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There is no evidence that dark matter even exists other than the fact that our universe is not expanding at the rate we think it should, based on our current th

There is no evidence that dark matter even exists other than the fact that our universe is not expanding at the rate we think it should, based on our current theory of gravity and how much mass we can account for based on the radiation currently reaching us (light). I think it's far more likely that our theory is incomplete rather than some whole new class of invisible matter/energy being conjured into existence just to counter-balance our 1) wrong equations and/or 2) wrong observations. So many non-scientists think dark matter is proven. It isn't. It's nothing but pure conjecture.

Pica_soO 10y ago
Eh, those galaxys would fling themselves apart without it? Is this irony?
- ClayFerguson 10y ago
  No one knows why space is expanding. The dark energy is just the 'label' they give to whatever is causing it to expand. Both dark matter and dark energy are postulated to be whatever shape of puzzle piece fits into the puzzle slot where the error in our formulas exists. What i'm saying is that it's more likely that the formula itself is wrong than it is that something magically exists to conveniently fit that slot.
  - raattgift 10y ago
    > The dark energy is just the 'label' they give to whatever is causing it to expand This depends on how you look at it. In the standard cosmology we define a preferred frame wherein an observer will see the matter (that's in the most general sense of "not the gravitational field", so it includes atoms and their components, photons, and various types of dark matter (e.g. neutrinos, which are "hot" dark matter, since they move relativistically and do not experience electromagnetism)) content of the universe as homogeneous and isotropic. This is physically reasonable since along every unobscured line of sight we see a lot of galaxies of various shapes, "tilts", sizes, surface brightnesses, and spectral lines. Observations also lead us to conclude that there is a relationship between redshifting of the spectral lines of common types of galaxies (and common radiative occurrences within them, like type A supernovas), and the change of the other observables (angular size on the sky, luminosity, etc.) that correlate with greater distance. This in turn led to the discovery of the Hubble "constant", and provoked ever deeper field telescopic studies to prove its value. So if we assume that along every line of sight, including obscured ones, we have much the same view of many many galaxies at a variety of distances, we can make a variation on the Friedmann equation that parameterize several things that would lead to the observables of galaxies when we model their known (and unknown) components as a set of perfect fluids. We can take the Hubble "constant" and put it into a Robertson-Walker vacuum spacetime. RW spacetimes can be grokked by dimensional reduction. Consider a cylinder that we slice (foliate) along its axis into a set of infinitesimally thin circles stacked on top of each other. We describe the radius of each circle with a function r(h) where h is the height of the circle from the base of the cylinder. Where r(h) is constant, we have a cylinder, but if r(h) increases with h, then we have something like a cone balancing on its apex; r(h) can describe a wide variety of shapes. For an 3+1 RW spacetime that is similar to our universe, we foliate on the timelike axis and define a function a(t) where t_0 == now with the spacelike coordinates set on a chosen observer (us here on Earth, for example). "t" counts upwards as we go into the past from t_0, and a(t) goes to zero as t increases. "t" is the lookback time and a(t) is the scale factor. The chosen observer is the special observer mentioned above, who sees the matter of the universe as isotropic and homogeneous at the largest scales. The most useful coordinates on such a spacetime are comoving, that is each gravitationally bound galaxy cluster stays at the same coordinates at every time t. If we mix together the Friedmann equations, the Lemaître idea of spacetime having a zero radius at some large lookback time, and the RW spacetime that can model that, we get the FLRW model of the standard cosmology. We take the RW case where there is no extrinsic curvature, that is, when we foliate on the timelike axis each spacelike hypersurface is spatially flat; that is similar to saying that when we slice up our dimensionally reduced solid along its height, we get a set of circles of the same radius (i.e. a cylinder rather than a cone). We absorb the expansion parameter into a(t) as another of the fluids. We then consider two types of fluid: those that dilute away as t -> t_0 -> future and those that do not dilute away. The former is "matter", including dark matter; the latter is "dark energy". When we consider them as components of an action, diluting-away fluids are attractive and non-diluting fluids are repuslive. When we consider them in terms of the matter tensor T in General Relativity, the diluting-away fluids have positive pressure and the non-diluting fluids have negative pressure. It's important to return to the point that this model has a preferred frame, and that translating the non-diluting fluid into "the same for all observers in all frames of reference" physics leads one to