[HN Gopher] Heavy particles don't explain gravitational lensing ...
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Heavy particles don't explain gravitational lensing oddities
Author : isaacfrond
Score : 116 points
Date : 2023-04-24 07:54 UTC (1 days ago)
(HTM) web link (arstechnica.com)
(TXT) w3m dump (arstechnica.com)
| jvanderbot wrote:
| Sticking to TFA, and not ancillary information, I found this
| frustrating:
|
| > Instead, the research team ran 75 different models with the
| initial conditions chosen at random. By chance, some of these
| created distortions similar to the ones seen in the real-world
| data, typically affecting only one of the four lensed images. So,
| the researchers conclude that the distortions in the lensed
| images are consistent with a dark matter halo structured by the
| quantum interference of axions.
|
| Essentially the best contender aside from WIMPs is, as subject of
| TFA, axions. And axions have near-random interference and
| standing waves, which essentially scatters about light bent by
| gravitational lensing due to uneven gravity in the lensing
| object.
|
| Essentially, if you randomize the field enough, you can
| eventually find, after enough trials, something that seems to
| match reality. And it just so happens axions are the nice random
| variables we needed to inject.
|
| That's where we're at: Just randomly select some parameters until
| the data fits, and lo and behold, it fits.
| zapov wrote:
| Its not just you: https://www.youtube.com/watch?v=lu4mH3Hmw2o
| jeroenvlek wrote:
| I knew this was Sabine Hossenfelder before clicking the link
| and not having seen the video before (I like her, fwiw)
| mkehrt wrote:
| I can't tell if she's a legitimate scientist or a crackpot.
| Does anyone know?
|
| (I haven't actually watched this video, but I've seen others
| of hers. They seem convincing in general to a nonspecialist.)
| kakapo88 wrote:
| Wild guess: Sabine is not a fan.
| l33tman wrote:
| They claim the parameters that go with canonical particle dark
| matter won't lead to a fit. This is the point, not necessarily
| that they could pinpoint axions in particular.
| jvanderbot wrote:
| So they searched for a set of parameters for axions that
| would essentially be a counter example to WIMPs? That makes
| more sense. But the article is structured as this being
| supporting evidence for axions, which doesn't make much sense
| to me.
| l33tman wrote:
| At least the headline says "No WIMPS! Heavy particles don't
| explain gravitational lensing oddities" and also inside the
| article (near the two colorful figures) they write
|
| "On cosmological scales, WIMPs continue to fit the data
| extremely well. But once you get down to the levels of
| individual galaxies, there are some oddities that don't
| work quite as well unless the dark matter halo surrounding
| a galaxy has a complicated structure. Similar things seem
| to be true when you try to map the dark matter of
| individual galaxies based on its ability to create a
| gravitational lens that warps space so that it magnifies
| and distorts background objects."
|
| and later
|
| "It's a relatively simple thing to do with the WIMPs, since
| there's only one pattern we'd expect: the gradual fall off
| of dark matter levels as you move away from the galactic
| core. The lensing predictions based on that distribution do
| a poor job of matching the real-world data of where the
| lensed images show up."
|
| So I guess the real article (which I haven't read) has the
| hypothesis that the WIMPs fit badly and they try to make a
| case for the axions. Still it is kind of a nice clue to
| show that the WIMP predictions fit badly even if their
| axion solution is weak..
| jmartrican wrote:
| When watching the history of how the idea of dark matter came to
| be, I do not understand how they jumped directly to matter being
| the solution to the gravitational oddities that were noticed.
|
| So here goes the story as I recall. A scientist noticed the
| galaxies spinning faster than they would have expected for the
| amount of gravity produced by the expected matter in that galaxy.
| The galaxies are spinning in a more uniform way throughout... aka
| the outside of the spiral galaxy is spinning faster than
| expected. Immediately I see this as being a gravitational issue.
| Maybe gravity works in ways we do not understand at this level.
| But instead scientists jump directly to "there must be matter
| here that we can't see".
|
| My question is, has the marketing of the phrase "dark matter" got
| ahead of the actual problem and has stuck and really is a synonym
| to "strange gravity issues we are seeing" or have we really
| jumped to matter being the culprit without taking a look at
| gravity behaving in unexpected ways? Is the issue matter or
| gravity, and if it is matter, why does the story of dark matter
| jump from fast spinning galaxies directly to "missing matter"
| without a detailed explanation of why it must be matter, and not
| gravity acting strange?
