Can someone knowledgeable weigh in: is the "dark object" here believed to be a localized blob of dark matter? A dark star or black hole? Or is "dark" being used generally to mean "not bright enough to see at this distance"?
In this context, “dark object” really does mean a localized blob of dark matter, not a black hole or a dim, normal-matter object.
The research team detected it only through its gravitational lensing effect — the way it slightly distorted the light from a more distant galaxy. There’s no emission at any wavelength (optical, infrared, or radio), and its gravitational signature matches a million-solar-mass clump of invisible mass rather than a compact point source like a black hole.
They specifically interpret it as a dark matter subhalo — one of the small, dense lumps that simulations of “cold dark matter” predict should pepper the universe’s larger halos. It’s too massive to be a single star, far too diffuse to be a stellar remnant, and not luminous enough to be a faint galaxy.
So “dark” here isn’t just shorthand for “too dim to see at this distance” — it’s used in the literal physical sense: matter that doesn’t emit or absorb light at all, detectable only via gravity.
Eventually, all the dark matter clumps into rings around galaxies, but since this one is so distant, ~10B light years, so we are seeing that clump as it was that long ago before it difused into it's ring shape we can see in the galaxies around us.
All matter (stuff interacting with gravity) is attracted toward other gravitational centres, however all matter also has momentum, which may tend to carry it away from that centre. Objects don't merely fall directly toward a gravitational centre, but, subject to their initial velocity, orbit it. You may find yourself thankful for this on reflection, as the body you're likely resting on has been in such an orbit for roughly 4.5 billions of years, and will continue to be so for roughly a similar period of time.
If you're sufficiently close to the mass, and/or its radius (relative to your own and your distance from it) is large, as with, say, a stone tossed from ground level on Earth, that orbit will intersect the surface rather quickly.
At astronomical distances, ranging from some significant fraction of the distance between the Earth and Moon to interstellar and intergallactic distances, it's far more likely that an attraction will result in some other form, generally an ellipse (typical of a captured orbit), circle (a perfectly non-eccentric ellipse), a parabola (object moving at escape velocity), or hyperbola (object moving faster than escape velocity).
Ring systems form as multiple masses interact around a larger mass, be that a moon, planet, star / quasi-stellar object, galaxy, or other mass. Until the tangential velocity is lost, the particles within the ring will continue their orbit. Occasional interactions and collisions, as well as radiated energy (including gravitational radiation) may cause a given particle to spiral inwards, or be ejected from, the ring system.
I believe that you have the order of operations misunderstood.
I probably don't know that much more than you about the subject, but from what I understand, the prevailing model suggests that these Halos formed early in the formation of the universe when spacetime had varying "pockets" of density that naturally led to these halos - the formation of the galactic disk therein was actually supported by the halo existing first, because baryonic matter (aka non-dark matter, the stuff that makes up planets, stars, etc) was still too energetic from the formation of the universe to become gravitationally bound to itself.
At this point my knowledge probably pales in comparison to skimming some Wikipedia articles, but my understanding is that there is just so much dark matter concentrated in these halos and inter-galactic structures of it that the gravitational effects of baryonic matter are negligible in comparison.
I believe dark matter comprises something like 80-85% of all matter in the universe.
it does, but it orbits the barycenter (usually the supermassive blackhole of the host galaxy), but since it can pass through itself its orbital energy doesn't decrease from "drag" as it's falling through itself and normal matter
Normal matter also makes halos or rings around the center of the galaxy. That's how gravity works. And since dark matter interacts less, it stays more spread.
Halo implies empty (or low density) at the center. The 'normal' matter is denser at the center of a galaxy. I'm trying to understand why the difference.
Did a bit more reading. I was thinking of a halo like an angel's halo, a disk with greater density near the edge and less at the center. But it seems that dark matter halos are roughly spherical with greatest sensity near the centre. In which case halo seems like a pretty poor name.
I wonder when exactly saints halos changed into rings in religious images. The older ones are all, well halo-shapped, but currently the only image people think of are the rings.
"Halos in religious art began transitioning from spherical or radiant forms to flat, ring-like discs during the early Renaissance, around the 14th to 15th centuries."
I'll be straight up and tell you we have no real idea... We don't entirely know what this is. Our theory of dark matter started as "We are missing a large amount of matter in order for galaxy's to form as they do". To "We can detect this matter in rings around galaxy's that bend light, and is acted on by gravity but nothing else." and now. "We can find these pre-"historic" clumps of dark matter before they are decreted into discs".
None of this tells us what this "matter" actually is.
Dark in the context of astrophysics means specifically that the object/matter does not interact directly with electromagnetic radiation (eg absorb an optical/microwave/radio photon). So it is probably dark matter, but probably unlikely to be a black hole because we can typically detect a black hole's effects in an indirect manner :P
"dim" implies "there is something normal there that is just not emitting light". "dark" in this astronomical sense means essentially "dim and completely transparent" which is not what you get with e.g. a cold gas cloud - those are opaque.
It's not just that. Remember that in space distance doesn't attenuate electromagnetic radiation. Given perfect line of sight, you could broadcast a 1mW 5GHz signal across the empty space between galaxies and have perfect reception (provided you're very patient)
One also has to consider that at this scale, you cannot have a normal interaction with the EMF and be dim. The normal physical processes of matter at the scale of 1 million suns ends up being quite loud. Black holes that aren't actively eating things form an exception, but black holes aren't normally dark either. Whatever this is it's peculiar, but I wouldn't write-off that it might be an issue with the model they developed for interpreting the data.