assume that dark energy is just a feature of the Lorentz-invariant vacuum. So dark energy arises in the cosmological model but corresponds to the ground state of the empty-of-matter spacetime in frames of reference other than the preferred one picked out by the cosmological model. That is, the statement that "dark energy drives the expansion (via negative pressure or repulsion)" is frame-dependent, and thus observer-dependent, and with a change of frames of reference (and even a change of coordinates on the preferred frame), the statement becomes untrue. What is true in all frames is that there is an intrinsic property of space in an expanding spacetime that has a constant energy-density no matter how large a volume of space is considered. The exact equation of state of dark energy is an area of active research and also tests to make sure that the assumptions that inevitably lead to it (isotropy at huge scales, homogeneity, spatial flatness, redshift-distance relations and other things implying expansion) are not blown up by evidence from ever finer observations. So it's not so much a 'label' as a phenomenon whose microscopic details have yet to be discovered. There are lots of those in physics, and we've had a good century of probing the microscopic details of phenomena discovered at the end of the 19th century and since, so this shouldn't really be causing anyone sleepless nights.
    - ClayFerguson 10y ago
      Your statement that there's "an expanding space-time that has a constant energy-density", gets to the nub of it for sure, but in reality no one knows if any of the fundamental field strengths are indeed constant. To me the need for "dark" stuff to be postulated, is more likely to indicate that gravity strength, or light speed, are more likely to NOT be constant over large space-time ranges, than the likelihood that there is a whole class of 'dark' particles and waves that currently don't interfere with any other matter in a provable way. So if someone asked me what's my 'evidence' that gravity or light-speed are not universally constant, my answer is simply: "The evidence for dark-energy/matter, is that exact same evidence". There should be some wave-function (possibly resulting from what we call a big bang), in terms of G and C, that once integrated out over the current life of the universe, will yield precisely that positions and velocities of the galaxies that we currently observe. To me it seems far less likely that there's an entirely new set of particles we cannot see, despite the Standard Model of particles being proven correct out to 40 decimal places.
      - Neeek 10y ago
        You already have the neutrino which interacts only via weak force and gravity, They show up much the same in bubble chambers as dark matter does in our telescopes; they don't. We only see where they interact with more observable matter, is it really that far fetched to imagine a particle that interacts through gravity only?
      - raattgift 10y ago
        I'm not sure what you're trying to argue here, or even why. I'll guess that you're interested in physical cosmology and would like a set of brief and not-too-technical reactions to your ideas (which I take to be implied questions) from someone who knows the \Lambda-CDM cosmology reasonably well. Dark energy it is a property of empty space that does not dilute away as more space appears in an expanding universe. The only way to abolish dark energy is to eliminate the expansion of the universe. It only has a "fundamental field strength" in particular chosen frames of reference in which one can represent it as a field with a constant energy-density. The comoving frame of the standard cosmology is one such frame of reference. However, in most other frames this energy density vanishes, and when that happens a general relativist will decide that the energy density was an artifact of the choice of frame of reference or system of coordinates and ditch the word "fundamental". Now, there are lots of ways we can complicate the action S_{\Lambda-CDM} in a Lagrangian formulation of the standard cosmology by introducing further repulsive terms into it beyond constant * \Lambda (as in the Einstein-Hilbert action or an expansion of it) or L_{repulsivematter} in a parameterization, and indeed there have been numerous attempts to do so. However, short of eliminating the expansion of space at all times and in all reference frames there is no way to get rid of a repulsive, non-diluting, non-concentrating (when time-reversed or if the critical density of the universe turns out to lead to a contraction of space in the future) term. But in the standard cosmology, we have \Lambda, which is just the cosmological constant, i.e., dark energy is a property of the ground state, which is the vacuum. When there's more vacuum, there's more dark energy. Mathematically, we start with an action that leads to the standard write-down of the EFEs. Physically, this matches observational tests at large scales, which is convenient because the standard write-down of the EFEs is almost the only way to match observational tests within our solar system (c.f. the parameterized post-Newtonian formalism). > gravity or light speed are not universally constant What exactly do you mean by "gravity ... not universally constant"? In particular, what do you mean by gravity? In General Relativity speeds are something that are extremely hard to talk about except in the