| plaguepilled wrote:
| I wasn't aware of the galaxy rotation argument for DM - I have
| always heard it argued for using the bullet cluster observation
| for galaxy collision
| (https://en.wikipedia.org/wiki/Bullet_Cluster).
|
| That's not to say DM is the only way to explain such data, but
| it is a good motivation for it.
| Sharlin wrote:
| Bullet cluster came much later and indeed the mass/visible
| matter discrepancy was discovered specifically because they
| were looking for evidence for dark matter. It's just one
| piece of additional evidence for the theory that dark matter
| is specifically matter.
| omneity wrote:
| Seen this way it seems DM theories are a failure of
| applying Occam's Razor on a scientifically complex topic.
| Whereby our current tools and theories cannot explain some
| observed discrepancies, we aim for a more complicated
| explanation for them (special types of matter) rather than
| limitations of our current techniques.
|
| At the same time science doesn't progress by just shrugging
| and saying "we can't know", so maybe it's an inherent
| conflict of interest at play here?
| Sharlin wrote:
| More complicated? That makes zero sense and is some sort
| of a strange cognitive bias against particles (or
| possibly against things we can't directly see?)
|
| When it was found that experiments didn't support
| Thomson's "raisin pudding" model of the atom, the
| solution was not to modify electromagnetism but to
| introduce a hitherto unknown particle, the proton. When
| it was found that the mass of a nucleus wasn't simply the
| sum of the masses of the protons, the solution was not to
| modify theories of inertial mass and gravity, but to
| introduce a hitherto unknown particle, the neutron. When
| it was found that in a beta decay some of the mass-energy
| was unaccounted for, the solution was not a modification
| to Einstein's equations but to introduce a hitherto
| unknown particle, the neutrino, whose existence was only
| experimentally proven quite a bit later.
|
| All our evidence from distinct sub-disciplines of
| astronomy suggest that some specific well-constrained
| type of matter that interacts via gravity but not via EM
| is needed to make predictions match observations. No
| competing hypothesis has been proposed that is as good a
| match. Scientific method 101 says that if something walks
| like a duck, swims like a duck, and quacks like a duck,
| we should assume that it is indeed a duck until and
| unless additional evidence rules out the duck model.
| wholinator2 wrote:
| The issue is that a modified gravitational theory means getting
| rid of general Relativity or at least _significantly_ modifying
| it. As i understand, no proposed modified gravity theory has
| ever been able to be more accurate than GR. Every theory that's
| been proposed (very many for over 100 years) to resolve galaxy
| rotation has then failed at some other scale or situation. Most
| of those theories still require dark matter to explain those
| discrepancies. The single most accurate and explainable model
| currently is general relativity with some extra dark mass
| hiding in extremely large structures.
|
| As well, to do away with GR means ridding ourselves of the most
| accurate gravitational predictions ever made. GR explains
| basically _everything else_. It predicts gravitational waves,
| black holes, the orbit of mercury and the precession of
| rotating bodies, as well as many other things I'm sure. And so
| far every big prediction has come out to be true. In physics,
| predictive power is everything. Most of the modified gravity
| theories can't even explain everything we already see let alone
| make accurate predictions about other things.
|
| It just comes down to math and data. I've had this conversation
| with many people and while i don't want to discourage
| questioning or curiosity, none of these people were very versed
| in the actual topics of modified gravity or modern dark matter
| theories (not saying this about you, but I've had this
| conversation _a lot_). You can Google a Wikipedia article about
| problems with modified gravity, MOND, etc. And read about it
| freely if curious.
|
| The issue i think is that it's too complicated. The science and
| especially the math of these theories is graduate level physics
| and these theories are being proposed and discounted by
| doctorates who've dedicated their entire lives to learning
| about this. It's extremely difficult for regular people with
| jobs and lives to have the time to truly understand the issues
| here, at least until someone comes along who can explain it
| much better than I.
|
| I must say, I'm also not a doctorate in gr or anything, but
| I've been around it a while and have seen the developments
| unfold over time. The short answer is that modified gravity
| theories have more problems than gr + dark matter, at least
| right now.
| naveen99 wrote:
| What about another force that shows up at larger scales that
| is to gravity what gravity is to the strong force ?
| empyrrhicist wrote:
| And what is gravity to the strong force, exactly? I'm
| pretty sure if you solve that one you might win a Nobel
| Prize.
