We Saw Nothing
October 30, 2013 10:46 AM   Subscribe

"We have entered the new millennium and yet we still have no idea what 95% of the universe is made of." The Large Underground Xenon experiment has failed to see a single particle of Dark Matter. Will the Lux Zeplin have better luck?
posted by billiebee (78 comments total) 14 users marked this as a favorite
 
Goddamn it people I have work to do.
posted by physicsmatt at 10:51 AM on October 30, 2013 [38 favorites]


It's turtles.
posted by The Card Cheat at 10:52 AM on October 30, 2013 [3 favorites]


considers a Led Zepplin physcis song parody, then considers better and walks away
posted by Samizdata at 10:56 AM on October 30, 2013 [5 favorites]


We are patiently waiting, physicsmatt.
posted by Mei's lost sandal at 10:57 AM on October 30, 2013


To correct a point in the Guardian article: Zwicky made his inference from movements of entire galaxies in a cluster, not rotation of galaxies. Vera Rubin did the first work on rotating galaxies if I recall correctly.
posted by edd at 10:57 AM on October 30, 2013 [2 favorites]


Also dark energy isn't the driving force behind the universe's expansion. It has only been accelerating it relatively recently but if you magically made it vanish the universe would continue expanding. It no more needs a driving force than a baseball needs a motor. Maybe something similar started it all off but it wasn't the dark energy we measure today, most likely.
posted by edd at 11:01 AM on October 30, 2013 [1 favorite]


If it's like 90 percent of everything else, it's made of crap.
posted by Bunny Ultramod at 11:02 AM on October 30, 2013 [17 favorites]


These people do not know how to spin a headline.
posted by goethean at 11:03 AM on October 30, 2013


Does Duct Tape cling to dark matter? Will WD-40 lubricate it?
posted by Greg_Ace at 11:07 AM on October 30, 2013 [1 favorite]


Maybe Dark Matter is just Very Private. I think physicists should respect Dark Matter more and stop behaving like a horde of paparazzi. Show some humanity!
posted by GenjiandProust at 11:09 AM on October 30, 2013 [3 favorites]


WD-40 is a dark matter dispersant.
posted by vbfg at 11:10 AM on October 30, 2013 [2 favorites]


Honestly, if we are just looking for materials that neither produce nor reflect illumination, I suggest undergraduate papers and Michael Bay plots would be fruitful venues for exploration.
posted by GenjiandProust at 11:12 AM on October 30, 2013 [8 favorites]


Is it a dot, or is it a speck?
When it's underwater does it get wet?
Or does the water get it instead?
Nobody knows. Dark matter man.
posted by Zonker at 11:12 AM on October 30, 2013 [11 favorites]


Well, a different result would have been earthshaking news. As a non-specialist, this is ... not surprising, I guess?
posted by RedOrGreen at 11:12 AM on October 30, 2013


Here's the paper.

The interesting plot is the inset of Figure 5. Quick version: All those shaded regions are the range of DM particle cross section vs. DM particle mass that some other experiments have kinda-sorta-maybe detected something. The LUX result argues that everything above the blue line is excluded. If the particle has properties above the blue line, LUX would have detected something, and it didn't. So now we got a conflict between the sorta-maybe detections and this non-detection. I'll let an actual particle person fill in the implications of that.
posted by kiltedtaco at 11:13 AM on October 30, 2013 [1 favorite]


It's actually Quantum Peanut Butter that holds the Space-Time Bread together, not Dark Matter.
posted by polywomp at 11:17 AM on October 30, 2013


The Card Cheat: "It's turtles."

Ah, but the turtles ultimately rest on a pile of sticks, and those sticks are carried by a little old dude who's all bent over.
posted by jquinby at 11:18 AM on October 30, 2013


To be a little more fair to LUX, Figure 4 is also "the interesting plot", in that if they were seeing dark matter, they would expect more events (black points) to fall inside the red dashed region. A small number do, but that number is consistent with spillover from the blue region, which is where events from one particular source of non-DM interactions are supposed to lie. That's really what shows the "non-detection" aspect of this.
posted by kiltedtaco at 11:22 AM on October 30, 2013


Why don't they just call it "undetectable matter" so they can stop looking for it?
posted by rocket88 at 11:25 AM on October 30, 2013 [5 favorites]


Quick and dirty (because for some TOTALLY UNRELATED reason I have a bunch of slides I need to change for a colloquium on Friday). No reason, just suddenly have to update things...

