As already said several times in this blog, dark matter consists in an appealing option to reconcile predictions with cosmological data. Consequently, there are numerous searches for dark matter, and new ideas are regularly proposed.
[image credits: @pab.ink]
Very recently, my attention got triggered by a new scientific article in which it was considered to scrutinise Earth to probe dark matter.
In a few words, Earth has the possibility to capture dark matter, which could then annihilate and release energy within the core of the planet. The heat output (or the increase of the temperature on the planet) cannot however exceed the observations.
Dark matter and its interactions - the classics
Whilst dark matter is highly evidenced by data, it still escapes direct detection.
[image credits: SLAC]
The principle behind these direct detection experiments is quite simple (see the image).
First, one needs to build a large detector. Dark matter rarely interacting with matter, the detection volume needs to be maximised to have a chance to see something.
Second, the detector has to be buried deep underground, to be protected from the backgrounds (cosmic rays, human activities, etc.).
Monitoring what is going on, physicists then track events in which dark matter particles (being part of the dark matter wind blowing on Earth) would hit the detector constituents and generate a recordable recoil.
However, the story is not always that simple…
Captures and annihilations on Earth
There are models in which dark matter will interact with the crust of the planet before reaching the detector (that is buried underground). Its kinetic energy getting subsequently reduced, dark matter is then not able to generate any visible recoil in the experiments.
[image credits: NASA]
This could sound dramatic as we could be insensitive to several dark matter models.
There is however a way out!
The velocity of the dark matter particles decreases, so that dark matter particles could get captured by the planet (due to gravity). The dark matter density inside Earth hence increases.
This increase in density implies that dark matter could annihilate efficiently within the planet.
This results in a potentially significant heat output, that contributes to rising the temperature on Earth.
Dark matter vs. the planet temperature
In this article, the dark matter contribution to the heat output of Earth has been calculated, the results depending on various dark matter properties. On the other hand, we know that Earth generates about 47 TW of energy.
We can therefore constrain the dark matter interaction rate (the y-axis in the figure below). For a given dark matter mass (the x-axis on the figure below, 1 GeV being the proton mass), this rate must be such that the total energy output of the planet does not exceed the observations.
[image credits: arxiv]
The two grey areas corresponds to constraints stemming from the properties of the fossil radiation left over from the Big Bang (upper area), and from the underground experiments described above (lower area).
The blue area corresponds to setups generating a too large heat output. We can notice that this new method nicely fills holes in the dark matter search program.
Finally, the red contour is obtained by trading Earth for Mars (which generates at most 3.5 TW of energy).
Take-home message: Earth and dark matter
Earth gets a constant wind of dark matter that goes through the planet. Losing energy in the process, dark matter is slowed down and could be captured through gravity. Such captured dark matter could then annihilate within the planet, and generate an energy flux capable to rise its temperature.
Combining predictions with observations on the heat output of Earth, we get a new handle to discover dark matter, that is actually complementary to other existing dark matter probes.
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