|
| Anyway, positing other forces is a good thought experiment,
| but ultimately that's more complex than LCDM, which is a
| pretty simple kind of thing to potentially exist relative
| to how much it explains.
| [deleted]
| Sharlin wrote:
| If I had a euro every time someone is skeptical of dark matter
| because they think it only predicts galaxy rotation curves, I'd
| have enough money for several beers (beer is expensive in
| Finland!) GRC observations were the original motivator for the
| DM hypothesis, but in the following decades its predictions
| (specifically, those of a specific cold dark matter model well-
| constrained by evidence) have been in entire agreement with
| many observations in several distinct subfields of astronomy,
| whereas no competing hypothesis has been able to achieve the
| same.
|
| Indeed DM, or something very much like it, is a required
| ingredient in cosmology to get a universe even remotely similar
| to ours! As far as we know, we wouldn't even have galaxies,
| much less the intricate foam-like structure of voids and
| filaments, without DM, and no competing hypothesis can predict
| the observed large-scale structure of the universe while also
| explaining galaxy rotation curves and everything else, for
| instance the Bullet Cluster, or the specific observed
| anisotropies in the cosmic microwave background.
| quantum_mcts wrote:
| I've long noticed that this idea - that scientists just "jumped
| to conclusion" that there must be some dark matter to resolve
| some observational inconsistencies - is very prevalent among
| non-specialists. It could not be further from the truth, of
| course - as soon as the rigorous statistical study of galaxy
| rotation curves was provided by Vera Rubin et. al. in the 80s,
| the MOND classes of alternatives to gravity started to appear.
|
| I tried to understand why this narrative is so prevalent, and
| it looks like it is due to its very appealing nature - both to
| the listener and to the speaker: "all those egghead scientists
| can't see an obvious solution to a problem they are fighting
| with for decades". I also noticed that when you get deeper into
| it, then you find that the source - the "speaker" part in the
| whole narrative - is some kind of science popularizer. Who at
| the same time (surprise) is a proponent of his own flavor of
| modified gravity theory.
|
| Truth is - _everyone_ , including me, had this idea as soon as
| they first heard about the DM. If you actually study all the
| evidence and discussions around the subject, you'll see that
| introducing an extra "dark" particle is the most obvious
| solution to the whole collection of observational
| inconsistencies. Modifying gravity in such a way so that it is
| self-consistent and consistent with all the observations... I
| actually don't think anyone completely finished that project so
| far.
| b33j0r wrote:
| When Sabine Hossenfelder and Sean Caroll agree on something,
| I pay attention.
|
| I do take some humility there. We have stunningly brilliant
| contrarians who would be the first to dismantle dark matter
| on the public stage, and instead they tell us, "no, it's a
| real thing. We just haven't seen it in a lab on earth much
| yet."
|
| Now if Eric Weinstein started saying dark matter is actually
| the consequence of a massively egocentric MOND, and everyone
| else is a WIMP, I might start believing that too ;)
| mr_mitm wrote:
| What do Hossenfelder and Carroll agree on? It was my
| understanding that Hossenfelder thinks we need both DM and
| MoND while Carroll is firmly in the conventional DM camp.
| b33j0r wrote:
| You caught me, in that she's 100% open to new evidence.
| Kinda why she is so effective.
|
| So, over the past decade, Hossenfelder made a series of
| videos and talks about "dark matter is real." And then
| "is dark matter real?"
|
| Her opinion evolves. But the conclusions are generally
| that the data points to a measurable thing along many
| lines of evidence.
|
| I find the cosmic web the most compelling, personally.
| Like, what else could that be besides a mystery? You
| can't draw the web in some other configuration, it's a
| real thing somehow.
|
| From my understanding. Her hesitation that she expresses
| is more linked to an underlying intuition: the universe
| is completely deterministic--we just can't see all of the
| rules being computed, and the initial conditions are
| intractable.
|
| I also find that pretty compelling.
| fartsucker69 wrote:
| except its not consistent with all the observations, which is
| why it's not accepted consensus.
| forgetfulness wrote:
| Everyone got drilled into them in school the stories of
| scientists and philosophers clinging to outdated models and
| introducing extraneous elements to extend their life in light
| of contradicting evidence, the piling up of epicycles to
| explain celestial movements by circular motions, or
| luminiferous aether for light to be wave in a fluid medium.
