On the other hand, the motion of the solar system is essentially why we may be able to measure dark matter at Earth. If dark matter is made up of WIMPs, then our ability to observe its direct (non-gravitational) effects depends heavily on the velocity distribution function of dark matter at Earth.
Because the non-dark matter component of the galaxy formed a disk while the dark matter halo will likely have remained roughly spherical, we expect that Earth is moving through the dark matter halo at O(230 km/s). This has a few important impliciations. First, it increases the expected kinetic energy of WIMPs in our frame, thus resulting in more energetic WIMP-nucleon interactions, making direct detection (which essentially consists of having a bunch of material and waiting for an unexplained interaction within it) significantly more feasible. Secondly, it leads to an annual modulation effect (as reportedly, controversially. observed by DAMA or COGENT) related to the Earth's motion around the sun. Thirdly, it means that recoils from interactions with WIMPs will have a preferred direction away from the direction of the Earth's propagation through the galaxy. Because no potential terrestrial background can have this signature, a number of groups, including the one I work with, are trying to develop detectors sensitive to the direction of low-energy recoils in order to potentially unambiguously detect WIMPs.
Interesting article, I was hoping that dark matter would prove to be observable in our solar system.
I'm more curious about dark energy though. Hopefully in my lifetime, we can figure out what dark energy is. I'm curious what dark energy does to time since it accelerates the expansion of space. The next golden age of science resides in our understanding of the unknown matter and energy that we are currently observing.
It's highly likely that within the next decade or two we will have a very good idea about what dark matter is actually made of and have direct observational evidence of it. The most likely candidate for dark matter is a hypothesized supersymmetric particle that happens to be weakly interacting, there are several good candidates for such. But these particles should be detectable in future particle colliders and should give rise to characteristic signals in x-ray/gamma ray emissions from galaxies as well as perhaps being directly detectable in "dark matter wind" type experiments (which leverage the significant difference in speed relative to the galactic disk the Earth experiences at different times of the year).
Dark energy is still a rather large mystery, but it's possible someone will make a big breakthrough at some point.
That's a common sentiment among people who aren't terribly familiar with the scientific background for dark matter. I would urge you to go take a look at the wikipedia entry on dark matter and read through it, it's a good overview of a lot of the lines of evidence. The fact is that scientists have always been reluctant to believe in the existence of some exotic, new form of matter that hasn't been seen or directly detected, but in trying to disprove its existence or to prove that it's really just something else they've consistently, time after time, been stuck with that as the only option that explains the evidence. And there is quite a lot of evidence, from cosmological simulations to gravitational lensing studies to galactic rotational curves and so much more.
If dark matter is what we think it probably is then it would actually make for a fairly elegant explanation. The overwhelmingly likely story is that dark matter is a kind of "supersymmetric" particle which requires very high energy conditions to create, conditions which do not generally exist much anymore, and which interacts very weakly or not at all with electromagnetic and strong nuclear forces. Highly energetic conditions were quite common in the early period of the Universe just after the big bang though, and if it were at all possible for such weakly interacting particles to exist then they would have been created in abundance, sapping a lot of the total mass/energy away from the Universe as they "fell out" of the energetic soup of the Universe. Eventually the Universe would have cooled enough to cause creation of such particles to halt, after which it would have gone on to create the more ordinary matter we know today (photons, electrons, protons, etc.)
Fortunately we're likely to get a lot more data on the subject in the next few decades. As I mentioned our capabilities of directly testing the existence of dark matter particles are increasing to the point where within one to two decades we will likely have direct confirmation of their existence.
I put the chances of dark matter being something other than a weakly interacting massive particle to be fairly low, though not impossible of course.
As for dark energy, the evidence for the accelerating expansion of the Universe is much newer and the hypotheses attempting to explain it much more circumspect, I wouldn't be surprised if it turned out to be something other than the crude guesses we've made at present.
Layman here, I can't help but notice the similarities of how 19th century scientists describe an aether with magical properties to help plug holes in their theories to dark matter.
1. "Dark matter is a hypothetical kind of matter that cannot be seen with telescopes but accounts for most of the matter in the Universe."
2. "It is hypothesized to be matter that does not react to light."
3. "Astrophysicists hypothesized dark matter because of discrepancies between the mass of large astronomical objects determined from their gravitational effects and the mass calculated from the observable matter (stars, gas, and dust) that they can be seen to contain."
