We haven't directly detected dark matter because we don't really know what we are looking for, and there are only a few things we can search for. Dark matter might not be structured in a way we can investigate with current technology.
Also, dark matter doesn't interact much with regular matter, which makes the search even harder.
> Also, dark matter doesn't interact much with regular matter, which makes the search even harder.
This is (one of the many places) where I get lost. It has mass, so it by definition interacts with anything with mass?
Are these particles supposed to be so small and so rare that they can't be measured even at the scale of solar system? (How much dark matter would be in the solar system? What would be the average density of the dark matter? what would be the mass of single dark matter particle?) Or what do I miss here?
As I understood it (I'm not a physicist so probably wrongly) dark matter is the proposed answer to reconcile General Relativity Theory (which is the current gravity theory) with actual observations of the universe.
So there is two possibilities:
- There is no dark matter and the general relativity is wrong.
- There is dark matter and the general relativity is correct.
And maybe a combination of both: there is something we don't detect but the theory is also wrong.
Super weird food for thought, but I used to think about the universe a lot as a kid, and back then we were under the assumption that the expansion of the universe was slowing. At some point that viewpoint changed and we now believe it is accelerating, hence the emergence of so called dark matter. Anyway, this led me to envision a fourth dimension, a sphere. Imagine that our universe began at any arbitrary point on the inside of this sphere, and then orient the sphere so that we are at the bottom (like a penny inside of an inflated balloon). Now imagine (don’t believe, just imagine) that our universe is expanding at a constant rate. As we approach the equator of this sphere, the area that we must cover grows larger, but once we pass the equator it begins to grow smaller. And so what might appear to us as slowing down and speeding up could just be the shape of space changing, and not the speed of expansion. Again, just food for thought.
I used to be a bit interested in astrophysics, so I can't vouch 100% for this being accurate, but it's my understanding:
The universe isn't the penny, it's the balloon. Physicists believe we are living on the surface of a hypersphere. One important consequence of this idea is that the big bang didn't occur at a specific point in our 3D space, but at the center of the sphere.
Furthermore, the concept of a balloon expanding vs. deflating is a bit of a misconception. The argument used to be that whether or not the balloon is expanding, depending on the rate of the expansion, gravity could eventually win out and cause the matter to collapse back together (big crunch scenario). The problem with that theory is that we now know that galaxies are speeding away from us at a growing speed that (not sure the exact details, probably based on red shift in light from nearby galaxies). So the idea of gravity winning out was not based on evidence, just one of a number of possibilities, but the evidence proved it wrong beyond a doubt.
Think of 4 dimensional spacetime as an oddly-shaped rectangle, which is much wider at one end than another. Maybe more like a pyramid with the top bit missing. All of matter and energy started as an explosion at the narrow end, and is traveling across towards the much broader end. The expansion of the universe is merely the expansion of the barrel, and the dark energy spreading apart the universe is really the slight trajectory differences caused by the shape of the charge that started the process.
To me, it's like 3d canon shot flying down a 4d barrel.
Or trace the worldlines of these particles. Any given point in time represents a 3d slice of the 4d pyramid-rectangle.
That’s a really awesome thought experiment! I got a related idea during undergrad. What if the apparent symmetry between the three special and one temporal dimensions was at one point complete? Ie time could at one point be swapped out with any of the other three dimensions. But somehow that symmetry collapsed- and maybe it’s collapse was the result or creation of mass-energy. I imagine a hypercube being “pulled down” into a confined region. But the confinement “pushed” the two directions of time into the one direction and mass-energy.
Wouldn't this result in relative acceleration which correlates to the angle of the body being observed relative to the viewer? Anything aligbed directly between the viewer and a pole would move at a constant speed while speeds would appear to increase as you approach perpendicular? I can't imagine how this would play out in a 3d projection of a 4d surface however.
This strikes me a lot like “flatland”, ie. there’s something there but we’re not equipped to perceive it. Our experiments are mostly like extreme extensions of our own senses - seeing and touching things. But could something exist entirely within a dimensional space that we can’t touch or see? And if so how would we ever figure out how to detect it ... and even more meta, what if it was impossible for us to perceive the detection?
The problem is that gravity, like EM, follows an inverse-square rule. If gravity had a 4th dimensional component, we would expect to see an inverse-cube situation. The equation is: 1/(1-dimensions)
Well, it has a gravitational effect, so it definitely exists inside of our normal spacetime.