|
| So of course when you first hear "so astronomers found out
| the equations don't work out with the matter we can detect,
| so they figured that there must be uhhh... more matter,
| invisible matter! That exists but you can't detect no matter
| what!" it sounds like those examples.
|
| If you dig in deeper, then you realize that it's not just
| fudging of equations and that there being mass that's very
| sparse and unaccounted-for makes sense.
| xeonmc wrote:
| can it be possible that the galaxy just _appears_ to be
| rotating faster due to time dilation, so from the POV of the
| galaxy itself it doesn 't seem like there is anything out of
| the ordinary?
| gus_massa wrote:
| [It's not my branch, so I have to guess.] They are using
| special and general relativity to make a lot of
| corrections, like red shift and gravitational lens, so
| there is 0% chance that all the community forgot to add
| that correction.
| kmm wrote:
| Galactic rotational velocities are on the order of 100
| km/s, which, though blazingly fast by human standards, is
| so slow compared to the speed of light that special
| relativistic effects are completely negligible.
|
| Moreover, it's not just the absolute magnitude of the
| velocity that is the problem, it's also the shape of the
| curve. Newtonian mechanics (even with the extremely tiny
| relativistic corrections taken into account) predict that
| the rotational speed of a galaxy would decrease the further
| you go out from the center, and we in fact find that this
| speed remains more or less constant.
| bena wrote:
| I mean we're trying to determine the mass of objects many light
| years away.
|
| The concept that we might not be able to perceive everything
| isn't a real huge leap. Far shorter a leap than "everything we
| know about gravity is wrong".
| magicalhippo wrote:
| > Immediately I see this as being a gravitational issue. Maybe
| gravity works in ways we do not understand at this level. But
| instead scientists jump directly to "there must be matter here
| that we can't see".
|
| You go for the most likely explanation first. When I go to my
| car and my keys aren't in my pocket, I go and check the counter
| where I usually put them when I get home. I don't start
| checking behind the toilet or similar.
|
| In this case we had a theory, general relativity, which worked
| very well for all the tests we'd done so far, and there was
| nothing in the theory to indicate it shouldn't be the same all
| over. Meanwhile astronomical observations are difficult and we
| knew, and still know, there's more to particle physics. Hence
| more mass is a more likely explanation.
|
| Keep in mind that trying to replace an existing, validated
| theory can be very hard, since you need to not just explain the
| new stuff, but also pass all the checks the existing theory
| did. So far nobody has been able to come up with a modification
| of general relativity that can do this.
|
| And it's not been for lack of trying[1].
|
| [1]:
| https://en.wikipedia.org/wiki/Alternatives_to_general_relati...
| [deleted]
| mkl wrote:
| There's a lot more to it than galaxy rotation curves.
| https://en.wikipedia.org/wiki/Dark_matter is worth a read,
| especially the Observational Evidence section.
|
| Also, general relativity has had considerable predictive
| success, so replacing it with something else that works the
| same in some situations but differently in others seems more
| complicated than "there's mass here in some form we don't
| understand yet", though people have of course tried to find
| such replacements. Physicists have been trying all the
| hypotheses they can think of and dark matter currently fits the
| evidence best, they didn't jump straight to dark matter or
| fixate on it without considering anything else.
| Beldin wrote:
| > _general relativity has had considerable predictive
| success, so replacing it with something else that works the
| same in some situations but differently in others seems more
| complicated than [...]_
|
| First: you're very correct - we shouldn't expect an easy
| solution.
|
| Second: _precisely_ the same argument applies to Newtonian
| gravity. It was phenomenally successful. The mismatch between
| observations and its model were more easily explained by lack
| in observations - and they were! We found Neptune because of
| that!
|
| That role (of plugging holes in observations) is currently
| being played by dark matter. So chances for dark matter
| existing in some form are good from a historical perspective.
| On the other hand, from that same perspective, chances of
| gravity theory being slightly(1) updated for extreme cases
| are also good.
|
| (1) "slightly" in the sense that the difference with the
| current theory, GR, would be nil or negligible in most cases.
| the__alchemist wrote:
| >general relativity has had considerable predictive success,
| so replacing it with something else that works the same in
| some situations but differently in others seems more
| complicated than "there's mass here in some form we don't
| understand yet", though people have of course tried to find
| such replacements.
|
| Do the models use GR? My understanding is that they make the
| assumption that Newtoninan gravity models are sufficient.
