When did dark matter arise?

Dark forces

Dark energy dominates our universe. But nobody knows exactly what the dark energy is. Scientists only know that it is distributed in perfect uniformity in the universe and that its pressure is negative. It could be Einstein's "cosmological constant" or a dynamic quantum field, the quintessence. Then a "fifth force" would be responsible for the dark energy. Christof Wetterich from the Institute for Theoretical Physics explains what the mysterious dark forces are all about.

The quintessence has always been shrouded in mystery. The Greeks of antiquity saw in it the fine primary material, the subtle and essential, the inconceivable fifth element in contrast to earth, water, air and fire. The alchemists of the Middle Ages tried to distill the quintessence as the purest elixir. Even for the cosmologists and astrophysicists of today, the quintessence is a great unknown. A dark energy dominates our universe - and we do not know what it is and what place it could take in the building of physics. It could be the cosmological constant invented by Einstein. But a dynamic quantum field is also possible - the quintessence. And so the age-old question is asked again: What is our universe made of?

Contrary to popular belief, our universe is not primarily made up of stars and gas clouds. The baryons - that is the matter of which we, our earth and the stars are essentially made - make up less than five percent of the energy density of the universe. Most of the substance that makes up the universe cannot be viewed with light, radio waves, or X-rays: it is dark.

We are at least indirectly informed of part of the dark secret. Dark matter clumps together in galaxies. Our galaxy - the Milky Way - also consists predominantly of dark matter. Where something clumps, gravity attracts other objects. Just as we can determine the mass of the sun from the orbits and speeds of the planets, we can measure particles and objects moving in and around a galaxy, and use this to determine the mass of this galaxy - including dark matter. This method can be used to estimate how large the concentration of dark matter is in galaxies and galaxy clusters. Together with our knowledge of the cosmic density of the baryons, this allows quite reliable conclusions about how much dark matter is present in our universe.

Lumps of dark matter can also be tracked down by using their ability to deflect rays of light. Like a lens, any mass concentration (i.e. baryons and dark matter) distorts the image of an object behind it. We also know such distorted images from everyday life, for example when observing an object through a wine glass. So-called gravitational lens images show how a foreground cluster of galaxies distorts a galaxy behind it so much that it becomes visible several times in the form of arc pieces.

Gravitational lensing effect: A galaxy cluster in the foreground distorts the image of a galaxy (blue) behind it so strongly that it becomes visible several times in the form of curved pieces.

Rays of light from a distant galaxy on the edge of our universe often traverse fluctuations in the gravitational potential of various mass concentrations on their journey of more than ten billion years. The mean distortion is only small, but can now be measured. You can see everything that clumps and find what the other measurements say here too: Dark matter contributes about a quarter of the energy density of the universe.

Clusters of galaxies in the depths of the universe

Today there is hardly any doubt that there is significantly more dark matter than baryons. Without dark matter, it would not be possible to understand quantitatively how cosmic structures, such as galaxy clusters and galaxies, were formed from what were originally tiny fluctuations in energy density. But the components of dark matter have not yet been identified by any particle accelerator in high-energy physics. Even the great efforts to detect weakly interacting particles in our solar system have so far not been successful.


As if that weren't enough of a mystery, most cosmologists today are convinced that most of the substance of the cosmos consists of an even more mysterious component: dark energy. In contrast to dark matter, it is completely structureless and distributed in perfect uniformity over the entire universe. The existence of dark energy follows a very simple calculation: We know very well the total energy density in the universe. If you find that the sum of all clumping forms of matter (= baryons and dark matter) only makes up about 30 percent of the total energy density, then the remaining 70 percent must be homogeneously distributed over the universe.

This puts cosmologists in a state of need of explanation: All particles with a sufficiently large mass must form clumps under the influence of gravity ("gravitational instability") and should therefore concentrate in galaxies or galaxy clusters. Massless or extremely light particles that can escape the gravity of the galaxy clusters are the photons and the neutrinos. However, neither are considered candidates for dark energy.

One may doubt whether the total energy density of the cosmos is actually as well known as the theorists claim it is. However, it is precisely this prediction that has been confirmed by impressive observations in recent years. The "Cobe" satellite photographed the Big Bang for the first time. More precisely, it is a photo of the universe at the age of 300,000 years, its early childhood. It was then that the extremely hot plasma became transparent to light rays for the first time. The successor satellite (WMAP) has succeeded in creating an exact "map" of the universe of this early epoch. The map shows small fluctuations in the temperature of the cosmic background radiation, which arose at a precise point in time throughout the universe. What we see today is the radiation from the surface of a sphere around us. Since their formation, the rays have traveled 13.7 billion light years. It is a perfect black body radiation with a temperature of 2.73 Kelvin. It comes almost isotropically with the same temperature from all directions - but only almost.

Background radiation anisotropy. The graphic shows the strength of the temperature fluctuations in the cosmic background radiation as a function of the angle between two observation regions. The maximum corresponds to an angle of one degree. For a total energy density much smaller than the critical energy density, this maximum would be shifted far to the right.

If you take a closer look at the temperature depending on the direction, you will see tiny fluctuations of less than one part in ten thousand. A characteristic size of the fluctuation regions can be seen on the map, corresponding to an angle in the sky of approximately one degree. This is crucial information about the total energy density of the universe: the angle is influenced by the geometry of the universe, the geometry in turn is directly related to the total energy density.

Despite all open questions, we have to come to terms with the existence of a homogeneously distributed energy density, about which we hardly know anything. The homogeneity of the dark energy seems to be one of its most essential properties. At the same time, it prevents any possible discovery in local systems, for example in galaxies. Something that is evenly distributed cannot exert force on other bodies. In which direction should the force pull? Nor can it deflect light: why should the light be deflected in one direction rather than another?

