It reads like a great example of how theory works.
You have some evidence (old planets), and a model that predicts it well (Newton/Kepler). Then some new evidence comes in (Uranus), and the theory is almost right, but not quite. So we modify it a little, or we start searching for an extra mass to explain the error.
Then some more experimental evidence comes in (galactic radial velocity), and we are thinking about modifying the theory a bit more (MOND). This is good for the radial velocity explanation, but not certain other things. We suppose there's some dark matter, and think about the consequences that would have. Then we see evidence consistent with that.
Of course this is a boiled down version. Real science is rarely so clean.
I'm no physicist but I've never liked dark matter and dark energy: they smell suspiciously similar to hacks (the bad kind) if a hack could be defined as over-fitting a solution to a problem.
It seems as though, recently, science has become rather bad at the words "I don't know." Things were staying together more than they should so we invented dark matter in order to explain that. Now, whenever something stays together more than it should we claim that dark matter has more evidence[1]: no, there is simply more evidence of the problem that dark matter seeks to solve. How do we even know it's a single problem (a.k.a. force)? How do we even know that dark energy is another problem?
It's like me claiming that little monsters called frombles are responsible for light and every time I see light I claim "see! There's light, so there are frombles!"
Not that dark matter is a bad solution, I just feel as though there is a disproportionate amount of people looking into it given that we've never actually observed a WIMP. Nor do I think that research into dark matter should cease, it has so far found a really good definition of the problem and, yes, WIMPs are a good candidate solution.
If we could have a few more people educated in the field start off at "I don't know," that would be great. You know, the "dirty" kind of science that used to be done when nobody really knew what was going on at all - that's when we learned the most: when we didn't have the "safe bet" research topics. When people came up with stupid ideas and tested those stupid ideas. Stupid ideas are the best ideas because it at least means that one person is thinking outside of the box.
Real science is ugly because there are a bunch of theories that kinda sorta fit, but not really well. I think Maxwell had something like 40 equations for electricity. Give it a hundred years and hundreds of thousands of grad student hours, and something quite clean emerges.
The unknown edges are messy, but as competing theories get winnowed out, merged and refined they become clear and concise.
> Then some more experimental evidence comes in (galactic radial velocity), and we are thinking about modifying the theory a bit more (MOND). This is good for the radial velocity explanation, but not certain other things. We suppose there's some dark matter, and think about the consequences that would have. Then we see evidence consistent with that.
Just to be clear, in case someone interprets that as being a time-ordered list, the existence of some "dark" matter was the proposed explanation prior to the development of MOND (though MOND followed very quickly).
Robust evidence for dark matter is generally (rightfully) attributed to the work of Vera Rubin (as in the parent article), but it is worth noting that Friz Zwicky also proposed dark matter in the 1930s to explain the velocities of galaxies in clusters.
I recommend you to read the work of an astrophysicist Alexander F. Mayer (Stanford): http://www.sensibleuniverse.net/ who proves that the theory of the universe as we know since Einstein is wrong.
These "MOND" gravity theories purport to explain observations better than the theory of dark matter, but they don't. They explain some things and leave gaping holes elsewhere. On the whole they are hugely inferior theories in terms of matching observational evidence. Their only advantage is that they lack the existence of massive, weakly interacting particles, which a lot of people seem to have strong objections to despite the fact that several types of such particles are already known to exist.
Throughout the entire history of the development of the theory of dark matter the bias has always been against the idea of true dark matter, unseen particles that make up most of the mass of the Universe. But at every single step all of the other possibilities have been eliminated as realistic explanations of the evidence. The cold WIMP dark matter theory is the only one that explains the structure of the cosmic microwave background radiation, the large scale structure of the Universe, and the structure and behavior of galaxies and galaxy clusters. At this point the theory of dark matter is remarkably well hemmed in, it would be absolutely shocking to an extreme degree if it did not truly exist in the form we think it does.
There's additionally a very elegant mechanism behind the formation of dark matter. It seems most likely to be composed of super-symmetric particles. Such particles would only be able to form in extreme, high-energy conditions. Precisely the sort of conditions that existed in the early era of the Universe immediately after the Big Bang. So the early Universe would have been creating huge quantities of such particles, which would then not interact much due to their weak interactivity. It wouldn't be until the Universe had expanded enough and cooled down enough to halt their production. This would neatly explain why so much of the mass of the Universe is in the form of dark matter rather than conventional matter.
> These "MOND" gravity theories purport to explain observations better than the theory of dark matter, but they don't.
MOND consistently fits the rotational velocity profile of galaxies better than dark matter. This isn't too surprising as MOND originated as an ad-hoc formula derived from observations of said rotational profiles, however MOND works very well over a very wide range of galactic profiles. The mass deficit observed in tidal dwarf galaxies is also quite strong evidence against just dark matter. I don't think anybody is seriously claiming that MOND is a complete theory, though of course there have been attempts by theorists at creating complete models, but they've failed to explain large-scale structural observation and the cosmic microwave background. This just means these attempts at creating a complete theory are probably wrong, not that there isn't something funky going on with galaxies that isn't completely accounted for by dark matter.