| rich_sasha wrote:
| I believe the rotational speed discrepancy in galaxies is
| simply the most intuitive example. There are many more cases
| where mass appears to be missing:
| https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
| . Crucially, these occur at vastly different scales, from
| individual galaxies to clusters and whole observable universe.
|
| > ...have we really jumped to matter being the culprit without
| taking a look at gravity behaving in unexpected ways?
|
| This is called Modified Gravity, or MOND:
| https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics and
| is well-researched. It stands as a niche but actively pursued
| alternative to Dark Matter. For the most part, Dark Matter fits
| observations better and more parsimoniously.
|
| But of course we don't have definitive proof/disproof for
| either.
| eterevsky wrote:
| Because it's the most economical assumption.
|
| We know that there are various different particles and there
| are some that we haven't discovered yet. There's no reason why
| some particle wouldn't only interact via gravity and weak
| interaction.
|
| Adding some "dark" mass in the simulations paints the picture
| that matches what we see.
|
| Some galaxies were found without these gravitational anomalies,
| which all but rules out a general systemic issues with the
| theory of gravity.
| wholinator2 wrote:
| I'll add, it paints the _entire_ picture (including smaller
| gravitational bodies and new found anomalies) much better
| than other theories
| ajross wrote:
| > I do not understand how they jumped directly to matter being
| the solution to the gravitational oddities
|
| This never happened. "Dark Matter" is just a name for a
| phenomenon. Galaxies rotate as if they have a bunch of matter
| in them that can't be accounted for by looking at the light
| coming from them. So it's "dark" matter. Get it?
|
| No one has a "solution" here. It's a problem, and it needs a
| name. What would you prefer?
| dragonelite wrote:
| Once again "naming things is hard" strikes again :p
| AnimalMuppet wrote:
| So WIMPS don't explain the pattern of lensing caused by one
| particular galaxy, _if_ you assume a vanilla distribution of the
| WIMPS. But we already have situations (the Bullet Cluster, e.g.)
| where we _don 't_ assume that. So... I'm not sure that this is
| all that strong an argument?
|
| And, they argue for axions, because it would be more normal for
| them to assume a more complicated distribution. But they can't
| predict _which_ distribution they would assume. So this looks
| like "it must be A rather than B, if we assume the distribution
| of B, because we _can 't_ assume the distribution of A". That's
| not all that convincing.
| Zamicol wrote:
| I'd like to see more work done with investigating field self-
| interaction.
|
| This 2020 paper by Alexandre Deur is very interesting. "[W]e find
| that relativistic corrections to the rotation curves of disk
| galaxies are significant at large galactic radii."
|
| "Relativistic corrections to the rotation curves of disk
| galaxies" - https://arxiv.org/pdf/2004.05905.pdf
|
| And a blog covering the paper:
| https://astrobites.org/2020/08/17/the-alternative-to-dark-ma...
| AnotherGoodName wrote:
| The fun part about this is that gravity has to either be self
| interacting or capable of propagating faster than light.
|
| If gravity wasn't self interacting it wouldn't follow the
| shortest path that gravity itself creates. Light does follow
| this path. Gravity would move in different directions and at
| different velocities to light across the universe which is full
| of gravitational field if it wasn't self interacting. We have
| seen zero evidence that gravity does propagate differently to
| light. In fact ligo and other gravity experiments have only
| confirmed it's the same.
|
| So gravity is almost certainly self interacting. What does this
| mean? Well it wouldn't change much at small scales. There's not
| enough effect at say planetary scales to really notice. But
| what happens when you have a field self interacting across vast
| distances and involving vast galaxy sized masses? Well your
| link looks at this and yes it matches some of the anomaly's we
| are at large scale.
|
| It's a good theory and I suspect despite being hard to argue
| against it will face opposition since it means a lot of
| scientists were chasing a dead end.
| [deleted]
| Zamicol wrote:
| In my mind, gravity must self interact due to moving at
| exactly the same speed as light (and light is affected by
| gravity). To put this in quantitative terms, the paper
| "Gravitational Waves and Gamma-rays from a Binary Neutron
| Star Merger: GW170817 and GRB 170817A"
| (https://arxiv.org/abs/1710.05834) constrains the "difference
| between the speed of gravity and the speed of light to be
| between -3x10^-15 and +7x10^-16 times the speed of light."
