Why don't Indians like the cosmos?

Hubble constant What's wrong with the expansion of the universe?

Bruno Leibundgut: "The Hubble constant is the current rate of expansion of the universe."

Adam Riess: "One of the few ways to find out something about the composition, age and future fate of our universe is to watch its expansion."

Dirk Lorenzen: "The Hubble constant is an old acquaintance of astronomy. It appears again and again, was contested for decades, then for 20 years it was thought that the problem had been solved."

Bruno Leibundgut: "I think people made themselves comfortable with this dark matter and the cosmological constants. It all somehow worked out nicely. But nobody said that this should really be the ultimate model."

Galaxies like raisins in a rising yeast dough

The Hubble constant. It is part of the equipment of every astronomer. It determines the fundamental relationship between the distance of a galaxy and the speed at which it moves away from us. According to the Big Bang theory, galaxies in space fare like raisins in a rising yeast dough. The universe expands and the galaxies move away from each other - and the faster the greater their distance. The Hubble constant provides this link.

Dir Lorenzen: "And now one observes better and better and there is just this discrepancy, this contradiction in the data."

What is the value of the Hubble constant, i.e. how fast the universe is expanding, that is currently the 100,000 dollar question of cosmology. According to the currently most popular theory for the structure of the world, the Hubble constant should be slightly smaller than the observations in the cosmos. The difference is around nine percent. That is too much to dismiss as a measurement inaccuracy.

"The discrepancy exists. This is a really great cosmological, scientific problem that we will certainly have to solve at some point. Also because there is of course the risk that we have not yet understood the universe correctly. That is okay. From that one learn. "

Standard model of building the world before it collapsed

Bruno Leibundgut from the European Southern Observatory ESO in Garching takes the looming "crisis" of cosmology calmly. He was already working on the Hubble constant during his studies over 30 years ago - and as one of the world's most prominent observing cosmologists, he has seen many a surprising turnaround in his field. Now the standard model of the structure of the world could be on the verge of collapse - because the Hubble constant is much more than just a cosmic speed factor. Adam Riess:

"The Hubble constant determines the size and age of the universe. Once its value was hotly debated. It was a factor of two apart. Today, however, the uncertainty is around ten percent."

Observing the expansion of the universe: US astronomer Adam G. Riess in his office at John Hopkins University (AP / Gail Burton / The John D. and Catherine T. MacArthur Foundation)

Adam Riess received the Nobel Prize in Physics in 2011 - at the age of only 42. He is tormented by the current deviation of almost ten percent in the measurements. Thirty years ago astronomers argued about much larger differences, but back then it was ultimately only about measurement errors. It was downright a religious war, recalls Bruno Leibundgut:

"Back then there were really these irreconcilable camps. There were groups that sometimes treated each other uncomfortably. I see that less today. Today, science is more in the foreground. People want to find the right answer and no longer their right answer. "

Established theory no longer fits

The astronomers no longer argue with each other. They struggle with the theory that they have developed and grown to love - but which now no longer seems to fit. It's not about whether the cosmos expands a little faster or slower, but about whether our idea of ​​the universe is correct. Adam Riess:

"We have a standard model of cosmology called Lambda CDM. It is very successful when it comes to building the universe. It explains perfectly how the universe developed from the Big Bang to the present day. It is based on the cosmic background radiation, the short after the Big Bang and was observed extremely precisely by the Planck satellite telescope. Using this data and the standard model, we can calculate how quickly the universe should expand today - and that brings us to a value of 67.4. "

Observes the cosmic background radiation: Satellite Planck (ESA)

67.4 is the Hubble constant expected from the analysis of the data from the ESA satellite Planck. This is a prediction based on the current Big Bang model, not a direct measurement. If this prediction is correct, it should match the speed at which the galaxies are actually flying apart today. This is exactly what Adam Riess and others measured around the world.

"With the Hubble space telescope, we observe Cepheid stars and supernova explosions. This enables us to determine the distance and speed of galaxies."

Nine percent deviation - tens of light years difference

Dozens of objects have now been evaluated. The database is solid. The result is clear. Adam Riess:

"Our data gives a Hubble constant of 73.5 - and that only fits the expected value with a probability of one in a million."

