Energy and matter are finite

On the trail of the dark side of the universe

Albert Einstein was finally able to complete his general theory of relativity in 1915 after years of work - but he still disliked his masterpiece, with which he established a connection between space, time, matter and gravity: there was a contradiction between the prediction of his theory and the observed Data.

The year is 1917 and the accepted opinion about the nature of the universe is that the universe has no beginning and no end. The observations indicate that the cosmos is static. Einstein was also a supporter of this so-called steady state theory. But unfortunately the field equations of his general theory of relativity described a universe that either expands or shrinks.

To get this discrepancy out of the way, Einstein set about adapting his equations accordingly. In the essay "Cosmological considerations on the general theory of relativity" he introduced an additional term with a size that he called the cosmological constant. This was mathematically possible without changing the previous predictions that his theory had made.

Einstein's "greatest donkey"

With and without cosmological constants, Einstein's theory could, for example, correctly calculate the movement of the planet Mercury. This was not possible with Isaac Newton's equations of motion. At the same time, Einstein was able to describe a static universe through the cosmological constant.

Einstein's joy at the introduction of the cosmological constant was short-lived, however. A few years later, observations by the astronomer Edwin Hubble made it clear that the universe cannot be static - it is expanding. The galaxies move away from us in all directions. And the further away they are, the faster they fly away. According to his colleague George Gamow, Einstein later described the introduction of the cosmological constant as his "greatest blunder". As we now know, the question of the cosmological constant is a lot more complex than Einstein assumed. More than a hundred years after its inception, it is still one of the great mysteries of cosmology.

A puzzling 95 percent

So what is it about? The story of the exploration of the cosmos is, in a sense, the story of an ongoing narcissistic mortification. First of all, we had to get used to the idea that the world is not flat. Then it was a matter of digesting that the earth is not in the center of the universe. Once we had accepted the sun as our cosmic center, we had to say goodbye to this notion. Neither our sun nor our galaxy are unique. And that's not all: As we now know, the conventional matter that we see with our telescopes - i.e. all stars and galaxies - makes up just five percent of the total energy content of the universe.

According to Einstein's theory, energy and mass are known to be equivalent quantities: Energy can be converted into mass and vice versa. But what about the remaining 95 percent of the energy and mass in the universe?

Persistent fix solution

While observing galaxy clusters, the Swiss astronomer Fritz Zwicky discovered in 1933 that the gravity of the visible mass was insufficient to keep the rotating galaxy clusters together. For the missing mass, he coined the term dark matter. It was a really crazy idea at the time, and even today it may still seem a bit like a fix for a fix: you're making an observation that can't be explained. In order to solve the riddle, one postulates an invisible something. Nonetheless, Zwicky's concept of dark matter has become an integral part of cosmology. Physicists assume that it makes up 25 percent of the total energy density in the universe.

Astrophysicists Vera Rubin and Kent Ford made a significant contribution to shedding light on the secrets of dark matter. It was a hot, clear, and dry summer night at the Kitt Peak Observatory in the mountains of southern Arizona in 1968 when the both wrote physics history. The conditions for the observations were perfect, the astronomers at least helped themselves a little by eating ice cream. Rubin and Ford had been a research duo for many years - he was primarily responsible for the instruments, she for the interpretation of the data.

Amazing discovery

That night, the two of them observed the stars of the Andromeda Galaxy again to determine their speed. For the first time, they should succeed. They were finally able to determine the speed of rotation of the stars in the center of the galaxy - in technical jargon one speaks of the rotation curve. But the result baffled the astronomers. "At the end of that night we were amazed at the shape of the rotation curve," Rubin later recalled.

In so-called spiral galaxies, which also include our Milky Way, there is a large, bright cluster of stars in the center, but it becomes emptier the further you move outwards. The mass we are familiar with is concentrated in the center, so one would expect that the outer regions of the galaxy would rotate less strongly than the more central ones.