4. "dark matter — that exerts a well-understood gravitational force but neither absorbs nor emits light, and doesn’t collide (as far as we can tell) with protons, neutrons or electrons. So while normal matter (in pink, below) slows down and can even stick together when it runs into other normal matter, dark matter (in blue, below) just passes right through both itself and all other forms of matter."
In the sense that dark matter is a theoretical entity that is invoked to explain a particular set of astronomical observations, this is a fair comparison. Late 19th century critics of aether might have compared it to caloric (a theoretical entity that explained a particular set of thermodynamic observations.) Caloric turned out not to exist.
By the same token, we might compare dark matter to neutrinos, which were a theoretical entity invoked to explain a particular set of observations on radioactivity (the shape of the beta spectrum.) Neutrinos turned out to exist.
As such, while the comparison to aether is superficially apt, it is not something we can draw any conclusions from, because the comparison to the neutrino is equally apt.
Science is the discipline of publicly testing ideas by systematic observation, controlled experiment and Bayesian inference. It is not the discipline of testing ideas by making analogies to other ideas. There is a reason for this: making analogies to other ideas has consistently proven to be almost completely useless for creating knowledge of reality, while the discipline of science has been wildly successful.
Nor are the properties of caloric, aether, neutrinos or dark matter "magical". They are merely the ones required of an entity that is able to explain our observations in each instance. In the case of caloric it turned out to have self-contradictory properties, when the full deductive closure of the theory was teased out. In the case of aether it turned out to have properties that made predictions that were false. In the case of neutrinos the required properties made predictions that were true.
In the case of dark matter: we don't know yet, and the only way we will ever know is if we continue on with our program of systematic observation, controlled experiment and Bayesian inference. There is no other way to know.
Your first two sentences is what I was trying to describe.
I may be addressing a straw man here, but anyways;
I would contest this that analogy is completely useless for creating knowledge of reality.
Analogy is a tool to translate facts into thoughts another person can understand. Physicists describe Dark matter as an analogy to matter with exceptions to make it easier for others to understand what they're looking for. "It is like matter, except in these ways". If it works as a tool of human communication, and creating knowledge of reality requires human communication, then it is a tool that can be potentially useful.
Humans, unlike machines, rely on human interaction to cooperate. It is one thing to say the scientific method is very useful in describing the underlying reality in which we all exist, and yet something else altogether to show disdain to all other kinds of thinking. The scientific method is a tool at humanity disposal, to believe human thinking and human knowledge can be reduced to Science with a capital "S" is to mock it.
I believe the parent was intentionally particular in its choice of words on this point, and perhaps you've glossed over that intentionality, as well as how it differs from your idea here.
The original:
>> "... making analogies to other ideas has consistently proven to be almost completely useless for creating knowledge of reality ..."
vs your statement:
>> "Analogy is a tool to translate facts into thoughts another person can understand ... If it works as a tool of human communication, and creating knowledge of reality requires human communication, then it is a tool that can be potentially useful."
The parent is talking about creating knowledge of reality by systematic observation, controlled experimentation, and Bayesian inference--and then repeating that process based on the results and predictions of the process to gradually create knowledge of reality previously unknown. However, you're pointing to communicating knowledge of reality in a more understandable and relatable format.
I do not think the parent was showing any "disdain to all other kinds of thinking"--because analogies are not another kind of thinking, but are a tool for communication.
Analogies cannot be tested and judged on their predictive capabilities. You cannot infer new knowledge of reality from analogies. The most rigorous test one can subject analogies to is determining whether they are, in fact, homologous. You can use analogies to [usually poorly] communicate knowledge of reality to those who do not already understand it, relating unknown knowledge to something already known, familiar, and understood. This can help a person grasp this knowledge where s/he previously did not. However, analogies do not allow for systematic study, testing, experimenting, predicting, etc.--all the things the scientific method allows. One cannot make Bayesian inferences from analogies--how exactly would such a thing work? The most one can do with analogies is use them to lubricate a conversation when one finds it necessary to help someone lacking the requisite verbal or technical understanding join the discussion.
If you'll grant that we can obtain both verbal and technical understanding of any given piece of knowledge, analogies help obtain the former, but can do nothing for the latter. That is, analogies help increase verbal understanding by facilitating easier communication, but they do nothing for arriving at technical understanding or discovering knowledge in the first place.
