You heard us saying "we do not know" quite a lot in this course. It all started in chapter 1, where we had to admit that we do not know what a particle is. Instead, we presented you with different well-established models that we use to describe the phenomena in our Universe. However, there are a few things we really, really do not know. Things for which we do not even have well-established models but only vague ideas that lie beyond the Standard Model of particle physics. This chapter is all about these open questions and vague ideas. As we pointed out before, we rely on the help of young, creative scientists to tackle these open questions in the future. Therefore, we will not provide key messages in this chapter. Instead, we will present our wish list for your future Nobel Prize-worthy discoveries.

The energy content of our Universe

Let us start with the most depressing fact right at the beginning to get it over with. Everything we think we understood about our Universe over the past few hundred years, and everything you will ever be able to learn if you start now, only accounts for a tiny proportion of our Universe. When we talk about our Universe in this context, we mean the total energy content of our Universe. That means all forms of energy we can find, for example, the energy associated with the rest mass of particles, but also all kinetic energies.

In this jar, you can see 100 chocolate drops in three different colours, blue, red and yellow. You might also notice that the different colours appear in different numbers. Indeed, we mixed the chocolate drops in this jar so that one type of chocolate represents the component of the energy content of our Universe that we understand kind of well. But the other two types of chocolate represent components of which we have no clue at all.

Now, which one do you think is the type of chocolate that represents what we think we understood? No, it is not the blue chocolate drops. It is also not the red chocolate drops. But here, from time to time, we can find yellow chocolate drops. In this jar with 100 chocolate drops, there are five yellow chocolate drops. And that is the proportion of our Universe we think we understand – more or less, tiny 5%. We often call this proportion of the energy content of our Universe "visible matter". This visible matter refers to all the objects in our Universe that we can observe with our eyes or telescopes, such as planets, stars, or gas clouds. All of these consist of elementary particles, which can be described by the Standard Model of particle physics. Why did I say we understand this 5% only "more or less"? Because even in this tiny fraction of the energy content of our Universe we still have many open questions! I will briefly mention two of them before moving on to the other 95% of our Universe.

Gravity

With the Standard Model of particle physics, we have an amazing theory that describes three fundamental interactions, including their associated charges and interaction particles. We can even fit the short version of this theory on a coffee mug. It is beautiful! But there is a big problem, gravity. If you ever want to see particle physicists in despair, you can either ask them "What is a particle?" or you ask them "What about gravity?". For a very long time now, particle physicists tried to integrate the gravitational interaction in the Standard Model of particle physics. We all hope that this interaction follows the same basic principles as the other three fundamental interaction. We even came up with a name for a hypothetical interaction particle of the gravitational interaction, the graviton. However, so far, no one has observed gravitons yet. Even worse, physicists are still struggling immensely to describe the gravitational interaction in a similar way to the other three fundamental interactions. This would require a quantum theory of gravity. And this is the first Nobel Prize idea of this chapter. Come up with a quantum theory of gravity!

The generation problem

The Standard Model of particle physics is a theory of interactions, charges, and particles. However, neither does the Standard Model of particle physics predict which types of particles we should observe in our laboratories nor which masses these particles should have. Instead, it only provides a set of rules for these particles. That means that all the particles we have discussed before are merely experimental observations. And the particle masses are also just measured in a laboratory, they are nothing we can calculate.

One incredibly annoying question is: Why are there precisely three generations of elementary particles? There is no reason why this must be the case. In principle, the Standard Model of particle physics would also work with only two, or even four or five generations. But we observe only three generations. And we always observe the same number of generations. For example, there are three generations of quarks and three generations of neutrinos, although these groups of particles are not connected. This is the second Nobel Prize idea of this chapter. Find an explanation of why there are exactly three generations of elementary particles!

Dark matter

Now we already have two good ideas for Nobel Prizes that have to do with the 5% of the Universe we think we kind of understand. But, what about the red chocolate drops in the jar? There are 27 of them. These red chocolate drops represent another component of the energy content of our Universe. It is something we do not know much about at all. But at least we gave it a name, we call it dark matter.

So, what is dark matter? We have no clue. No one knows what dark matter is. We do not even know if dark matter consists of particles. We use this term "dark matter" really just as a mental placeholder for unknown physics. The only thing we are pretty sure about is that there are large amounts of something in our Universe that has a mass but is invisible to our eyes and our telescopes. Indeed, in the past, we have usually used our eyes and telescopes to study the Universe. However, seeing something in this way is only possible if the electromagnetic interaction is involved. If something does not interact via the electromagnetic interaction, it remains invisible to our eyes and our telescopes. For something to be visible, we simply need photons, the interaction particles of the electromagnetic interaction. Therefore, we call this type of matter that has a mass but does not interact via the electromagnetic interaction dark matter.

How do we know dark matter is there if we do not see it? Well, even though we cannot observe dark matter directly, we can observe the gravitational effect dark matter has on visible matter. For example, we noticed that galaxies rotate in an unexpected way. In particular, stars in the outer regions of spiral galaxies rotate very fast around the centre of the galaxy, much faster than we would expect if we calculate the gravitational force caused by all the visible matter in the galaxy. So, our observation indicates that there must be way more matter out there than we can see. Indeed, this unexpected rotation can be explained best if we assume that there is more matter than we can observe directly. We explain this phenomenon with large amounts of dark matter. This is the third Nobel Prize idea of this chapter. Find out what dark matter is!

Dark energy

Finally, let us look at the blue chocolate drops in the jar. Out of 100 chocolate drops, we had 5 yellow chocolate drops representing visible matter, and another 27 red chocolate drops representing dark matter. The remaining 68 blue chocolate drops represent something we call dark energy. Again, the word "dark energy" is just a placeholder for something we think must be there, but we have really no clue about. Indeed, dark energy is even more theoretical than dark matter. We describe dark energy as an unknown form of energy that somehow affects the Universe.

Yes, we are aware that this sounds very vague. So, how do we know dark energy exists at all? After the big bang, our Universe continued expanding. We can even measure its expansion rate. What is weird is that our Universe expands faster and faster, its expansion is accelerating. And the best way to explain this expansion behaviour is to assume that there is a yet unknown form of energy, which we call dark energy. This is the fourth Nobel Prize idea of this chapter. Find out what dark energy is!

By now, we have listed four open questions. If you find an answer to any one of them, the next Physics Nobel Prize would be yours. Guaranteed! Of course, there are many more open questions in Physics, and in Science in general. That is why we rely on future generations of creative scientists to help us push forward the frontiers of human knowledge. One last note, if you succeed and you solve one of the open questions, please do not forget to mention us in your speech in Stockholm when you collect your Nobel Prize.

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Bend it like dark matter!