Invisible cohesion

Contrary to what was believed, however, Rubin and Ford discovered that the Andromeda Galaxy’s rotation curve was flat, meaning that the stars on the outside of the galaxy rotated at the same speed as the stars near the center of the galaxy. Even more disturbing, the outer stars were rotating so fast that they should have flown away: the visible stars simply didn't have enough mass to hold the galaxy together. So was there another invisible mass? Zwicky's concept of dark matter was used again - if you couldn't explain the phenomenon, at least you had a name for it.

The Andromeda Galaxy was the first to discover the missing mass. Many more observations by Rubin and Ford in numerous galaxies followed. The two of them contributed significantly to our current knowledge that there is much more out there than our eyes and telescopes perceive. From a bold hypothesis, the observations of Rubin and Ford made dark matter a recognized professional opinion. As we now know, we need dark matter to explain how galaxies were formed, how they behave and also why they do not fly apart.

Difficult search for clues

But what exactly dark matter is is still unclear today. At least some statements can be made about what it is not: It does not consist of ordinary matter particles such as protons, neutrons or electrons. Neither from the atoms we know, let alone from the elements in the periodic table. In the end, dark matter has little to do with familiar matter, with one exception: both dark and "normal" matter exert a gravitational force.

Despite the numerous unanswered questions, dark matter has long been accepted by the majority of physicists, but not all agree with the concept. "It could be that it behaves like the steady-state theory," said Nobel laureate in physics, George Smoot. "Dark matter will have prevailed completely when its opponents have died out," is Smoot's somewhat macabre prognosis at the end of the criticism.

The followers of dark matter are currently pursuing the goal of tracking down dark matter directly or producing it in the laboratory and measuring its properties. Opinions differ on how promising this venture is. One of the optimists when it comes to dark matter is the theoretical physicist and Nobel Prize winner David Gross. "I am sure that there will be a Nobel Prize for Dark Matter in the next ten years," he said recently during a discussion on the dark universe at the Nobel Laureate Meeting in Lindau on Lake Constance.

Enigmatic energy

The dark universe? The phenomenon of dark matter is quite puzzling, but another part of the universe - the so-called dark energy - is even more mysterious. At 70 percent, it makes up the main part of the energy in the universe. But while we can at least indirectly observe dark matter using a variety of methods, we lack almost all clues for dark energy.

But then why do we even know that it is there? Almost exactly 30 years after the Rubin and Ford discoveries, US astronomers set out to measure something that no one expected to come as a surprise. It was about the rate of expansion of the universe, and how it changed over time - and it was supposed to be known what would come of it.

Afraid of embarrassing mistakes

Because the expert opinion was that the universe would have to expand more and more slowly over time, because ultimately the gravitational force counteracts the expansion. But when the astronomers Adam Riess, Brian Schmidt and Saul Perlmutter wanted to use new measuring methods to determine what the current rate of expansion of the universe is and at what rate it used to expand, their results showed exactly the opposite result: the expansion of the universe took place over time not from, but to. In other words: the universe is expanding at increasing speed!

At first, the astronomers assumed that they must have made an embarrassing measurement error. "Adam and I discussed how idiot we were and where we were wrong," reported Schmidt at the Lindau Nobel Laureate Meeting. After a few months, at the beginning of 1998, it was clear that the universe was actually expanding at an accelerated rate. "We had checked our results thoroughly and systematically. When you get to a point in your work where you achieve a result that goes against the best assumptions despite being meticulous, you have to go out with your result and hope that you don't get too much is laughed at and has not made a stupid mistake, "says Riess, looking back on the publication.

Interest instead of scorn

In contrast to earlier experiments to determine the rate of expansion of the universe, Riess, Schmidt and Perlmutter had new technical methods available in the late 1990s to determine the distance of distant supernovae. It is the bright glow of massive stars at the end of their lifetimes. For the first time, physicists were able to use digital detectors with two and later four megapixels. This technique was around a hundred times more efficient than the analog film that had previously been used for such observations, Schmidt reports.

The professional world received the surprising result with a lot of interest and without scorn. "The theoretical physicists were very enthusiastic about it, the experimental physicists were a bit more skeptical, but they didn't laugh at us either," says Schmidt. The theoretical astrophysicist Michael Turner soon coined a new term to describe the mysterious phenomenon: Following Fritz Zwicky's suggestion, Turner called the unknown force that is driving the universe apart ever faster, dark energy.

