Assuming this is true, how do we have any hope for directly detecting dark matter at all? How could any of the experiments even have a possibility of detection?
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.
Assuming this is true, how do we have any hope for directly detecting dark matter at all? How could any of the experiments even have a possibility of detection?
Oh that's a good point. That just says the odds for just one particle, enough particles and even unimaginable odds like those aren't terrible. I guess the question then is how do dark matter bits...
Oh that's a good point. That just says the odds for just one particle, enough particles and even unimaginable odds like those aren't terrible. I guess the question then is how do dark matter bits compare in number to neutrinos then. Do we even know how we'd detect these guys if they did happen to interact? For that matter how do we detect neutrinos? How do we know it's a neutrino that just made that blip and not something else?
I want to understand dark matter, but it's a concept I just cannot seem to wrap my head around. Are there any good books written for the general public that serve as a good intro to the topic?
I want to understand dark matter, but it's a concept I just cannot seem to wrap my head around.
Are there any good books written for the general public that serve as a good intro to the topic?
A while back there was a pretty good post on reddit detailing the different types of evidence for dark matter. I'll repost it here, but if you're unsure about something feel free to ask questions...
Exemplary
A while back there was a pretty good post on reddit detailing the different types of evidence for dark matter. I'll repost it here, but if you're unsure about something feel free to ask questions -- I might be able to help, though I'm hardly an expert. That said, I've added a few notes for clarification.
Below is basically a historical approach to why we believe in dark matter. I will also cite this paper for the serious student who wants to read more, or who wants to check my claims against the literature.
In the early 1930s, a Dutch scientist named Jan Oort originally found that there are objects in galaxies that are moving faster than the escape velocity of the same galaxies (given the observed mass) and concluded there must be unobservable mass holding these objects in and published his theory in 1932.
Evidence 1:Objects in galaxies often move faster than the escape velocities but don't actually escape.
Zwicky, also in the 1930s, found that galaxies have much more kinetic energy than could be explained by the observed mass and concluded there must be some unobserved mass he called dark matter. (Zwicky then coined the term "dark matter")
Evidence 2:Galaxies have more kinetic energy than "normal" matter alone would allow for.
Vera Rubin then decided to study what are known as the 'rotation curves' of galaxies and found this plot. As you can see, the velocity away from the center is very different from what is predicted from the observed matter. She concluded that something like Zwickey's proposed dark matter was needed to explain this.
Evidence 3:Galaxies rotate differently than "normal" matter alone would allow for.
[Note: Specifically, the reason for the discrepancy comes from Kepler's third law. Period squared is inversely proportional to mass, which gives us a prediction for the speed of objects. Objects move faster than they should given the observable matter in the galaxy, suggesting there must be more matter there than just what's visible.]
In 1979, D. Walsh et al. were among the first to detect gravitational lensing proposed by relativity. One problem: the amount light that is lensed is much greater than would be expected from the known observable matter. However, if you add the exact amount of dark matter that fixes the rotation curves above, you get the exact amount of expected gravitational lensing.
Evidence 4:Galaxies bend light greater than "normal" matter alone would allow. And the "unseen" amount needed is the exact same amount that resolves 1-3 above.
[Note:Einstein rings are a particularly striking example. Although these would still exist without the presence of dark matter, the presence of dark matter changes the amount of distortion.]
By this time people were taking dark matter seriously since there were independent ways of verifying the needed mass.
MACHOs were proposed as solutions (which are basically normal stars that are just to faint to see from earth) but recent surveys have ruled this out because as our sensitivity for these objects increase, we don't see any "missing" stars that could explain the issue.
[Note: My understanding is that MACHOs as a solution for dark matter has fallen out of favor. Rather, physicists believe that weakly interacting massive particles -- WIMPS -- are the prime candidate for dark matter, as discussed in the OP. ]
Evidence 5:Our telescopes are orders of magnitude better than in the 30s. And the better we look then more it's confirmed that unseen "normal" matter is never going to solve the problem
The ratio of deuterium to hydrogen in a material is known to be proportional to the density. The observed ratio in the universe was discovered to be inconsistent with only observed matter... but it was exactly what was predicted if you add the same dark mater to galaxies as the groups did above.
Evidence 6:The deuterium to hydrogen ratio is completely independent of the evidences above and yet confirms the exact same amount of "missing" mass is needed.
The cosmic microwave background's power spectrum is very sensitive to how much matter is in the universe. As this plot shows here, only if the observable matter is ~4% of the total energy budget can the data be explained.