Scientists have explored alternative models of gravity or spacetime with additional spatial dimensions and things like that. As far as I know, none of them are very promising. There was an interesting PBS space time video about one example recently: https://youtu.be/3HYw6vPR9qU
Of course, it is definitely interesting to think about :)
Nice article, especially showing how much experimental data about dark exists. Dark matter isn't just a crazy concept to "fix" gravity. As nicely described in the article, there are several experiments clearly showing some invisible matter. We know a lot about it, except which unknown particle causes it.
As I was walking the dog last night I was thinking about SEO and how even the "experts" don't really know Google's secret sauce or how the recipe will change. There are best-practices that seem to work but with Google's algo hidden and changing it seems funny to be an expert. But certainly they know more than their clients. That led my thoughts to dark matter.
Expert: I am an expert on Dark Matter.
Me: What is it?
Expert: I have no idea but I think it exists.
Me: Ok, you must be really smart.
My glib little dialog contains no sarcasm. No doubt they are very smart. Expertise is measured differently in different fields.
Expert on Oak Island knows all the rumors and theories but not where the treasure is.
Expert flat-earther knows all the wrong facts.
Expert politician might know .0001% due to the vastness of government.
Experts on religion know the experts in other religions are wrong.
Expert MLB hitters fail more than succeed.
I guess it gives me hope I may one day be an expert at something.
I'm guessing that "guessing in the dark" is the way experiments for particles in the standard model have been done for a while now. I would assume that a single success would set off a chain reaction as the author said.
I'd say it's not completely in the dark. The experiments try to test theories that suggest where to look. For instance if a theory predicts a previously un-discovered particle, with some rules for how it interacts, then you can refine your search based on those rules -- what range of masses you're looking for, what decay patterns, and so forth.
The simplest “proof” of dark matter - Kepler’s Laws dictate that objects with smaller orbits (closer to center) move faster (in terms o fangular velocity). Our solar system works like that. Our galaxy doesn’t - its spiral shape indicates that the angular velocity varies minimally with radius.
The spiral structure isn't fixed to specific matter, it's a wave moving "through" the galaxy. But broadly yes: you can look at orbital velocities of galactic matter directly (via doppler shifts of their spectra) and make a map of the mass distribution in the galaxy. And then you can add up all the mass we can see as stars, and they don't remotely add up.
This carries the implicit assumption that all forces fall off monotonically with distance. Maybe gravity doesn't? Maybe it has a curve similar to a meteor impact crater: raised in the middle, a deep valley, and raised at the outer edge?
Of course, with more evidence, such as the observations of the Bullet cluster, these simple explanations fall apart. But at any rate, it's not enough to take one observation and assume it holds at all scales.
Look at tiny water droplets. They don't behave at all like large bodies of water.
Why are there so many people who trot out their theories that they've put about 30 seconds worth of thinking into and are based on the assumption that professional scientists fundamentally don't know what they are doing and are completely clueless?
You need more than this. The alternative is modified gravity, meaning gravity rules are different at different scales. Most people arguing against dark matter use that one example as their battle ground, aha, I can get the same result with just a tweak to Newton's laws.
It's when you add in all the other problems that a little tweak to Newton's won't be enough.
This may be a dumb question, but could dark matter just be regular matter, like planetoids? What if interstellar space had a light sprinkling of things like Oumuamua?
On an interstellar scale, maybe this could create the lensing effects and explain some other phenomena. Is it unreasonable that 5/6 of the mass of the universe is like the stuff that makes up planets, but isn't lit up as brightly as a star?
This is the "MACHO" (massive compact halo objects) theory, which has been tested and disproven. The missing mass can't be gas or dust clouds because that would have certainly observational traces which we have looked for and don't exist. It can't be planetoids because that would cause a frequency of gravitational micro-lensing of stars in other galaxies that we do not observe.
This got me thinking about the difference between "direct" and "indirect" observation. What is the difference?
For example: in these experiments, what would "direct" observation be? We have instruments that detect changes in certain variables, and we look for changes that align with our expectations of how a particle affects these variables. But we're not directly observing the particle... we're observing the particle's effects on these variables. So there's always an intermediary between us and the phenomena we're attempting to explore.
It seems to me that this intermediary must exist for all phenomena that cannot be perceived by our 5 senses. So how do we determine when an intermediary is "direct" vs "indirect"?