> They explain some things and leave gaping holes elsewhere.
This is a false dichotomy. MOND is not incompatible with dark matter. This "us vs them" mentality is really quite harmful. A combination of MOND and dark matter matches most observations very well, far better than dark matter in isolation.
> Their only advantage is that they lack the existence of massive, weakly interacting particles, which a lot of people seem to have strong objections to despite the fact that several types of such particles are already known to exist.
While I'm sure there are quite a few people for whom this is the case, you shouldn't dismiss the possiblity there are people who take MOND seriously because it legitimately fits the data better in some circumstances.
I graduated physics, but I'm almost a layman when it comes to general relativity. Still, I wonder if another possible explanation for stuff that don't match theory could be the basic assumption about our current cosmology. That the space is flat where there's no matter. Since there's a relation between the geometry of the space and the matter within, would it be unreasonable to believe that space itself could be curved independently of the matter we can observed? Maybe for other reasons, yet unknown?
Something has orbits around galaxies
failing to follow the law of gravity
from ordinary matter. The orbits
are as if there
there is some more mass with
gravity, mass that doesn't interact with
light.
Something is in/near galaxies is bending
light as if there were more mass there
than there is from just ordinary matter.
So, for that extra mass, call it dark
matter.
Seems simple enough.
"Flaw in the theory of gravity?" Maybe,
but that seems to be the hard way
to get an answer.
The orbits around galaxies fail to follow the law of gravity of ordinary matter. The orbits are as if our gravity calculations, which seem to work well otherwise, are flawed.
Something in/near galaxies causes them to bend light as if gravity were different.
So, for that region, call it a flaw in the theory of gravity. Seems simple enough.
"Unseen, never-before-detected form of matter which doesn't interact with any other matter except gravitationally?" Maybe, but that seems to be the hard way to get an answer.
This is tongue-in-cheek of course, but it's hard to trust intuition in these realms, especially when one person's intuition is another person's "hard way" :)
Exactly. Dark matter and dark energy is just a symptom of the standard model of physics being incorrect.
>Dark matter neither emits nor absorbs light or any other electromagnetic radiation at any significant level. According to the Planck mission team, and based on the standard model of cosmology, the total mass–energy of the known universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.[2][3] Thus, dark matter is estimated to constitute 84.5% of the total matter in the universe, while dark energy plus dark matter constitute 95.1% of the total mass–energy content of the universe.[4][5][6]
"it must make new predictions that can be experimentally or observationally tested, and confirmed or refuted, that are unique to this new theory"
("MOND fails spectacularly, either offering no predictions...")
I fail to understand the necessity for new predictions. This simply may or may not come out of new theories as a paradigm shift consequence and it is therefore just optional, not a must. In fact, I fail to fully understand even the first requirement (about reproducing all the successes of previous leading theory) - the current leading theory's explanation may happen to explain things somehow, but that may not be necessarily correct. I wouldn't dismiss a competing theory even with holes in it if there is a slightest chance that further work on it or on something closely related to it may come to cover those shortcomings in time.
I think the point is that, for a new theory to become more favored than the current leading theory, the new theory must be consistent with at least as much scientific evidence as the leading theory. From there, the value of predictions is that they give a path for strengthening the theory further.
I think you're right in saying that these are not hard-and-fast rules.
Interesting, I always thought of dark matter as just a lot of extra "dust", the dust that clumps to make the stars that make up the universe. The asteroid belt, the extra mass in our kuiper or oort clouds, etc. The dust that blocks parts of our view of the Milky way.
This article (and others I'm now skimming) point out that dark matter needs other magical properties however. Why is that the case? Why does simple "dark dust" not fit?
There's nothing magical about the required properties, so I'm not sure why anyone would describe them that way. They are no different than the properties imputed to neutrinos or the aether: http://www.tjradcliffe.com/?p=1801
The important thing about extra-galactic dark matter is there is too much of it to be made of ordinary particles: protons and neutrons and electrons. We know how many ordinary nucleons there are because we have a pretty reasonable estimate of the primoridal ratio of hydrogen to helium. As the Big Bang cooled quarks eventually cooled down enough to make protons and neutrons. Free neutrons only live about fifteen minutes so they only had a short time to capture onto protons and make deuterium nuclei which could further colide to make helium. The ratio of He/H in the early universe is therefore a sensitive measure of the denisty of the universe at the time of nucleon condensation, and we know the radius from the temperature, so we can calculate the total number of particles. There aren't enough to make up extra-galactic dark matter. There are enough to make up galactic dark matter, so there may be two totally unrelated dark matter problems.
When physicists talk about "dark matter" we are sliding back and forth between meanings according to context, but this is all lost on outsiders, unfortunately. It's needlessly confusing, but unfortunately it's the way it is.