| AnotherGoodName wrote:
| Yep I feel like ligo and Virgo are the Michelson Morley
| experiment of this century.
|
| They have shown something that is not well accounted for in
| current theories. Just as MM showed light is always
| observed travelling at the same velocity Ligo has shown
| gravity is self interacting. Now we need to change our
| models and build mathematical frameworks to account for
| this. It is not acceptable to have a theory that doesn't
| account for this post Ligo.
| HardlyCurious wrote:
| I'm not convinced of that interpretation of MM. MM found
| a signal below the margin of error. I think if they spun
| the table faster the signal would grow in strength. But
| subsequent experiments stopped rotating the assembly at
| all and just relied on the rotation of earth, and thus
| had even slower rotation and even smaller signals.
|
| But at this point nobody is going to revisit MM,
| attempting to do so would immediately ruin your
| scientific reputation.
| oneshtein wrote:
| Some of us have no scientific reputation at all, so we
| are free from the reputation trap.
| HardlyCurious wrote:
| I would love to revisit MM myself, but I don't have the
| expertise to actually setup the experiment. I might be
| able to resolve that gap, but I definitely don't have the
| funding either, and that is a gap I don't know how to
| solve.
| oneshtein wrote:
| My bet is that difference between gravitational waves and
| gamma rays due to the effect known as "surfing". A
| gravitational wave pushes light back from peak, where light
| is slower, to bottom, where the speed of light is normal
| again.
|
| The difference between gravitational wave and gamma rays is
| more dependent on initial magnitude of gravitational wave,
| than on distance traveled.
| dilyevsky wrote:
| There are some papers on this subj from Alexandre Deur.
| Here's a good explanation -
| http://dispatchesfromturtleisland.blogspot.com/p/deurs-
| work-...
|
| It really is a beautiful theory if correct
| codethief wrote:
| > If gravity wasn't self interacting it wouldn't follow the
| shortest path that gravity itself creates.
|
| I don't know if this is the right way to think about this.
| Gravity follows _all_ paths. In a sense, it _is_ the path.
| Or, as Hawking described it[0]:
|
| > [Gravity] shapes the arena in which it acts, unlike other
| fields which act in a fixed spacetime background.
|
| Back to your comment:
|
| > So gravity is almost certainly self interacting.
|
| I'm confused by the "almost certainly". I think this has long
| been established[1], but for entirely different reasons: The
| Einstein field equations are highly non-linear, so of course
| there will be self-interaction.
|
| Or, if you prefer to think of the Einstein-Hilbert action:
| The Lagrangian density L = R * sqrt(det|g|) behaves as L ~
| g-1 * Ric * g2, and the Ricci tensor is Ric ~ ([?]g)[?](g-1)
| + g-1 [?]2g + (g-1)2([?]g)2, so if we view gravity as a
| quantum field theory (effective or whatever), there are
| plenty of self-interaction terms that will prevent the
| N-point function from vanishing for N > 2. Already in the
| weak field approach (where you approximate g = 1 + h) you get
| a triple graviton vertex, see p. 21 of
| https://arxiv.org/abs/1702.00319 .
|
| Maybe we are talking about different definitions of "self-
| interaction here"?
|
| [0]: Hawking: The Nature of Space and Time (1994),
| https://arxiv.org/abs/hep-th/9409195
|
| [1]: See e.g.
| https://physics.stackexchange.com/questions/293873/do-
| gravit...
| nimish wrote:
| Yep, there's a lot of stuff that properly using general
| relativity would address in a much cleaner way than dark
| matter.
|
| Ignoring the relativistic nature and nonlinearity of GR is a
| very large problem with FLRW and other toy models, but they
| have become embedded in cosmology to the point where it's
| heresy to question them.
| jstanley wrote:
| Does that mean some WIMPS Heavy particles _do_ explain
| gravitational lensing oddities?
| gjm11 wrote:
| I _think_ it means that the HN software auto-removes
| exclamation marks from titles, and in this case the original
| title began "No WIMPS!".
|
| (I don't _know_ that the HN software does this. Maybe the
| submitter made a typo.)
| photochemsyn wrote:
| This 2014 Fermilab post on why physicists are hopeful that dark
| matter has some kind of weak-force interaction is a good read:
|
| https://www.fnal.gov/pub/today/archive/archive_2014/today14-...
