[HN Gopher] New dark matter map reveals cosmic mystery
___________________________________________________________________
New dark matter map reveals cosmic mystery
Author : tooltower
Score : 66 points
Date : 2021-05-27 17:29 UTC (5 hours ago)
(HTM) web link (www.bbc.com)
(TXT) w3m dump (www.bbc.com)
| fifticon wrote:
| I am aware I have almost no knowledge about dark matter in
| physics, but whenever it's mentioned, I can't help but think that
| our modern 'dark matter', is the 'ether of the 21st century'.
| That is, an interrim scientific theory that will eventually be
| completely replaced by revisions of our physics.
| caymanjim wrote:
| I think most academics studying dark matter are pretty upfront
| about not knowing what it is. The theories with the most fans
| tend to not be very specific about the source, but agree that
| it's most likely an "unknown something" with mass that causes
| gravity. There are some theories like MOND that instead
| postulate that there's no missing mass, but that our
| understanding of gravity needs to be altered at galactic
| scales.
|
| In any case, it's a placeholder, not a specific theory. There
| are many candidate theories, but so far they're either
| untestable or the tests have had negative outcomes.
| UnpossibleJim wrote:
| Just to put it in context, I'll share one of my favorite
| (even though I think it's one of the more outlandish and less
| accepted):
|
| In the Multiverse/Multidimensional universe theory, all of
| these dimensions aren't necessarily in different universes.
| Some are of different dimensions, laid over top of ours, but
| we can't see them because we can't perceive the dimensions
| they exist in. The only dimension that bleeds into ours, that
| we can track so far, is gravity. This is what we call "Dark
| matter". It is actually gravity leaking over from an "Nth"
| dimension laid over our own that we can't perceive.
|
| Again, as to the veracity I can't say. And I wish I
| remembered to author, but I don't =/ maybe I'll look it up
| later and make an annotation.
| 867-5309 wrote:
| sounds like Brane Cosmology
| https://en.wikipedia.org/wiki/Brane_cosmology
| Hemospectrum wrote:
| You've got plenty of scientists as company. There are cute
| names like WIMP and MOND for different types of hypotheses
| about what the heck dark matter actually is, or how to explain
| what's going on if it doesn't exist at all. Each of these is a
| huge headache for astrophysicists in one respect or another.
| [deleted]
| passivate wrote:
| But in modern science we differentiate between
| facts/hypotheses/laws/theories. What you're referring to is a
| hypothesis - and those should be replaced with a more concrete
| explanation - once we have it. I don't quite understand your
| point about connecting it to ether though. I can't think of any
| established theory in modern science that was completely
| replaced - but I'm not a physicist either.
| interestica wrote:
| > 'ether of the 21st century'
|
| Or 'caloric' or 'phlogiston'
|
| https://en.wikipedia.org/wiki/Superseded_theories_in_science
| tigerlily wrote:
| Phlogiston, now there's a word I haven't seen in a while. A
| lot of big names on that list of superseded theories by the
| way :)
| kiba wrote:
| Dark matter is not a theory, it's a name for an unexplained
| phenomena.
| hitpointdrew wrote:
| I also have almost no knowledge about dark matter in physics,
| and maybe I am a complete moron. But what do you think is more
| likely, there is this matter out there that no one has been
| able to find in decades of searching, or the mainstream
| assumptions/math on gravity are wrong/incomplete? I am going
| with the later.
| mr_mitm wrote:
| It's interesting that this sentiment almost exclusively comes
| from people with almost no knowledge about dark matter.
|
| What do you think is more likely, there is a theory that
| makes all observations look exactly like there is invisible
| matter without actually being invisible matter, or that there
| is invisible matter? I am going with the latter.
|
| I also do not understand why laypeople are of the opinion
| that all massive particles must interact electromagnetically.
| If you are open to the idea that there is a massive particle
| that doesn't, you got dark matter.
| salty_biscuits wrote:
| As a counterpoint, Neptune and Pluto were "dark matter" prior
| to confirmation with observations. They were theorized to exist
| due to their perturbations to observed motion of known planets.
| dnautics wrote:
| Well by that token so was the discrepancy in the precession
| of mercury (which was not, as it turns out,matter).