Looks like it's time for me to revisit Philosophy of Science...
"The theory. This tells us that around every galaxy and cluster of galaxies, there should be an extremely large, diffuse halo of dark matter. This dark matter should have practically no “collisions” with normal matter — upper limits indicate that it would take light-years of solid lead for a dark matter particle to have a 50/50 shot of interacting just once — there should be plenty of dark matter particles passing undetected through Earth, me and you every second, and dark matter should also not collide or interact with itself, the way normal matter does."
I've always heard that the lack of interactions was an observation, not a deduction: we can't see it, therefore it doesn't interact.
Anyone trying to use machine learning to make a "black box" mathematical framework? Feeding known measurements as "learning" (including known anomalies and discrepancies)?
Yep, it's been done before by Cornell researchers, in 2009. You provide a bunch of data to a program and it hums for awhile and derives some equations from the observation. In the story written around the time, it took positions of a pendulum swing and derived laws of motion from it. See https://www.wired.com/2009/04/newtonai/ (and lots of other articles around the time.)
You could feed a spreadsheet into and then it would iterate for awhile, converging toward most accurate equations describing the system. Or in my case, diverging from what I was sure was the solution. It was a hit and miss with most data (honestly, an eyeball and a few braincells did better with some data sets), but all the same I used it to lazily approximate position equations for some programs I was writing.
The desktop program was called Eureqa which then got ported to the cloud for obvious capacity increases, and relabeled into a company/product called Nutonian. Most recently bought by "DataRobot" and it's now being sold for sales optimizations. Because science.
Can someone that knows a bit more about this explain why we believe that this dark matter substance must exist and not simply that we have an incorrect model of gravitation?
Because such a minimal change to the model ("there's just some more stuff we can't see") fixes a number of problems in one fell swoop (off the top of my head, galactic rotation curves, observations of gravitational lensing, a cosmological model compatible with obervations of CMB fluctuations, structure formation, cosmic evolution).
Theories of modified gravity can fix those individually, but as far as I'm aware, no alternative has been shown to be viable once taken in combination.
No one has been able to come up with a compelling alternative model of gravity. All the attempts so far have largely failed (their have been many), as the proposed theories break in one or more critical ways.
I think their may be a small number of languishing models that aren't totally disproven but don't look very promising. But I'm not sure.
Astronomers are scientists, they don't make shit up just out of boredom, they only believe theories when they've withstood rigorous testing through observational evidence.
It's important to understand the history of dark matter / "missing matter". It's a multi-decade history that originally started out with a small amount of intriguing but seemingly persistent observational data that couldn't be explained easily (namely that when you measure how much galaxies weigh by looking at how fast the stars are orbiting that figure differs a lot from the weight you get by calculating up the contribution from all the stars and gas and dust and whatnot that we can see (the "light" matter)). In response a huge number of different "theories" (more properly hypothesis) were brought up to try to explain the mystery, while new ways of studying the problem were thought up as well.
And over the many years after the initial evidence came to light (in the '70s) considerably more evidence came to light from a wide diversity of observations. I won't list them here but I'll point out that the wikipedia page on dark matter has a good run down. Anyway, the fascinating twist to the story here is that as this evidence came to light it started eliminating various hypotheses about this missing matter until finally only one was left: the current theory of dark matter. It's not just a crazy idea, it's a crazy idea that fits all of the evidence when nothing else did.
So don't look at the WIMP dark matter theory as though it's just some half-baked idea, it's a hard fought veteran of numerous campaigns to kill it, but it just keeps going because by all the evidence it seems to be the only theory that explains reality.
As to specifically the "modified gravity" theories that compete with dark matter, they have a hard time explaining several observed phenomena, especially things like the famed "Bullet Cluster". There are a few examples where we can observe collisions between galaxy clusters and through different techniques we can map out the distribution of stars and of gas and of mass in the collision. What we observe in the Bullet Cluster as well as some others is that the gas is in a completely different place than the stars (because stars in galaxies mostly pass through one another whereas gas clouds squish together) and the center of mass of the stars of the galaxies is separated from the center of the non-visible mass of the galaxies. This absolutely cannot be explained by any of the modified gravity theories.
The article describes the rationale, as does the "observational evidence" section of the Wikipedia article on dark matter[0].
tl;dr no alternative models for gravity can sufficiently explain all of the data, but general relativity and "dark matter being particles" does, so that's what we're going with.
