Gravity Without Mass: UAH Study Proposes Alternative to Dark Matter
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> Wouldn’t these black holes radiate some Hawking radiation, making them potentially detectable?
Yes, according to widely accepted theory, they do. However, Hawking radiation is incredibly weak. With current technology, we are not able to detect such a faint signal.
> And how long would they exist for?
This depends on the size of the black hole. They will last much longer than our solar system, many billions of years, until they eventually evaporate.
> Also, what keeps these black holes from interacting with normal matter and clumping in the center of galaxies?
They do interact with normal matter via gravitation. If primordial black holes exist, many of them would fall into the supermassive black hole at the center of our galaxy and other galaxies. However, there are also those that orbit around the center. The problem is that our telescopes currently cannot detect a tennis ball-sized black object with the mass of a few Earths, it is an incredible tiny black dot in front of a black background. Hence, they are literally dark matter.
PS I have zero knowledge in this area, it’s a serious question.
https://www.youtube.com/watch?v=qy8MdewY_TY&t=67s
Still possible, but increasingly unlikely due to the narrowing size range in which these black holes are allowed to exist in.
Dark matter is non-collisional, doesn't decay, and doesn't strongly scatter radiation. The last is about gravitational microlensing. You address the first two in another comment, but focused on falling into central black holes of galaxies and Hawking radiation. Black holes merge (cold dark matter doesn't clump, and certainly the absorption cross-section of CDM does not grow with clumping in the event there is some "dark chemistry" in the CDM sector; black holes absorb more radiation as their masses increase, including via merger). The MACHO (massive compact halo object) models which use small black holes have to keep them very sparse and on a restricted set of orbits in order to keep them from merging in galaxy and galaxy-cluster halos where they would be detectable. It's also hard to have galaxy-cluster distributions of small black holes: the density profiles get very different as tiny black holes interact with each other. Additionally, in distributions around the central-bright-galaxy of clusters, it's hard to suppress inverse compton scattering X-rays from dust scattering around (or forming accretion structures around) tiny black holes. So the signature radiation a tiny black hole MACHO model has to worry about is less Hawking than the behaviour of normal matter moving around black-hole-infused space in clusters like Abell 2764 <https://www.esa.int/ESA_Multimedia/Images/2024/05/Euclid_s_n...> or Abell 2744 <https://esahubble.org/images/heic1506c/> <https://en.wikipedia.org/wiki/Abell_2744> for example. It's really hard for simulators to arrive at this kind of structure using mainly MACHOs, and there seem to be lots of these structures. Lots and lots.
It's somewhat easier to do with ~solar mass and bigger black holes (they can be sparser, their orbits are thus less contrived to avoid collisions -- after all star-star collisions are rare unlike dust-dust collisions), although larger-mass BHs run into limits form gravitational microlensing. In particular, BHs of 100 or more solar masses would tend to disrupt wide stellar binaries, particularly in at the outer margins of galaxies (including our own, easy enough to look for out of the plane of the disc). And there are strong limits on BHs between about 10 and 2000 solar masses from ultra-faint dwarf galaxies, where domination at that mass of BH would expand the visible part of those galaxies (especially bright star clusters like the central one in the Eridanus II dwarf satellite <https://en.wikipedia.org/wiki/Eridanus_II>) via dynamical heating.
The MACHO idea was certainly plausible, and projects like OGLE and MOA developed microlensing-observation techniques which are now used for finding low-mass and wide-orbit exoplanets, and "orphan" Neptune-like planets ejected from their birthplace star systems. See for example https://kmtnet.kasi.re.kr/~ulens/
However, there seems to be very little hope that black holes are a large fraction of whatever the detailed description of dark matter might be.
That there are plausible mechanisms for forming primordial black holes at arbitrary masses does not mean they are actually there.
Why have we concluded that it is matter and not dark energy.
Let's momentarily ignore the fact that the name dark energy was already taken for the force causing the universe to expand.
But can't both be attractive force and expansionary energy be different types of dark energy?
Radiation redshifts as the universe expands, but "matter" "just" "spreads".
Imagine a basketball player bouncing a ball up and down at a fix frequency. The wavelength is inversely proportional to the frequency.
The wave here is the ball bouncing up and down.
Now imagine the moment the ball hits the ground they start moving away from you at a fixed speed.
Because they have accelerated, light now needs to travel some additional distance since the ball at the top is further away from you than the ball at the bottom was.
But their new frequency remains the same because the displacement is now accounted for in the wavelength.
If they stop moving, opposite happens and it blueshifts.
If they keep accelerating, the distance increases and so does the redshifting.
... and behaves like matter
But it doesn't sound that convincing to replace dark matter due to lack of direct evidence, with something even more exotic which also has no direct evidence
Its not a valid concept, but its a new idea, something to test, anything is better, then collecting data and watching the model drift away every day like a ghost in the night.
Sure, this could be just coincidence of very similar galaxies, need more observations.