Evidence 7:Independent of all observations of stars and galaxies, light from the big bang also calls for the exact same amount of "missing" mass.
This image may be hard to understand but it turns out that we can quantify the "shape" of how galaxies cluster with and without dark matter. The "splotchiness" of the clustering from these SDSS pictures match the dark matter prediction only.
Evidence 8:Independent of how galaxies rotate, their kinetic energy, etc... is the question of how they cluster together. And observations of clustering confirm the necessity of vats of intermediate dark matter"
One of the recent most convincing things was the bullet cluster as described here. We saw two galaxies collide where the "observed" matter actually underwent a collision but the gravitational lensing kept moving un-impeded which matches the belief that the majority of mass in a galaxy is collisionless dark matter that felt no colliding interaction and passed right on through bringing the bulk of the gravitational lensing with it.
Evidence 9:When galaxies merge, we can literally watch the collisionless dark matter passing through the other side via gravitational lensing.
In 2009, Penny et al. showed that dark matter is required for fast rotating galaxies to not be ripped apart by tidal forces. And of course, the required amount is the exact same as what solves every other problem above. Evidence 10:Galaxies experience tidal forces that basic physics says should rip them apart and yet they remain stable. And the amount of unseen matter necessary to keep them stable is exactly what is needed for everything else.
There are counter-theories, but as Sean Carroll does nicely here is to show how badly the counter theories work. They don't fit all the data. They are way more messy and complicated. They continue to be falsified by new experiments. Etc... To the contrary, Zwicky's proposed dark matter model from back in the 1930s
continues to both explain and predict everything we observe flawlessly across multiple generations of scientists testing it independently. Hence dark matter is widely believed. Evidence 11:Dark matter theories have been around for more than 80 years, and not one alternative has ever been able to explain even most of the above. Except the original theory that has predicted it all.
Conclusion: Look, I know people love to express skepticism for dark matter for a whole host of reasons but at the end of the day, the vanilla theories of dark matter have passed literally dozens of tests without fail over many many decades now. Very independent tests across different research groups and generations. So personally I think that we have officially entered a realm where it's important for everyone to be skeptical of the claim that dark matter isn't real. Or the claim that scientists don't know what they are doing.
Also be skeptical when the inevitable media article comes out month after month saying someone has "debunked" dark matter because their theory explains some rotation curve from the 1930s. Skeptical because rotation curves are one of at least a dozen independent tests, not to mention 80 years of solid predictivity.
So there you go. These are some basic reasons to take dark matter seriously.
Who is expressing this skepticism, other than flat-earth types? I thought it was the current prevailing theory. A lot of money has been spent already trying to detect the presumed particle.
Who is expressing this skepticism, other than flat-earth types? I thought it was the current prevailing theory. A lot of money has been spent already trying to detect the presumed particle.
The alternative explanations to dark matter require modifying how gravity behaves at long ranges (eg, MOND, first proposed in 1984). MOND had support among some physicists (and certainly there are...
The alternative explanations to dark matter require modifying how gravity behaves at long ranges (eg, MOND, first proposed in 1984). MOND had support among some physicists (and certainly there are still physicists working on modified theories of gravity), but while it could solve some of the problems dark matter attempts to address, it can't solve all of them.
A lot of money has been spent already trying to detect the presumed particle.
To be fair, there isn't a single candidate for dark matter. Sometimes physics requires a scattershot approach: astronomers searching for MACHOs, particle physicists searching for WIMPs, and theorists finagling with general relativity. As it turns out, searches for MACHOs and modifying gravity have proven unfruitful; therefore, WIMPs are now the most likely explanation resolving the problems listed above.
There likely are, but I don't have any recommendations there. However, the article itself does a decent job of explaining the basics: we know from observation that only 1/5 of the mass of the...
There likely are, but I don't have any recommendations there. However, the article itself does a decent job of explaining the basics: we know from observation that only 1/5 of the mass of the universe is comprised of particles that we know exist, that make up the Standard Model of particle physics. It would be like if you stood in front of a trampoline and watched a person drop a baseball onto it: you know the mass of a baseball, yet the trampoline surface shows a huge indentation, just from that baseball. Something's missing, something's making the baseball (which, let's say, represents a galaxy here) have much more mass than it should, by a large amount. That something is theorized to be dark matter.
Heh, don't feel bad - you're on par with every physicist on the planet right there. All we know for sure is that there's a lot more gravity operating out there than what we expect to see based on...
Heh, don't feel bad - you're on par with every physicist on the planet right there.