« On 25 August 2016, astronomers reported that Dragonfly 44, an ultra diffuse galaxy (UDG) with the mass of the Milky Way galaxy, but with nearly no discernable stars or galactic structure, may be made almost entirely of dark matter.[6][7][8] »
So my takeaway from this article is: where there is a concentration of mass, there is dark matter. (Quote: "According to models and simulations, all galaxies should be embedded in dark matter halos, whose densities peak at the galactic centers.")
Wiiild speculation: perhaps mass is actually a complex number with real and imaginary parts (analogous to how quantum mechanics describes fields), and what we're able to measure directly (by weighing) is the real part. The imaginary part ("dark matter") is some yet unknown interaction with gravitational force and what we measure indirectly with gravitational lensing is the complex magnitude.
I have no chops whatever in this field, but it's fun to think of experiments we might do:
If I understand current theory, dark matter only interacts with itself and with ordinary matter through gravity. Hence the importance of Vera Rubin's observations [1] that some galaxies were rotating too darned fast to hold together based on the ordinary matter we could see. Gotta be something invisible generating more gravitational force.
If so, couldn't we expect larger masses to attract more dark matter than lesser masses? And mightn't very high resolution measurements of those masses' gravitational forces disclose a discrepancy attributable to more dark matter clustering around a larger mass?
I think of constructing two spheres, one of lithium (density = 0.534 g/cm^3), the other of platinum (density = 21.45 g/cm^3). Both have equal diameters, and very different masses. Park them out in space - maybe in an orbit inclined 90° to the ecliptic so there's some time when they're far away from the complicating effects of planetary masses.
Then release test objects with accurately known masses (think the silicon spheres made for Gravity B Probe's gyroscopes [2]) near each of the two spheres. Minimum approach speeds would be given by the assumption of only ordinary matter in the spheres, no dark matter present. If there is dark matter, and if it accumulates according to gravitational interactions, the test mass approach speeds should be greater than calculated from ordinary matter gravitational force. Also, the larger mass should attract more dark matter and exhibit a greater deviation from ordinary matter force.
Depending on the local density of dark matter, one might see the results change over time with differential accumulation of dark matter around the two spheres.
Do we have the measurement capabilities to do something like this? I recall LIGO measures distances four orders of magnitude less than the width of a proton.
Your experiment has several fundamental flaws. Firstly, dark matter is weakly interacting (even with itself) which is why it doesn't "clump" the way atomic matter does. The only "clumping" of dark matter happens on galactic scales, because the typical speeds of dark matter particles are in the range of orbital speeds around galaxies. Our measurements of the dark matter mass in galaxies don't have nearly enough precision to be able to determine the relationship you posit with much certainty.
As to your own experimental design, using large spheres of matter, it's completely non-workable. As mentioned above, dark matter particles have velocities in the range of orbital speeds around the galaxy, which is 100s of km/s. So most of the dark matter particles in the vicinity of Earth are going to simply pass through your spheres without stopping, and certainly without increasing their mass.
Additionally, you seem to be confused about the scales of densities here. Dark matter is distributed in a somewhat uniform density on interstellar scales (there are density gradients across the galaxy however). Near the Sun the density of dark matter is about 0.0025 solar masses per cubic light-year, or about 5e-20 kg per cubic meter. So, there really is not a lot of dark matter passing through objects around Earth, mass wise. There's only maybe 6 kg within the entire volume of the Earth at any given time, for example.
Despite recent some initial evidence to the contrary,[1] I'm still a fan of the idea that dark mater is actually concentrations of matter/energy outside of our universe.
If string theory is true, gravity does leak into the multiverse, and we survive the Great Filter, then someday we may be able to use gravity to communicate universes outside of our own.
> And that’s okay! Unless dark matter happens to be of a certain mass with a certain interaction cross-section, none of the designed experiments are going to see it. That doesn’t mean dark matter isn’t real, it just means that dark matter is something else than what our experiments are optimized to find.
That's the rather underwhelming conclusion. And it's wrong insofar the size argument is not certain, but depends trivially on the distance of observation. Otherwise I'd like to know what magical number we are talking about.
Slightly off topic, but is there a resource for slightly less scientific minds to understand the universe as seen by those who do. I get an impression reading here and there that are theories to describe start and end of the universe (Or the limitations of human mind to grasp the concept of it), but I give up the moment I see too much of mathematics.