local neighbourhood around a single point. In fact, many general relativists would argue that talking about "universal" speeds violates the spirit of general relativity. However, if our universe continues to be modellable as a smooth manifold with a Lorentz metric, we get a sort of quasi-universality of "c" in that at every point one can construct an infinitesimal region of spacetime in which "c" is a parameter in the action of matter and takes on the same _locally measured_ value in each such region for an observer in that region. There is ample evidence that favours this up to energy scales accessible to us on Earth and visible with observational platforms on and near our planet. There are certainly metric theories other than GR that allow for different sources to couple to different metrics, but most of these have to undergo a phase change to an effective single metric with universal coupling in the early universe or we would see clear evidence for them (in particular, the distribution of heat in the early dense phase of the universe still has to produce the Standard Model at lab energies and also stars and galaxies and labs). Some productive and well-regarded physical cosmologists have proposed these types of theories, even recently (e.g. Afshordi & Magueijo), and they explictly reject a universal value of "c" (in particular c approaches infinity in their model's extremely early universe). But in the standard cosmology we have General Relativity, and we don't vary the value of G or of c when grinding through the Einstein Field Equation; we take those values as given to us by nature, and have no evidence for them varying in the observable universe (and a pretty substantial amount of evidence against such variations, in particular including petabytes of spectroscopic data from objects in the sky). > "There should be some wave-function (possibly resulting from what we call a big bang), in terms of G and C, that once integrated out... I'm sorry, I don't understand this. Could you explain further? > To me it seems far less likely that there's an entirely new set of particles we cannot see, despite the Standard Model of particles being proven correct out to 40 decimal places. Dark energy is not particles. As I said above, it's a feature of the vacuum, and the most fundamental feature of the vacuum is that it is empty of particles. A tl;dr here is that we don't know what creates more space rather than less space, but we know that when more space is created there's more dark energy in the comoving frame but not more particles. Are you thinking of dark matter here, rather than dark energy? If so, we already have hot dark matter in the form of neutrinos. Neutrinos are "hot" because they move relativistically, and thus are prone to carry momentum far away from galaxies very quickly. Cold dark matter is "cold" because it moves non-relativistically and so the momentum CDM carries lingers in place in and around galaxy clusters. While it would be convenient if CDM were like heavy neutrinos, there is no reason in the standard cosmology that CDM has to be particles at all, or even just one species; the only requirement is that it be almost entirely collisionless and non-radiative so that we don't see it and so that it doesn't release its trapped momentum (e.g. by converting some of it into hot dark matter or light or standard model particles). Finally, the Standard Model is mute on gravity, and yet you -- made up of Standard Model particles -- are feeling it right now. So I'm not sure how your argument about the correctness of the Standard Model fits with your argument.
platz 10y ago
> no evidence The bullet cluster is hard to explain without some form of dark matter.
millstone 10y ago
What about the evidence from the bullet cluster? The gravitational center of mass is offset from the the visible center of mass. So something must be gravitating that is not visible - i.e. dark matter.
- CuriouslyC 10y ago
  Matter isn't the only thing that produces gravity. Energy also produces gravity. Given that the bullet cluster is also incredibly hot, it seems entirely plausible that its unusual gravitational morphology is somehow related to its energy content.
  - russdill 10y ago
    You'd expect it to be incredibly hot where the gas is, no? And the interesting thing about the bullet cluster is that the excess of matter is found where there is no gas and stars. Plus, if the gas were super hot, it would emit lots of light, including x-rays.
    - CuriouslyC 10y ago
      My understanding was that it is in fact incredibly hot where the gas is, and it is emitting lots of x-rays. And you're jumping ahead of yourself to say that it is matter, we just know that there is gravitational lensing. My main issue is that the bullet is anomalous in multiple ways, and dark matter doesn't explain the heat issue. If it did, then it wouldn't be useful as an explanation for the rotation curve problem in other galaxies. I feel that a more parsimonious explanation would cover both the lensing and the heat.
      - russdill 10y ago
        Ah, ok, I was going to say, if the excess energy in heat was sufficient to explain the gravitational lensing, you'd have several problems. You're just pointing out that there are other significant anonymous things going on we don't understand. Until they are also explained, the solution is not a complete one.
russdill 10y ago
