Primordial killers - excluding common dark matter with black holes

3개월 전

Weakly-interacting massive particles (aka WIMPs) are amongst the most popular candidates for dark matter. However, dark matter could also be made of primordial black holes, a special class of black holes that were formed right after the big bang.

[image credits:]

In a recent publication, physicists have explored the strongly-motivated idea of coexisting black-hole and WIMP dark matter. Their results are striking: observing a primordial black hole could rule out most popular extensions of the Standard Model of particle physics!

Primordial back holes and WIMPs

While it explains a large set of cosmological observations, dark matter still escapes detection on Earth. Consequently, there exists a multitude of dark matter models, some of them featuring WIMP and others primordial black hole dark matter.

[image credits: NASA (public domain)]

WIMPS are particles that rarely interact with normal matter and that appear in many theories extending the Standard Model of particle physics.

The weakness of their interactions allows them to exactly match the present dark matter abundance, and justifies that dark matter has not been detected yet.

Another appealing option for dark matter consists in primordial black holes formed in the early days of the universe, as resulting from gravitational collapses in regions where the (inhomogeneous) matter density was large.

Such black holes can however only account for a subdominant fraction of dark matter. In other words, they could be dark matter provided that dark matter has another component. Moreover, the latter could be a WIMP.

Primordial back holes as bullets for new phenomena in particle physics

Let’s focus on black hole detection for a moment.

[image credits: The SXS Project (CC BY-SA 4.0)]

Many experiments, like LIGO, Virgo or the Einstein Telescope, aim to track gravitational waves in a close future. They have hence the potential to directly observe primordial black holes.

On the other hand, we all know that black holes accrete matter.

The WIMP density around a primordial black hole is therefore enhanced, so that WIMP annihilation can take place. This however induce the production of gamma rays in a way strongly excluded by data.

Consequently, if we observe primordial black hole dark matter, WIMP annihilation must simultaneously be small enough to forbid the production of too many of these un-observed gamma rays.

This yields the results below.

[image credits: arxiv]

On this figure, the x-axis corresponds to different WIMP mass hypotheses, whilst the y-axis consists in the WIMP contribution to the dark matter annihilation rate (that is connected to the dark matter abundance).

The viable parameter space (containing all Standard Model extensions featuring a WIMP candidate) lies between the two grey dashed lines. Anything either above the horizontal dashed line at the top or below the diagonal dashed line at the right-bottom is irrelevant.

Let’s now assume that VIRGO/LIGO observe 1 primordial black hole. Any WIMP setup above the solid blue line turns out to be excluded. Taking the Einstein Telescope, anything above the solid orange line is this time excluded.

We can however get crazier. If VIRGO/LIGO observe 80 black holes, then anything above the dashed blue line would be excluded. This means that the entire viable parameter space is almost excluded!

Gravitational waves have hence an immense potential for particle physics!

Take-home message

Primordial black holes consist in a potential candidate for dark matter, provided that dark matter contains another component, like a WIMP. This multicomponent dark matter setup yields the production of gamma rays visible in the entire universe. Such gamma rays are however experimentally excluded.

On different lines, there are options for the potential observation of primordial black holes in a close future, thanks to gravitational-wave detection.

Consequently, observing a handful of primordial black hole could potentially entirely rule out the WIMP hypothesis (and thus a plethora theories extending the Standard Model and featuring a WIMP), as this would imply the existence of experimentally-excluded gamma rays.

Let us thus keep an eye on future gravitational wave observations!

PS: This article has been formatted for the front-end. Please see here for a better reading.


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I'm glad I waited (pressing schedule) a couple of days to read and comment on your latest post. I feel as though I'm sitting in on a senior seminar. The questions and answers are really quite provocative and enlightening.

I don't have a question I can clearly formulate...I'm just going to continue to read about wimps (I love that), Einstein telescopes, and the relationship between dark matter and the production of gamma rays.

Your posts are always mind-opening adventures into terrain I otherwise would not explore.

Thank you!


As said above (or below, it depends on which frontend one looks at ;) ), I really enjoy the comments. I am even starting to feel scary when the first one takes its time to appear ^^

If you have more questions later on, feel free to shoot! I will always be there to answer :)


I might take you up on that :) Thank you!


So, gamma-rays are only considered to be a byproduct of WIMP annihilation if they are undetectable, and, therefore, possible to exclude from data?

Forgive me for this possibly mad question. I was very happy with my understanding until I read:

observing a handful of primordial black hole could potentially entirely rule out the WIMP hypothesis, as this would imply the existence of experimentally-excluded gamma rays.

I need some iced-tea now! :P


Hey Abi! Thanks for passing by (and for this first comment to my post)!

So, gamma-rays are only considered to be a byproduct of WIMP annihilation if they are undetectable, and, therefore, possible to exclude from data?

There are many ways to generate gamma rays in the universe. Dark matter annihilation is one of them.

The point here is that when induced by WIMP annihilation, gamma-ray production has specific properties that could be observed. But we have not observed them. Consequently, such a production mechanism can only occur with a tiny suppressed rate (connected to the y-axis of the last figure).

To rephrase what I said::

  1. Primordial black hole could be dark matter, but under the condition that we have a second dark matter particle.
  2. We assume that this second dark matter particle is a WIMP.
  3. In this case, it has been demonstrated that WIMPs generate many special gamma rays that should have been observed.
  4. We didn't observe those gamma rays.
  5. If WIMP and back hole dark matter coexist, then we have huge constraints on WIMPs as they would generate something unobserved. In other words, the value of the parameter displayed on the y-axis of the last figure has to be small enough to suppress this gamma ray production.

is it clearer?

such a production mechanism can only occur with a tiny suppressed rate (connected to the y-axis of the last figure).

Ok, ok, ok! That’s better.

The numbered list cleared all my doubts and confusion 😃 I got a bit confused today ! Thank you for taking the time to explain things further @lemouth. I truly appreciate it!

Bye for now & happy weekend for the family! ☀️


I am happy this clarified!

Enjoy your week-end as well! Here, it seems fall is starting to really be there... It is coldish (so perfect for construction and gardening) :)


I love the drawing, especially your labcoat! :)


I somehow got an official illustrator ;)

Thanks for sharing amezing blog.

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