According to current estimates, dark energy makes up around 70 percent of the total energy content of the universe - it is therefore the most important and largest component of the cosmos. It is all the more unpleasant for physicists that we have hardly known anything concrete about it so far.

Monsters in the dark

Even for eminent astrophysicists like Adam Riess, who has been dealing with dark energy for decades and was at the beginning of its history of discovery, the dark universe is like "monsters in a dark room that we walk into", as he explains in an interview with STANDARD .

DEFAULT: What do we know so far about the dark universe?

Riess: For me, the dark universe means seeing the universe through gravity and not through light. If you are a creature who wants to understand the universe and who can choose their eyes, one should possibly choose gravitational eyes rather than light eyes. Most of what we see in the universe we see through gravity. We see dark matter because it speeds up the rotation of galaxies. And because clusters of galaxies stick together more than they should without dark matter. We see dark energy through the accelerated expansion of the universe. The dark universe is like monsters in a dark room that cannot be seen, you only hear their sounds, and sometimes you walk into them. In any case, it is very important that we test our knowledge of the dark universe. Surprises are very likely. We live in a special time!

DEFAULT: You received the Nobel Prize for your contributions to accelerating the expansion of the universe. How did you come across it?

Riess: We compared the earlier rate of expansion of the universe with today's rate. Everyone thought the expansion was going to decrease because of gravity, but we saw it accelerate. That was a big shock. It suggests that something exists that acts very differently from matter. Instead of the gravity that matter has, it acts like repulsive gravity. That something was called dark energy. It could be something very similar to what Einstein called the cosmological constant.

But not a donkey?

One thrust of the considerations is that the dark energy has to do with Einstein's considerations about his "greatest donkey". Possibly it is the same phenomenon that Einstein described with the cosmological constant. As it turned out, the theory of the cosmological constant is well compatible with the data that have been obtained for the accelerated expansion of the universe. Physicists have already developed their own cosmic model from this, the so-called Lambda CDM model. This allows the development of the universe since the Big Bang to be described with six parameters. This makes it the simplest cosmological model that is also in agreement with the measurement results. The lambda CDM model is therefore called the standard model of cosmology.

If the cosmological constant is actually behind the accelerated expansion of the universe, Einstein's proposal draws a wider circle: he originally introduced it to describe a static universe with his theory. Then he had discarded it to allow an expanding universe. Decades later it was rehabilitated because it was consistent with new data on the accelerated expansion of the universe.

If this is actually the case, it should also be possible to determine its exact value, which has not yet been achieved. "It's a number that is very close to zero, on the order of 10 ^ -120," says David Gross. "A number like that is very difficult to understand, and one of our greatest challenges is to understand it."

Since no other direct or indirect observations are known apart from the accelerated expansion of the universe, the physical properties of dark energy have so far been largely literally in the dark. Its most important physical property is that it exerts something like a negative gravitational force or a repulsion - an effect that is known neither from dark nor from "normal" matter.

Always trouble with Hubble

But it is getting even more mysterious: New indications are increasingly suggesting that we are encountering an apparently fundamental problem if we want to determine the current rate of expansion of the universe. This is known in technical jargon as the Hubble constant.

So far, researchers have pursued two strategies to measure the rate of expansion of the universe: On the one hand, the Hubble constant can be calculated from the energy and structure of the cosmic background radiation. As the universe expanded, this primeval radiation also expanded, making it a witness to the expansion. The Planck satellite of the European space agency Esa measured the cosmic background radiation with high accuracy. A Hubble constant of 67.4 kilometers per second per megaparsec can be determined from this. Parsec is an astronomical unit that is approximately 3.26 light years, or 30.9 times 1015 Meters.

On the other hand, the Hubble constant can be determined by measuring supernovae. This is the path that Adam Riess has been following with his group for years. When determining the Hubble constant with the help of supernovae, however, physicists repeatedly came to a value of 74.22 kilometers per second per megaparsec with an error bar of only 1.91 percent.