I believe you are thinking of dark energy. The evidence for dark matter is typically things like galaxy rotation curves.
M_Grey 10y ago
You've confused the logic for inferring dark energy or some other force at work, with dark matter which is involved in lensing experiments, galaxy rotation curves, etc.
- ClayFerguson 10y ago
  read the rest of my comments. I clarify why "dark stuff" is all the same.
lutusp 10y ago
> There is no evidence that dark matter even exists other than the fact that our universe is not expanding at the rate we think it should ... You're thinking of dark energy, the net effect of which is to change the overall expansion profile of the universe as a whole. Dark Matter was first proposed when galactic rotation profiles failed to meet theoretical expectations -- matter far from the center of each galaxy had a higher velocity than it would if visible matter had been the only factor. > I think it's far more likely that our theory is incomplete rather than some whole new class of invisible matter/energy being conjured into existence Occam's razor (the simplest explanation tends to be the right one) suggests that a new unobserved particle is more likely than abandonment of F = GMmr^-2 . This doesn't mean the equation must be correct, it's a question of reaching for the low-hanging fruit. > So many non-scientists think dark matter is proven. It isn't. It's nothing but pure conjecture. First, nothing is ever proven true in science, only false. Second, dark matter is more than pure conjecture, since there is observational evidence. A pure conjecture would be an idea about reality having no observational support at all (unicorns, Bigfoot). Dark matter is a hypothesis crafted to explain observations, but so far there's no persuasive theory to explain it, and no observations of candidate particles either.
- ClayFerguson 10y ago
  I should have been clear that I'm not making a distinction between energy and matter. I'm referring to the postulated "dark" forms of both. Both of them are pure speculation, and is nothing but the equivalent of taking whatever observable "error" there is in our formulas and labeling it "dark". The only reason there's two kinds of "dark" is because both the space aspect (matter) of our math is wrong AND the time aspect (energy) is wrong. GR and SR are correct but incomplete, for representing spacetime. Just like Newtonian rules are correct but incomplete. And as for your last sentence, trust me I understand the scientific method, and how proof, evidence, and fact interrelate with knowledge. Nice philosophical observations, but having nothing to do with this discussion.
  - digler999 10y ago
    the same thing could have been said about the discovery of Neptune [1]: "there's no evidence a planet is there, it's pure speculation. The changes in Uranus' orbit must be the result of mathematical error and not another planet." [1] https://en.wikipedia.org/wiki/Neptune https://en.wikipedia.org/wiki/Neptune
magicalist 10y ago
> There is no evidence that dark matter even exists other than... Most of our theories about the universe are based on evidence from secondary, tertiary, etc effects. It's often the best we can do. I don't think you'd find anyone who would object to the idea that "our theory is incomplete" ("all models are wrong", etc), so it seems like your main objection seems to be that we've given a name to a family of theories that attempt to explain the phenomenon we've grouped under "dark matter". > I think it's far more likely that our theory is incomplete rather than some whole new class of invisible matter/energy being conjured into existence just to counter-balance our 1) wrong equations and/or 2) wrong observations It's rather human to make an estimate of likelihood based on how long ago something was conjectured to exist :) Moreover "wrong equations" and/or "wrong observations" are part of the very theories attempting to explain our current state of knowledge. > So many non-scientists think dark matter is proven. It isn't. It's nothing but pure conjecture. Actually, usually the issue with these discussions are with scientifically literate folks who read a Scientific American article on dark matter in 1998 and have engaged with that as a strawman ever since. Somewhat tongue-in-cheek, but it is odd to me how many people are convinced they're bringing light to the darkness with these kinds of comments.
- ClayFerguson 10y ago
  Your first point of contention is invalid, because there ARE two distinct and DIFFERENT interpretations of what "dark stuff" is really telling us: Either 1) there are genuinely a whole class of particles that remain completely invisible to us except for their influence on the entire universe mass/energy balance, and yet still fit the current model of SR/GR --OR-- 2) the current model of space-time (GR/SR) is fundamentally broken (as if relativity, and even spacetime is 'emergent', from something more fundamental), and there is genuinely NO SUCH THING as dark energy or dark matter. All leading physicists agree those are the two scenarios that are very likely. You may be familiar with Einstein's "Cosmological Constant" which he called his greatest blunder. It's along those lines of reasoning that I go for door #2 above.
solipsism 10y ago
Completely false. Look up gravitational lensing.
- ClayFerguson 10y ago
  I'm a 50yo engineer. Chances are I've known the nuances of GR/SR several decades before you were even born, just based on HackerNews demographic.