> But that’s indirect; we know there’s supposed to be a particle associated with it, and that’s what the hunt is all about.
I think is the is the most important line in the article. Why does it have to be a particle? That's a large assumption. We've conceptualized everything in our models so far as waves/particles. Maybe we need a different concept.
[+] [-] perlgeek|7 years ago|reply
We haven't directly detected dark matter because we don't really know what we are looking for, and there are only a few things we can search for. Dark matter might not be structured in a way we can investigate with current technology.
Also, dark matter doesn't interact much with regular matter, which makes the search even harder.
[+] [-] beefield|7 years ago|reply
This is (one of the many places) where I get lost. It has mass, so it by definition interacts with anything with mass?
Are these particles supposed to be so small and so rare that they can't be measured even at the scale of solar system? (How much dark matter would be in the solar system? What would be the average density of the dark matter? what would be the mass of single dark matter particle?) Or what do I miss here?
[+] [-] luxcem|7 years ago|reply
So there is two possibilities:
- There is no dark matter and the general relativity is wrong.
- There is dark matter and the general relativity is correct.
And maybe a combination of both: there is something we don't detect but the theory is also wrong.
[+] [-] thx4allthestuff|7 years ago|reply
[+] [-] brian-armstrong|7 years ago|reply
[+] [-] isaachier|7 years ago|reply
The universe isn't the penny, it's the balloon. Physicists believe we are living on the surface of a hypersphere. One important consequence of this idea is that the big bang didn't occur at a specific point in our 3D space, but at the center of the sphere.
Furthermore, the concept of a balloon expanding vs. deflating is a bit of a misconception. The argument used to be that whether or not the balloon is expanding, depending on the rate of the expansion, gravity could eventually win out and cause the matter to collapse back together (big crunch scenario). The problem with that theory is that we now know that galaxies are speeding away from us at a growing speed that (not sure the exact details, probably based on red shift in light from nearby galaxies). So the idea of gravity winning out was not based on evidence, just one of a number of possibilities, but the evidence proved it wrong beyond a doubt.
[+] [-] stephengillie|7 years ago|reply
To me, it's like 3d canon shot flying down a 4d barrel.
Or trace the worldlines of these particles. Any given point in time represents a 3d slice of the 4d pyramid-rectangle.
[+] [-] ianai|7 years ago|reply
[+] [-] monkeydreams|7 years ago|reply
Wouldn't this result in relative acceleration which correlates to the angle of the body being observed relative to the viewer? Anything aligbed directly between the viewer and a pole would move at a constant speed while speeds would appear to increase as you approach perpendicular? I can't imagine how this would play out in a 3d projection of a 4d surface however.
[+] [-] bordercases|7 years ago|reply
[+] [-] burlesona|7 years ago|reply
It’s all really fascinating.
[+] [-] stephengillie|7 years ago|reply
Youtube video that explains this in depth: https://www.youtube.com/watch?v=3HYw6vPR9qU&t=726s
[+] [-] dwaltrip|7 years ago|reply
Scientists have explored alternative models of gravity or spacetime with additional spatial dimensions and things like that. As far as I know, none of them are very promising. There was an interesting PBS space time video about one example recently: https://youtu.be/3HYw6vPR9qU
Of course, it is definitely interesting to think about :)
[+] [-] _ph_|7 years ago|reply
[+] [-] allcentury|7 years ago|reply
[+] [-] bensonn|7 years ago|reply
Expert: I am an expert on Dark Matter. Me: What is it? Expert: I have no idea but I think it exists. Me: Ok, you must be really smart.
My glib little dialog contains no sarcasm. No doubt they are very smart. Expertise is measured differently in different fields.
Expert on Oak Island knows all the rumors and theories but not where the treasure is. Expert flat-earther knows all the wrong facts. Expert politician might know .0001% due to the vastness of government. Experts on religion know the experts in other religions are wrong. Expert MLB hitters fail more than succeed.
I guess it gives me hope I may one day be an expert at something.
[+] [-] xVedun|7 years ago|reply
[+] [-] analog31|7 years ago|reply
[+] [-] tomp|7 years ago|reply
[+] [-] ajross|7 years ago|reply
[+] [-] chongli|7 years ago|reply
Of course, with more evidence, such as the observations of the Bullet cluster, these simple explanations fall apart. But at any rate, it's not enough to take one observation and assume it holds at all scales.
