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Comment by awb | Hacker News Reader

Or, maybe dark matter and black holes are the same thing?

https://www.sciencedaily.com/releases/2021/12/211220120813.h...

Maybe these disperse galaxies have little to no black holes?

Dark matter made out of black holes would be upsetting to cosmologists, because they need for the universe to be only 4% "hadronic" matter. If the black holes are made of hadrons, then there is too much of that.

But maybe the stock of primordial black holes started as Dark Matter particles. They then have the problem of explaining why so much of the Dark Matter started out in or close to a black hole. Maybe each Dark Matter particle is so dense it is a black hole all by itself?

You have to be careful talking about what a black hole is made of. Stuff that falls in only has a description before it falls in. After it falls in, it can't be thought of as made out of anything, anymore. So if black holes were part of the initial universe, there is no saying which flavor of matter they represent. After things have settled down, these black holes could proceed to swallow any amount of regular matter, and any amount of "other stuff", if there is any.

The assumption is generally that the "other stuff" has difficulty cooling off enough to be sucked wholesale into black holes, unlike regular matter that is happy to radiate away a big fraction of its energy as photons, leaving the rest able to collapse and form planets and stars, and thence perhaps be gathered into the nearest black hole. Meanwhile, the Dark Matter can mill around a black hole's vicinity indefinitely, maybe swooping close and getting ejected, but otherwise hardly bothered unless it blunders straight in.

Where does the 4% number come from?

This is how (https://news.ucr.edu/articles/2020/09/28/scientists-precisel...):

[Dr. Mohamed Abdullah] explained that one well-proven technique for determining the total amount of matter in the universe is to compare the observed number and mass of galaxy clusters per unit volume with predictions from numerical simulations. Because present-day galaxy clusters have formed from matter that has collapsed over billions of years under its own gravity, the number of clusters observed at the present time is very sensitive to cosmological conditions and, in particular, the total amount of matter.

That article links to paper, co-authored by Abdullah, Anatoly Klypin, and Gillian Wilson, which concludes that "matter makes up 31.5±1.3% of the total amount of matter and energy in the universe" and "80% of matter is actually dark matter". So 20% of 31.5±1.3% is 6.3±0.26%, which is how much normal matter is in the total amount of matter and energy in the universe.

I realize that's different than the 4% number given by the other commenter. This is the latest estimate I could find. Maybe the estimated ratio of dark matter has changed with newer measurements, or maybe the other commenter was working from older estimates.

Yes, the number -- 4%, 6%, what have you -- has bounced all over in recent years. Probably 6% is more current than 4%.

One of the problems with the standard model of the early universe is that whenever an observation -- e.g. mix of light-element isotopes, expansion rate, simulations -- produces a number different from its predictions, its prediction has been easily adjusted to match.

That's how the scientific method works. You come up with a hypothesis, you test it, you adjust it accordingly, and you re-test it. If you can't come up with tests for it, or if your measurements never match the hypothesis, you throw the hypothesis out. That's why string theory and MOND are getting crapped on in this thread.

If they're using baryon acoustic oscillations and the cosmic microwave background, theyre describing the amount of hadrons in the universe after these extra ones would have already become black holes, and be counted separately as "starting" as dark matter.

A black hole enthusiast then needs to figure out why those black holes would form before big bang expansion

Thanks for this. The galaxy formation seems to give the 30% matter, where does the 80% dark matter come from? I get that you measuring the density of stuff that has gravity to see how many galaxies form, but how do we know there's not more dust than we'd expect to make up for that missing 80%?

The 80% number comes from the Dark Energy Survey (https://astronomynow.com/2017/08/05/survey-reveals-most-accu...):

DES scientists used two methods to measure dark matter. First, they created maps of galaxy positions as tracers, and second, they precisely measured the shapes of 26 million galaxies to directly map the patterns of dark matter over billions of light-years, using a technique called gravitational lensing.

To make these ultraprecise measurements, the DES team developed new ways to detect the tiny lensing distortions of galaxy images, an effect not even visible to the eye, enabling revolutionary advances in understanding these cosmic signals. In the process, they created the largest guide to spotting dark matter in the cosmos ever drawn (see image). The new dark matter map is 10 times the size of the one DES released in 2015 and will eventually be three times larger than it is now.

There's more info here:

https://news.fnal.gov/2021/05/dark-energy-survey-releases-mo...

And here:

https://www.darkenergysurvey.org/

Dust accounting for the missing 80% of matter and made out of normal matter would be visible in these surveys. It would block, reflect, or emit light. But it isn't visible despite bending light around it—and that's the dictionary definition of dark matter.

Awesome, thanks so much for the informative answers!

You're welcome.

Something I've always been curious about is how energy relates to black hole formation with regards to Schwarzschild radius and the early universe. How did the early universe not collapse into one giant black hole? Are mass-energy and density the only factors affecting formation or does the type of energy have an effect? For example, suppose I have a mass more than sufficient to collapse into a black hole. Now suppose that the mass is split in half and each half has a very high momentum energy directly away from the other half. Do they fly apart or does the momentum energy simply increase the mass-energy of the black hole?