A dilemma: the measured value is nine percent higher than the theoretically expected. At first, most professionals thought it was simply a measurement error that would go away with time and more data. However, around a dozen specialist articles have appeared in the past year. Although they use different methods, they all come to the same conclusion. Apparently the cosmos is expanding faster than expected today. The question is: why?

Dirk Lorenzen: "The Hubble constant has this bizarre unit of kilometers per second, per megaparsec. The megaparsec is 3.26 million light years."

Bruno Leibundgut: "It is a very, very difficult measurement. The difficulty with the Hubble constant is that you make an absolute measurement."

Dirk Lorenzen: "In the course of this expansion of the universe, the speed of a galaxy increases by 74 kilometers per second, for every 3.26 million light-years of distance. So the greater the distance to a galaxy, the faster it is."

Bruno Leibundgut: "What happens with the Hubble constant: You have to take measurements at great distances."

Dirk Lorenzen: "You then very quickly come to thousands or tens of thousands of kilometers per second with which these galaxies move away from us due to the expansion of the universe."

Bruno Leibundgut: "And if you want to measure this cosmic expansion of space, then you have to measure cosmic distances. And then you only have galaxies as objects or, in our case, supernovae. But they are very, very rare you wait until you can watch them again. "

Observe Cepheid stars and supernova explosions: Hubble Space Telescope (imago / Zuma Press)

Reliable distance measurement points are rare

The magic word for measuring the Hubble constant is "standard candles". Distance measurements are not difficult in themselves. All you need are objects that are known to shine brightly on site. The distance can then be calculated from the brightness observed in our sky. But that is easier said than done. Because stars are usually not normalized. Some are bright emitters, others weak twinkles. Adam Riess explains that only a few objects in space are actually suitable for a cosmic 100 watt lamp - for example a special type of star explosion.

"Type Ia supernovae are exploding white dwarfs that form a double star with a companion. Matter flows from the companion onto the compact white dwarf. If it has accumulated so much material that it has reached the limit of 1.4 solar masses, it becomes unstable and there is a very luminous and, by astronomical standards, always the same type of explosion. "

(NASA) "Mother of Hubble": Nancy Roman, NASA's first female astronomer
Nancy Grace Roman was born 95 years ago in Nashville, Tennessee. As a child she was an avid sky watcher and joined an astronomy association at the age of eleven.

Every day a number of supernovae flare up somewhere in the cosmos. But only a very few are of the coveted variety Ia. They reveal themselves through a very specific course of the light curve and through special features in their light spectrum. If the astronomers observe an Ia supernova, they immediately know how brightly it appears on site. The comparison with the brightness with which it can be seen in the sky gives the distance. Adam Riess:

"The only problem with the Ia supernovae is that they are relatively rare. We don't see them in every galaxy we would like to have one in - and certainly not in galaxies whose distance we already know in order to calibrate them. So we have to use other stars as well. "

Looking for better distance measurement ideas

In addition to supernova explosions, Cepheids are the best objects for determining distances in space. These are pulsating stars that regularly change their brightness. In 1912 Henrietta Leavitt discovered that the Cepheids shine the brighter the longer their flickering lasts. Leavitt did an excellent job, but at the time, as a woman at Harvard University, she was only allowed to be employed as an assistant. To this day, the Cepheids are an indispensable cosmic measuring tape for determining distances - and thus also the Hubble constant. Here, too, the value is above instead of below 70.

Our cosmological model may need to be changed (ESO)

Not all possibilities for measuring our universe have yet been exhausted - it is possible that even better observations and new, independent tools such as galaxy lenses or gravitational waves will expose hidden errors of thought in the next few years and save our world model. But so far it doesn't look like it. Now good ideas are needed. Adam Riess:

"Could we live in a large gap in the cosmos, in which there is less matter than elsewhere, and in which the expansion is therefore faster? Some see this as a way out. There are indeed areas with more and less matter. But these empty spaces could only influence the Hubble constant by about half a percent, not by nine percent. This large deviation cannot be explained at all. "

Our cosmological model - simply wrong?