| da_chicken wrote:
| I mean, we need to have a convenient term for a phenomenon even
| if we don't understand it. What's the alternative? "The unnamed
| phenomenon were some astronomic bodies behave as though a vast
| quantity of matter were affecting them but that matter can't be
| otherwise detected or identified"?
|
| In medicine, they often call it a "syndrome". A collection of
| symptoms that together form a known pattern that is often
| closely associated with a disease or disorder but has no
| understood cause or origin (although some syndromes continue to
| be called that after they are better understood).
|
| How do you think we arrived at terms like "planets"? We noticed
| that some stars in the sky moved around while the rest didn't,
| and we called them "wandering stars". We still call them
| "atoms" even though that word means "indivisible".
|
| Weather is full of such names: rain, thunder, lightning, wind,
| morning glory, dew, tornado, rainbow, etc. All these words for
| material phenomenon existed long before we knew how they
| worked.
|
| So what's wrong with "dark matter" as a name?
| colordrops wrote:
| I didn't read a negative judgment of the term in the
| grandparent comment, just an observation.
| da_chicken wrote:
| Yes, and I'm challenging that observation. "Making an
| observation" doesn't mean you're not subject to criticism
| or questioning.
| colordrops wrote:
| But you were reading criticism of the naming in the
| original comment, which wasn't there.
| burnished wrote:
| You haven't meaningfully challenged anything they said,
| though. Your thing was an odd rant about the need to name
| things (not a hot take), and theirs seemed more focused,
| well
|
| >>That is, an interrim scientific theory that will
| eventually be completely replaced by revisions of our
| physics.
| [deleted]
| dnautics wrote:
| > So what's wrong with "dark matter" as a name?
|
| it encodes a normative, biased prejudgement about what we
| think explains the syndrome.
| karmakaze wrote:
| Exactly, the fact that it's a phenomenon gets lost--all we
| have to do is find the missing mass (X-Y problem) that's
| causing the gravitational effects.
| tracedddd wrote:
| What would you prefer the concept be called?
| dnautics wrote:
| Gravitational anomalies, gravitational discrepancy,
| unexplained gravity effect? Hell, dark gravity, if you
| want.
|
| Until the point where the observations are multimodal.
| Then I'd be more comfortable assigning provisionally
| normative names. Till then it should probably explicitly
| reference the fact that all observations are
| gravitational in nature.
| mr_mitm wrote:
| Except the very first experiment that was supposed to confirm
| the ether showed that there is no ether, while every
| astrophysical observation we made on any scale [1] confirmed
| that there is dark matter.
|
| [1]
| https://www.twitter.com/seanmcarroll/status/1371141258236764...
| dnautics wrote:
| We don't have any confirmation that it exists, we have more
| observations that Newton's law is wrong. Moreover, these
| observations collectively are unimodal; to date we only have
| observations of more gravitational discrepancies, not
| corroborating phenomena which correlate to the existing
| discrepancies.
| generalizations wrote:
| I mean, the name is literally 'the stuff we can't see'. Dark
| matter is very much just a question mark looking for an
| explanation.
| iainmerrick wrote:
| The word "stuff" there is exactly the point. It's a lot more
| specific than "just a question mark" -- the hypothesis is
| that it's some exotic form of matter.
|
| If something like MOND gains popularity and there's no longer
| a need to postulate invisible matter, it would be fair to say
| that dark matter is incorrect and outdated, just like ether
| or phlogiston.
| generalizations wrote:
| That would be fair. However, from what I've read MOND is
| relatively unlikely. If its proponents can address the
| biggest difficulties (e.g. not every galaxy appears to have
| "dark matter") and make it somewhat more feasible, then I'd
| agree the name is insufficiently broad.
| dnautics wrote:
| Not really, MOND and the like falls into the category of
| "dark matter theory", and that's the problem with the name.
| keithnz wrote:
| Sabine is a great science (physics) communicator to the
| "masses", she recently did an update on dark matter.
| https://www.youtube.com/watch?v=4_qJptwikRc
| Hypx_ wrote:
| Summary: Both dark matter and modified gravity are true.
| Could be caused by wave-particle duality.
| claytongulick wrote:
| Even "Ether" has come back around a bit in certain circles.
| It's not called that anymore, I think it's called the "Grid" or
| something similar. Milo Wolff discusses some of this in his WSM
| theories, IIRC.
| grenoire wrote:
| It looks fascinatingly close to Perlin noise in its cloudiness.
