Science Dark Energy

What is dark energy?

ESA's Euclid probe and NASA's Roman probe jointly explore dark energy

In 367 B.C., a young man entered the Academy of the philosopher Plato in Athens. His name was Aristotle. He went on to become one of the most important philosophers and natural scientists in history. In his physics of the time, he rejected the existence of completely empty space, arguing:

"No one would be able to give a reason why that which has been set in motion (in it) should once remain stationary somewhere."

This seemed nonsensical to him. In doing so, he anticipated Isaac Newton's law of inertia, which he formulated as an axiom some 2000 years later:

"Without an acting force, a body remains in its state of motion."

Newton imagined space as empty, without any matter or substance in it. For him, it was a kind of "container". But does the completely empty container really exist? Or was Aristotle on the right track?

Today, the exploration of space has led to a surprising discovery: the container is expanding, faster and faster - constantly - everywhere. This expansion can be traced back. It began at a single point 13.6 billion years ago . This is where the universe was created in a big bang

Ein Bild, das Electric Blue (Farbe), Solarzelle, Kunst enthält.

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2 Diagram: Diameter of the of the universe (maximum value) and time (right value). You can see: the diameter is increasing

Something unknown is driving the universe apart faster and faster. The average distance between visible stars and galaxies is increasing. This is happening against the mutual attraction of the celestial bodies due to their gravity. The "momentum" that existed after the Big Bang is not enough to do this. An enormous amount of energy must be the ever-accelerating expansion. We do not know this energy. That's why we call it "dark energy".


With uniform expansion, on the other hand, you would expect something like this:

3 Uniform extension

It would be a universe in which the redshift of the emitted light.., caused by "escape velocities", would follow exactly the HUBBLE-LEMAÎTRE law everywhere: Double the distance means, strictly speaking double the redshift and double the escape velocity, valid for all distances. But that is not what we measure. Moreover, it would require a constant flow of energy to compensate for the gravitational pull. Even if it is less and less energy over time.

Under the influence of gravity only, the expansion should be roughly as follows:

4 Gravity only

As we assume today, however, we only see this at the very beginning. If we compare both with the image of the expansion actually measured today (although the image above is also only schematic), we get the impression that there may have been two phases in the expansion: A first phase in which gravity was dominant, and then a second phase in which dark energy became more and more important. This caused the universe to expand faster and faster.

Many researchers today assume that it is "empty space" itself that provides the necessary energy. So it is not empty, it is not a container, as Newton thought. Nor is it just a container with length, width, height and time as the fourth dimension, as Albert Einstein described it.

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5 Everything and nothing: the amazing science of empty space


Empty space is something completely different, something unknown.

In a source you wouldn't expect, Amazon Prime Video in Germany, you can watch a movie in English. It was produced by the BBC in 2011 under the direction of cosmologist Professor "Jim" Al-Kalili from the University of Surrey in the UK.

The movie explains what might be hidden behind the modern concept of empty space in a vivid - and physically correct - way: The constant creation and destruction of tiny pairs of matter and antimatter particles - all the time and everywhere - in empty space.

It is Werner Heisenberg's uncertainty principle that makes this possible. It is a fundamental principle of quantum mechanics. It states that certain pairs of physical quantities, such as position and momentum or energy and time, cannot be measured simultaneously with arbitrary accuracy. The more precisely one quantity is measured, the less precise the measurement of the other quantity becomes. The product of energy times time must therefore also never go below a certain minimum value. For short times, the energy of a energy of a system is therefore so indeterminate that energy can be taken from the system which, seen over a longer period of time, is not available at all.

According to this uncertainty principle, virtual particles can be created "out of nothing" for a very short time and then disappear again, as long as the energy-time uncertainty principle is observed. The greater the energy of the virtual particle, the shorter the time in which it can exist. In a large empty space, many of these particles can be created in the same period of time, as there are many different states (one could say habitats) that they can temporarily occupy.

The Casimir effect is a fascinating example of how virtual particles cause measurable physical effects even though they cannot be observed directly: Between two closely spaced, uncharged, conducting plates in a vacuum, the virtual particles create a force that pushes the plates together slightly.

This is due to the fact that not all states are possible between the plates. In particular, not those with wavelengths that are longer than the distance between the plates. In other words, there is no room for these waves between the plates. This means that there are fewer free states and therefore a lower density of particle pairs between the plates. Outside the plates, however, there is no such restriction.

This inequality in the density of the particle pairs creates the pressure on the plates.

The British physicist Paul Dirac made a valuable theoretical contribution to the study by formulating his Dirac equation and introducing the concept of antiparticles.


6 Memorial plaque. Since November 1995, a stone in the floor of Westminster Abbey near Newton's grave commemorates Paul Dirac. His famous formula is engraved on the gravestone. It looks simple, but it is not.

The Dirac equation, formulated in 1928, describes the behavior of particles such as electrons. It is the first theory to integrate both quantum mechanics and special relativity. The equation allowed the calculation - not the measurement - of the electron spin and predicted the existence of the positron, the antiparticle of an electron, which was then discovered in 1932.

Dirac laid the foundation for the later development of quantum field theory, which describes particles and their interactions in terms of quantum fields, with his equation and prediction of antiparticles. Dirac's work has had a profound influence on 20th century and modern physics, leading to a deeper understanding of matter.

His calculations also produced particles with negative energy. However, since negative energy had never been observed, Dirac's theory was that the vacuum was a Dirac lake, in which every conceivable state of negative energy was already occupied, so that further electrons could only have positive energies.

The idea of a Dirac lake is now considered obsolete. It has been replaced by the Feynman-Stückelberg interpretation: In this theory, antiparticles are particles that move in a backward direction in time. In this view, a positron (the anti-particle of the electron) is itself an electron that is moving backward in time. This interpretation allows for a more unified and consistent description of how particles and antiparticles interact in quantum field theory. However, it seems strange.) .

7 ESA's Planck 2009 – 2013

Data from the Planck spacecraft now indicate …

·        … that 68.3 % of all energy and matter in the universe is this dark energy.

·        Next at 26% is the equally mysterious dark matter - matter that does not interact with light.

·        And only 4.9 % consists of normal visible matter. We ourselves are part of the normal matter.

      And we would like to know: What is dark energy?...

(… and dark matter as well.)