Good News: LUX is the most sensitive search for dark matter on the planet. It has placed limits on a broad class of possible dark matter scenarios that are a factor of 20 times better than anything previous, and did so with only 85 days of running. One of the best remaining candidates for dark matter is dark matter interacting through the recently discovered Higgs boson. LUX today did not reach sensitivities good enough to eat deeply into the region of parameter space, but it proved that it is working brilliantly, and so will be able to tell us new and interesting things about such possible theories of dark matter in a year or two after their long run is completed.

In addition, and somewhat more disappointingly, they did not see any hint of 10 GeV dark matter, as had been reported by the direct detection experiments CDMS-II and CoGeNT. A previous experiment, built on similar technology, XENON100, had also failed to see any sign of this kind of dark matter. However, there were possible loopholes in the experimental calibration and analysis that allowed some wiggle room. LUX today did a great job of demonstrating that they had closed those loopholes (though it's early yet, and I couldn't hear answers to some technical questions, and so I'm sure someone can find something new to worry about). However, the target material for LUX and XENON100 is liquid xenon, and the target materials for the other materials was germanium in CoGeNT and silicon in CDMS-II. So it is possible that dark matter has different interaction with protons and neutrons and just interacts much more weakly with xenon than these other materials, or produces smaller signals than expected for other reasons. At this point, that's still possible, but it's getting more constrained. Things look bad for these signals of 10 GeV dark matter, but I need to ruminate a bit about exactly how bad.

But again, LUX tried to do something and succeeded amazingly. We still don't know what dark matter is, but it's a slow game, and LUX is making great progress.
posted by physicsmatt at 11:26 AM on October 30, 2013 [19 favorites]


DARK MATTER
15.08% plutonium
18.06% tantalum
27.71% xenon
24.02% promethium
10.62% dialium
3.94% mercury
0.57% unknown element TAR
posted by not_on_display at 11:30 AM on October 30, 2013 [6 favorites]


"We have entered the new millennium and yet we still have no idea what 95% of the universe is made of."

there was a deadline? crap.
posted by quonsar II: smock fishpants and the temple of foon at 11:30 AM on October 30, 2013 [5 favorites]


The deeper they looked, the more nothing they found.
posted by dephlogisticated at 11:33 AM on October 30, 2013 [3 favorites]


No matter.
posted by Kabanos at 11:35 AM on October 30, 2013 [3 favorites]


Why isn't it... here? For lack of a better word. And I guess I'm talking about dark matter specifically, rather than dark energy.

So, for example, is it that "We think a gallon of water weighs about eight pounds, but really it only weighs about a pound and a half with the rest of the mass being from dark matter rather than water"?

Or "It's all around us (on a local scale) and so it's pulling us in all directions and the forces essentially cancel each other out"?

Or "It only exists in environments with extremely rarified baryonic matter"?

Or "Force from it is inversely proportional to distance from it"?

Or what?

I understand that the answer may very well be "We don't know", but do we have any hints or guesses on this sort of thing, based on either the existing data or the prominent guesses for what dark matter is (WIMPS or whatever)?
posted by Flunkie at 11:36 AM on October 30, 2013 [2 favorites]


One thing that physicsmatt brings up that I was wondering is "what's different" about this than the other experiments. Location, obviously is one thing that might account...

But his point about Xenon being used vs Germanium and Silicon makes me wonder what it was about Xenon that made them choose it as the material. It looks like Ge and Si are stacked on top of each other on the table of elements. (Carbon is above that). Xenon is in a different column (noble gases) and a completely different row than any of them.

I don't know enough about Chemistry, so I wonder what traits Ge & Si had that made them detect the candidates, and Xe that might not have let them do so, and why Xe might be a better choice (if it is indeed a better choice).

I'm wondering if choosing something that's less interactive means they expect it to be more reactive to alternative events?