So amongst the perfectly ordinary, non-magical properties that extra-galactic dark matter has to have is to be something other then ordinary neutrons and protons. There are some other contraints on its various interaction strengths that come from the scale of galaxy formation and larger scale structures in the universe too. Again, these are perfectly ordinary properties, inferred based on the evidence in exactly the same way the properties of aether and neutrinos were.
Dark matter may or may not exist (neutrinos do, aether does not) but there is absolutely nothing out of the ordinary about it. It is just normal science doing what science normally does.
"Dark dust" would, in many circumstances, have at least an infrared signature (or, rather, a signature that originates in the infrared, even if it has shifted considerably downward by the time we see it). This stuff is "dark" in the sense that the only effects we can see at all (so far, at least) are gravitational. In terms of what we think we do know, that implies mass - but mass that doesn't interact with light except insofar as it distorts spacetime.
Because dust would emit something, especially while being hot from nearby stars radiation. It would also absorb waves from background, to some degree. Dust and gasses are pretty familiar guys, comparing to dark matter.
It's "dark", in that it doesn't interact with electromagnetic forces (photons and electrons). Neutrinos are one example of this kind of matter, so it seems reasonable that there are other kinds that are even more difficult to observe.
There'd have to be so much of it that we'd see it. There's way more dark matter than normal matter in the universe (stipulating that dark matter theories as we currently understand them are at a coarse level correct).
If it were "dark dust," there would be enough of it that we would be able to detect it through means other than gravitational ones.
In theory, dark matter is composed of particles that have mass (and can therefore produce gravitational effects that are noticeable if you have a bunch of them), but they do not interact with the particles that compose atoms via electromagnetic forces or via the "strong nuclear force" (the force that holds the nucleus of an atom together).
This means that dark matter is not like the "physical matter" that you're used to seeing. Dark matter doesn't absorb or emit light, so it doesn't block our view of some parts of the Milky Way. The stuff that blocks our view is basically "space dust", and that "space dust" is made of atoms, not dark matter.
According to the theories of dark matter, it can affect our view of other places in the universe, but only via "gravitational lensing". "Gravitational lensing" is where there are enough particles with lots of mass all together (like a galaxy) that they noticeably distort light as it goes by. There is compelling evidence for dark matter from observations of gravitational lensing effects near galaxies that do not have enough mass from their observable stars to account for the degree to which light is distorted as it goes past the galaxy--in other words, to produce that effect, some galaxies must have significant amounts of mass from particles which are not visible.
Why couldn't that extra mass be from "space dust"? Because there is an enormous amount of mass that we can detect via gravitational lensing, but which is not emitting or absorbing light. "Space dust" does not make up much mass for each galaxy compared to the stars and black holes. It's nowhere near the quantity required to produce the observed gravitational lensing effects.
Dark matter & gravitational lensing:
http://m.phys.org/news/2014-07-large-dark-peaks-gravitationa...
- This article notes that dark matter is estimated to comprise about 80% of the mass of the universe. If dark matter was made of visible particles, that would be like having four times as much visible material floating around in galaxies as the stars and black holes which comprise the galaxies, and somehow we don't see any of it.
The current best explanation for all of the observational evidence is a "cold dark matter" (CDM) theory, where the dark matter is composed of "WIMPs" (weakly interacting massive particles). Neutrinos are WIMPs that we know exist, and compose some small fraction of dark matter, but neutrinos are "hot", they travel at relativistic speeds, rather than much slower speeds.
There have been many theories of what the "unseen mass" could have been made of before the CDM/WIMP theory gained ground, but those competing theories have been eliminated by observational evidence which contradicts their predicted effects. Large amounts of dust would block visible light and impose characteristic changes on the light that passes through it. We see dust in other galaxies and our own, but we don't not see nearly enough dust in the right locations and in the right quantities to account for the missing mass. So it can't be dust. Large gas clouds are another idea. Huge, transparent, whispy clouds of gas. But these too would affect the light passing through them, adding the spectral absorption signature of the gas to the light coming from the other side of the cloud. We can observe several large concentrations of gas around galaxies, and often the mass of that gas is larger than the total mass of stars in the galaxy, but it's still far too low, by an order of magnitude or more, to be the missing mass. We also see cases, such as the bullet cluster, where we can map the location of the stars, gas, and the overall mass (through gravitational lensing), and we see that the mass is not where the gas is. Another theory is that it could be not dust but bigger chunks of stuff, like planets or brown dwarfs, so called "MaCHOs" (Massive Compact Halo Objects). But to make up the missing mass there would have to be a great many of them. We can collect statistics on how common such objects are in our own galaxy and nearby galaxies using gravitational micro-lensing surveys. Because there are a huge number of stars in the sky, and if there were lots of MaCHOs floating around then every once in a while one would happen to be precisely lined up along the line of sight to another star and that arrangement would slightly brighten the remote star. We can monitor a huge number of stars using modern digital imaging systems and survey the MaCHO population this way. We've found several such micro-lensing events but again the observations put an upper limit on the MaCHO population which is far, far below what could possibly account for all of the missing mass.