This value is therefore higher than that which was determined by means of the cosmic background radiation. The measurements were repeated over and over again, but the discrepancy could not be resolved. Finally, a third measurement method based on red giants was presented. The team led by astrophysicist Wendy Freedman recently announced that their method achieved a value for the Hubble constant of 69.8 kilometers per second per megaparsec.In the meantime, the terms "Hubble Trouble", "Hubble Tension" or "Hubble Constant Problem" have become commonplace for the problem.

Indications of a new physics

At first, the researchers thought that it must be due to a systematic measurement error of one or the other determination method. But the teams' last publications this summer with even more precise measurements confirmed the discrepancy again. "I think we got to the point this summer where the discrepancies are too big to be ignored. My feeling is that there is definitely something going on that we have overlooked," said Charles Bennett, physicist at from the Johns Hopkins University in Baltimore, who recently attended a conference on Kurt Gödel's legacy at the University of Vienna.

For him, the long-standing Hubble Trouble points to a new, as yet unknown physics. "It's a puzzling situation and people don't really know what theory we can get on here. If the data shows we're missing something, then we're really missing something," says Bennett.

It is regrettable that we have so far only so few clues about dark energy, because it is the decisive factor when it comes to our long-term fate. Whether the universe ultimately collapses or everything falls apart, depends crucially on the properties of the dark energy. The observations made so far allow at least some speculation, as Adam Riess explains:

DEFAULT: What are the consequences of dark energy for the future of our universe?

Riess: Until we understand dark energy, it is hard to be sure what the future of our universe will be like. Through the accelerated expansion, we know that there is obviously an energy in empty space that we do not yet know. We don't even know if this energy changes over time. But we would have to know that in order to be able to make a reliable statement about the future of the universe. To really understand dark energy, we may need a quantum theory of gravity that theoretical physicists have been looking for for many years. Right now, it seems most likely that the expansion of the universe will continue.

DEFAULT: What scenarios could this result for the universe?

Riess: Two interesting scenarios could arise. The first is called Big Rip, where we get more dark energy per volume than binding energy from anything else. That could tear everything apart as the dark energy gets stronger. But if it weakens or even turns into something attractive, it could collapse the universe again. We have no evidence for any of these scenarios, they are just theoretical possibilities.

DEFAULT: To illustrate the expansion of the universe, the analogy of a balloon being inflated is sometimes used. What do you think of this picture?

Riess: It's good, but I prefer a large loaf of raisin bread right now.

DEFAULT: Raisin bread?

Riess: I hope you like raisin bread. Because when I say "big", it is possibly an infinitely large raisin bread that keeps rising. The galaxies are like raisins - when the dough rises between them, they drift further and further apart. It also doesn't matter which raisin you are on: all other raisins move away from you. The balloon was a really good analogy as long as we thought the universe was curved. However, the latest data has shown that the room is relatively flat. So now I prefer the raisin bread.

DEFAULT: What speaks for the fact that the space is infinite, or, to stay with the analogy, that the raisin bread is infinitely large?

Riess: If we look into the dough of the raisin bread, the maximum distance we can see results from the speed of light times the age of the universe. We cannot look further, we call it the horizon. What we see at this distance is still raisin bread. It is a metaphysical question if you want to know what is beyond the horizon - because in principle we can never look beyond the horizon.

DEFAULT: How can you imagine an infinity that keeps growing? Where should it grow when it is already infinite?

Riess: When I was a child, my father asked me the following riddle: What is greater than infinity? I had just learned the definition of infinity and I said, "Nothing is greater than infinity." He said: "Yes. The infinity + 1." The expanding universe is infinity + 1. Something is added to something that is possibly already infinite.

DEFAULT: If the galaxies move further and further away from us, will we ever see fewer stars in the sky?

Riess: If the universe continues to expand as we currently assume, then at some point it will expand faster than light. So you would have to move faster than the speed of light to continue to see a galaxy. That means the horizon will shrink. In the accelerated expanding universe, galaxies will fall out of the horizon at some point. Ultimately, we will no longer be able to see anything but a handful of galaxies tied to us by gravity like the Andromeda Galaxy.