Look at tiny water droplets. They don't behave at all like large bodies of water.
[+] [-] InclinedPlane|7 years ago|reply
[+] [-] mabbo|7 years ago|reply
It's when you add in all the other problems that a little tweak to Newton's won't be enough.
[+] [-] tomkat0789|7 years ago|reply
On an interstellar scale, maybe this could create the lensing effects and explain some other phenomena. Is it unreasonable that 5/6 of the mass of the universe is like the stuff that makes up planets, but isn't lit up as brightly as a star?
[+] [-] dTal|7 years ago|reply
https://en.wikipedia.org/wiki/Dark_matter#Baryonic_matter
[+] [-] InclinedPlane|7 years ago|reply
[+] [-] md224|7 years ago|reply
For example: in these experiments, what would "direct" observation be? We have instruments that detect changes in certain variables, and we look for changes that align with our expectations of how a particle affects these variables. But we're not directly observing the particle... we're observing the particle's effects on these variables. So there's always an intermediary between us and the phenomena we're attempting to explore.
It seems to me that this intermediary must exist for all phenomena that cannot be perceived by our 5 senses. So how do we determine when an intermediary is "direct" vs "indirect"?
Looks like it's time for me to revisit Philosophy of Science...
[+] [-] mcguire|7 years ago|reply
I've always heard that the lack of interactions was an observation, not a deduction: we can't see it, therefore it doesn't interact.
[+] [-] jumpywizard|7 years ago|reply
Using techniques such as deep learning for program synthesis. https://www.microsoft.com/en-us/research/blog/deep-learning-...
[+] [-] _kccq|7 years ago|reply
You could feed a spreadsheet into and then it would iterate for awhile, converging toward most accurate equations describing the system. Or in my case, diverging from what I was sure was the solution. It was a hit and miss with most data (honestly, an eyeball and a few braincells did better with some data sets), but all the same I used it to lazily approximate position equations for some programs I was writing.
The desktop program was called Eureqa which then got ported to the cloud for obvious capacity increases, and relabeled into a company/product called Nutonian. Most recently bought by "DataRobot" and it's now being sold for sales optimizations. Because science.
[+] [-] darawk|7 years ago|reply
[+] [-] cygx|7 years ago|reply
Theories of modified gravity can fix those individually, but as far as I'm aware, no alternative has been shown to be viable once taken in combination.
[+] [-] dwaltrip|7 years ago|reply
I think their may be a small number of languishing models that aren't totally disproven but don't look very promising. But I'm not sure.
[+] [-] InclinedPlane|7 years ago|reply
Astronomers are scientists, they don't make shit up just out of boredom, they only believe theories when they've withstood rigorous testing through observational evidence.
It's important to understand the history of dark matter / "missing matter". It's a multi-decade history that originally started out with a small amount of intriguing but seemingly persistent observational data that couldn't be explained easily (namely that when you measure how much galaxies weigh by looking at how fast the stars are orbiting that figure differs a lot from the weight you get by calculating up the contribution from all the stars and gas and dust and whatnot that we can see (the "light" matter)). In response a huge number of different "theories" (more properly hypothesis) were brought up to try to explain the mystery, while new ways of studying the problem were thought up as well.
And over the many years after the initial evidence came to light (in the '70s) considerably more evidence came to light from a wide diversity of observations. I won't list them here but I'll point out that the wikipedia page on dark matter has a good run down. Anyway, the fascinating twist to the story here is that as this evidence came to light it started eliminating various hypotheses about this missing matter until finally only one was left: the current theory of dark matter. It's not just a crazy idea, it's a crazy idea that fits all of the evidence when nothing else did.
So don't look at the WIMP dark matter theory as though it's just some half-baked idea, it's a hard fought veteran of numerous campaigns to kill it, but it just keeps going because by all the evidence it seems to be the only theory that explains reality.
As to specifically the "modified gravity" theories that compete with dark matter, they have a hard time explaining several observed phenomena, especially things like the famed "Bullet Cluster". There are a few examples where we can observe collisions between galaxy clusters and through different techniques we can map out the distribution of stars and of gas and of mass in the collision. What we observe in the Bullet Cluster as well as some others is that the gas is in a completely different place than the stars (because stars in galaxies mostly pass through one another whereas gas clouds squish together) and the center of mass of the stars of the galaxies is separated from the center of the non-visible mass of the galaxies. This absolutely cannot be explained by any of the modified gravity theories.