No matter how you slice the observations, you still end up with a lot of missing mass and a lot of inconsistent observations. Unless you accept the possibility of WIMP dark matter. When you do that then all of the observations fall into place. Not just the above but also things like the large scale structure of the universe, the structure of the cosmic microwave background radiation, and so forth.
The constraints from big bang nucleosynthesis (see e.g. http://ned.ipac.caltech.edu/level5/Sept09/Einasto/Einasto4.h...) say normal (baryonic) matter only makes up a small fraction of the mass in the universe. Therefore the remaining matter has to be non-baryonic (not normal gas or dust).
You can distribute dark matter in space to shape it in any pattern you like and give the dark matter any kind of property just to create a mathematical model that is consistent with observations.
Dark matter is just a theoretical concept - for now or forever.
Most of concepts we are conditioned to take for granted does not exist outside our head.)
The essence of scientific method is not in piling up concepts or in building up a support for a fancy theory. On the contrary, it is in exactly opposite - it is to refute and remove everything "mental" to see what remains, something which cannot be shaken by mere speculations or dismissed by any experience. That, perhaps, has something to do with what we call Truth.
According to the ancients, who practiced this method, there is nothing but different forms of light.
The question "what is between two Photons" makes no sense, because two Photons have nothing in common exept an imagination of an observer. Tracing them back "in time" makes no sense either, because time as a category is irrelevant to the light. It doesn't change.
I found the most interesting part of the article to be this image of a 1919 newspaper illustrating "Starlight bent by the sun's attraction": The Einstein theory.
Perhaps our observations are a result of existing within two gravitational space-time distortions, one trivial and one not. The trivial one being the Sun and the non-trivial being the galaxy which we inhabit.
If curvature of space-time theorized by General Relativity is accepted, then observing non-local energy will be affected by gravity in which the observation is being undertaken. Accounting for the Sun's influence has largely (if not completely) been addressed, but there lies a more difficult "elephant in the room" problem of determining what effect the gravitational curvature of the Milky Way has on any observations we can make.
According to the University of Oregon[1], the observable mass of the Milky Way is 200,000,000,000 Solar masses. The same material indicates that visible stars account for 30% of that. Throw in a bit for singularities and lets call it "less than 50%" for the sake of discussion. In any event, there exists enough mass in this galaxy to plausibly state that the space-time we inhabit is affected. Yet a non-trivial amount considered "unaccounted for."
Using the canonical 3d inverse-cone visualization of matter's effect on space-time, imagine our solar system exists on some point within the galaxy's gravitational cone yet not near its center. Making intra-galactic calculations may be influenced by compressed space-time instead of the presence of dark matter. And observations regarding other galaxies colliding (as mentioned in the article) migth be explained by gravitational phase shifting.
Though all of this likely will be rebuked, it is why I originally said "sure."
For those who are interested, physicist John Hartnett has developed a model that does not require dark matter. His work builds on that of Moshe Carmeli, who published "Cosmological Special Relativity". Hartnett's book is "Starlight, Time, and the New Physics" ISBN 978-0-949906-68-7. Disclaimer: I am not a physicist, and the math is way over my head, so I cannot double-check it. Hartnett has not made much, if any, headway with mainstream scientific publications, probably because he is a Biblical Creationist.
I'm no physicist but I've asked myself this question. It's a bit like string theory. String theory is a mathematical answer that solves some (all?) of the inconsistencies between quantum theory and gravity but has some implications that have no experimental evidence (eg extra dimensions).
Dark matter always struck me as a kludge to explain the universe fit our model. There are many theories on it but no observed or experimental effects of dark matter to date.
We say that there are four fundamental forces in the universe: weak nuclear, strong nuclear, electromagnetism and gravity. The first three are pretty adequately explained by quantum mechanics (AFAICT). The last of course isn't but is described pretty darn well by relativity.
Thing is, what's always struck me as odd is the other forces, I believe, can both attract and repel. There is no observation or experimental evidence of "antigravity".
It has always made me wonder if we're not making an error by trying to fit gravity into the Standard Model. Perhaps it's not a force like the others are and simply the result of the nature of space-time.
Anyway, like I said: complete layman here. What I'm saying might be completely nonsensical to anyone with half a clue about physics.
I know the LHC has looked and is looking for any evidence at the edges of the Standard Model of some violation that will hint at something that might explain all this. I also know minds far brighter than mine have spent a lot of time thinking about this.
I'm still left wondering if dark matter/energy isn't simply another "string theory".
Dark matter is no more a kludge than phlogiston, or the lumineferous aether, or neutrinos. All of them were introduced as novel entities to explain experimental phenomena. The first two turn out not to exist, the latter does: http://www.tjradcliffe.com/?p=1801
Calling them kludges is to mistake the basic process of science for a hack.