Pica_soO 10y ago

Eh, those galaxys would fling themselves apart without it? Is this irony?

ClayFerguson 10y ago
No one knows why space is expanding. The dark energy is just the 'label' they give to whatever is causing it to expand. Both dark matter and dark energy are postulated to be whatever shape of puzzle piece fits into the puzzle slot where the error in our formulas exists. What i'm saying is that it's more likely that the formula itself is wrong than it is that something magically exists to conveniently fit that slot.
- raattgift 10y ago
  > The dark energy is just the 'label' they give to whatever is causing it to expand This depends on how you look at it. In the standard cosmology we define a preferred frame wherein an observer will see the matter (that's in the most general sense of "not the gravitational field", so it includes atoms and their components, photons, and various types of dark matter (e.g. neutrinos, which are "hot" dark matter, since they move relativistically and do not experience electromagnetism)) content of the universe as homogeneous and isotropic. This is physically reasonable since along every unobscured line of sight we see a lot of galaxies of various shapes, "tilts", sizes, surface brightnesses, and spectral lines. Observations also lead us to conclude that there is a relationship between redshifting of the spectral lines of common types of galaxies (and common radiative occurrences within them, like type A supernovas), and the change of the other observables (angular size on the sky, luminosity, etc.) that correlate with greater distance. This in turn led to the discovery of the Hubble "constant", and provoked ever deeper field telescopic studies to prove its value. So if we assume that along every line of sight, including obscured ones, we have much the same view of many many galaxies at a variety of distances, we can make a variation on the Friedmann equation that parameterize several things that would lead to the observables of galaxies when we model their known (and unknown) components as a set of perfect fluids. We can take the Hubble "constant" and put it into a Robertson-Walker vacuum spacetime. RW spacetimes can be grokked by dimensional reduction. Consider a cylinder that we slice (foliate) along its axis into a set of infinitesimally thin circles stacked on top of each other. We describe the radius of each circle with a function r(h) where h is the height of the circle from the base of the cylinder. Where r(h) is constant, we have a cylinder, but if r(h) increases with h, then we have something like a cone balancing on its apex; r(h) can describe a wide variety of shapes. For an 3+1 RW spacetime that is similar to our universe, we foliate on the timelike axis and define a function a(t) where t_0 == now with the spacelike coordinates set on a chosen observer (us here on Earth, for example). "t" counts upwards as we go into the past from t_0, and a(t) goes to zero as t increases. "t" is the lookback time and a(t) is the scale factor. The chosen observer is the special observer mentioned above, who sees the matter of the universe as isotropic and homogeneous at the largest scales. The most useful coordinates on such a spacetime are comoving, that is each gravitationally bound galaxy cluster stays at the same coordinates at every time t. If we mix together the Friedmann equations, the Lemaître idea of spacetime having a zero radius at some large lookback time, and the RW spacetime that can model that, we get the FLRW model of the standard cosmology. We take the RW case where there is no extrinsic curvature, that is, when we foliate on the timelike axis each spacelike hypersurface is spatially flat; that is similar to saying that when we slice up our dimensionally reduced solid along its height, we get a set of circles of the same radius (i.e. a cylinder rather than a cone). We absorb the expansion parameter into a(t) as another of the fluids. We then consider two types of fluid: those that dilute away as t -> t_0 -> future and those that do not dilute away. The former is "matter", including dark matter; the latter is "dark energy". When we consider them as components of an action, diluting-away fluids are attractive and non-diluting fluids are repuslive. When we consider them in terms of the matter tensor T in General Relativity, the diluting-away fluids have positive pressure and the non-diluting fluids have negative pressure. It's important to return to the point that this model has a preferred frame, and that translating the non-diluting fluid into "the same for all observers in all frames of reference" physics leads one to assume that dark energy is just a feature of the Lorentz-invariant vacuum. So dark energy arises in the cosmological model but corresponds to the ground state of the empty-of-matter spacetime in frames of reference other than the preferred one picked out by the cosmological model. That is, the statement that "dark energy drives the expansion (via negative pressure or repulsion)" is frame-dependent, and thus observer-dependent, and with a change of frames of reference (and even a change of coordinates on the preferred frame), the statement becomes untrue. What is true in all frames is that there is an intrinsic property of space in an expanding spacetime that has a constant energy-density no matter how large a volume of space is considered. The exact equation of state of dark energy is an area of active research and also tests to make sure that the assumptions that inevitably lead to it (isotropy at huge scales, homogeneity, spatial flatness, redshift-distance relations and other things implying expansion) are not blown up by evidence from ever finer observations. So it's not so much a 'label' as a phenomenon whose microscopic details have yet to be discovered. There are lots of those in physics, and we've had a good century of probing the microscopic details of phenomena discovered at the end of the 19th century and since, so this shouldn't really be causing anyone sleepless nights.
  - ClayFerguson 10y ago
    Your statement that there's "an expanding space-time that has a constant energy-density", gets to the nub of it for sure, but in reality no one knows if any of the fundamental field strengths are indeed constant. To me the need for "dark" stuff to be postulated, is more likely to indicate that gravity strength, or light speed, are more likely to NOT be constant over large space-time ranges, than the likelihood that there is a whole class of 'dark' particles and waves that currently don't interfere with any other matter in a provable way. So if someone asked me what's my 'evidence' that gravity or light-speed are not universally constant, my answer is simply: "The evidence for dark-energy/matter, is that exact same evidence". There should be some wave-function (possibly resulting from what we call a big bang), in terms of G and C, that once integrated out over the current life of the universe, will yield precisely that positions and velocities of the galaxies that we currently observe. To me it seems far less likely that there's an entirely new set of particles we cannot see, despite the Standard Model of particles being proven correct out to 40 decimal places.
