In my last post, I introduced an important concept connected to dark matter: the relic density of dark matter. This relic (from the ancient times) is roughly the density of dark matter that is left in the universe today.
[image credits: Wikipedia]
Physicists have developed varied mechanisms that explain correctly the amount of dark matter that is observed today, and all relate the relic in one way or the other to the amount of dark matter of the early days of our universe.
Thermally freezing-out dark matter consists of one of these options, and I have largely detailed how this works in my previous post.
However, the correct relic density of dark matter can also be obtained by other means. For instance, instead of freezing-out, dark matter could freeze-in.
In any case, winter is coming, and everything must thus be frozen and dark ;)
This freeze-in is what I will talk about in this post, after having briefly summarized what is dark matter and how it freezes-out (so that the differences with the freeze-in case will be easier to point).
DARK MATTER IN A NUTSHELL
Before introducing the concepts of freezing-out and freezing-in dark matter, let me briefly recall what dark matter is. It consists of a type of matter that does not interact electromagnetically, which is the reason why it is called dark, and that interacts gravitationally, which is the reason why it is called matter.
[image credits: Wikipedia]
Dark matter has been introduced almost 85 years ago to explain the dependence of the velocity of distant galaxies on the distance.
With the help of classical mechanics, it was shown that some invisible mass was required, so that one could have enough gravitational interactions to get an agreement between theory and experiment.
Without this invisible mass, it is indeed clear that the predictions (the dashed line on the left) disagree with data (the yellow and blue markers).
This invisible mass is what is called dark matter.
Furthermore, we know today that dark matter is also required to explain many many many (many?) observations, and this is why it is considered so seriously by physicists.
The only missing item on the agenda consists of its direct detection, at least for now.
FREEZING-OUT DARK MATTER
Before detailing how dark matter could freeze-in, let me recall what it does when it freezes-out. We can start from the figure of my older post, on which I have added colors to make it more visual below.
During the history of the universe, dark matter particles can undergo two reactions. Pairs of dark matter particles can be created from the collision of lighter particles (that are accelerated, like at the LHC, as the universe is super hot) and pairs of dark matter can be destroyed to give back pairs of lighter particles of the Standard Model.
In short, this is the same reaction that occurs from the left to the right and from the right to the left.
[image credits: homemade from Inspire]
At the origins of the universe (the yellow region on the figure), both reactions occur at the same rate.
The amount of dark matter that is created by the first reaction is thus equal to the amount of dark matter that is annihilated by the second reaction, so that the density of dark matter (that is what is represented on the y-axis of the figure) is constant.
The universe is however in expansion and its temperature goes down. As a result, lighter particles are at some point not energetic enough to create any new pair of dark matter particles, and one is left with a single reaction. Dark matter annihilates.
As a consequence, the density of dark matter goes down. This corresponds to the orange zone of the figure.
Finally, the universe continuing its expansion, dark matter starts to be too diluted to be able to annihilate. A single dark matter particle cannot find any neighbour anymore to annihilate together.
As a consequence, both reactions have stopped, and the dark matter density is constant, as on the blue region of the curve. This constant corresponds to the density of dark matter observed today.
FREEZING-IN DARK MATTER
In the freeze-out case, we have seen that one starts with a large amount of dark matter, that this amount is reduced during the history of the universe before getting constant at some point by virtue of a too cold and too expanded universe.
This is not the only way to produce the right amount of dark matter. A freeze-in mechanism is also possible, where one starts with very little dark matter at the time of the big-bang.
In the freeze-in context, the dark matter particle is a very feebly interacting guy that interacts very little with the other particles in the early universe.
In other words, dark matter is decoupled from the cosmic soup that originated from the big bang. It lives its life on its own, somehow.
[image credits: homemade from arXiv]
Once in a while, additional dark matter particles can however produced, either through collisions of other particles or via the decay of heavier particles.
Consequently, the density of dark matter increases more and more with time. This is illustrated by the yellow region on the figure.
We need here to check the dashed lines and forget about the solid lines that represent the usual freeze-out case.
At some point, the universe will have already cooled down enough, so that there is not enough energy to produce more dark matter, as in the freeze-out case by the way.
The density of dark matter thus becomes constant (the orange area on the figure).
TAKE HOME MESSAGE
In this post, I came back to this freeze-out mechanism and re-discuss (hopefully more clearly) this mechanism that can successfully explain the amount of dark matter that is observed today. On the other hand, dark matter can also freeze-in instead of freezing-out.
The freeze-in mechanism can be seen as the opposite of the freeze-out mechanism. We start from almost no dark matter in the early days of the universe, and one slowly and slowly gets to the quantity of dark matter that is observed today. On the other hand, in the freeze-out case, one starts with a lot of dark matter in the early days of the universe, and dark matter is then annihilated until we get the right amount of dark matter observed today.
The only missing item on this dark matter agenda is of course its direct observation. Even if we have no direct observation of dark matter today, we must keep in mind that the set of indirect evidences is pretty large. And dark matter can be trickier that we think it is: there are still theoretically motivated ways for it to escape detection. However, the future experiments will definitely close this gap :)
This is, in my opinion, one of the things that make particle physics exciting.
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