This ruled out the most elegant solution to the problem of high Hubble values. The most boring solution would be that the astronomers so far overlook some errors when observing that make the Hubble constant appear only so high - for example when calibrating the distances of Cepheids and supernovae. And then there remains the revolutionary solution, which Bruno Leibundgut is quite relaxed about.

"If it stays that way, then it is of course clear that the cosmological model that we are currently using, that we also like to have and like, is either incomplete or wrong. So it just has to be improved. That is not really tragic . "

Dirk Lorenzen: "According to the standard model, which almost everyone finds good, the cosmos only consists of about five percent of the normal, as they say, baryonic matter, of which the earth, the sun, everything we see in space are made of . And then there is 25 percent dark matter made of a very ominous substance. And about 70 percent is this dark energy. That means that 95 percent of the cosmos is completely unknown.

According to the current world model, our matter is cosmically exotic. At best, astronomers see five percent of the universe - 95 percent are in principle unobservable, can only be guessed indirectly and so far cannot be explained physically. Nevertheless, they play a dominant role in the calculations of cosmologists. Bruno Leibundgut:

"I think people made themselves too comfortable. It all somehow worked out nicely. But nobody said that this should really be the ultimate model."

Cosmology lateral thinker Fritz Zwicky

Dirk Lorenzen: "A really very remarkable person in the history of science is Fritz Zwicky, who dared a lot, who often had an outsider opinion, was completely cranky in many ways. Today we would say that he probably wasn't able to work in a team either, but who simply had great ideas and who dared to really think and then continue to pursue things about which everyone else said: It can't be. "

The bold idea of ​​dark matter: Fritz Zwicky (Caltech)

That Fritz Zwicky, a Swiss astronomer of Hungarian descent who worked in California, had studied the movement of galaxies in the constellation Haar der Berenike in 1933. In doing so, he discovered that there is probably a lot more attracting matter than what can be seen in the telescopes. The bold idea of ​​dark matter was born, even if hardly anyone took it seriously at the time.

Dirk Lorenzen: "And then in the 60s and 70s came the great cosmologist Vera Rubin, who then took a very close look at individual galaxies and how fast the stars move there. And that's when she saw, just about Explaining the movement of the stars at the edge of the galaxy also requires a great deal of dark matter. "

(Max Planck Institute for Nuclear Physics) XENON experiment - reference to the search for dark matter
Dark matter makes up around eighty-five percent of the substance of the universe. The XENON experiment searches for new elementary particles and potential carriers of dark matter.

This was the breakthrough for dark matter. To this day, nobody knows what it is made of. It is only clear that it is not the familiar particles of matter. Dark matter cannot be seen because it does not shine and does not swallow any light. But it betrays itself indirectly through its attraction to visible matter. Fritz Zwicky has a legendary reputation among astronomers to this day - and so Adam Riess was recently asked after a scientific lecture what Fritz Zwicky would think of the contradictions in the Hubble constant.

Adam Riess: "Oh, it's always difficult to say what Zwicky would say. First of all, he would certainly offend my intelligence, which would be okay. And then? He wasn't afraid to say things that didn't fit into the general picture . I think he would say that things are going wrong. The world model no longer works. He would argue that much more emphatically. "

Search for dark matter was unsuccessful

The astronomers have come to terms with this model - called Lambda Cold Dark Matter. Accordingly, dark matter and dark energy dominate the universe. Their effect fits perfectly with the observations: around 14 billion years ago the cosmos emerged from an extremely dense and hot state, the Big Bang. As the universe continued to expand, the gas cooled and formed stars, galaxies - and at some point also planets like our earth. So far, so good - or bad. It is precisely this cosmic idyll that predicts a Hubble constant that is lower than that measured by astronomers - and relentlessly directs the focus to a number of questions that experts have largely ignored so far. Bruno Leibundgut:

"Dark matter, is that really a particle that we haven't discovered yet? Or is it really a change in general relativity? And that's still open at the moment. I would say."

(dpa / picture alliance / Landov) Dark matter - alternative gravitation theories are becoming socially acceptable In space, dark matter has still not been found. That is why more and more researchers are considering that the force of attraction works differently than previously assumed.