The only the matter portion of the universe is 85% dark matter (26.8% of mass-energy content of the universe).
As for the question; I don't think it's right to assume that dark matter really is exotic matter. It's just a set of observations. It's something that behaves like matter (with respect to density scaling) but only seems to interact gravitationally.
Some theories posit that it's some unknown particles, others that they are primordial black holes, and some that gravity just behaves differently (MOND).
I really enjoyed Dr Angela's videos on Dark Matter [1,2]
[1] https://www.youtube.com/watch?v=qS34oV-jv_A [2] https://www.youtube.com/watch?v=PbmJkMhmrVI
I've never heard anyone but Collier put forward this perspective on "dark matter", and I can't recall any other physicist using the term in a way consistent with that perspective. Frankly I think she was off base. Her videos are fun but her takes on terminology are often pretty bad.
It’s a set of observations about “something” that behaves like matter[1], and interacts only gravitationally. There are many theories proposed but none has any real answers or solutions. The following is taken from Cern[2]:
> Many theories say the dark matter particles would be light enough to be produced at the LHC. If they were created at the LHC, they would escape through the detectors unnoticed. However, they would carry away energy and momentum, so physicists could infer their existence from the amount of energy and momentum “missing” after a collision. Dark matter candidates arise frequently in theories that suggest physics beyond the Standard Model, such as supersymmetry and extra dimensions. One theory suggests the existence of a “Hidden Valley”, a parallel world made of dark matter having very little in common with matter we know. If one of these theories proved to be true, it could help scientists gain a better understanding of the composition of our universe and, in particular, how galaxies hold together.
And thus at least whoever wrote this and the people who approved that publication agree with the notion that it is observations for which no concrete theory exists that explains them.
[1] i used “behaves like matter” in the sense that follows this definition:
> In standard cosmological calculations, "matter" means any constituent of the universe whose energy density scales with the inverse cube of the scale factor, i.e., ρ ∝ a−3 . This is in contrast to "radiation", which scales as the inverse fourth power of the scale factor ρ ∝ a−4 , and a cosmological constant, which does not change with respect to a (ρ ∝ a0). The different scaling factors for matter and radiation are a consequence of radiation redshift: For example, after gradually doubling the diameter of the observable Universe via cosmic expansion of General Relativity, the scale, a, has doubled. The energy of the cosmic microwave background radiation has been halved (because the wavelength of each photon has doubled);[51] the energy of ultra-relativistic particles, such as early-era standard-model neutrinos, is similarly halved.[d] The cosmological constant, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration.[52][e] In principle, "dark matter" means all components of the universe which are not visible but still obey ρ ∝ a−3 . In practice, the term "dark matter" is often used to mean only the non-baryonic component of dark matter, i.e., excluding "missing baryons". Context will usually indicate which meaning is intended.
She agrees that Dark Matter is observations or a problem for something that seems to be matter from our definitions of matter and its interactions.
She didn’t claim that MOND is the only theory or answer — in fact she made a video about this exactly, that she believes she failed to adequately argue that DM is what we are discussing because people put too much emphasis on MOND or thought of it as “the only solution to the DM problem”.
She did that too.
She didn’t claim Mond is true, neither did I, and as far as I can understand you, neither did you.
If you assume me or her did that, then you’ve grossly misunderstood mine or her remarks.
Well I'm glad we're clear on that. But then why did you feel the need to say
> She didn’t claim that MOND is the only theory or answer
As if that was a question anyone was asking?
https://en.wikipedia.org/wiki/No-hair_theorem
applies, once the black hole is formed, you can't tell which kind of matter was involved in its creation.
If it could interact it would be able to shed its momentum through collisions like normal matter, and thus fall towards the center of the galaxy and clump like normal matter.
Instead the gravitational evidence, like galaxy rotation curves, indicates that the dark matter is spread out in a huge diffuse halo around the galaxies, several times the radius of the visible galaxy.
Thus in the same way, dark matter would be unable to form a "dark accretion disk" around a black hole, where it could shed it's momentum in order to lose enough orbital velocity to fall into the black hole.
Thus the only dark matter that would enter would be that captured during formation and that which hits it directly like a meteor, so the ratio would be significantly lower.
This of course assumes dark matter is a particle (or several), which is the current leading hypothesis but it's by no means settled yet.
At least that's my understanding.
But yes. If there's a negative mass we learn to control, possibility of FTL drive goes from "Absolutely not" to "Umm, maybe?".
Is "negative mass" a real thing in Physics or just some wild theory?
https://huntsvillebusinessjournal.com/news/2023/07/30/solvin...
(Generally, not Stage 1 denialists, but various later stages of "it's not so bad" or "it's not our fault". At least one is also a creationist.)
There's a suprisingly large number of people in theoretical physics dabbling about with gravity, quantuum gravity, dark matter, et al.
Not all of them have a uTube channel.