From the paper, it could be the dark-matter halo of an otherwise too faint dwarf galaxy. They state that a “more definitive statement on what type of object [it] is will require deep optical/infrared observations to detect any potential EM emission”.
I'm an amateur but I feel confident enough to answer -- hopefully not a mistake!
They're explicitly looking for "Dark Matter", which doesn't "interact" with normal ("baryonic") matter or electromagnetic radiation (e.g. light). So it's not a black hole for sure, as those are composed of regular ol' matter.
RE:"dark star", that's really up in the air, I'd say! AFAICT the only academic reference to that term is for normal stars influenced by dark matter[1], but kinda the whole problem here is that we don't know much about what dark matter is composed of or into. Certainly it's not going to be a star in the traditional sense as it can't emit light, but I'm not aware of any reason this object can't end up being a giant sphere.
FWIW, Wikipedia says "One of the most massive stars known is Eta Carinae, with 100–200 [solar masses]", whereas this object "has a mass that is a million times greater than that of our Sun". If we're going to use metaphors, I think "dark dwarf galaxy" might be more appropriate?
I was always surprised that when we talk about BHs mass, charge, and spin that we really mean U(1) (electromagnetic) gauge charge and not charges from global symmetries. (If BHs had global charge, you could at least say that this or that black hole was made out of N baryons, or whatever.)
But it's really so---according to GR, black holes don't have global charges. So even if you see a star made out of baryons collapse into a black hole, once the BH settles down into a steady state you can't say it's "really" got baryons inside: the baryon number gets destroyed.
(Of course, a different model of gravity that preserves unitarity might upset this understanding.)
Basically, the event horizon is the event horizon is the event horizon. If two non-steady-state matter/anti black holes merge (like before everything has hit the singularities), it will cause explosions inside the BH, but energy is mass is energy is mass, so for an external observer it's indiscernable. It will look no different from two all-matter BHs merging.
I mean, I included a disclaimer... But regardless, you appear to be wrong on both counts (or at least contradicting Wikipedia):
1. "The presence of a black hole can be inferred through its interaction with OTHER MATTER and with electromagnetic radiation such as visible light." https://en.wikipedia.org/wiki/Black_hole
1. Your argument is about the grammar of a sentence about black holes on Wikipedia? This isn’t some kind of gotcha.
2. I missed the dwarf part, but think about what you’re arguing: the mass range of a loosely defined category (the lower bound of a few thousands is not one I’ve ever heard, btw) that has nothing to do with the paper in question. Collections of stars of any kind produce light. This doesn’t. What are you saying?
which doesn't "interact" with normal ("baryonic") matter
I think you mean it doesn't interact electromagnetically with either matter or radiation. It does interact with normal matter via gravity -- that's pretty much the strongest (only?) argument for its existence.
I'm not aware of any reason this object can't end up being a giant sphere
AIUI, most theories posit that solid spheres of dark matter are very unlikely because matter accretion is governed by electromagnetism in addition to gravity, and dark matter is not supposed to obey the former. Most models assume that dark matter is organized in gaseous clouds (halos); strictly speaking that's still a giant sphere, just not in the same way that Jupiter or the Sun or even the Oort Cloud is.
This confused me too from all those solar object size comparisons I’ve seen. Turns out there are stars that are 1000s of times bigger than the sun, but they aren’t the same density.
I'm unaware of any stars in the 1000 Msun range. Wikipedia puts 291 Msun of R136a1 at the largest. After that, 195 M of R136a2 is the next. A star at 100 Msun would be in the most massive stars known.
“ A number of the "stars" listed below may actually be two or more companions orbiting too closely for our telescopes to distinguish, each star possibly being massive in itself but not necessarily "supermassive" to either be on this list, or near the top of it. “
“ More globally, statistics on stellar populations seem to indicate that the upper mass limit is in the 120-solar-mass range,[1] so any mass estimate above this range is suspect. “
There are good theoretical reasons why a star shouldn’t normally get as big as the ones on the top of the list. Long story short: they’d very quickly shed mass due to their intense luminosity. Some of them might even be boiling with bubbles of pure radiation.
Definitionally, yes. It’s inert but lenses light around it.
The paper is more about the technical achievement of detecting it, IIUC. It’s not the first dark matter inference we’ve had, and doesn’t really tell us anything new about the stuff.
It challenges warm dark matter and ultralight dark matter theories because they'd be less likely to clump into something so small. Similarly MOND would have trouble explaining a completely isolated chunk of it at this size (any baryonic matter trapped in a region this small would almost certainly emit enough light to detect).
I’m admittedly a few years out of date in this, but weren’t those already kinda ruled out? I’ve never met anyone who took MOND seriously - it looks like it’s a pet project of a small number of people who cite each other, and people in different subfields have always been saying it doesn’t work for them (diffuse galaxies, etc.).
I know the current models favor cold DM, I thought the hot DM model was abandoned already when it became clear that clusters of any size exist?
Your assessment is spot on, but for whatever reason it's extremely popular with amateur physicists. The HN crowd likes it a lot, too. Almost every thread about dark matter has at least one comment that goes like "I'm not a physicist but dark matter always seemed like a cop out to me and will go the way of the luminiferous aether". I'm surprised that we're not seeing them here, perhaps because MOND can't explain this.
And yes, hot dark matter has largely been ruled out, but there are still extensions into warm dark matter and ultralight dark matter that seem more manufactured but are still plausible. The observations in this paper creates some additional challenges for those theories.
But yes, CDM is what most researchers expect, by a large margin.
They found a statistical anomaly that they're trying to atrribute to new physics, using some novel maths. So a tiny speck of evidence towards a new theory of matter (i know nothing about astro, just my supposition)