All we know for sure is that there's a lot more gravity operating out there than what we expect to see based on all of the material we can see. Somehow, we're only seeing a tiny fraction of the universe, and all that extra gravity is telling us that there are still some grand mysteries for us to unravel.
What is it? A unique class of particles that are effectively ghosts, never interacting with us? That might be thinking too small. String theory suggests that gravity itself is the only force that isn't bound to our own universe, so for all we know, all that extra gravity is seeping in from the dimensions next door, which would hint at a much grander overall structure to the universe than we've ever dreamed.
Don't worry about not knowing what dark matter is. Worry that someday, we'll live in a universe that hasn't got any mysteries left for us. ;)
Assuming this is true, how do we have any hope for directly detecting dark matter at all? How could any of the experiments even have a possibility of detection?
I don't think that's that off from what it takes to stop a neutrino, and we can detect those.
Oh that's a good point. That just says the odds for just one particle, enough particles and even unimaginable odds like those aren't terrible. I guess the question then is how do dark matter bits compare in number to neutrinos then. Do we even know how we'd detect these guys if they did happen to interact? For that matter how do we detect neutrinos? How do we know it's a neutrino that just made that blip and not something else?
I want to understand dark matter, but it's a concept I just cannot seem to wrap my head around.
Are there any good books written for the general public that serve as a good intro to the topic?
A while back there was a pretty good post on reddit detailing the different types of evidence for dark matter. I'll repost it here, but if you're unsure about something feel free to ask questions -- I might be able to help, though I'm hardly an expert. That said, I've added a few notes for clarification.
Below is basically a historical approach to why we believe in dark matter. I will also cite this paper for the serious student who wants to read more, or who wants to check my claims against the literature.
In the early 1930s, a Dutch scientist named Jan Oort originally found that there are objects in galaxies that are moving faster than the escape velocity of the same galaxies (given the observed mass) and concluded there must be unobservable mass holding these objects in and published his theory in 1932.
Evidence 1: Objects in galaxies often move faster than the escape velocities but don't actually escape.
Zwicky, also in the 1930s, found that galaxies have much more kinetic energy than could be explained by the observed mass and concluded there must be some unobserved mass he called dark matter. (Zwicky then coined the term "dark matter")
Evidence 2: Galaxies have more kinetic energy than "normal" matter alone would allow for.
Vera Rubin then decided to study what are known as the 'rotation curves' of galaxies and found this plot. As you can see, the velocity away from the center is very different from what is predicted from the observed matter. She concluded that something like Zwickey's proposed dark matter was needed to explain this.
Evidence 3: Galaxies rotate differently than "normal" matter alone would allow for.
[Note: Specifically, the reason for the discrepancy comes from Kepler's third law. Period squared is inversely proportional to mass, which gives us a prediction for the speed of objects. Objects move faster than they should given the observable matter in the galaxy, suggesting there must be more matter there than just what's visible.]
In 1979, D. Walsh et al. were among the first to detect gravitational lensing proposed by relativity. One problem: the amount light that is lensed is much greater than would be expected from the known observable matter. However, if you add the exact amount of dark matter that fixes the rotation curves above, you get the exact amount of expected gravitational lensing.
Evidence 4: Galaxies bend light greater than "normal" matter alone would allow. And the "unseen" amount needed is the exact same amount that resolves 1-3 above.
[Note: Einstein rings are a particularly striking example. Although these would still exist without the presence of dark matter, the presence of dark matter changes the amount of distortion.]
By this time people were taking dark matter seriously since there were independent ways of verifying the needed mass.
MACHOs were proposed as solutions (which are basically normal stars that are just to faint to see from earth) but recent surveys have ruled this out because as our sensitivity for these objects increase, we don't see any "missing" stars that could explain the issue.
[Note: My understanding is that MACHOs as a solution for dark matter has fallen out of favor. Rather, physicists believe that weakly interacting massive particles -- WIMPS -- are the prime candidate for dark matter, as discussed in the OP. ]
Evidence 5: Our telescopes are orders of magnitude better than in the 30s. And the better we look then more it's confirmed that unseen "normal" matter is never going to solve the problem
The ratio of deuterium to hydrogen in a material is known to be proportional to the density. The observed ratio in the universe was discovered to be inconsistent with only observed matter... but it was exactly what was predicted if you add the same dark mater to galaxies as the groups did above.
Evidence 6: The deuterium to hydrogen ratio is completely independent of the evidences above and yet confirms the exact same amount of "missing" mass is needed.
The cosmic microwave background's power spectrum is very sensitive to how much matter is in the universe. As this plot shows here, only if the observable matter is ~4% of the total energy budget can the data be explained.