I know HN frowns on this type of comment, but this is a first class response to a question I think a lot of people have, both in this area of cosmology and more generally about science as a framework for developing understanding. Thanks for taking the time, and to the parent for asking a question I wasn't brave enough to voice.
I have a question--please let me know if I'm way off base here. Gravity distorts space and we postulate the existence of dark matter from the distortion of space without corresponding mass/matter. Why must space without matter/mass be non-distorted to begin with?
> Why must space without matter/mass be non-distorted to begin with?
It doesn't have to be. De Sitter came up with solutions to the equations of GR that were free of ordinary matter but included a cosmological constant that is the effect of an "inherent" curvature of space-time: https://en.wikipedia.org/wiki/De_Sitter_universe
However, the kind of curvature you get from a cosmological constant is not sufficient to explain everything that is observed, so even though modern cosmologies typically have a "dark energy" term (which is another name for a cosmological constant) they require an extra matter term (dark matter) to describe the universe we see.
Great responses--different observations and types of curvature to explain. I realize now that my question is inherently flawed--where space 'is' distorted and we don't find ordinary matter we still need some explanation matterlike or otherwise. I also found out about MOND and TeVeS "which try to account for the anomalous observations without requiring additional matter" although minor, does probe in a different direction altogether.
Evidence for dark matter is not just from gravitational lensing. As for your question, I'm no cosmologist, but my understanding is that we have not observed any curvature unassociated with mass ( we believe the universe to be flat on large scales).
18 comments
[ 2.9 ms ] story [ 42.0 ms ] threadBecause the non-dark matter component of the galaxy formed a disk while the dark matter halo will likely have remained roughly spherical, we expect that Earth is moving through the dark matter halo at O(230 km/s). This has a few important impliciations. First, it increases the expected kinetic energy of WIMPs in our frame, thus resulting in more energetic WIMP-nucleon interactions, making direct detection (which essentially consists of having a bunch of material and waiting for an unexplained interaction within it) significantly more feasible. Secondly, it leads to an annual modulation effect (as reportedly, controversially. observed by DAMA or COGENT) related to the Earth's motion around the sun. Thirdly, it means that recoils from interactions with WIMPs will have a preferred direction away from the direction of the Earth's propagation through the galaxy. Because no potential terrestrial background can have this signature, a number of groups, including the one I work with, are trying to develop detectors sensitive to the direction of low-energy recoils in order to potentially unambiguously detect WIMPs.
I'm more curious about dark energy though. Hopefully in my lifetime, we can figure out what dark energy is. I'm curious what dark energy does to time since it accelerates the expansion of space. The next golden age of science resides in our understanding of the unknown matter and energy that we are currently observing.
Dark energy is still a rather large mystery, but it's possible someone will make a big breakthrough at some point.
If dark matter is what we think it probably is then it would actually make for a fairly elegant explanation. The overwhelmingly likely story is that dark matter is a kind of "supersymmetric" particle which requires very high energy conditions to create, conditions which do not generally exist much anymore, and which interacts very weakly or not at all with electromagnetic and strong nuclear forces. Highly energetic conditions were quite common in the early period of the Universe just after the big bang though, and if it were at all possible for such weakly interacting particles to exist then they would have been created in abundance, sapping a lot of the total mass/energy away from the Universe as they "fell out" of the energetic soup of the Universe. Eventually the Universe would have cooled enough to cause creation of such particles to halt, after which it would have gone on to create the more ordinary matter we know today (photons, electrons, protons, etc.)
Fortunately we're likely to get a lot more data on the subject in the next few decades. As I mentioned our capabilities of directly testing the existence of dark matter particles are increasing to the point where within one to two decades we will likely have direct confirmation of their existence.
I put the chances of dark matter being something other than a weakly interacting massive particle to be fairly low, though not impossible of course.
As for dark energy, the evidence for the accelerating expansion of the Universe is much newer and the hypotheses attempting to explain it much more circumspect, I wouldn't be surprised if it turned out to be something other than the crude guesses we've made at present.
1. "Dark matter is a hypothetical kind of matter that cannot be seen with telescopes but accounts for most of the matter in the Universe."
2. "It is hypothesized to be matter that does not react to light."
3. "Astrophysicists hypothesized dark matter because of discrepancies between the mass of large astronomical objects determined from their gravitational effects and the mass calculated from the observable matter (stars, gas, and dust) that they can be seen to contain."
