Is completely empty space possible

Above all else, the universe is empty

The most exciting objects in the universe are not wormholes, quasars or dark matter halos: its emptiness, the vacuum, determines the fate of all of us

The question of what is empty space and whether there is such a thing at all has preoccupied researchers for centuries. According to Aristotle, the space beyond the lunar sphere was filled with quintessence, Latin for "fifth element". Unlike fire, water, earth and air, this "ether" should be the only element that should be immortal.

Later, it was the action at a distance - for example of light - that required a transport medium in the ideas of classical physics. Huygens, who formulated the first wave theory of light, assumed, for example, an ether of light that penetrates solid matter as well as space, and on which the light waves propagate.

It was only in the special theory of relativity that Einstein was able to do without the ether because he said goodbye to the idea of ​​absolute space. For a short time the vacuum was considered to be absolutely empty, even if the space of the general theory of relativity (GTR) naturally has physical properties, so it is not a matter of "nothing".

The universe grows

The universe was initially imagined as static; but when one noticed its expansion, one needed again a kind of vacuum energy for a suitable solution of the equations of the GTR, which one gave the name cosmological constant.

With the introduction of quantum field theories at the latest, things got complicated. On the smallest scale, quantum physics requires and this has been successfully proven, subatomic particles must constantly jump out of nowhere into existence in order to say goodbye again in the shortest possible time, so that on average the conservation of energy applies.

These virtual particles only exist for a short time, but in their short life they naturally contribute to the energy content of the vacuum. But if you add up the energy of all virtual particles at a certain point in time, you get a value of 1.4 * 10128 GeV / m3.

But what average vacuum energy does the room have to have so that the universe grows as it is in practice? The experimentally confirmed value is about 4 GeV / m3. There are over 100 orders of magnitude between theory and practice!

The problem of the cosmological constant

That is the problem of the cosmological constant, an open question, it can hardly be more open. The problem is that the vacuum has a much more complicated structure than one could naively assume by simply looking into the void of space. It shimmers and bubbles, it changes depending on the perspective from which you look at it.

The electromagnetic vacuum behaves differently than the vacuum of gravity, this differs from the vacuum of the weak interaction (which can be imagined as a condensate of non-localized Higgs particles distributed throughout space) and the vacuum of the strong interaction (the one Lake of quark-gluon matter in the ground state).

You can think of it as a lake filled with different liquids. The vacuum is the surface of the lake. Matter, on the other hand, is the wave-like excitation that arises when you throw marbles of different sizes into it. How this wave actually looks depends on the one hand on the mass of the thrown ball and on the other hand on the viscosity of the lake. On a lake of water, different stimuli arise than on a lake made of oil, tar or jelly.

Now all you have to do is make a tiny mental leap and imagine the lake as a quantum-physical overlay of all possible lake variants, which are also constantly transforming into one another - then you have a realistic idea of ​​how the vacuum is built up. Done? Congratulation! I always get out of the jelly and go to the fridge.

"Running Vacuum" models

The good news: In order to anticipate the evolution of the universe to some extent, we don't even need to know the exact structure of the vacuum. It is sufficient if we find a sufficiently precise model that describes the past and present within the scope of the measurement accuracy. Then we can assume that the statements made by the model about the future should also be reliable.

Physicists also deal with this in a meaningful way, for example in a paper in the Astrophysical Journal. Here three Spanish researchers are particularly interested in cosmological models which - unlike the ΛCDM model, the standard model of cosmology - assume a vacuum energy density that changes over time and depending on the Hubble "constant".

Such "running vacuum" models even assume that the gravitational constant G changes - or they even dispense with the conservation of mass. The current work of the Spaniards is not concerned with discussing the physical durability of these models. Instead, they simply take the given formulas and parameters and compare them with the actual values, following the motto that a theory is not good when we understand it, but when it describes reality.

The result is encouraging for the theorists behind the running vacuum models: The tested models with variable cosmological "constants" are clearly superior to the standard theory in reproducing reality as it looks according to our own measurements. However, the accuracy of the comparison is not quite sufficient to speak of a reliable result. The researchers therefore hope for more precise measurements of the parameters they use. (Matthias Matting)

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