----------
So then I thought of some really weird idea. I know it's not actually feasible, I think, because the mechanism of explanation is actually quite simple: Heisenberg's Uncertainty Principle... The idea that we aren't really sure what dark matter is, it's elusive, it's mysterious right now.

Well we posit that particles carry forces (a la Higgs, w, z quarks, electrons, etc...) so I almost wonder if dark matter is a boson of some sort, and then I pondered if it is carrying whatever makes the Uncertainty Principle work, but then I remembered that that's merely due to the levels of energy required to detect an item merely adds energy to the object, which pushes it out of it's quantum state/location by adding force. That is - it's just a reflection of the energies involved...

So, of course, that's a dumb idea in particular, but...

What sort of doors open up when we discover Dark Matter? And why do we think of Dark Matter being a certain specific configuration? Clearly, we infer things based upon the little bit we do know...

My theory is M'gubgub. I'm gonna keep pushing this one until it is standard orthodoxy.
posted by symbioid at 11:58 AM on October 30, 2013


Flunkie: There have been *lots* of dark matter candidates floated over the past few decades. The bucket of water you're carrying still weighs 8kg because it contains (mostly) 8kg of water however.

Dark matter probably is here, but to be sufficiently dense to make up 95% of the matter in the universe there doesn't need to be very much of it: the mean density of a galaxy is pretty low.
posted by pharm at 12:10 PM on October 30, 2013


This reminds me of something I read about relativity and the orbit of Mercury. Basically, Kepler's equations relating to planetary orbits correctly matched the observed orbits of all planets, except Mercury. For hundreds of years the discrepancy was unexplained until Einstein's theories quantified the distortion of space-time by the Sun's mass that was causing the error factor. The new equations matched the observed orbit exactly.
Isn't it likely that something similar is happening here? Instead of inventing what's basically a fudge-factor to make the equations match the observations, which is really what Dark Matter is, I (not a physicist) would tend to question whether the equations even apply to things as massive as galaxies and galactic clusters.
posted by rocket88 at 12:10 PM on October 30, 2013


They picked xenon because many classes of dark matter interactions work "coherently." That is, they "see" the entire nucleus, rather than individual protons and neutrons, and so interact with all of them at once. This means that the rate of interaction goes as the square of the number of nucleons A^2, rather than as the number of nucleons A, which it would if it interacted incoherently.* This is handy, as you want the biggest rate possible. Now, maybe dark matter doesn't interact coherently, but it's a very generic property, so you want to aim for materials with large A. Xenon is mass number A = 137, I believe.

You also want to pick a target nucleus that has approximately the same mass as the dark matter you're searching for. Think of the interaction at two billiard balls hitting each other. One's at rest (the target), and one's moving (the dark matter). After the interaction, the kinetic energy of the moving guy is approximately evenly split between the two, due to the equal masses. Now, if you throw a pea at the stationary billiard ball (meaning, lighter dark matter than target), the pea reflects off and doesn't share much energy. If you throw a bowling ball at a billiard ball (heavier dark matter than target), the billiard ball will move off, but with much less energy than the bowling ball retains. You'd do better from an experimental point of view with bowling ball sized targets. Since we though that dark matter was likely to be around 100 GeV we picked nuclei with approximately 100 nucleons in them (each nucleon is about 1 GeV in mass: 0.938 and 0.940 for protons and neutrons).

Finally, we need targets that are one or more of the following:
Cheap.

Easily instrumented (so that's easy for solid state targets made up of semiconductor materials like silicon and germanium).

Easily separable from radioactive isotopes. These experiments have to be CLEAN. You are far to radioactive to get anywhere near one of these experiments when it is running; the background radiation you contain from eating bananas and breathing radium swamps the signal. Plus, as you live on the surface of the Earth, you have been "activated" by cosmic rays: high energy particles from space rain down and turn some of your atoms into unstable isotopes (this is where carbon-14 comes from). Building these super-clean experiments, they often go after things like "Roman lead," which is lead mined by the Romans and sunk or buried for 1000+ years, as that has had time to lose its cosmic activation. Or, they use the steel and metal from the WWI German ships sunk at Scapa Flow, as that metal predates the nuclear testing that gave metal worked after 1945 a low-level radioactive contamination that's hard to get rid of.