There are any number of confirmatory observations that fit the dark matter model, just as there were any number of confirmatory observations that fit the neutrio model. It's just that we haven't observed any dark matter particle interactions directly, just as we hadn't observed any neutrino interactions directly before Reines' experiment.
It may be that dark matter turns out not to exist, and then we will know it was wrong. Until then it is perfectly ordinary science working in perfectly ordinary ways as it has been for over a century.
> There are many theories on it but no observed or experimental effects of dark matter to date.
FTFA:
> And most spectacularly, you get an entirely new prediction: that when you get two clusters of galaxies colliding, the gas inside should heat up, slow down and emit X-rays (in pink, above), while the mass that we can see through gravitational lensing (in blue, above) should follow the dark matter, and be displaced from the X-rays. This new prediction has been borne out observationally and held up over the past decade, a spectacular indirect confirmation of dark matter.
[+] [-] lordnacho|10 years ago|reply
You have some evidence (old planets), and a model that predicts it well (Newton/Kepler). Then some new evidence comes in (Uranus), and the theory is almost right, but not quite. So we modify it a little, or we start searching for an extra mass to explain the error.
Then some more experimental evidence comes in (galactic radial velocity), and we are thinking about modifying the theory a bit more (MOND). This is good for the radial velocity explanation, but not certain other things. We suppose there's some dark matter, and think about the consequences that would have. Then we see evidence consistent with that.
Of course this is a boiled down version. Real science is rarely so clean.
[+] [-] zamalek|10 years ago|reply
I'm no physicist but I've never liked dark matter and dark energy: they smell suspiciously similar to hacks (the bad kind) if a hack could be defined as over-fitting a solution to a problem.
It seems as though, recently, science has become rather bad at the words "I don't know." Things were staying together more than they should so we invented dark matter in order to explain that. Now, whenever something stays together more than it should we claim that dark matter has more evidence[1]: no, there is simply more evidence of the problem that dark matter seeks to solve. How do we even know it's a single problem (a.k.a. force)? How do we even know that dark energy is another problem?
It's like me claiming that little monsters called frombles are responsible for light and every time I see light I claim "see! There's light, so there are frombles!"
Not that dark matter is a bad solution, I just feel as though there is a disproportionate amount of people looking into it given that we've never actually observed a WIMP. Nor do I think that research into dark matter should cease, it has so far found a really good definition of the problem and, yes, WIMPs are a good candidate solution.
If we could have a few more people educated in the field start off at "I don't know," that would be great. You know, the "dirty" kind of science that used to be done when nobody really knew what was going on at all - that's when we learned the most: when we didn't have the "safe bet" research topics. When people came up with stupid ideas and tested those stupid ideas. Stupid ideas are the best ideas because it at least means that one person is thinking outside of the box.
[1]: http://www.iflscience.com/physics/researchers-claim-have-fou...
[+] [-] jfoutz|10 years ago|reply
The unknown edges are messy, but as competing theories get winnowed out, merged and refined they become clear and concise.
[+] [-] privong|10 years ago|reply
Just to be clear, in case someone interprets that as being a time-ordered list, the existence of some "dark" matter was the proposed explanation prior to the development of MOND (though MOND followed very quickly).
Robust evidence for dark matter is generally (rightfully) attributed to the work of Vera Rubin (as in the parent article), but it is worth noting that Friz Zwicky also proposed dark matter in the 1930s to explain the velocities of galaxies in clusters.
[+] [-] niutech|10 years ago|reply
[+] [-] InclinedPlane|10 years ago|reply
These "MOND" gravity theories purport to explain observations better than the theory of dark matter, but they don't. They explain some things and leave gaping holes elsewhere. On the whole they are hugely inferior theories in terms of matching observational evidence. Their only advantage is that they lack the existence of massive, weakly interacting particles, which a lot of people seem to have strong objections to despite the fact that several types of such particles are already known to exist.
Throughout the entire history of the development of the theory of dark matter the bias has always been against the idea of true dark matter, unseen particles that make up most of the mass of the Universe. But at every single step all of the other possibilities have been eliminated as realistic explanations of the evidence. The cold WIMP dark matter theory is the only one that explains the structure of the cosmic microwave background radiation, the large scale structure of the Universe, and the structure and behavior of galaxies and galaxy clusters. At this point the theory of dark matter is remarkably well hemmed in, it would be absolutely shocking to an extreme degree if it did not truly exist in the form we think it does.
There's additionally a very elegant mechanism behind the formation of dark matter. It seems most likely to be composed of super-symmetric particles. Such particles would only be able to form in extreme, high-energy conditions. Precisely the sort of conditions that existed in the early era of the Universe immediately after the Big Bang. So the early Universe would have been creating huge quantities of such particles, which would then not interact much due to their weak interactivity. It wouldn't be until the Universe had expanded enough and cooled down enough to halt their production. This would neatly explain why so much of the mass of the Universe is in the form of dark matter rather than conventional matter.
