9 ms·

A Natural Law for Rotating Galaxies: What Does This Mean for Dark Matter?

145 points by altstar 10y ago

Animats 10y ago
The observation that the visible galaxies all behave similarly, as if they have a dark matter halo, clashes with the observation that our galaxy doesn't have any detectable dark matter in the neighborhood.[1] [1] http://www.space.com/15333-dark-matter-missing-sun.html http://www.space.com/15333-dark-matter-missing-sun.html
- vorg 10y ago
  According to that article (2012), you'd better clarify that as "...our galaxy doesn't have any detectable dark matter in the neighborhood of our sun." Perhaps the phenomenon is the result of new relativistic dynamical laws instead of dark matter, e.g. "All large-scale rotating structures appear to be indistinguishable from a single rotating object when viewed from any observer's frame of reference" for some definition of "large-scale".
conistonwater 10y ago
https://arxiv.org/abs/1609.05917 https://arxiv.org/abs/1609.05917
josephagoss 10y ago
I thought that Dark Matter and Dark Energy are simply placeholders for our lack of explanation of observed phenomenon? In which case these types of discoveries are to be expected as we fill in the missing blanks. If so then I am happy to see progress being made. If I am wrong about my assumptions about Dark Matter and Dark Energy I wouldn't mind some starters :) Thanks!
- conistonwater 10y ago
  My understanding is that the difference between observed and predicted galaxy rotation rates is real, and the thing that is thought to cause it is given the name of dark matter. But that doesn't make it a placeholder! Any more, for example, than when people detected the effects of Mercury or Neptune on the orbits of other planets in the solar system, before they could see those planets directly, they could give them a name without being able to see them: yet those weren't placeholders, rather their effects on orbits was evidence of their existence. OTOH, maybe everything in science is a kind of a placeholder, but that sort of thing can get unpleasantly philosophical.
  - hanoz 10y ago
    It seems to me there's a big difference between an explanation of an expected/obsevered mismatch which says there must be another massive object involved - and here's where to point your telescopes to look for it, and one which says well there must be some other mass of some form somewhere.
    - conistonwater 10y ago
      For a pretty long time, for both those planets, they didn't know where to point their telescopes. This was also true for the outer planets, and Pluto too. A similar sort of thing, to a different/lesser extent, used to be the case with gravitational waves not having been directly observed despite other evidence. There were similar problems with the early discussions of atomic nuclei, electrons, neutrinos, neutrons, where all kinds of things were posited based on their effects only. I know it's quite a bit more difficult, and less direct, with dark matter, but still. (Really, I was objecting to the use of the word placeholder, because I found it unfair.)
  - samatman 10y ago
    Mercury is visible to the naked eye. Consequently, its existence has been known since prehistory.
    - conistonwater 10y ago
      You're right. I think I must have misremembered something I read about https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) But I think I got the example of Neptune right: https://en.wikipedia.org/wiki/Discovery_of_Neptune#Irregularities_in_Uranus.27s_orbit https://en.wikipedia.org/wiki/Discovery_of_Neptune#Irregular...
- Avshalom 10y ago
  Sort of, "dark matter" would just be matter that doesn't interact with photons. We're pretty dang sure dark matter isn't neutrinos but for reference neutrinos don't interact with photons either, so it's not like dark matter is actually all that exotic. We just can't constrain every thing about it. What it comes down to, and this paper doesn't change this at all, is that either there's a bunch of matter we can't see or force carrying particles don't obey basic geometry (or the universe has much weirder geometry than we think). Most of us are a lot more comfortable with basically invisible matter. https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/MACS_J0025.4-1222 https://en.wikipedia.org/wiki/MACS_J0025.4-1222 https://en.wikipedia.org/wiki/Dragonfly_44 https://en.wikipedia.org/wiki/Dragonfly_44
  - lisivka 10y ago
    It was called "Luminiferous aether". Sagnac effect shows that speed of light is not constant: http://www.conspiracyoflight.com/Sagnac/SagnacEarth.html http://www.conspiracyoflight.com/Sagnac/SagnacEarth.html . My understanding is that our Universe is endless in all 4 dimensions and in scale. Every point of Universe contains infinite number of small, even infinite small, particles. These particles are forming large objects, even infinite large objects. And these objects are oscillating endlessly: condensing and evaporating, so whole Universe cannot be sink into singularity, even in endless time. IMHO, we are in large bulb of plank-scale particles, which are called "physical vacuum", which are forming medium for all other particles and forces to travel. This medium is not regular: it denser in center (so it causes red shift), it denser around massive objects.
    - ifdefdebug 10y ago
      IMHO it's just turtles all the way down
      - Joof 10y ago
        At least that means it converges and we can estimate it to some useful degree.
peter303 10y ago
Dark matter is used to explain other observations - the accelerated movement of galaxy clusters, lensing of objects behind galaxies, the spatial frequency off the background radiation.
- Avshalom 10y ago
  The authors have for some reason decided to leave "the amount of visible matter [is] directly related to the amount of dark matter" out of their list of possible conclusions but I suspect that if their data/methods are legitimate than that is exactly the conclusion we're going to end up with.
  - nshepperd 10y ago
    That isn't on the list because two of them items on their list are possible explanations for why the visible matter would be directly related to the dark matter. The fact that visible and dark matter are related is not much of a conclusion by itself.
infectoid 10y ago
I'm a bit simple, but is this saying that dark matter (at least at a galactic scale) is in some way proportional to visible matter and that this is something that had not been tested for until now?
