Over time, it has happened that some solution in one area of physics could also be used in a quite different area. Or, at least, inspired the solution. Unfortunately, this does not always work. Even quite often it happened that when reaching the finer points it turns out that something promising did in the end not work. Thus, it pays off to be always careful with such a transfer, and never believe a hype. Still, in some cases it worked, and even lead to brilliant triumphs. And so it is always worthwhile to try.

Such an attempt is precisely the content of my latest paper. In it, I try to transfer ideas from my research on electroweak physics and the Brout-Englert-Higgs effect to quantum gravity. Quantum gravity is first and foremost still an unsolved issue. We know that mathematical consistency demands that there is some unification of quantum physics and gravity. We expect that this will be by having a quantum theory of gravity. Though we are yet lacking any experimental evidence for this assumption. Still, I also make the assumption for now that quantum gravity exists.

Based on this assumption, I take a candidate for such a quantum gravity theory and pose the question what are its observable consequences. This is a question which has driven me since a long time in particle physics. I think that by now I have an understanding of how it works. But last year, I was challenged whether these ideas can still be right if there is gravity in the game. And this new paper is essentially my first step towards an answerhttps://arxiv.org/abs/1908.02140. Much of this answer is still rough, and especially mathematically will require much work. But at least it provides a first consistent picture. And, as advertised above, it draws from a different field.

The starting point is that the simplest version of quantum gravity currently considered is actually not that different from other theories in particle physics. It is a so-called gauge theory. As such, many of its fundamental objects, like the structure of space and time, are not really observable. Just like most of the elementary particles of the standard model, which is also a gauge theory, are not. Thus, we cannot see them directly in an experiment. In the standard model case, it was possible to construct observable particles by combining the elementary ones. In a sense, the particles we observe are bound states of the elementary particles. However, in electroweak physics one of the bound elementary particles totally dominates the rest, and so the whole object looks very similar to the elementary one, but not quite.

This works, because the Brout-Englert-Higgs effect makes it possible. The reason is that there is a dominating kind of not observable structure, the so-called Higgs condensate, which creates this effect. This is something coincidental. If the parameters of the standard model would be different, it would not work. But, luckily, our standard model has just the right parameter values.

Now, when looking at gravity around us, there is a very similar feature. While we have the powerful theory of general relativity, which describes how matter warps space, we rarely see this. Most of our universe behaves much simpler, because there is so little matter in it. And because the parameters of gravity are such that this warping is very, very small. Thus, we have again a dominating structure: A vacuum which is almost not warped.

Using this analogy and the properties of gauge theories, I figured out the following: We can use something like the Brout-Englert-Higgs effect in quantum gravity. And all observable particles must still be some kind of bound states. But they may now also include gravitons, the elementary particles of quantum gravity. But just like in the standard model, these bound states are dominated by just one of its components. And if there is a standard model component it is this one. Hence, the particles we see at LHC will essentially look like there is no gravity. And this is very consistent with experiment. Detecting the deviations will be so hard in comparison to those which come from the standard model, we can pretty much forget about it for earthbound experiments. At least for the next couple of decades.

However, there are now also some combinations of gravitons without standard model particles involved. Such objects have been long speculated about, and are called geons, or gravity balls. But in contrast to the standard model case, they are not stable classically. But they may be stabilized due to quantum effects. The bound state structure strongly suggests that there is at least one stable one. Still, this is pure speculation at the moment. But if they are, these objects could have dramatic consequences. E.g., they could be part of the dark matter we are searching for. Or, they could make up black holes very much like neutrons make a neutron star. I have no idea, whether any of these speculations could be true. But if there is only a tiny amount of truth in it, this could be spectacular.

Thus, some master students and I will set out to have a look at these ideas. To this end, we will need to some hard calculations. And, eventually, the results should be tested against observation. These will be coming form the universe, and from astronomy. Especially from the astronomy of black holes, where recently there have been many interesting and exciting developments, like observing two black holes merge, or the first direct image of a black hole (obviously just black inside a kind of halo). These are exciting times, and I am looking forward to see whether any of these ideas work out. Stay tuned!

## Wednesday, August 7, 2019

### Making connections

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