[+] [-] dkbrk|10 years ago|reply
MOND consistently fits the rotational velocity profile of galaxies better than dark matter. This isn't too surprising as MOND originated as an ad-hoc formula derived from observations of said rotational profiles, however MOND works very well over a very wide range of galactic profiles. The mass deficit observed in tidal dwarf galaxies is also quite strong evidence against just dark matter. I don't think anybody is seriously claiming that MOND is a complete theory, though of course there have been attempts by theorists at creating complete models, but they've failed to explain large-scale structural observation and the cosmic microwave background. This just means these attempts at creating a complete theory are probably wrong, not that there isn't something funky going on with galaxies that isn't completely accounted for by dark matter.
> They explain some things and leave gaping holes elsewhere.
This is a false dichotomy. MOND is not incompatible with dark matter. This "us vs them" mentality is really quite harmful. A combination of MOND and dark matter matches most observations very well, far better than dark matter in isolation.
> Their only advantage is that they lack the existence of massive, weakly interacting particles, which a lot of people seem to have strong objections to despite the fact that several types of such particles are already known to exist.
While I'm sure there are quite a few people for whom this is the case, you shouldn't dismiss the possiblity there are people who take MOND seriously because it legitimately fits the data better in some circumstances.
[+] [-] ci5er|10 years ago|reply
[+] [-] ajkjk|10 years ago|reply
[+] [-] cristianpascu|10 years ago|reply
[+] [-] graycat|10 years ago|reply
Something is in/near galaxies is bending light as if there were more mass there than there is from just ordinary matter.
So, for that extra mass, call it dark matter.
Seems simple enough.
"Flaw in the theory of gravity?" Maybe, but that seems to be the hard way to get an answer.
[+] [-] dandelany|10 years ago|reply
The orbits around galaxies fail to follow the law of gravity of ordinary matter. The orbits are as if our gravity calculations, which seem to work well otherwise, are flawed.
Something in/near galaxies causes them to bend light as if gravity were different.
So, for that region, call it a flaw in the theory of gravity. Seems simple enough.
"Unseen, never-before-detected form of matter which doesn't interact with any other matter except gravitationally?" Maybe, but that seems to be the hard way to get an answer.
This is tongue-in-cheek of course, but it's hard to trust intuition in these realms, especially when one person's intuition is another person's "hard way" :)
[+] [-] smil|10 years ago|reply
>Dark matter neither emits nor absorbs light or any other electromagnetic radiation at any significant level. According to the Planck mission team, and based on the standard model of cosmology, the total mass–energy of the known universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.[2][3] Thus, dark matter is estimated to constitute 84.5% of the total matter in the universe, while dark energy plus dark matter constitute 95.1% of the total mass–energy content of the universe.[4][5][6]
Science is literally religious faith.
[+] [-] raverbashing|10 years ago|reply
[+] [-] restalis|10 years ago|reply
...
"it must make new predictions that can be experimentally or observationally tested, and confirmed or refuted, that are unique to this new theory"
("MOND fails spectacularly, either offering no predictions...")
I fail to understand the necessity for new predictions. This simply may or may not come out of new theories as a paradigm shift consequence and it is therefore just optional, not a must. In fact, I fail to fully understand even the first requirement (about reproducing all the successes of previous leading theory) - the current leading theory's explanation may happen to explain things somehow, but that may not be necessarily correct. I wouldn't dismiss a competing theory even with holes in it if there is a slightest chance that further work on it or on something closely related to it may come to cover those shortcomings in time.
[+] [-] drpre|10 years ago|reply
I think the point is that, for a new theory to become more favored than the current leading theory, the new theory must be consistent with at least as much scientific evidence as the leading theory. From there, the value of predictions is that they give a path for strengthening the theory further.
I think you're right in saying that these are not hard-and-fast rules.
[+] [-] mixmastamyk|10 years ago|reply
This article (and others I'm now skimming) point out that dark matter needs other magical properties however. Why is that the case? Why does simple "dark dust" not fit?
[+] [-] BurningFrog|10 years ago|reply
But not matter you can touch. Which makes it not really matter in normal English. You probably can't see it either.
So "Invisible Untouchable Gravitation Sources" would be a more descriptive name. I don't think it'll catch on.
[+] [-] tjradcliffe|10 years ago|reply
The important thing about extra-galactic dark matter is there is too much of it to be made of ordinary particles: protons and neutrons and electrons. We know how many ordinary nucleons there are because we have a pretty reasonable estimate of the primoridal ratio of hydrogen to helium. As the Big Bang cooled quarks eventually cooled down enough to make protons and neutrons. Free neutrons only live about fifteen minutes so they only had a short time to capture onto protons and make deuterium nuclei which could further colide to make helium. The ratio of He/H in the early universe is therefore a sensitive measure of the denisty of the universe at the time of nucleon condensation, and we know the radius from the temperature, so we can calculate the total number of particles. There aren't enough to make up extra-galactic dark matter. There are enough to make up galactic dark matter, so there may be two totally unrelated dark matter problems.