I love science.

*This explaination might also be described as such.
posted by physicsmatt at 12:13 PM on October 30, 2013 [17 favorites]


rocket88: at least some dark matter consists of some kind of matter that carries mass: the evidence from gravitational lensing by chunks of the universe that don't appear to carry any visible matter are fairly convincing (see, eg the bullet cluster).
posted by pharm at 12:14 PM on October 30, 2013 [1 favorite]


rocket88, you just reinvented Modified Gravity (MOND). I definitely don't have time at this particular moment to recap all of the evidence for dark matter. There are several separate lines, and all converge on the same answer for a particulate component of the Universe that composes 25% of all energy-density and is not nuclear or electromagnetically interacting (as was "born cold" in that it was not relativistic at a certain point in the Early Universe). Beyond that we don't know what it is.

Sorry for the lack of detailed reply. I think I have several comments in my history on metafilter that might help answer why people like me believe that dark matter is the correct solution to certain problems in our understanding of the Universe. Please check those out and I'll hopefully have time later to answer additional questions.
posted by physicsmatt at 12:16 PM on October 30, 2013 [1 favorite]


Also, the fact that you reinvented MOND should not be taken as a dig. There are still physicists working right now who disagree with me about dark matter and believe in MOND. I think they're wrong, but you came to a perfectly valid conclusion, given your knowledge of the experimental results, and it's a conclusion that many people have come to over the years. Fewer these days, but that's because, in order to answer "complaints" such as yours, we had to do better at defending the dark matter idea with experimental evidence. It took many decades after its original proposal for anyone to really take dark matter seriously.
posted by physicsmatt at 12:20 PM on October 30, 2013 [2 favorites]


rocket88: "Instead of inventing what's basically a fudge-factor to make the equations match the observations, which is really what Dark Matter is, I (not a physicist) would tend to question whether the equations even apply to things as massive as galaxies and galactic clusters."

Here's a link that does a pretty good job at explaining some of the evidence for dark matter based on observed vs expected rotation curves for galaxies and the odd distribution of dark matter implied by the evidence (amount of dark matter increasing with distance from galaxy centers):

http://www.astro.cornell.edu/academics/courses/astro201/rotation_curves.htm
posted by Hairy Lobster at 12:20 PM on October 30, 2013 [1 favorite]


Samizdata: D'ark Mat'er
(errr sorry)
posted by lbebber at 12:32 PM on October 30, 2013 [1 favorite]


Kabanos: No matter.

Never mind.
posted by history_denier at 12:47 PM on October 30, 2013 [1 favorite]


The answer to this all is obvious: Before the machine elves ascended to a higher plane of existence, they formed an empire that spanned countless galaxies, enveloping each star system they settled in in massive (pun intended) Dyson spheres to collect every last photon output from their parent stars. Such was their efficiency and ingenuity, that we have no hope of detecting the untold number of stars that exist in the seemingly vast expanses of "empty" space. When Voyager impacts one of the shells surrounding the main sequence stars that lie just beyond the edge of Sol's Oort Cloud many, many years from now we'll discover this simple fact. If any humans are still alive by then.
posted by Thoughtcrime at 1:12 PM on October 30, 2013 [4 favorites]


0.57% unknown element

posted by not_on_display

What are you hiding, not_on_display? 95% of everything, perhaps?
posted by It's Raining Florence Henderson at 1:29 PM on October 30, 2013 [1 favorite]


the machine elves worked very fast then. The most accurate information we have about dark matter comes from the first 11 or so minutes of the Universe, where we know that the amount of "matter" (stuff that feel gravity and was non-relativistic) is much greater than the amount of "baryons" (matter that also participates in the synthesis of heavy elements out of protons and neutrons). We also know from the CMB, some 300,000 years later, that dark matter existed and was non-baryonic.