    - Neeek 10y ago
      You already have the neutrino which interacts only via weak force and gravity, They show up much the same in bubble chambers as dark matter does in our telescopes; they don't. We only see where they interact with more observable matter, is it really that far fetched to imagine a particle that interacts through gravity only?
    - raattgift 10y ago
      I'm not sure what you're trying to argue here, or even why. I'll guess that you're interested in physical cosmology and would like a set of brief and not-too-technical reactions to your ideas (which I take to be implied questions) from someone who knows the \Lambda-CDM cosmology reasonably well. Dark energy it is a property of empty space that does not dilute away as more space appears in an expanding universe. The only way to abolish dark energy is to eliminate the expansion of the universe. It only has a "fundamental field strength" in particular chosen frames of reference in which one can represent it as a field with a constant energy-density. The comoving frame of the standard cosmology is one such frame of reference. However, in most other frames this energy density vanishes, and when that happens a general relativist will decide that the energy density was an artifact of the choice of frame of reference or system of coordinates and ditch the word "fundamental". Now, there are lots of ways we can complicate the action S_{\Lambda-CDM} in a Lagrangian formulation of the standard cosmology by introducing further repulsive terms into it beyond constant * \Lambda (as in the Einstein-Hilbert action or an expansion of it) or L_{repulsivematter} in a parameterization, and indeed there have been numerous attempts to do so. However, short of eliminating the expansion of space at all times and in all reference frames there is no way to get rid of a repulsive, non-diluting, non-concentrating (when time-reversed or if the critical density of the universe turns out to lead to a contraction of space in the future) term. But in the standard cosmology, we have \Lambda, which is just the cosmological constant, i.e., dark energy is a property of the ground state, which is the vacuum. When there's more vacuum, there's more dark energy. Mathematically, we start with an action that leads to the standard write-down of the EFEs. Physically, this matches observational tests at large scales, which is convenient because the standard write-down of the EFEs is almost the only way to match observational tests within our solar system (c.f. the parameterized post-Newtonian formalism). > gravity or light speed are not universally constant What exactly do you mean by "gravity ... not universally constant"? In particular, what do you mean by gravity? In General Relativity speeds are something that are extremely hard to talk about except in the local neighbourhood around a single point. In fact, many general relativists would argue that talking about "universal" speeds violates the spirit of general relativity. However, if our universe continues to be modellable as a smooth manifold with a Lorentz metric, we get a sort of quasi-universality of "c" in that at every point one can construct an infinitesimal region of spacetime in which "c" is a parameter in the action of matter and takes on the same _locally measured_ value in each such region for an observer in that region. There is ample evidence that favours this up to energy scales accessible to us on Earth and visible with observational platforms on and near our planet. There are certainly metric theories other than GR that allow for different sources to couple to different metrics, but most of these have to undergo a phase change to an effective single metric with universal coupling in the early universe or we would see clear evidence for them (in particular, the distribution of heat in the early dense phase of the universe still has to produce the Standard Model at lab energies and also stars and galaxies and labs). Some productive and well-regarded physical cosmologists have proposed these types of theories, even recently (e.g. Afshordi & Magueijo), and they explictly reject a universal value of "c" (in particular c approaches infinity in their model's extremely early universe). But in the standard cosmology we have General Relativity, and we don't vary the value of G or of c when grinding through the Einstein Field Equation; we take those values as given to us by nature, and have no evidence for them varying in the observable universe (and a pretty substantial amount of evidence against such variations, in particular including petabytes of spectroscopic data from objects in the sky). > "There should be some wave-function (possibly resulting from what we call a big bang), in terms of G and C, that once integrated out... I'm sorry, I don't understand this. Could you explain further? > To me it seems far less likely that there's an entirely new set of particles we cannot see, despite the Standard Model of particles being proven correct out to 40 decimal places. Dark energy is not particles. As I said above, it's a feature of the vacuum, and the most fundamental feature of the vacuum is that it is empty of particles. A tl;dr here is that we don't know what creates more space rather than less space, but we know that when more space is created there's more dark energy in the comoving frame but not more particles. Are you thinking of dark matter here, rather than dark energy? If so, we already have hot dark matter in the form of neutrinos. Neutrinos are "hot" because they move relativistically, and thus are prone to carry momentum far away from galaxies very quickly. Cold dark matter is "cold" because it moves non-relativistically and so the momentum CDM carries lingers in place in and around galaxy clusters. While it would be convenient if CDM were like heavy neutrinos, there is no reason in the standard cosmology that CDM has to be particles at all, or even just one species; the only requirement is that it be almost entirely collisionless and non-radiative so that we don't see it and so that it doesn't release its trapped momentum (e.g. by converting some of it into hot dark matter or light or standard model particles). Finally, the Standard Model is mute on gravity, and yet you -- made up of Standard Model particles -- are feeling it right now. So I'm not sure how your argument about the correctness of the Standard Model fits with your argument.