When physicists talk about "dark matter" we are sliding back and forth between meanings according to context, but this is all lost on outsiders, unfortunately. It's needlessly confusing, but unfortunately it's the way it is.
So amongst the perfectly ordinary, non-magical properties that extra-galactic dark matter has to have is to be something other then ordinary neutrons and protons. There are some other contraints on its various interaction strengths that come from the scale of galaxy formation and larger scale structures in the universe too. Again, these are perfectly ordinary properties, inferred based on the evidence in exactly the same way the properties of aether and neutrinos were.
Dark matter may or may not exist (neutrinos do, aether does not) but there is absolutely nothing out of the ordinary about it. It is just normal science doing what science normally does.
[+] [-] stan_rogers|10 years ago|reply
[+] [-] deepsun|10 years ago|reply
[+] [-] bashinator|10 years ago|reply
[+] [-] aetherson|10 years ago|reply
If it were "dark dust," there would be enough of it that we would be able to detect it through means other than gravitational ones.
[+] [-] djokkataja|10 years ago|reply
This means that dark matter is not like the "physical matter" that you're used to seeing. Dark matter doesn't absorb or emit light, so it doesn't block our view of some parts of the Milky Way. The stuff that blocks our view is basically "space dust", and that "space dust" is made of atoms, not dark matter.
According to the theories of dark matter, it can affect our view of other places in the universe, but only via "gravitational lensing". "Gravitational lensing" is where there are enough particles with lots of mass all together (like a galaxy) that they noticeably distort light as it goes by. There is compelling evidence for dark matter from observations of gravitational lensing effects near galaxies that do not have enough mass from their observable stars to account for the degree to which light is distorted as it goes past the galaxy--in other words, to produce that effect, some galaxies must have significant amounts of mass from particles which are not visible.
Why couldn't that extra mass be from "space dust"? Because there is an enormous amount of mass that we can detect via gravitational lensing, but which is not emitting or absorbing light. "Space dust" does not make up much mass for each galaxy compared to the stars and black holes. It's nowhere near the quantity required to produce the observed gravitational lensing effects.
Related: Space dust: https://en.m.wikipedia.org/wiki/Cosmic_dust
Dark matter & gravitational lensing: http://m.phys.org/news/2014-07-large-dark-peaks-gravitationa... - This article notes that dark matter is estimated to comprise about 80% of the mass of the universe. If dark matter was made of visible particles, that would be like having four times as much visible material floating around in galaxies as the stars and black holes which comprise the galaxies, and somehow we don't see any of it.
[+] [-] InclinedPlane|10 years ago|reply
There have been many theories of what the "unseen mass" could have been made of before the CDM/WIMP theory gained ground, but those competing theories have been eliminated by observational evidence which contradicts their predicted effects. Large amounts of dust would block visible light and impose characteristic changes on the light that passes through it. We see dust in other galaxies and our own, but we don't not see nearly enough dust in the right locations and in the right quantities to account for the missing mass. So it can't be dust. Large gas clouds are another idea. Huge, transparent, whispy clouds of gas. But these too would affect the light passing through them, adding the spectral absorption signature of the gas to the light coming from the other side of the cloud. We can observe several large concentrations of gas around galaxies, and often the mass of that gas is larger than the total mass of stars in the galaxy, but it's still far too low, by an order of magnitude or more, to be the missing mass. We also see cases, such as the bullet cluster, where we can map the location of the stars, gas, and the overall mass (through gravitational lensing), and we see that the mass is not where the gas is. Another theory is that it could be not dust but bigger chunks of stuff, like planets or brown dwarfs, so called "MaCHOs" (Massive Compact Halo Objects). But to make up the missing mass there would have to be a great many of them. We can collect statistics on how common such objects are in our own galaxy and nearby galaxies using gravitational micro-lensing surveys. Because there are a huge number of stars in the sky, and if there were lots of MaCHOs floating around then every once in a while one would happen to be precisely lined up along the line of sight to another star and that arrangement would slightly brighten the remote star. We can monitor a huge number of stars using modern digital imaging systems and survey the MaCHO population this way. We've found several such micro-lensing events but again the observations put an upper limit on the MaCHO population which is far, far below what could possibly account for all of the missing mass.
No matter how you slice the observations, you still end up with a lot of missing mass and a lot of inconsistent observations. Unless you accept the possibility of WIMP dark matter. When you do that then all of the observations fall into place. Not just the above but also things like the large scale structure of the universe, the structure of the cosmic microwave background radiation, and so forth.
[+] [-] xioxox|10 years ago|reply
[+] [-] Retra|10 years ago|reply
[+] [-] madaxe_again|10 years ago|reply
[+] [-] 0xe2-0x9a-0x9b|10 years ago|reply
Dark matter is just a theoretical concept - for now or forever.