| mturk wrote:
| When generating cosmological initial conditions for use in
| simulations of large-scale structure, one common method is to
| utilize random numbers (distributed about a known power
| spectrum) in k-space and then transforming back to real space.
| Some details can be found here:
| https://enzo.readthedocs.io/en/enzo-2.3/_downloads/makeics.p...
| miohtama wrote:
| What is the projection used to display "the Universe" maps? What
| is the left edge, the right edge, etc. They seem to be similar in
| all articles but I have no idea how map relates to e.g. a night
| sky?
| motloch wrote:
| Check out Fig 10 in https://www.darkenergysurvey.org/wp-
| content/uploads/2021/05/...
| Sharlin wrote:
| The illustration in the article ([1]) seems to be a (slightly
| cropped) Mollweide projection [2], in familiar equatorial
| coordinates such that the north pole is top center, south pole
| is bottom center, and the equator is a (imaginary) horizontal
| line in the middle. The region of acquired dark matter data is
| located in the southern hemisphere sky, which makes sense given
| that the observatories used were in Chile. At the bottom you
| can see the Magellanic Clouds, satellite galaxies of the Milky
| Way which can only ever be seen from the southern hemisphere.
|
| [1]
| https://ichef.bbci.co.uk/news/976/cpsprodpb/F39E/production/...
|
| [2] https://en.wikipedia.org/wiki/Mollweide_projection
| tshaddox wrote:
| I believe that image in the news article is cropped from this:
| https://www.darkenergysurvey.org/wp-content/uploads/2021/05/...
|
| I found that image on this page, which seems to the be web page
| created by this research group about this project:
| https://www.darkenergysurvey.org/des-year-3-cosmology-result...
|
| From what I can tell, it's a projection of a view from the
| night sky from one particular telescope on Earth. They sampled
| a bunch of galaxies in an area about one eighth of the night
| sky.
| Sharlin wrote:
| The background starfield appears, in fact, to be a projection
| of the entire celestial sphere, all 360degx180deg of it, so
| only a half of it at most can be seen at a time from any
| point on Earth.
| mellosouls wrote:
| Mostly mysterious here is the detail from unconnected parties as
| to the significance of the supposed deviation.
|
| It reads mostly like a puff piece from an academia PR person. Way
| too vague in the article itself.
| _rpd wrote:
| To be fair, there were 30 new papers published:
|
| https://www.darkenergysurvey.org/des-year-3-cosmology-result...
|
| The overview paper appears to be this one:
|
| https://www.darkenergysurvey.org/wp-content/uploads/2021/05/...
|
| (the first published paper at the bottom of the page. I'd have
| posted the title and abstract, but they've disabled copy-
| paste.)
| motloch wrote:
| Yeah, as a cosmologist my reading of the DES results is exactly
| the opposite than what the article mentions - new DES results
| seem to show that one mystery (low values of S_8 in the low
| redshift measurements) was actually resolved..
|
| See the bottom of their Fig 14 in
| https://www.darkenergysurvey.org/wp-content/uploads/2021/05/...
|
| The problem was that the green (CMB/Planck) and gray (weak
| lensing) curves in the past were not overlapping as well as
| they are now. To me the tension seems to mostly go away now.
| gtsop wrote:
| The only mysteries here are:
|
| 1. The certainty that an infinite entity (universe) can be
| determined to have a very specific percentage (80%) of something
| (dark matter)
|
| 2. The fact that the big bang is still being considered and cited
| as the beginning (?) of the universe
|
| 3. That we have accepted terms such as "dark" matter and "dark"
| energy in science as if some witchcrafty comes into play, as if
| lord voldemort created the cosmos.
| junon wrote:
| The real mystery here is your point.
| floxy wrote:
| Off topic a little bit, but has anyone checked the work of the
| guy who was trying to explain galactic rotation curves using
| general relativity and gravitomagnetism? Which would lessen the
| need for dark matter.
|
| https://link.springer.com/article/10.1140/epjc/s10052-021-08...
|
| https://news.ycombinator.com/item?id=26442021
| knzhou wrote:
| Gravitomagnetism is a well-understood and experimentally
| measured effect. It is also a very small effect, of the order
| v^2 / c^2 where v is the speed of the sources. In the galaxy,
| stars move with v/c ~ 1/1000, which means the gravitomagnetic
| correction is one in a million. So while N-body simulations do
| sometimes account for general relativistic corrections like
| these, they're not nearly large enough to remove the
| requirement for dark matter.