It seems like the dark energy is hidden in the most perfect way. The fact that it determines the development of the cosmos as a whole still gives us the chance to track it down. Perhaps its most amazing property is that it drifts the universe apart, interacting with gravity, like after an explosion. We also know that dark energy was far less important in earlier epochs of cosmological evolution. In earlier times, dark matter dominated the universe, and for the first 100,000 years radiation dominated events. These "normal" forms of matter slow down the expansion of the universe through their gravitational attraction. Until a few years ago, almost all scientists believed that the slowdown in the rate of expansion continues today. Dark energy, on the other hand, has exactly the opposite tendency: its pressure becomes negative if it does not change too much over time. If our universe "recently" - that is a few billion years for cosmologists - fell under the rule of dark energy, then the slowed expansion should have turned into an accelerated expansion by now. This was exactly the result of measuring the luminosity and redshift of distant supernova explosions. If the accelerated expansion continues, future observers will no longer be able to see the most distant galaxies observable today. We know that the dark energy is distributed homogeneously and that it exerts a negative pressure. We don't know much more. Here the question arises: If cosmologists are so in the dark, how do they get the certainty that their thoughts are not completely alien to reality?

In contrast to the alchemists' unsuccessful quintessential search, we have a number of other impressive pieces of evidence that indicate that dark energy dominates our universe. The dark energy not only influences the position of the first maximum of the anisotropies of the cosmic background radiation. It also influences the entire characteristic course of the curve with maxima and minima. From the tiny anisotropies in the early universe, the galaxies and galaxy clusters later emerged. We can measure the strength of the fluctuations 13.7 billion years ago with the background radiation and then calculate for a given cosmological model how the galaxies observed today are distributed. For models with dark energy, the observation agrees well with the calculations. The age of the universe calculated from the anisotropies of the background radiation also coincides with our knowledge of the oldest stars in the Milky Way. If there is dark energy, this also fits the observed value of the Hubble parameter. Now what is the dark energy? Is it the cosmological constant introduced by Einstein?

Although he later dismissed it as "the greatest ass of my life", it has haunted cosmology for decades, but is a major headache for theoretical physicists. In naive calculations, it comes out 120 orders of magnitude larger than observed - there are only a few estimates that are even more off the mark. The alternative would be a dark energy that changes dynamically: the quintessence. 16 years ago I suggested that a scalar field - the cosmon - is responsible for a cosmological "constant" that changes over time. The homogeneously distributed potential and kinetic energy of the scalar field - later called the quintessence - decreases over cosmological times similar to the other forms of energy. The tiny amount of dark energy can then be explained by the enormous age of the universe. For a large class of such models, the correct magnitude of the dark energy then follows from the existence of "cosmic attractor solutions" for which the dark energy adapts to the other forms of matter. For a realistic cosmology, the quintessence must trump dark matter in today's epoch.

Can the alternatives quintessence and cosmological constant be distinguished by observations? An essential difference lies in the time behavior. The cosmological constant can be completely neglected in the early stages of the evolution of the universe. In contrast to this, quintessence may have played a certain role at the time of nucleosynthesis, the emergence of cosmic background radiation, or during structure formation. The acceleration of the expansion of the universe could also be a little less for Quintessenz than for a cosmological constant. Previous measurements already rule out some of the proposed quintessence models.

However, a particularly interesting class of quintessence models can only be distinguished from a cosmological constant if the precision of the observations is further increased. A great flood of exciting data is expected here in the coming years from new satellites and a detailed survey of the sky with powerful telescopes.

Quintessence is based on the quantum field theory for the scalar cosmon field. A force is conveyed through the exchange of virtual cosmon quanta - just as the electrical and magnetic forces come about through the exchange of virtual photons. Instead of the "fifth element" of antiquity, we are now dealing with a new "fifth force". In contrast to the cosmological constant, quintessence implies a new fundamental macroscopic interaction. On Earth or in our solar system, this interaction is significantly weaker than gravity - but its existence would have quite spectacular consequences, which could be proven with future precision measurements.

In particular, the equivalence principle is violated. Bodies with the same mass but different composition no longer fall at the same speed even in a vacuum. The difference in their relative acceleration is only tiny - typical models come to a value of 10-14 - but the "Microscope" satellite will be able to test this. Perhaps even more spectacular is that Quintessence predicts a time dependence of the fundamental "constants". The electrical charge and mass of the proton did not have exactly the same value in early cosmology as it is today. The reason for this is simple: the fundamental "constants" depend on the value of the cosmon field, and this changes over time. Fortunately, this effect is only tiny - otherwise statements about the state of the early universe would be much more complicated.

Perhaps the most precise way to search for a time change in the coupling constants in the early universe is to measure the change in the fine structure constants through absorption lines in the light of distant quasars. The evaluation of this measurement showed with high statistical significance that the fine structure constant in the very early universe was a little less than a hundred thousandth smaller than it is today. Systematic errors must always be expected when making an initial observation. If the group were right, however, another pillar of a static worldview would shake: Even the fundamental couplings change. As the ancient Greeks already knew: "panta rei" - "everything flows". The fundamental laws of the world become time-dependent - if only a little. They depend on the state of the cosmon field. And that is almost something like the fine original material, the ether, of the ancient Greek philosophers.

Prof. Dr. Christof Wetterich
Institute for Theoretical Physics
Philosophenweg 16, 69120 Heidelberg
Telephone (0 62 21) 54 93 40
e-mail: [email protected]