DEFAULT: But that is a lonely prospect.

Riess: Yes, it is really sad!

DEFAULT: When could it be so far?

Riess: In about 30 billion years. When we have found out more about dark energy, it could also be that we realize that everything will develop differently than we currently assume.

Last hope

Another way of saying it: The extensive ignorance about dark energy is still our greatest hope so far that we will not end up in a dark, lonely universe. And of course the fact that our sun is only predicted to have a life expectancy of another six billion years. If by then we are not already a multiplanetary species that also settles in other solar systems, we will no longer notice the great darkness anyway.

The fact that ultimately the future depends on everything also explains why Hubble Trouble and dark energy are currently among the biggest unsolved questions in physics - also because the latter makes up the majority of the energy in the universe. But how can we find out more about it?

A new cosmic sense

One possibility that has only emerged in the past few years is gravitational waves. This is also a theoretical prediction by Albert Einstein. Here, too, he rejected his own idea several times and then affirmed it again. His last word on gravitational waves was: And they do exist, but we will never be able to observe them. With the latter assessment he should be wrong again.

Why is? As a result of his general theory of relativity, Einstein deduced that accelerated masses could cause perturbations in the structure of space and time. These would propagate in waves at the speed of light. As early as the 1950s, the US physicist Joseph Weber tried to prove it with the help of aluminum cylinders. Its supposedly successful detection later turned out to be a fallacy.

The first actual measurement of gravitational waves was only announced in February 2016. Researchers at the US gravitational wave observatory Ligo had the appropriate equipment for 50 years. The first detected gravitational waves were created when two black holes merged 1.3 billion years ago. Overall, such events can be measured every one and a half weeks on average.

A value for the Hubble constant can also be determined by means of gravitational wave measurements. At the moment this is still too imprecise to be able to seriously compare it with the other two measurements. However, there is hope that gravitational waves could also provide a precise value for the Hubble constant in the next few years. They could confirm one or the other measurement or even provide completely new information. Although Adam Riess does not work with gravitational waves, he has high hopes for this new window to the cosmos.

DEFAULT: Do you think that gravitational waves will allow us to find out more about the dark universe?

Riess: Yes, I would be surprised if it were different. Because it is such an effective technology and a new window into the cosmos. It's almost like having a sixth sense added to the five senses we have. If you ask yourself: Will I learn a lot from it, the answer is: Of course, each of our senses teaches us so much about the world.

DEFAULT: Where do we come from?

Riess: The dark side of the universe does not only allow us to make physically informed speculations about the future of the universe. Dark energy and dark matter were also the driving forces behind the formation of the galaxies as we know them today, our solar system, the planet earth and finally life on earth and us. Astronomy has already found out astonishingly much and in great detail about this, as Riess reports.

DEFAULT: After we have already talked a lot about the future of the cosmos: can we take a look into the past, right up to the origin of the universe?

Riess: Sure, but I have to say: I wasn't there.

DEFAULT: Maybe you still know about our best theories about the first seconds of the universe?

Riess: With our cosmology, we are very good at looking back in time and describing what happened once, because everything follows the normal rules of physics. But if you look back to 10-34 seconds after the Big Bang, the energy scales are so large that you would need a quantum theory of gravity to describe the processes. We therefore do not know exactly what happened in the first 10-34 seconds after the Big Bang. In any case, the universe must have been unimaginably hot and unbelievably dense during this period, with unimaginably high energies. Things moved apart incredibly quickly. The universe has doubled over and over again in a tiny fraction of a second.

DEFAULT: How could we, in the end, also emerge from this hot, thick broth?

Riess: The universe was pretty uniform at this early stage, but there were tiny fluctuations where it was a little denser. Then a process occurred that could be described as the story of the rich getting richer and the poor getting poorer: regions that were a little denser were more successful in attracting even more matter. The regions that were less dense have lost mass as a result. This process created galaxies in some places and emptiness in others. (Tanja Traxler, David Rennert, September 16, 2019)


This article was published in the current STANDARD science magazine FORSCHUNG. The magazine is available in the STANDARD online shop for € 5.90.