are apparently galaxy clusters at such a great distance from us that act as strong gravitational lenses that their existence and their ability to produce a strong lens effect indicate that the essential development of the galaxy cluster population took place at a comparatively very early cosmic time, which again becomes clearer Way points to the dark energy. We'll see what the next few years bring.
Cosmological simulation calculations and the analysis of observation data will have to go hand in hand. The proximity to the observing astronomers on the Königstuhl on the one hand and the interest in theoretical physics on the other hand will be of great help to us.
Beyond the gravitational lensing, we are looking for other visible effects of violent evolution in galaxy clusters. This includes that galaxy clusters are penetrated by large-scale magnetic fields in which extremely high-energy electrons emit radio radiation. When a galaxy cluster undergoes a violent change, these magnetic fields are both strengthened and deformed and correspondingly high-energy electrons are generated. We therefore expect that the radio emission of the galaxy clusters will offer another diagnostic tool for analyzing their state of development. The X-rays emitted by the hot gas in galaxy clusters also become more intense during certain development phases of the galaxy clusters. If we combine the strong and weak gravitational lensing with these other observations in the best possible way, we should begin to track down dark matter and dark energy.
Prof. Dr. Matthias Bartelmann
Center for Astronomy, Institute for Theoretical Astrophysics
Albert-Überle-Strasse 2, 69120 Heidelberg
Telephone (0 62 21) 54 48 17
e-mail: [email protected]
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