# Black holes are obvious answer to dark matter, why is that wrong? [duplicate]

Primordial Black Holes as dark matter candidate. I think this is interesting and new, in that there is a window of PBH masses (link from Count Iblis) And the first GW event from Ligo is right in that window.

This seems to be back in the news, (Well my latest copy of science news) Searching brings up previous papers (all behind pay walls.)

There have been some previous question here, and, more.

This seems like a nice solution, I'm mostly looking for other arXiv articles on the subject. Or reasons why it can't be so.

I edited my title to be a question.
Here's a link from one of my references.

## marked as duplicate by ACuriousMind♦, heather, John Rennie black-holes StackExchange.ready(function() { if (StackExchange.options.isMobile) return; $('.dupe-hammer-message-hover:not(.hover-bound)').each(function() { var$hover = $(this).addClass('hover-bound'),$msg = $hover.siblings('.dupe-hammer-message');$hover.hover( function() { $hover.showInfoMessage('', { messageElement:$msg.clone().show(), transient: false, position: { my: 'bottom left', at: 'top center', offsetTop: -7 }, dismissable: false, relativeToBody: true }); }, function() { StackExchange.helpers.removeMessages(); } ); }); }); Sep 1 '16 at 6:15

• Can you summarize the latest information and ask a question? Not supposed to be simply asking for more references. – Bob Bee Aug 31 '16 at 16:12
• @BobBee, thanks, I certainly don't know enough about the subject to summarize it. I changed my title to ask a question. – George Herold Aug 31 '16 at 23:22
• You have not really explained how this is not a duplicate of Why can't dark matter be black holes? - you mention the question as a previous question, but it's not clear to be how this one is different. – ACuriousMind Aug 31 '16 at 23:44
• @CuriousMind The new papers he refers to claim that 20 to 100 solar mass BHs would evade the constraints noted in that question around 2012. The new papaers seem to be somewhat motivated by the fact that the merged BH that was observed in 2015 was from BHs in that mass range. The new claims have observational consequences like that mergers of those kinds of BHs come more from DM areas (halos, etc) than visible matter areas. A few years to invalidate. The source of those also requires some tuning on inflation, see the reference from Iblis below. Possible, for me still threading the needle. – Bob Bee Sep 1 '16 at 3:50
• @ACuriousMind, reading this paper, arxiv.org/abs/1603.00464, it seems to suggest that there is a window of BH masses that could still exist. That is interesting mostly because the first GW event was of two ~30 solar masses. Obviously we need more data, and a few more LIGO's spread around the world would give us some directional information. – George Herold Sep 1 '16 at 14:59

As the article points out, there is a window between 20 to 100 solar masses available for black hole dark matter to evade the microlensing constraint mentioned in Sean Lakes's answer. Now, the possibility of primordial black hole dark matter has traditionally been considered for very small black hole masses. The idea was that after the big bang small mass black holes could have formed. Black hole masses that are very light, the mass of the Earth or lighter, would be strong gamma ray emitters via the Hawking process. If most of the dark matter mass were in the form of such light black holes, then that would inconsistent with the observed gamma ray background, so we can rule that out.

Larger mass black holes are ruled out via the microlensing constraint, but this leaves open the window between 20 and 100 solar masses. The question is then how such black holes could have formed. You cannot start out with the conventional low mass primordial black holes to get most of the dark matter mass in the 20 to 100 solar mass window via merger processes. But there are possible mechanisms to directly form heavy black holes during the inflationary era, see e.g. this article.

• Your reference ('this article') still seems to have significant constraints. As they state towards the end, the 20 to 100 solar masses possibility evades the constraints known if the counts of those were established by mergers, form a factor of 1000 times fewer. The merger mechanism for the factor of 1000 seems to me to be a large unexplained source uncertainty. Is this correct or did I miss something? – Bob Bee Sep 1 '16 at 3:27

The class of dark matter theory you're talking about are called Massive Compact Halo Objects (MACHOs). The short version is: when a MACHO passes in front of a star in a background galaxy, primarily the LMC and SMC, the gravity of the MACHO will lens (or microlense) the light, causing a brief magnification. When we watch for events like this, we don't see them as often as we'd expect for dark matter to be made from objects in the range of masses that astrophysical processes we understand can produce.

• So there is still a window for big black holes in the halo, a simple ground search for bigger/fewer microlense (ing) events might rule this out too. – George Herold Sep 1 '16 at 1:15