platz 10y ago

> no evidence The bullet cluster is hard to explain without some form of dark matter.

millstone 10y ago

What about the evidence from the bullet cluster? The gravitational center of mass is offset from the the visible center of mass. So something must be gravitating that is not visible - i.e. dark matter.

CuriouslyC 10y ago
Matter isn't the only thing that produces gravity. Energy also produces gravity. Given that the bullet cluster is also incredibly hot, it seems entirely plausible that its unusual gravitational morphology is somehow related to its energy content.
- russdill 10y ago
  You'd expect it to be incredibly hot where the gas is, no? And the interesting thing about the bullet cluster is that the excess of matter is found where there is no gas and stars. Plus, if the gas were super hot, it would emit lots of light, including x-rays.
  - CuriouslyC 10y ago
    My understanding was that it is in fact incredibly hot where the gas is, and it is emitting lots of x-rays. And you're jumping ahead of yourself to say that it is matter, we just know that there is gravitational lensing. My main issue is that the bullet is anomalous in multiple ways, and dark matter doesn't explain the heat issue. If it did, then it wouldn't be useful as an explanation for the rotation curve problem in other galaxies. I feel that a more parsimonious explanation would cover both the lensing and the heat.
    - russdill 10y ago
      Ah, ok, I was going to say, if the excess energy in heat was sufficient to explain the gravitational lensing, you'd have several problems. You're just pointing out that there are other significant anonymous things going on we don't understand. Until they are also explained, the solution is not a complete one.

russdill 10y ago

I believe you are thinking of dark energy. The evidence for dark matter is typically things like galaxy rotation curves.

M_Grey 10y ago

You've confused the logic for inferring dark energy or some other force at work, with dark matter which is involved in lensing experiments, galaxy rotation curves, etc.