[+] [-] dschiptsov|10 years ago|reply
The essence of scientific method is not in piling up concepts or in building up a support for a fancy theory. On the contrary, it is in exactly opposite - it is to refute and remove everything "mental" to see what remains, something which cannot be shaken by mere speculations or dismissed by any experience. That, perhaps, has something to do with what we call Truth.
According to the ancients, who practiced this method, there is nothing but different forms of light.
The question "what is between two Photons" makes no sense, because two Photons have nothing in common exept an imagination of an observer. Tracing them back "in time" makes no sense either, because time as a category is irrelevant to the light. It doesn't change.
[+] [-] morsch|10 years ago|reply
https://d262ilb51hltx0.cloudfront.net/max/847/1*LV-wtp9IclSx...
As for the rest of the article, well, Betteridge's law holds, though with the "as best we know"-suffix implicit to science and science journalism.
[+] [-] ianamartin|10 years ago|reply
Hello, people. Dark matter is the eflourescent ether theory of the 21st century.
There are things that we don't know how to explain right now.
We have some ideas about how they can be explained. But they are really really bad.
We will be laughing at dark matter theory in a few years.
[+] [-] Sharlin|10 years ago|reply
[+] [-] PhantomGremlin|10 years ago|reply
The quip I'm fond of is:
But, right now, dark matter is the best explanation we have of what we've observed.[+] [-] socceroos|10 years ago|reply
[+] [-] AdieuToLogic|10 years ago|reply
Sure.
[+] [-] AdieuToLogic|10 years ago|reply
Perhaps our observations are a result of existing within two gravitational space-time distortions, one trivial and one not. The trivial one being the Sun and the non-trivial being the galaxy which we inhabit.
If curvature of space-time theorized by General Relativity is accepted, then observing non-local energy will be affected by gravity in which the observation is being undertaken. Accounting for the Sun's influence has largely (if not completely) been addressed, but there lies a more difficult "elephant in the room" problem of determining what effect the gravitational curvature of the Milky Way has on any observations we can make.
According to the University of Oregon[1], the observable mass of the Milky Way is 200,000,000,000 Solar masses. The same material indicates that visible stars account for 30% of that. Throw in a bit for singularities and lets call it "less than 50%" for the sake of discussion. In any event, there exists enough mass in this galaxy to plausibly state that the space-time we inhabit is affected. Yet a non-trivial amount considered "unaccounted for."
Using the canonical 3d inverse-cone visualization of matter's effect on space-time, imagine our solar system exists on some point within the galaxy's gravitational cone yet not near its center. Making intra-galactic calculations may be influenced by compressed space-time instead of the presence of dark matter. And observations regarding other galaxies colliding (as mentioned in the article) migth be explained by gravitational phase shifting.
Though all of this likely will be rebuked, it is why I originally said "sure."
1 - http://hendrix2.uoregon.edu/~imamura/123cs/lecture-2/mass.ht...
[+] [-] HillOBeans|10 years ago|reply
[+] [-] gnufied|10 years ago|reply
[+] [-] mpc755|10 years ago|reply
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[+] [-] cletus|10 years ago|reply
Dark matter always struck me as a kludge to explain the universe fit our model. There are many theories on it but no observed or experimental effects of dark matter to date.
We say that there are four fundamental forces in the universe: weak nuclear, strong nuclear, electromagnetism and gravity. The first three are pretty adequately explained by quantum mechanics (AFAICT). The last of course isn't but is described pretty darn well by relativity.
Thing is, what's always struck me as odd is the other forces, I believe, can both attract and repel. There is no observation or experimental evidence of "antigravity".
It has always made me wonder if we're not making an error by trying to fit gravity into the Standard Model. Perhaps it's not a force like the others are and simply the result of the nature of space-time.
Anyway, like I said: complete layman here. What I'm saying might be completely nonsensical to anyone with half a clue about physics.
I know the LHC has looked and is looking for any evidence at the edges of the Standard Model of some violation that will hint at something that might explain all this. I also know minds far brighter than mine have spent a lot of time thinking about this.
I'm still left wondering if dark matter/energy isn't simply another "string theory".
[+] [-] tjradcliffe|10 years ago|reply
Calling them kludges is to mistake the basic process of science for a hack.
There are any number of confirmatory observations that fit the dark matter model, just as there were any number of confirmatory observations that fit the neutrio model. It's just that we haven't observed any dark matter particle interactions directly, just as we hadn't observed any neutrino interactions directly before Reines' experiment.
It may be that dark matter turns out not to exist, and then we will know it was wrong. Until then it is perfectly ordinary science working in perfectly ordinary ways as it has been for over a century.
[+] [-] deciplex|10 years ago|reply
FTFA:
> And most spectacularly, you get an entirely new prediction: that when you get two clusters of galaxies colliding, the gas inside should heat up, slow down and emit X-rays (in pink, above), while the mass that we can see through gravitational lensing (in blue, above) should follow the dark matter, and be displaced from the X-rays. This new prediction has been borne out observationally and held up over the past decade, a spectacular indirect confirmation of dark matter.