Sorry to run the fun, but just wanted to say that we actually know a good deal about this mysterious stuff. Just not, you know, what it is.
posted by physicsmatt at 1:32 PM on October 30, 2013 [3 favorites]


we actually know a good deal about this mysterious stuff

Its darkness, for example, has been firmly established.
posted by dephlogisticated at 1:37 PM on October 30, 2013 [2 favorites]


physicsmatt: Building these super-clean experiments, they often go after things like "Roman lead," which is lead mined by the Romans and sunk or buried for 1000+ years, as that has had time to lose its cosmic activation. Or, they use the steel and metal from the WWI German ships sunk at Scapa Flow, as that metal predates the nuclear testing that gave metal worked after 1945 a low-level radioactive contamination that's hard to get rid of.

I love it. The sheer cleverness of this blows my mind, and makes me wonder what future generations will be mining from our garbage dumps for their ultra-precise quantum gravity tests.

Maybe the lunar rocks in museums will be critical to compare against fresh samples from the moon?
posted by RedOrGreen at 1:46 PM on October 30, 2013


Dark matter is cat poop. It's dark and smells like no regular matter could ever smell. Also, nobody I know that's made from baryonic matter wants to interact with it. Proof!
posted by Hairy Lobster at 1:47 PM on October 30, 2013


This post/comments is like a random generator of band names -- Lux Zeplin, Dark Matter, The Uncertainty Principle, Machine Elves, Quantum Peanut Butter, Space-Time Bread.

I'm placing dibs on Sol's Oort Cloud.
posted by CosmicRayCharles at 1:50 PM on October 30, 2013 [2 favorites]


Is there a good recent popular science book on cosmology, specifically covering the science about dark matter? Or maybe a good long article? I last read a book like that maybe 20 years ago and feel out of date. I'd totally buy physicsmatt's book if he had one.
posted by Nelson at 1:57 PM on October 30, 2013 [1 favorite]


GenjiandProust: "Maybe Dark Matter is just Very Private. I think physicists should respect Dark Matter more and stop behaving like a horde of paparazzi. Show some humanity!"

Look, extreme shyness is NOT something to joke about.
posted by Samizdata at 1:59 PM on October 30, 2013


jquinby: "The Card Cheat: "It's turtles."

Ah, but the turtles ultimately rest on a pile of sticks, and those sticks are carried by a little old dude who's all bent over.
"

Nope. Wrong. The turtles rest on other turtles...

Can we NOT get our cosmology posts right?
posted by Samizdata at 2:00 PM on October 30, 2013


lbebber: "Samizdata: D'ark Mat'er
(errr sorry)
"

No apologies needed. You haven't seen some of the punchlines I go for...
posted by Samizdata at 2:06 PM on October 30, 2013


Samizdata: "Nope. Wrong. The turtles rest on other turtles..."

They're just trying to find the bridge. Have you seen the bridge?

WHERE'S THE CONFOUNDED BRIDGE?
posted by jquinby at 2:17 PM on October 30, 2013 [2 favorites]


Hey hey Mama, I like the way you move, gonna make you sweat, gonna make you reveal the secrets of the universe by bombarding you with neutrinos.
posted by joelf at 2:29 PM on October 30, 2013 [2 favorites]


Dark matter? We don't even know why mass creates gravity even though everyone experiences it everyday all the time.
posted by three blind mice at 2:31 PM on October 30, 2013


They're going about this all wrong. They should be searching for Nibblonians.
posted by Foosnark at 2:36 PM on October 30, 2013


Flunkie: If dark matter exists, your option #2 is closest.

"It's all around us (on a local scale) and so it's pulling us in all directions and the forces essentially mostly cancel each other out"?

If you look around the galaxy and add up all the matter that we can detect, then calculate how much matter should exist in the galaxy based on how fast the stars are rotating around the galactic center, it seems like there's a LOT of matter that must be there that we can't see.

The explanations of that are either
1) There is a mysterious type of matter that we can't yet detect (aka 'dark matter')
2) Our theory of gravity is in some way incorrect
3) We have made some kind of error, such as not accounting for all the baryonic matter (unlikely)
posted by Salvor Hardin at 2:49 PM on October 30, 2013 [1 favorite]


Aether pls.
posted by turbid dahlia at 2:52 PM on October 30, 2013 [1 favorite]


Will WD-40 lubricate it?

