RE: O Dark Matter, Where Art Thou?

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Great article on an incredibly fascinating topic.

So dark matter ignores light as you say, but at the same time bends light due its gravitational interactions which it has in common with its sibling, the standard matter. In a way, similar to what standard matter black holes do, right? Hence the gravitational lenses caused by an otherwise invisible substance. So here is my question. By which observation can it be ruled out that dark matter is nothing but a gigantic network of countless standard matter black holes? If those had formed early on in huge quantities and scales, couldn't that explain the inhomogeneities wich we observe in the visible universe too?

Very much looking forward to this special event ;)

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Thanks for passing by and the question.

So dark matter ignores light as you say, but at the same time bends light due its gravitational interactions which it has in common with its sibling, the standard matter.

In fact, I was a bit sloppy here, for simplifications reasons. Two things are in fact mixed.

  • First, dark matter ignores light in the electromagnetic sense. It is totally insensitive to electromagnetism and therefore does not absorb, reflect or emit light.
  • Second, dark matter interacts gravitationally, and in this way modifies the structure of spacetime (as any massive object does). This is this modification that impacts the trajectories followed by light when it travels into space.

This second point applies to black holes too. They will modify the structure of spacetime and hence (drastically) impact the trajectory of light traveling into space.

Now let's move on with your question.

By which observation can it be ruled out that dark matter is nothing but a gigantic network of countless standard matter black holes? If those had formed early on in huge quantities and scales, couldn't that explain the inhomogeneities wich we observe in the visible universe too?

This hypothesis has been made, and studied. This is what is called primordial black holes. This assumption is not ruled out, but black holes enough are not sufficient to explain all observations. We indeed need a second source of dark matter in this context. On the other hand, the "no black hole" hypothesis is not rejected (at least "yet").

I hope this answers the question and clarifies the situation. Please let me know.

PS: This actually extends what I answered @agmoore, who I tag so that she could get more information.

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Thank your for your answer. I must say I'm almost shocked that this idea had been brought up and discussed already as a serious hypothesis. I can only assume that I some how picked this up when watching one of the many documentaries about black holes without actually remembering. Very interesting.

Right after sending my initial comment, I thought about the following. I assume that after the big bang, the universe first had to cool down below a certain threshold so that black holes could even start to form. Therefore, a precondition for the primordial black hole theory would be that the inhomogeneities can not have started to materialize before the temperature hat fallen below that temperature. So my next question would therefore be, do our computational models produce enough temporal resolution to be sure regarding the order of these events? Or does this thesis still require to simply assume that black hole formation preceded the inhomogeneities?

EDIT: I just skimmed the wikipedia article which you had linked in your answer. Thanks for that! So we are talking about events within the first second of the universe's existence. That's beyond mind-blowing. 😅

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We should never underestimate the wealth of ideas for physics beyond the Standard Model. It is definitely a very wild area ;)

I will try to answer your question, but please note that this lies a bit far from what I do, so that my knowledge on this topic is very partial.

Right after sending my initial comment, I thought about the following. I assume that after the big bang, the universe first had to cool down below a certain threshold so that black holes could even start to form. Therefore, a precondition for the primordial black hole theory would be that the inhomogeneities can not have started to materialize before the temperature hat fallen below that temperature.

That's correct. We need some gravitational collapses for the primordial black holes to form. Note however that this happens well before stars were formed, so that this has nothing to do with the way "usual" black holes are formed. We need obviously a trigger, that either materalises itself as a quantum fluctuation of the energy density of the universe, or relies on a more fancy models (those even exist ;) ).

So my next question would therefore be, do our computational models produce enough temporal resolution to be sure regarding the order of these events? Or does this thesis still require to simply assume that black hole formation preceded the inhomogeneities?

I am afraid I didn't get this question. Sorry... Do you mind clarifying / elaborating?

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