|
| The main thing the paper should do is explain why they think
| the correction is a million times larger than the back of the
| envelope estimate. But they don't. Instead, they try to solve
| everything analytically, never plugging in numbers or reasoning
| about what's big or small, leading to a forest of long
| combinations of special functions. That's a reliable recipe for
| making a mistake.
|
| That is the simple reason the paper has been ignored by
| everyone in the scientific community and rejected from decent
| journals. Of course, this hasn't stopped hundreds of fluffy pop
| articles being written on it, or it getting posted every week
| on HN. The blind leading the blind.
| the8472 wrote:
| Thanks, I assume the same cricitcism also applies to other
| applying-GR-corrections papers? E.g. I saw one about
| gravitational self-interaction leading to concentrating
| gravity inside galaxies and starving the outside or something
| like that.
| [deleted]
| raattgift wrote:
| "Has anyone checked the work?"
|
| tl;dr: no, probably not.
|
| (I am aware of informal comments which have raised questions
| about whether the disk is a singularity, which would destroy
| the possibility of solving an initial value problem, unlike
| already-in-use approaches. Additionally, the rotating disk
| developed in the paper is clearly not present in non-
| axisymmetric elliptical galaxies dominated by radial motion,
| and so cannot replace dark matter in them; the paper only deals
| with disk-like approximations of spiral and axially-rotating
| spheroidal galaxies. There are plenty of galaxies where there's
| no common rotational axis, but there's still a rotational curve
| problem for stars and hydrogen gas clouds moving inwards vs
| outwards. Lastly, the paper only claims to be a good
| approximation in the limit of weak gravitational fields, so
| very dense galaxy clusters (which will include the future
| collision of the Andromeda galaxy with our own) are not covered
| by the work in this paper: it makes no claim to be able to
| predict the outcome of that collision, which breaks the Vlasov
| condition. When you collide the dust and gas in these galaxies,
| or galaxies like them in our sky, you will see lots and lots of
| X-Rays and the like, while some fraction of _actually
| collisionless_ matter would deform the galaxy and red
| /blueshift its component spectra.)
|
| This is the work in question, fed into Google Scholar (sauce
| for the goose as for the gander: Springer adds Google Scholar
| links to each of the author's references) using the first URL
| you supply:
|
| https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=http...
|
| We then hit "Cited by 3"
|
| https://scholar.google.com/scholar?cites=8505897096354068536...
|
| Not much there, and two are author self-cites, with no
| citations on the newer papers, and no collaborators on these
| papers.
|
| Glancing _briefly_ through the two newer ones, I got distracted
| by one inconsistency which strikes me as glaring because the
| "novel form" (author's words) the author builds strongly hangs
| off it: I found "pseudo tensor", "pseudotensor" and "pseudo-
| tensor" at the very least, and promptly gave up reading more
| deeply. Choose just one, please, or don't try to use them at
| all [1].
|
| Aside: EPJ+ charges _authors_ a USD 3280 fee to publish each
| article. It 's also not a journal working cosmologists or
| extragalactic astrophysicsts would follow closely.
|
| Fortunately, _readers_ without institutional access can find
| (again, via Google Scholar) essentially the same material on
| researchgate (which says nothing either way about quality) so
| one can glance without handing Springer $30+ for the two newer
| articles. Even more fortunately, the link you supplied is open-
| access, and can be read there (the PDF is nicely formatted)
| without paying a fee.
|
| Finally, we can look the author up and see numerous papers with
| collaborators in (mainly terrestrial applications of) plasma
| physics, but the only papers on astrophysics (in the broadest
| sense) are those three most recent ones.
|
| https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=%22G...
|
| My comments above are nothing at all like "checking the work",
| but rather an excuse for why I personally would be in no rush
| to do so.