Dammit people, it's a rust inhibitor not a lubricant, if we base our science on incorrect assumptions we'll never get anywhere.
posted by doctor_negative at 2:59 PM on October 30, 2013 [4 favorites]


Maybe Zelazny was right and we are surrounded but a shadow.
posted by ryoshu at 3:07 PM on October 30, 2013 [1 favorite]


On the Mercury/gravity being wrong issue... for every perihelion advance of Mercury that is explained by a better theory of gravity, there's a momentum non-conservation that is explained by a new particle called the neutrino.

Sometimes suggesting a new particle really is the right thing to do over modifying the laws of physics as you currently understand them.
posted by edd at 3:31 PM on October 30, 2013


"Also dark energy isn't the driving force behind the universe's expansion. It has only been accelerating it relatively recently but if you magically made it vanish the universe would continue expanding. It no more needs a driving force than a baseball needs a motor. Maybe something similar started it all off but it wasn't the dark energy we measure today, most likely."

I thought that without "dark energy," the mass of the universe would cause it to collapse upon itself. A baseball is maybe not the best metaphor — baseballs need to be thrown to move, otherwise they fall to the ground.
posted by klangklangston at 3:41 PM on October 30, 2013


Hairy Lobster's reply to rocket88, where rocket88 has said:
Instead of inventing what's basically a fudge-factor to make the equations match the observations, which is really what Dark Matter is, I (not a physicist) would tend to question whether the equations even apply to things as massive as galaxies and galactic clusters.
reminds me that one of the really fascinating things about MOND as an alternative is that it is a model with just one number (one fudge factor arguably, as it is theoretically rather poorly motivated with a relativistic version arriving very late on the scene) - it sets just one number, an acceleration, where Newtonian dynamics stops working and something else takes over. Whatever else is going on, it will be very interesting to figure out why, if dark matter is right, MOND as a theory works so well on galaxy rotation curves. From a purely phenomenological point of view, it's fascinating and demands explanation, and will hopefully lead to insight even if gravity works pretty much as Einstein suggested.
posted by edd at 3:44 PM on October 30, 2013 [1 favorite]


klangklangston: well it might have, or it might not. Without dark energy the analogy would be that either the universe would recollapse eventually (if there's enough matter) - and we'd be a baseball currently in flight and doomed to hit the field at some point in the future, or the universe wouldn't and we would be a baseball hit by Clark Kent and doomed to leave the solar system forever.
With dark energy, things are not so straightforward, and the motor analogy isn't terribly poor - either the baseball is fitted with a rocket to get it out of the solar system or the motor is puny and the baseball will still hit the field, but either way it acts differently to your bog-standard baseball and surprises everyone because noone expected a baseball with a motor.

OK, I'm clearly pushing the analogy a bit too far now.
posted by edd at 3:48 PM on October 30, 2013


Edd: it will be very interesting to figure out why, if dark matter is right, MOND as a theory works so well on galaxy rotation curves.

Offered for your consideration, with no opinions about correctness or even "not even wrong"-ness:

Reconciliation of MOND and Dark Matter theory
Man Ho Chan

I show that Modified Newtonian Dynamics (MOND) is equivalent to assuming an isothermal dark matter density profile, with its density related to the enclosed total baryonic mass. This density profile can be deduced by physical laws if a dark matter core exists and if the baryonic component is spherically-symmetric, isotropic and isothermal. All the usual predictions of MOND, as well as the universal constant a0, can be derived in this model. Since the effects of baryonic matter are larger in galaxies than in galaxy clusters, this result may explain why MOND appears to work well for galaxies but poorly for clusters. As a consequence of the results presented here, MOND can be regarded as a misinterpretation of a particular dark matter density profile.