Just the fact that this idea is circulating would have been almost unimaginable ten or 20 years ago. Michael Krämer, Professor of Theoretical Physics at RWTH Aachen University, is also somewhat disillusioned with his subject:

"Dark matter has always been the standard paradigm and the search has so far been unsuccessful, it must be said clearly. It is not now conclusive that it does not exist. But if you search in a certain direction and find nothing, then you should sometimes start looking left and right to have alternatives in mind. "

A "modified Newtonian dynamics"

And so more and more researchers are now devoting themselves to the second possibility, in order to understand the movement of stars and galaxies in space. Maybe something is wrong with the well-known gravity - an idea that the Israeli physicist Mordehai Milgrom from the Weizmann Institute in Rehovot came up with in the early 1980s:

"I have long disagreed with mainstream science. I don't think there is much dark matter in galaxies. Rather, we are using the wrong physics. The movement of galaxies can be easily explained if we add gravity to something change, if the force of attraction works a little differently than described by Newton. "

Developed the alternative MOON theory: Mordehai Milgrom from the Weizmann Institute of Science (Weizmann Institute of Science)

Mordehai Milgrom, in a way a soul mate of Fritz Zwicky, studied the dynamics of galaxies as a young researcher. He was bothered by the fact that, according to the widely accepted theory, all galaxies should be embedded in very similar clouds of dark matter - while the galaxies themselves look very different. So he came up with the idea that the movement of galaxies could perhaps also be explained differently than with the gravitational pull of hypothetical dark matter. He developed an alternative theory called the MOON.

"This abbreviation stands for Modified Newtonian Dynamics. Galaxies move very slowly - compared to the speed of light. You don't need a theory of relativity. You just have to apply Newtonian mechanics very precisely and adapt something. The basic idea is that the force of attraction between the Celestial bodies change something when they are very weak. In the meantime there are also relativistic extensions of the theory that are needed for other phenomena.

Alternative theories of gravity

In the MOON theory, the strength of the gravitation changes with the distance between the masses, while in the relativity theory the curvature of space is decisive. It doesn't matter in our everyday life, but it does play a role on the edge of galaxies and in the vastness of the cosmos. In addition to the MOON theory, there are now a number of alternative theories of gravity, as experts call it. Indeed, these approaches beyond Einstein explain some phenomena better than dark matter models can. On the other hand, they often fail to explain the large-scale structure of the cosmos. Milgrom:

"I always emphasize that we are still almost at the very beginning. There are still large gaps in the theory. We have to expand them to describe the cosmos as a whole. I hope that now people from other fields and with other technical Participate skills with us and bring in new ideas. "

The alternative theories of gravity, once ridiculed at best, are now at least taken seriously - and unlike in the past, research organizations are now funding projects to improve the MOON theory. Michael Krämer is looking forward to the next few years with joy and excitement - because the current relative standstill in physics can only be ended with a big step forward: Either particles of dark matter can still be traced - or it shows that gravity actually works differently than expected.

"That would be a very, very exciting form of new physics, because if these theories of modified gravity are further elaborated, and also explain cosmological things plausibly, then they would also be attractive as an alternative to dark matter. That then has an indirect influence on what we investigate in particle physics. "

Repairing cracks in the world model

But most astrophysicists don't want to let it get that far. Instead of giving up the old theory and developing a completely new one, they are struggling to patch up the cracks in the world model. "Model X" is intended to provide a way out of the Hubble dilemma. It changes the conditions shortly after the Big Bang in such a way that the predicted value for the Hubble constant increases, preferably in ranges well above 70.

"You have to take the universe as it is": Bruno Leibundgut (ESO)

But even there, reports Bruno Leibundgut, it is about previously unknown physical phenomena:

"That would of course also be exciting if you then either have to add a sterile neutrino or have to add another component to the universe or early dark energy. Then the behavior of the dark energy would be different. Sure, everything is okay. Man must take the universe as it is. "

Whether a previously unknown elementary particle is at work, the early cosmos was full of changing dark energy or dark matter interacts with other particles: These approaches are hardly sufficient to close the gap between the predicted and observed Hubble value. The theorists regret that the constant would rise from 68 to 70 at best in the models, but not further. The observations, if they are confirmed in the future, simply do not fit the theory.