|
| > "So why would scientists postulate that dark matter experiences
| the weak force? One answer is that if it doesn't, we'll never
| detect it. But that's not a very good answer. A better answer
| involves the Higgs boson. Because the Higgs field gives mass to
| ordinary matter..."
|
| Current status of the Higgs boson decaying to invisible particles
| is, no such evidence has yet been found (2020):
|
| https://atlas.cern/updates/briefing/probing-dark-matter-higg...
|
| This apparent failure (so far) of that hypothesis seems to have
| caused scientists in the field to cast around for other ideas,
| like primordial black holes:
|
| https://news.yale.edu/2021/12/16/black-holes-and-dark-matter...
| d--b wrote:
| I always wonder if dark matter isn't simply made of wrinkles in
| space-time that just don't go back to flat after some weird event
| happens
| jfengel wrote:
| It would have to be a galaxy sized, smooth spherical wrinkle
| that happened to coincide with practically every galaxy. And
| also to fail to interact with other galaxies when they collide.
|
| While we don't know the details of dark matter, we do know a
| lot of things about it. Putative explanations have to cover a
| lot of ground.
| candiodari wrote:
| That wouldn't be so weird as it sounds. From simulations we
| know that it was dark matter that came first, _then_ came the
| galaxies.
|
| So if these wrinkles (they don't have to be very smooth)
| simply had a (large) preferred size and clumped up faster
| than normal matter in the early universe, that would be
| enough.
|
| Neither of those would be particularly weird. When dark
| matter clumped up normal matter was essentially a very low
| density hydrogen cloud, devoid of stars, much less dense than
| a cloud in the sky, which would take (without dark matter)
| some 15-50 billion years to clump up by itself.
|
| As for the size of the dark matter structures, well we don't
| know what they are, so you can postulate any size for them.
| throwawaymaths wrote:
| In the case that dark matter doesn't interact via any force
| other than gravity, then there will be an epistemological
| crisis in physics as your model will be indistinguishable from
| one in which dark matter is a particle.
| XorNot wrote:
| It's the classic failing of everyone's casual "alternative to
| dark matter". It's not X because we can't detect it, what if
| instead it's an undetectable form of Y?
|
| Far too many people get hung up on the idea that in science
| what things "are" matters: because it doesn't. What matters
| is predictive power for future observations - if whatever
| you're proposing doesn't predict anything different to
| "invisible matter" then it's exactly as bad an explanation as
| "invisible matter" which so far is a perfectly good
| explanation for current observations.
| 01648599160 wrote:
| [dead]
| signalToNose wrote:
| Variable speed of light must be considered
| vintagedave wrote:
| I've heard this theory for years but never understood why it
| should be considered viable / possible. Part of me wants to
| believe because it would be really cool, but the rest of me is
| skeptical :)
|
| Can you explain why it's reasonable and what it answers or how?
| Ie why must it be considered here?
| blincoln wrote:
| Does "WIMPS don't explain observed data" also exclude "matter in
| other M-Theory branes exerts a gravitational influence on matter
| in our own" as an explanation for dark matter?
|
| The main argument I've seen against this is that matter
| interacting gravitationally across branes would cause a sort of
| n-body problem condition where everything in a particular sphere
| of influence would orbit continuously around the center of mass
| instead of settling into a lump, but maybe that explains why some
| planet-scale (or larger) objects have spinning cores?
| addaon wrote:
| I don't believe anyone's casting around too hard for an
| alternate explanation for conservation of angular momentum,
| which is a consequence of rotational symmetry of physical laws,
| a relatively intuitive and desired property.
| icapybara wrote:
| "When you hear hoofprints, think horses, not zebras."
|
| You can always contrive a more complicated structure to explain
| some observation. There is no evidence branes exist. We are
| looking for an explanation to the "dark matter problem" that
| doesn't require asserting that a ton of new and unproven
| physics exists.
| sleeplessworld wrote:
| This may point to a key problem in the hypotheses, theories
| and conclusions. When you are in Africa, a "think horses"
| paradigm, may lead you very astray in your thinking.
| peteradio wrote:
| What happened with general relativity somehow explaining the
| "dark matter" observations?
| mkl wrote:
| It didn't? Dark matter in its modern conception was discovered
| after general relativity was established. Dark matter was
| hypothesised _because_ general relativity alone couldn 't
| explain some observations.
| peteradio wrote:
| I was under the impression that it was not fully rigorously
| accounted for because its not exactly a simple calculation.