Put a pile of sand on large deflated ballon. Inflate ballon. Stuff will get further apart and thinned out.

It’s more that we had infinite density. Then space itself spread out.

Oversimplified disclaimer.

Space is "able to" expand/inflate faster than light. So, the universe started as a black hole, but space expanded/inflated fast enough to push almost everything out past the event horizon. Maybe there wasn't enough matter left in any one place to be a black hole anymore? Or, any already sufficiently dense clumps kept their event horizons, and became the primordial black holes.

Or, maybe there wasn't anything until there was enough room for it to be in. Then you don't need a black hole.

Cosmologists like to toss about "expanding universe" and "inflation" as if they mean something, but they really are like gravity to Newton: a name for a process that can be described, but is otherwise wholly incomprehensible.

Not an expert; please correct me if anything is wrong.

My understanding is:

1. there needs to be somewhere to escape to, for the schwarzschild radius to make sense, and there's no "outside" to the universe

2. we don't know how big the universe is or what it looks like; if it's infinite and uniform then there won't be any central point for a singularity

3. as others have mentioned, inflation is a get-out-of-black-hole card

4. the physics of the early universe aren't necessarily the same as physics now

This was one of the leading theories for what dark matter is for quite some time. The term is "MACHO" or Massive Compact Halo Object. MACHO doesn't include just black holes, but also a sea of objects small enough to be very difficult to observe but massive enough so that in aggregate they can cause what we observe as dark matter.

However, the evidence against MACHOs is pretty strong. Once we had a theory of what to look for, experiments were designed to detect them. The thing we looked for is microlensing: Focus a telescope at some stars, and watch for a sudden, short jump in brightness. This can be caused by a massive object passing by in front of the objects, gravitationally lensing the light. We performed these studies and the number of microlensing events that we found were not sufficient to explain dark matter. So MACHOs as an explanation for dark matter isn't the answer.

It's a good suggestion though, and like I said it was a leading theory for a while until we were able to test it.

https://en.wikipedia.org/wiki/Massive_compact_halo_object

The big problem with that hypothesis is that you have to explain why we see so little matter falling into these blackholes and they don't seem to: dark matter you can basically pass right through and have your trajectory disrupted a little, whereas a black hole once you hit it becomes a zone of infinite attraction - i.e. a black hole passing near a star is pretty likely to setup in orbit around it because any matter that gets close enough inevitably gets sucked in. Dark matter - having finite mass and no singularity - doesn't have this problem.

It doesn't matter, you are assuming a dark matter theory to begin with here. You are just assuming the dark matter is made up of black holes. This is an established idea of dark matter that has been given serious attention at times, though it's never been a leading candidate that I know of.

Black holes as dark matter has been mostly ruled out by lensing surveys in my understanding.

That's right. Those are called massive compact halo objects (MACHOs). From "Dark Matter: A Primer" (https://arxiv.org/abs/1006.2483):

To hunt for these objects two collaborations, the MACHO Collaboration and the EROS-2 Survey, searched for gravitational microlensing (the changing brightness of a distant object due to the interference of a nearby object) caused by possible MACHOs in the Milky Way halo. (Other collaborations have studied this as well, such as MOA, OGLE, and SuperMACHO.) The MACHO Collaboration painstakingly observed and statistically analyzed the skies for such lensing; 11.9 million stars were studied, with only 13-17 possible lensing events detected. In April of 2007, the EROS-2 Survey reported even fewer events, observing a sample of 7 million bright stars with only one lensing candidate found. This low number of possible MACHOs can only account for a very small percentage of the non-luminous mass in our galaxy, revealing that most dark matter cannot be strongly concentrated or exist in the form of baryonic astrophysical objects. Although microlensing surveys rule out baryonic objects like brown dwarfs, black holes, and neutron stars in our galactic halo, can other forms of baryonic matter make up the bulk of dark matter? The answer, surprisingly, is no, and the evidence behind this claim comes from Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB).

That paragraph references "The SuperMACHO Microlensing Survey," "The MACHO project: microlensing results from 5.7 years of large magellanic cloud observations," and "Limits on the MACHO content of the galactic halo from the EROS-2 survey of the magellanic clouds," if you feel like reading a bunch of physics research papers.

Machos are a certain mass of black holes, aren't they? Or does it include small but dense ones too? I suppose we'd see those on the microwave background though if hawking radiation holds true for small black holes

Not just black holes, but neutron stars and brown dwarfs, too. These studies ruled out MACHOs with masses in the range of 10^(−8) to 100 solar masses. That's 1/3rd the mass of the Moon to five times the mass of Betelgeuse.

For black holes to be visible due to Hawking radiation against the cosmic microwave background, they would have to be pretty small, about 0.8% of the mass of Earth or about the mass of the Moon (https://arxiv.org/abs/astro-ph/9911309). I don't think anyone's found such a small black hole anywhere, yet. If they did, it would be a pretty big discovery since such a small black hole would have to be primordial.

As an aside, "dense" doesn't really make sense in the context of a black hole, which has infinite density.

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