(Accepted by Physical Review D. I'm sure that last line in the abstract is going over real well with MOND people.)
posted by RedOrGreen at 3:49 PM on October 30, 2013 [1 favorite]


Natural xenon is a mix of several different stable and ultra-long lived isotopes. That makes it unlikely that there's some xenon-specific effect going on. On the other hand it's possible that there's a silicon or germanium-specific effect, but if you look at what should have been detected according to CDMS II or CoGeNt, versus the detection limits established by LUX, there's a ratio of several hundred involved. Something mighty peculiar would have to be occurring.
posted by topynate at 4:42 PM on October 30, 2013


Maybe this is a stupid question, but if dark matter interacts with normal matter via gravitation, then shouldn't it impart momentum to it? If there's so much dark matter around, then why isn't everything being heated by these interactions? I.e., the dark matter tugs on normal matter, which hits more normal matter and then releases some of the energy as heat.
posted by Joe in Australia at 6:20 PM on October 30, 2013


It only exists in black holes and other largely aggregated objects so it only acts on objects within range, like the rotational orbit of galaxies around their supermassive black hole centers.

/making shit up entirely
MSUE
posted by lordaych at 6:24 PM on October 30, 2013


Whenever I read about this kind of experiment, I'm always reminded of this.
posted by lalochezia at 6:27 PM on October 30, 2013


AIUI, our observational evidence of dark matter basically is that it is tugging on baryonic matter—e.g., when we look at galactic rotation speeds. But I don't follow your next thought that this tugging of dark matter on baryonic matter would cause more baryon-baryon collisions…
posted by jepler at 6:33 PM on October 30, 2013


Well, suppose that we have an isolated bit of baryonic matter floating around. A particle of dark matter whizzes past, imparting some velocity to the baryonic matter. The dark matter has been slowed down (would "cooled" be appropriate here?) and the baryonic matter accelerated ("heated"). But, the baryonic matter can interact normally with other baryonic matter. It's now moving faster, so it's more likely to hit more baryonic matter, and when it does it will radiate some of that energy away.This means that over aeons all the matter will tend to cool down, both baryonic and dark.

Now, consider the dark matter that should be flowing through us as we speak. Each dark particle imparts a small tug on the atoms we're composed of, but it won't do it evenly: the atoms closest to its path are affected most strongly, stretching the molecular bonds a bit. So there should be this constant heating effect due to a flow of dark matter. I suppose it wouldn't amount to much, but there's a lot of dark matter and a lot of time for it to interact.
posted by Joe in Australia at 7:01 PM on October 30, 2013


It's socks.

One falls through a wormhole, lost to us forever. However, it remains entangled with its partner from across the universe. This creates a massive amount of energy, which acts as matter, but you can't see it, because it's dark inside socks.
posted by obiwanwasabi at 1:21 AM on October 31, 2013 [1 favorite]


Yeah, not convinced by anything I've read about dark matter that it's anything other than a misapplication of Newtonian assumptions on the behaviour of a massively decentralised disc of stars that shouldn't be expected to behave like a solar system.

In our solar system, 97+% of mass is The Sun. So sure, planets closer to the center will travel faster (depending on mass) than those farther out. So we know a planet of mass x in the vicinity of Mercury spins rapidly around the sun to avoid falling in or out of orbit, while the same planet out near Pluto would lazily orbit the sun and still stay in orbit.

The basis of predicting dark matter is that galaxies don't follow the same 'expected' Newtonian pattern. Why aren't the stars way out in the 'sparse' fringe of the galaxy orbiting slower than the ones orbiting the 'massive central bulge' centred on a super-massive black hole?

Maybe because mass distribution is more like a dinner plate with a pea in the middle in a galaxy, rather than the very massive sun and teeny-tiny planets model of our solar system.

In a galaxy, local stars and clouds of dust have a far greater influence on other nearby stars, keeping them together in rotation, while the vastly distant galactic centre's effect is negligible.
posted by panaceanot at 3:52 AM on October 31, 2013


"Well panaceanot... if Star x is halfway from the center of the galaxy, there's far more cumulative mass from all the stars inside its rotation around the galactic center than there are stars outside it pulling it away from the galaxy so it should behave the same way as a solar system, otherwise dark matter"