| So galaxies were still being modeled with newtonian physics
| in at least part of the calculations which neglects field
| self-interactions.
| pfdietz wrote:
| Ordinary matter in the galaxies cannot explain the rotation
| curves. Something dark (that doesn't interact with
| electromagnetic radiation) is needed.
|
| Also, the mass in the universe cannot be all baryonic,
| since that would make primordial nucleosynthesis
| inconsistent with observed abundances of various light
| isotopes.
| Maursault wrote:
| > Ordinary matter in the galaxies cannot explain the
| rotation curves.
|
| The observations of Fritz Zwicky and Jan Oort were of
| spiral galaxies in isolation, which led to the need for
| Dark Matter to explain the rotation curives. Once the
| entire galaxy cluster is considered, it becomes evident
| that a spiral galaxy is not an isolated reference frame
| and that Dark Matter is no longer necessary to explain
| galaxy rotation curves.[1]
|
| [1] https://www.youtube.com/watch?v=PL0ewiwqoTw
| AnIdiotOnTheNet wrote:
| Hey I recognize that name. He's the guy who wrote a book
| about some new cosmology he came up with from making a
| small modification to relativity that he claimed, among
| other things and IIRC, explained the Ganymede Mass
| Anomaly, that black holes were time-reversed white-holes
| from the other (time reversed from us) side of a
| hyperspherical universe, that the expansion of the
| universe is an illusion created by the curvature of said
| hpyersphere, and probably some other stuff I don't
| remember.
|
| His work is attractive to laymen like myself, but any
| rational person has to wonder why, if his claims have any
| validity, he has been ignored completely by the wider
| physics community (which is to say, people who actually
| know shit, unlike most of us).
|
| Your call to "watch the video and you'll never talk about
| galaxy rotation curves again" sounds a _whole lot_ like
| similar statements made about such nonsense as "There
| are no forests on Earth", so if you honestly believe this
| guy should be taken seriously for some reason I would
| suggest a change in approach.
| peteradio wrote:
| You are repeating the same dominant theory and don't
| address that at least some of the excess can be accounted
| for by more rigorous computation...
| pfdietz wrote:
| I should be clear: ordinary matter cannot explain the
| rotation curves with conventional gravity. One can come
| up with ad hoc modified theories of gravity, but none
| appears to be consistent with all the evidence.
| Maursault wrote:
| See my comment above. Watch the whole video. Once you
| understand it, which you will, you will never talk about
| galaxy rotation curves again. Dark Matter may still be
| necessary, as a lot of unexplained has been dumped into
| the theory, but it is no longer necessary to explain
| galaxy rotation curves as a much simpler explanation
| exists, namely, gravitational tidal forces created by
| other galaxies in a cluster, which is very well
| understood and entirely explains curves of galaxy
| rotation and solves the winding problem.
| pfdietz wrote:
| Is there a paper on that? The talk was ten years ago, so
| I assume the idea would have appeared in print by now, if
| the idea actually worked.
|
| I looked under his name on arxiv and didn't find anything
| on this topic.
| Maursault wrote:
| Will a paper be more convincing? I think there's a book.
| Write the author and ask him.[1] But the simple and
| unambiguous explanation of galaxy cluster tidal forces is
| devastating to Dark Matter Theory in regards to galaxy
| rotation curves. Why anyone is still talking about Dark
| Matter as a viable explanation for galaxy rotation curves
| likely has to do with the paradigmatic nature of science
| which resists any change, plus that there are myriads
| that have wasted entire academic and scientific careers
| on Dark Matter.
|
| [1] alex@SensibleUniverse.net
| pfdietz wrote:
| A paper would tell one the thing passed peer review, and
| also let us look for papers publishing rebuttals. If it
| sank without a trace in the refereed literature, that's a
| screaming red flag.
|
| I am very dubious the idea is correct. If objects at
| large distances from the galaxy center have these large
| relative velocities induced by external objects they
| wouldn't be bound to the galaxy.
| bdhcuidbebe wrote:
| Bro just trust youtube kkek
| peteradio wrote:
| You still haven't engaged on the idea that we haven't
| been doing full fidelity calculations based on vanilla
| general relativity and doing so yields non-negligible
| impacts on the lensing apparently. If you are using
| models which were meant to have their numbers crunched on
| an 80s pc then its possible it got too dumbed down. The
| assertion is that the theoretical error bars were
| improperly computed and should have been much larger.
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