4. "dark matter — that exerts a well-understood gravitational force but neither absorbs nor emits light, and doesn’t collide (as far as we can tell) with protons, neutrons or electrons. So while normal matter (in pink, below) slows down and can even stick together when it runs into other normal matter, dark matter (in blue, below) just passes right through both itself and all other forms of matter."
It sounds to me like a kind of Aether 2.0.
By the same token, we might compare dark matter to neutrinos, which were a theoretical entity invoked to explain a particular set of observations on radioactivity (the shape of the beta spectrum.) Neutrinos turned out to exist.
As such, while the comparison to aether is superficially apt, it is not something we can draw any conclusions from, because the comparison to the neutrino is equally apt.
Science is the discipline of publicly testing ideas by systematic observation, controlled experiment and Bayesian inference. It is not the discipline of testing ideas by making analogies to other ideas. There is a reason for this: making analogies to other ideas has consistently proven to be almost completely useless for creating knowledge of reality, while the discipline of science has been wildly successful.
Nor are the properties of caloric, aether, neutrinos or dark matter "magical". They are merely the ones required of an entity that is able to explain our observations in each instance. In the case of caloric it turned out to have self-contradictory properties, when the full deductive closure of the theory was teased out. In the case of aether it turned out to have properties that made predictions that were false. In the case of neutrinos the required properties made predictions that were true.
In the case of dark matter: we don't know yet, and the only way we will ever know is if we continue on with our program of systematic observation, controlled experiment and Bayesian inference. There is no other way to know.
I may be addressing a straw man here, but anyways;
I would contest this that analogy is completely useless for creating knowledge of reality.
Analogy is a tool to translate facts into thoughts another person can understand. Physicists describe Dark matter as an analogy to matter with exceptions to make it easier for others to understand what they're looking for. "It is like matter, except in these ways". If it works as a tool of human communication, and creating knowledge of reality requires human communication, then it is a tool that can be potentially useful.
Humans, unlike machines, rely on human interaction to cooperate. It is one thing to say the scientific method is very useful in describing the underlying reality in which we all exist, and yet something else altogether to show disdain to all other kinds of thinking. The scientific method is a tool at humanity disposal, to believe human thinking and human knowledge can be reduced to Science with a capital "S" is to mock it.
The original:
>> "... making analogies to other ideas has consistently proven to be almost completely useless for creating knowledge of reality ..."
vs your statement:
>> "Analogy is a tool to translate facts into thoughts another person can understand ... If it works as a tool of human communication, and creating knowledge of reality requires human communication, then it is a tool that can be potentially useful."
The parent is talking about creating knowledge of reality by systematic observation, controlled experimentation, and Bayesian inference--and then repeating that process based on the results and predictions of the process to gradually create knowledge of reality previously unknown. However, you're pointing to communicating knowledge of reality in a more understandable and relatable format.
I do not think the parent was showing any "disdain to all other kinds of thinking"--because analogies are not another kind of thinking, but are a tool for communication.
Analogies cannot be tested and judged on their predictive capabilities. You cannot infer new knowledge of reality from analogies. The most rigorous test one can subject analogies to is determining whether they are, in fact, homologous. You can use analogies to [usually poorly] communicate knowledge of reality to those who do not already understand it, relating unknown knowledge to something already known, familiar, and understood. This can help a person grasp this knowledge where s/he previously did not. However, analogies do not allow for systematic study, testing, experimenting, predicting, etc.--all the things the scientific method allows. One cannot make Bayesian inferences from analogies--how exactly would such a thing work? The most one can do with analogies is use them to lubricate a conversation when one finds it necessary to help someone lacking the requisite verbal or technical understanding join the discussion.
If you'll grant that we can obtain both verbal and technical understanding of any given piece of knowledge, analogies help obtain the former, but can do nothing for the latter. That is, analogies help increase verbal understanding by facilitating easier communication, but they do nothing for arriving at technical understanding or discovering knowledge in the first place.
https://en.wikipedia.org/wiki/Tests_of_general_relativity#De...
(I didn't downvote you)
It doesn't have to be. De Sitter came up with solutions to the equations of GR that were free of ordinary matter but included a cosmological constant that is the effect of an "inherent" curvature of space-time: https://en.wikipedia.org/wiki/De_Sitter_universe
However, the kind of curvature you get from a cosmological constant is not sufficient to explain everything that is observed, so even though modern cosmologies typically have a "dark energy" term (which is another name for a cosmological constant) they require an extra matter term (dark matter) to describe the universe we see.