I say, inverse-square law at inter-solar distances means that maybe the combined mass of the rest of the universe (and nearby galaxies) is counteracting that, and that's precisely why galaxies exist in slowly spinning plate formation...
posted by panaceanot at 4:04 AM on October 31, 2013


panaceanot: It's easy to calculate the cumulative gravitational forces on a star from any reasonable distribution of matter that we see. It simply doesn't fit with Newton's laws.
On top of this you have to explain why gravitational lensing sees higher total galaxy masses (we can calibrate lensing off the Sun, where the lensing effect matches the Sun's known mass), why galaxy clusters don't show the right dynamics, why the cosmic microwave background properties are as they are, and why nucleosynthesis in the early universe points to a low fraction of the universe being ordinary matter. And other lines of evidence I haven't bothered to mention. It's not a misapplication of known laws of physics.
posted by edd at 6:02 AM on October 31, 2013


Joe in Australia: the term you want to google is probably 'dynamical friction'. It does happen, but probably not to the extent you expect.
posted by edd at 6:06 AM on October 31, 2013 [1 favorite]


edd: Apologies for potentially coming across as a crank (I'm quite aware there's vast amounts of scientific inquiry I'm not privy to or would even be able to parse, and I'm simply not mathematically talented enough to explore on my own)

However, unlike so many scientific fields, the study of cosmology *must* admit to a high degree of uncertainty. We can't triangulate from our dot in this intergalactic spacetime, the way we can for climate/evolution/biology etc.

The mathematical basis for dark matter has surely been rigorously explored, but you start extrapolating locally consistent maths to the entire Universe and I scratch my head and think... we're a pale blue dot and I don't think we've explored all possible creative theories before resorting to analytics.

I'm an atheist, sceptic, climate change worrier and all round 'go scientists!' guy
posted by panaceanot at 6:30 AM on October 31, 2013 [1 favorite]


Example: the lensing effect matches the Sun's known mass

Yeah, so?
posted by panaceanot at 6:34 AM on October 31, 2013


panaceanot: Not at all crank-like, it's entirely reasonable to ask if dark matter is the right explanation or if it is a modification to the laws of physics as we understand them. We certainly have good tests of general relativity on solar system scales, but much less so on scales larger than that. It's definitely a problem with either a missing ingredient to the universe, or a problem with the laws of physics as we understand them though, and not our ability to calculate the results of the laws of physics.
posted by edd at 6:34 AM on October 31, 2013


They haven't seen anything because God doesn't want them to. Intelligent Design proven as fact!!!
posted by Thorzdad at 6:57 AM on October 31, 2013


All this study and they haven't proved the 95% isn't GOD ITSELF!



*ducks*
posted by billiebee at 7:12 AM on October 31, 2013 [1 favorite]


Is there a good recent popular science book on cosmology, specifically covering the science about dark matter? Or maybe a good long article? I last read a book like that maybe 20 years ago and feel out of date.

Einstein's Telescope helped me understand why "dark matter" really does have to be taken seriously. The evidence for its existence is actually not that abstract. Some THING is warping the light from certain distant galaxies in a specific way.
posted by General Tonic at 8:29 AM on October 31, 2013 [2 favorites]


the term you want to google is probably 'dynamical friction'.

Thanks, Edd. I see there are papers on its applicability to dark matter, although I haven't found a discussion of its heating effect on normal matter. I also found it interesting that the effect follows an inverse square law: faster particles are affected much less strongly than slow ones. I think this should mean that you end up with two broad populations of dark matter particles, fast ones and slow ones, and the fastest ones still have most of their original velocity! Mind-boggling.
posted by Joe in Australia at 2:56 PM on October 31, 2013


panaceanot: There's nothing wrong with reasonable skepticism, but you're stepping over into woo territory here IMO.
I don't think we've explored all possible creative theories before resorting to analytics.
At a certain level, "analytics" (by which I take to mean: extrapolating from known physics to the wider universe) is all we've got. Sure, you can go down the rabbit hole of asserting all kinds of weird and wonderful modifications to existing theories and physicists have suggested many themselves. Dark matter is merely the most parsimonious of the proposed explanations for the observed phenomena at this point in time.

Jumping in and asserting that they "haven't been creative enough" is a little insulting, especially coming from someone who hasn't even bothered to familiarise themselves with the literature on the topic, which is extensive.
posted by pharm at 3:06 AM on November 1, 2013


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