[+] [-] kamaal|7 years ago|reply
[+] [-] unknown|7 years ago|reply
[deleted]
[+] [-] krapp|7 years ago|reply
tl;dr no alternative models for gravity can sufficiently explain all of the data, but general relativity and "dark matter being particles" does, so that's what we're going with.
[0]https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
[+] [-] The_rationalist|7 years ago|reply
Meta-source: https://en.m.wikipedia.org/wiki/Dark_galaxy
[+] [-] The_rationalist|7 years ago|reply
[+] [-] wwarner|7 years ago|reply
[+] [-] zvrba|7 years ago|reply
Wiiild speculation: perhaps mass is actually a complex number with real and imaginary parts (analogous to how quantum mechanics describes fields), and what we're able to measure directly (by weighing) is the real part. The imaginary part ("dark matter") is some yet unknown interaction with gravitational force and what we measure indirectly with gravitational lensing is the complex magnitude.
[+] [-] ridgeguy|7 years ago|reply
If I understand current theory, dark matter only interacts with itself and with ordinary matter through gravity. Hence the importance of Vera Rubin's observations [1] that some galaxies were rotating too darned fast to hold together based on the ordinary matter we could see. Gotta be something invisible generating more gravitational force.
If so, couldn't we expect larger masses to attract more dark matter than lesser masses? And mightn't very high resolution measurements of those masses' gravitational forces disclose a discrepancy attributable to more dark matter clustering around a larger mass?
I think of constructing two spheres, one of lithium (density = 0.534 g/cm^3), the other of platinum (density = 21.45 g/cm^3). Both have equal diameters, and very different masses. Park them out in space - maybe in an orbit inclined 90° to the ecliptic so there's some time when they're far away from the complicating effects of planetary masses.
Then release test objects with accurately known masses (think the silicon spheres made for Gravity B Probe's gyroscopes [2]) near each of the two spheres. Minimum approach speeds would be given by the assumption of only ordinary matter in the spheres, no dark matter present. If there is dark matter, and if it accumulates according to gravitational interactions, the test mass approach speeds should be greater than calculated from ordinary matter gravitational force. Also, the larger mass should attract more dark matter and exhibit a greater deviation from ordinary matter force.
Depending on the local density of dark matter, one might see the results change over time with differential accumulation of dark matter around the two spheres.
Do we have the measurement capabilities to do something like this? I recall LIGO measures distances four orders of magnitude less than the width of a proton.
[1] https://en.wikipedia.org/wiki/Vera_Rubin [2] https://einstein.stanford.edu/TECH/technology1.html
[+] [-] InclinedPlane|7 years ago|reply
As to your own experimental design, using large spheres of matter, it's completely non-workable. As mentioned above, dark matter particles have velocities in the range of orbital speeds around the galaxy, which is 100s of km/s. So most of the dark matter particles in the vicinity of Earth are going to simply pass through your spheres without stopping, and certainly without increasing their mass.
Additionally, you seem to be confused about the scales of densities here. Dark matter is distributed in a somewhat uniform density on interstellar scales (there are density gradients across the galaxy however). Near the Sun the density of dark matter is about 0.0025 solar masses per cubic light-year, or about 5e-20 kg per cubic meter. So, there really is not a lot of dark matter passing through objects around Earth, mass wise. There's only maybe 6 kg within the entire volume of the Earth at any given time, for example.
[+] [-] slacka|7 years ago|reply
If string theory is true, gravity does leak into the multiverse, and we survive the Great Filter, then someday we may be able to use gravity to communicate universes outside of our own.
https://www.sciencenews.org/article/gravity-doesnt-leak-larg...
[+] [-] posterboy|7 years ago|reply
That's the rather underwhelming conclusion. And it's wrong insofar the size argument is not certain, but depends trivially on the distance of observation. Otherwise I'd like to know what magical number we are talking about.
[+] [-] sanmon3186|7 years ago|reply
[+] [-] prewett|7 years ago|reply
[+] [-] gauravphoenix|7 years ago|reply
[+] [-] golemotron|7 years ago|reply
I think is the is the most important line in the article. Why does it have to be a particle? That's a large assumption. We've conceptualized everything in our models so far as waves/particles. Maybe we need a different concept.