Evidence 7: Independent of all observations of stars and galaxies, light from the big bang also calls for the exact same amount of "missing" mass.
This image may be hard to understand but it turns out that we can quantify the "shape" of how galaxies cluster with and without dark matter. The "splotchiness" of the clustering from these SDSS pictures match the dark matter prediction only.
Evidence 8: Independent of how galaxies rotate, their kinetic energy, etc... is the question of how they cluster together. And observations of clustering confirm the necessity of vats of intermediate dark matter"
One of the recent most convincing things was the bullet cluster as described here. We saw two galaxies collide where the "observed" matter actually underwent a collision but the gravitational lensing kept moving un-impeded which matches the belief that the majority of mass in a galaxy is collisionless dark matter that felt no colliding interaction and passed right on through bringing the bulk of the gravitational lensing with it.
Evidence 9: When galaxies merge, we can literally watch the collisionless dark matter passing through the other side via gravitational lensing.
In 2009, Penny et al. showed that dark matter is required for fast rotating galaxies to not be ripped apart by tidal forces. And of course, the required amount is the exact same as what solves every other problem above.
Evidence 10: Galaxies experience tidal forces that basic physics says should rip them apart and yet they remain stable. And the amount of unseen matter necessary to keep them stable is exactly what is needed for everything else.
There are counter-theories, but as Sean Carroll does nicely here is to show how badly the counter theories work. They don't fit all the data. They are way more messy and complicated. They continue to be falsified by new experiments. Etc... To the contrary, Zwicky's proposed dark matter model from back in the 1930s
continues to both explain and predict everything we observe flawlessly across multiple generations of scientists testing it independently. Hence dark matter is widely believed.
Evidence 11: Dark matter theories have been around for more than 80 years, and not one alternative has ever been able to explain even most of the above. Except the original theory that has predicted it all.
Conclusion: Look, I know people love to express skepticism for dark matter for a whole host of reasons but at the end of the day, the vanilla theories of dark matter have passed literally dozens of tests without fail over many many decades now. Very independent tests across different research groups and generations. So personally I think that we have officially entered a realm where it's important for everyone to be skeptical of the claim that dark matter isn't real. Or the claim that scientists don't know what they are doing.
Also be skeptical when the inevitable media article comes out month after month saying someone has "debunked" dark matter because their theory explains some rotation curve from the 1930s. Skeptical because rotation curves are one of at least a dozen independent tests, not to mention 80 years of solid predictivity.
So there you go. These are some basic reasons to take dark matter seriously.
Who is expressing this skepticism, other than flat-earth types? I thought it was the current prevailing theory. A lot of money has been spent already trying to detect the presumed particle.
The alternative explanations to dark matter require modifying how gravity behaves at long ranges (eg, MOND, first proposed in 1984). MOND had support among some physicists (and certainly there are still physicists working on modified theories of gravity), but while it could solve some of the problems dark matter attempts to address, it can't solve all of them.
To be fair, there isn't a single candidate for dark matter. Sometimes physics requires a scattershot approach: astronomers searching for MACHOs, particle physicists searching for WIMPs, and theorists finagling with general relativity. As it turns out, searches for MACHOs and modifying gravity have proven unfruitful; therefore, WIMPs are now the most likely explanation resolving the problems listed above.
There likely are, but I don't have any recommendations there. However, the article itself does a decent job of explaining the basics: we know from observation that only 1/5 of the mass of the universe is comprised of particles that we know exist, that make up the Standard Model of particle physics. It would be like if you stood in front of a trampoline and watched a person drop a baseball onto it: you know the mass of a baseball, yet the trampoline surface shows a huge indentation, just from that baseball. Something's missing, something's making the baseball (which, let's say, represents a galaxy here) have much more mass than it should, by a large amount. That something is theorized to be dark matter.
Heh, don't feel bad - you're on par with every physicist on the planet right there.
All we know for sure is that there's a lot more gravity operating out there than what we expect to see based on all of the material we can see. Somehow, we're only seeing a tiny fraction of the universe, and all that extra gravity is telling us that there are still some grand mysteries for us to unravel.
What is it? A unique class of particles that are effectively ghosts, never interacting with us? That might be thinking too small. String theory suggests that gravity itself is the only force that isn't bound to our own universe, so for all we know, all that extra gravity is seeping in from the dimensions next door, which would hint at a much grander overall structure to the universe than we've ever dreamed.
Don't worry about not knowing what dark matter is. Worry that someday, we'll live in a universe that hasn't got any mysteries left for us. ;)