ClayFerguson 10y ago
read the rest of my comments. I clarify why "dark stuff" is all the same.

lutusp 10y ago

> There is no evidence that dark matter even exists other than the fact that our universe is not expanding at the rate we think it should ... You're thinking of dark energy, the net effect of which is to change the overall expansion profile of the universe as a whole. Dark Matter was first proposed when galactic rotation profiles failed to meet theoretical expectations -- matter far from the center of each galaxy had a higher velocity than it would if visible matter had been the only factor. > I think it's far more likely that our theory is incomplete rather than some whole new class of invisible matter/energy being conjured into existence Occam's razor (the simplest explanation tends to be the right one) suggests that a new unobserved particle is more likely than abandonment of F = GMmr^-2 . This doesn't mean the equation must be correct, it's a question of reaching for the low-hanging fruit. > So many non-scientists think dark matter is proven. It isn't. It's nothing but pure conjecture. First, nothing is ever proven true in science, only false. Second, dark matter is more than pure conjecture, since there is observational evidence. A pure conjecture would be an idea about reality having no observational support at all (unicorns, Bigfoot). Dark matter is a hypothesis crafted to explain observations, but so far there's no persuasive theory to explain it, and no observations of candidate particles either.

ClayFerguson 10y ago
I should have been clear that I'm not making a distinction between energy and matter. I'm referring to the postulated "dark" forms of both. Both of them are pure speculation, and is nothing but the equivalent of taking whatever observable "error" there is in our formulas and labeling it "dark". The only reason there's two kinds of "dark" is because both the space aspect (matter) of our math is wrong AND the time aspect (energy) is wrong. GR and SR are correct but incomplete, for representing spacetime. Just like Newtonian rules are correct but incomplete. And as for your last sentence, trust me I understand the scientific method, and how proof, evidence, and fact interrelate with knowledge. Nice philosophical observations, but having nothing to do with this discussion.
- digler999 10y ago
  the same thing could have been said about the discovery of Neptune [1]: "there's no evidence a planet is there, it's pure speculation. The changes in Uranus' orbit must be the result of mathematical error and not another planet." [1] https://en.wikipedia.org/wiki/Neptune https://en.wikipedia.org/wiki/Neptune

magicalist 10y ago

> There is no evidence that dark matter even exists other than... Most of our theories about the universe are based on evidence from secondary, tertiary, etc effects. It's often the best we can do. I don't think you'd find anyone who would object to the idea that "our theory is incomplete" ("all models are wrong", etc), so it seems like your main objection seems to be that we've given a name to a family of theories that attempt to explain the phenomenon we've grouped under "dark matter". > I think it's far more likely that our theory is incomplete rather than some whole new class of invisible matter/energy being conjured into existence just to counter-balance our 1) wrong equations and/or 2) wrong observations It's rather human to make an estimate of likelihood based on how long ago something was conjectured to exist :) Moreover "wrong equations" and/or "wrong observations" are part of the very theories attempting to explain our current state of knowledge. > So many non-scientists think dark matter is proven. It isn't. It's nothing but pure conjecture. Actually, usually the issue with these discussions are with scientifically literate folks who read a Scientific American article on dark matter in 1998 and have engaged with that as a strawman ever since. Somewhat tongue-in-cheek, but it is odd to me how many people are convinced they're bringing light to the darkness with these kinds of comments.

ClayFerguson 10y ago
Your first point of contention is invalid, because there ARE two distinct and DIFFERENT interpretations of what "dark stuff" is really telling us: Either 1) there are genuinely a whole class of particles that remain completely invisible to us except for their influence on the entire universe mass/energy balance, and yet still fit the current model of SR/GR --OR-- 2) the current model of space-time (GR/SR) is fundamentally broken (as if relativity, and even spacetime is 'emergent', from something more fundamental), and there is genuinely NO SUCH THING as dark energy or dark matter. All leading physicists agree those are the two scenarios that are very likely. You may be familiar with Einstein's "Cosmological Constant" which he called his greatest blunder. It's along those lines of reasoning that I go for door #2 above.

solipsism 10y ago

Completely false. Look up gravitational lensing.

ClayFerguson 10y ago
I'm a 50yo engineer. Chances are I've known the nuances of GR/SR several decades before you were even born, just based on HackerNews demographic.