"Unholy" marriage of quantum physics and relativity

You can't change data, but you can change theories. The contradictions in the Hubble constant could cause a major upheaval in cosmology. It wouldn't be the first. Adam Riess, a young scientist at the time, was right there at the last sensation:

"In 1998 two research groups found that the expansion of the cosmos does not slow down - as expected - as a result of the mutual attraction of matter. On the contrary: The universe accelerates, it expands ever faster. Apparently there are large quantities of repulsive ones in the universe Component. This is what we call dark energy today. "

The teams wanted to measure how much gravity slows the expansion of the cosmos. In fact, however, they found the completely mysterious Dark Energy. Suddenly, 70 percent of the cosmos consisted of something that is driving it apart faster and faster, but of whose properties astronomers have little idea. This dark energy is added to the already puzzling dark matter - and brought Adam Riess and two colleagues the Nobel Prize in Physics. However, the discovery has one flaw:

"We don't understand the physics behind it. Why is the universe accelerating? This leads to an unholy marriage of quantum physics and the theory of relativity. According to this, there could be a vacuum energy that acts like gravity, but which does not attract, but repels. However, the prediction deviates Quantum field theory by 120 orders of magnitude from the observations. "

95 percent in the world model remains a mystery

This corresponds to a 1 with 120 zeros. The value of dark energy observed in space differs by this factor from the predicted value. Critics scoff at this as the worst forecast of all time. Cosmology absurd: With the Hubble constant, astronomers are fighting for a deviation of nine percent. In the case of dark energy, a gigantic difference is accepted with a shrug. But perhaps the speed of expansion of the cosmos will help to learn a little more about its structure. Perhaps the good old Hubble constant will give new momentum to dark matter and dark energy. Bruno Leibundgut:

"It's actually presumptuous to feel like you've found the ultimate cosmological model."

Dirk Lorenzen: "In the world model, 95 percent are unclear. That is mathematically beautiful, but physically you cannot explain anything or almost nothing. 95 percent are still a very, very big puzzle."

Bruno Leibundgut: "Okay, that was not the case 20 years ago. It was not the case 50 years ago. And why should it be the case now?"

Dirk Lorenzen: "That reminds a bit of the epicycles of antiquity. Back then, these circular orbits were used to somehow explain the movements of the planets. With this fundamentally false assumption: the earth must be in the center of the world was wrong. And then at some point you noticed: The sun is in the center of the world, so you don't need the epicyclics. "

Bruno Leibundgut: "It would actually be disappointing if we say that we have understood the universe."

Dirk Lorenzen: "And maybe you are now gradually there that you notice. With this 95 percent unknown content of the universe you don't really get any further."

Bruno Leibundgut: "In this respect, these problems are exciting. It's a great thing."

Dirk Lorenzen: "Maybe someday someone will have the really big idea and say: No, no, it's not like that at all. You have to think about that here, and suddenly it dissolves."

Hope for the "other, better idea"

Adam Riess sweeps most astronomers away and encourages them to see the conflict in the Hubble constant as an opportunity, not a problem or danger. For him, cosmology could be on the verge of an epoch-making turning point again:

"Can we really believe these measurements, even without any idea for a theoretical explanation? Many times in the history of science it was obvious that a model had failed or that something had not been understood - simply because of very precise observations. That was it For example, during the precession of Mercury's orbit. Sometimes the measurement data provide an indication, but it takes a while before we are smart enough to understand it.

For decades, researchers pondered a minimal deviation in the orbit of the innermost planet Mercury. Hardly any physicist or cosmologist took the matter seriously - who cares about a cosmic crumb?

Dirk Lorenzen: "I hope we will live long enough to at least experience this next step. That would be great if you can see what will become of dark matter and dark energy."

In the end, the course of the smallest planet in the solar system could only be understood with the help of general relativity. The supposedly small problem caused one of the great upheavals in the history of physics. Adam Riess:

"We just have to look at the facts. If our theories are wrong, then that's the way it is. Perhaps the universe is smarter than we are today. At some point, a better idea will come up."