|
| - --
|
| [1] _Tensors_ (not _pseudo-_ tensors) are enormously useful in
| General Relativity, because a _tensor_ solution solved in one
| set of coordinates is solved in _all_ systems of coordinates
| (including no coordinates at all). Matter should be specified
| as tensors. This has been done (e.g. the (Faraday)
| https://en.wikipedia.org/wiki/Electromagnetic_tensor). This
| lets us arrive at an understanding that works for you standing
| on your part of Earth using local notions of up, down, left,
| right, forward backward; and me standing somewhere else on
| Earth using my local notions of the same directions. It also
| works for people in the ISS who have to pick a conventional
| up/down, and whose local clocks tick fast (from _our_ standing-
| on-Earth 's-surface perspective). It also works for coordinates
| covering the solar system, the Milky way, or the entire cosmos.
| But not all physical systems have fully developed _tensor_
| theories. For them we may choose to introduce a _pseudo-_
| tensor, which are valid only for certain coordinate systems. A
| solution in one of those will work for some other systems of
| coordinates but not _all_ , and if one is not careful in
| choosing coordinates, one can get things spectacularly wrong,
| like wildly wrong recoveries of the components of the energy-
| momentum 4-vector ("In some coordinate systems you think there
| is energy present when there is no energy in other coordinate
| systems" is a common symptom, and in this context means "you
| think you do not need a _generator_ of curvature beyond the
| dust encoded in the _pseudo-_ tensor"). If one is inconsistent
| about the spelling of _pseudo-_ tensor, I think that's a bad
| sign about the extreme care in keeping different coordinate
| systems mutually and fully consistent when using them.
|
| One runs into _pseudo-_ tensor inconsistencies fairly often in
| cosmological contexts, so much so that it was dealt with in a
| USENET sci.physics FAQ entry:
| https://math.ucr.edu/home//baez/physics/Relativity/GR/energy...
| nimish wrote:
| The math is correct. Linearized GR and Newtonian gravity are
| qualitatively and quantitatively different and at
| "relativistic" scales we should probably see a difference!
|
| I don't have the cosmology background to evaluate more than
| that unfortunately; it's just that the "Standard Model" of
| cosmology uses a toy solution of GR (FLRW + newton).
|
| That said, it'd be wild if any major case of dark matter is
| just an artifact of incorrect approximations.
| OldGoodNewBad wrote:
| Dark matter is the name for compound mathematical errors based
| upon poor assumptions. We can't re-examine fundamental constants
| or established theories, so we'll be kludging around forever with
| this.
|
| It's not really important though because the practical
| application of cosmology is nil.
| ggggtez wrote:
| > practical applications are nil
|
| It's 80% of mass on the universe. You don't think that has
| applications in space travel? Maybe it doesn't effect your day
| to day life, but it's rather ridiculous to say there is no
| applications at all.
| Analog24 wrote:
| This is incredibly ignorant and pretty arrogant to actually
| think that you know better than thousands of researchers who
| presumably know far more about this than you. There are
| numerous independent and unrelated pieces of evidence that
| point to the existence of dark matter. You honestly think
| they're all the result of mathematical errors that just so
| happen to all point to the same conclusion? That would be quite
| a coincidence.
| aaroninsf wrote:
| Pet theory: "dark matter" is gravitation leaking from adjacent
| universes, in specific, from the juxtaposition of many divergent
| universes.
|
| Most diverge in trivial ways, so dark matter concentration is
| correlated with where matter is in our universe.
|
| Some relatively few diverge more drastically, so there is a
| background.
|
| As someone outside the field I remain curious if there are
| reasons this is inconsistent with observations to date, for some
| models of the multi-world hypothesis.
| hef19898 wrote:
| Flying through hyperspace is not like flying over fields. Said
| one famous pilot doing the Kessel Run in less than 12 parsecs.
| Maybe Dark Matter is the real space shadow of stuff in
| hyperspace the same way real space stuff has shadows in
| hyperspace.
|
| Seriously, I cannot judge this level of physics, or just
| understand them. It would make a nice addition to Star Wars
| lore so.
| tooltower wrote:
| Another article in the Guardian:
| https://www.theguardian.com/science/2021/may/27/astronomers-...
| chadcmulligan wrote:
| My theory is dark matter is the shadow of angels, there's more of
| it where new galaxies are forming because there's more angels
| there building the galaxies :-).
|
| Here's a movie of it in progress https://youtu.be/1x3RRrqJWKA
|
| I know this will get downvoted, but wouldn't it be funny if it
| turned out to be angels and we're looking at heaven. It's
| possible.
___________________________________________________________________
(page generated 2021-05-27 23:00 UTC)