- Avshalom 10y ago
  Yeah. There's no obvious reason it should be but this data says it is. The actual paper also says that but then ignores the fact that they just said it so they can suggest MOND.
- Joof 10y ago
  As far as I can tell it has only been observed in stars. Whether it is proportional in the same way to other objects remains to be seen.
joe_the_user 10y ago
I am not sure why this isn't MOND, Modified Newtonian Dynamics in different garb [1] - instead of modifying Newton's equations, just modify the predictions of those equation - ie, the laws of planetary motion. And MOND is an interesting theory that's been explored but hasn't gone anywhere. [1] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
hzhou321 10y ago
This is good. Dark matter has always in my eye so similar to the God hypothesis that we conjure something that is mysterious with undefined but whatever necessary to explain away the discrepancy or mystery. Acknowledging that we don't know -- not even assume it is matter that fits our current understanding -- is the start of understanding. Now it appears we do know it fits some mathematical equation ... PS: the God hypothesis could well be correct, but that should be the last resort -- It is not really an explanation, so it is not necessary until the explanation is needed for reasons of other than explaining.
- pharrington 10y ago
  If that's what bugs you, then just call dark matter "Fred" instead. Its functionally equivalent. e: I'm agreeing with you that putting "matter" in its name is fairly misleading. Fortunately, that doesn't confuse physicists.
  - ab5tract 10y ago
    I'm sorry, but scientists rarely talk about dark anything as if it is an unknown. They use it all of the time as a central component of their explanations. It's as baseless as the ether of the 19th century and as ubiquitous in its invocation. But mention this and get downvoted.
    - ifdefdebug 10y ago
      That's not entirely fair, because treating it as "matter" gives a quite good mathematical fit for current observations so far, while the first experimental observation of light speed ever made immediately falsified the ether hypothesis.
      - hzhou321 10y ago
        It took a while even after the experiment was repeated a couple times. And even after the experiment only very few could abandon the idea of ether, and only after abandon the idea of ether, they see the new physics. Now with dark matter, we have no experimental idea and it is out-of-reach. It is there to protect the consistency (cover up the inconsistency) of our current understanding -- familiar?
gh1 10y ago
First we had Newton's laws of gravity. It worked quite well in the solar system scale and everyone was happy. Then Einstein formulated the special theory of relativity and Newtonian gravity was found to be incompatible with that. So Einstein formulated the General Theory of Relativity (Einsteinian Gravity) which solves that problem. As a bonus, his theory could also explain the eccentric behavior of Mercury's orbits, which Newtonian Gravity couldn't. It also predicted that galaxies should be able to bend light (lensing) and this was confirmed by Arthur Eddington. However, for many galaxies, the amount of lensing is way off from what the Einsteinian theory predicts. Similarly, rotational velocities of stars in a galaxy are also way off from what is predicted by both Einsteinian and Newtonian theory. In addition, when you apply the Einsteinian theory to the entire universe, you can't explain the observable geometry of the universe. That is also way off. Dark matter and Dark Energy are just a very clever way of saying "we have no idea what's going on with gravity in the large scale (beyond solar system scales)". No one has ever observed any dark matter or dark energy so far. It is entirely possible that Einsteinian theory of Gravity is just an approximation that works well for small scales and that we need a new theory of gravity at the galactic scale or the scale of the universe. It is also entirely possible that Dark Matter and Energy don't actually exist.
- danharaj 10y ago
  > In addition, when you apply the Einsteinian theory to the entire universe, you can't explain the observable geometry of the universe. https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtr... "The Friedmann–Lemaître–Robertson–Walker (FLRW) metric is an exact solution of Einstein's field equations of general relativity; it describes a homogeneous, isotropic expanding or contracting universe that is path connected, but not necessarily simply connected." > No one has ever observed any dark matter http://chandra.harvard.edu/press/06_releases/press_082106.html http://chandra.harvard.edu/press/06_releases/press_082106.ht... "NASA finds direct proof of Dark Matter" > or dark energy so far. This is probably the only part of your post that actually came close to the truth. Dark energy is far more of a placeholder than dark matter at this point.
  - 10y ago
slicktux 10y ago
http://blogs.scientificamerican.com/critical-opalescence/is-dark-matter-a-glimpse-of-a-deeper-level-of-reality/ http://blogs.scientificamerican.com/critical-opalescence/is-...
nonbel 10y ago
One thing that bugs me about dark matter is that all the assumptions being made do not seem to have been fully investigated before "accepting" the idea as mainstream: "The Duhem–Quine thesis, also called the Duhem–Quine problem, after Pierre Duhem and Willard Van Orman Quine, is that it is impossible to test a scientific hypothesis in isolation, because an empirical test of the hypothesis requires one or more background assumptions (also called auxiliary assumptions or auxiliary hypotheses). In recent decades the set of associated assumptions supporting a thesis sometimes is called a bundle of hypotheses." https://en.wikipedia.org/wiki/Duhem%E2%80%93Quine_thesis https://en.wikipedia.org/wiki/Duhem%E2%80%93Quine_thesis For example, when predicting the rotation curves you can instead assume the mass is log-normally distributed and get the flat curves: "The generation of the 37 galaxy velocity profiles presented in this paper assumed that most of the galactic mass is in the disk, and gravity is Newtonian. A best-fit algorithm was used to generate the curves of Figures 1 and 3 – 38 using a truncated log-normal surface density distribution function. The log-normal models closely matched the shape of observational rotation velocities, and the predicted masses for these curves fitted a baryonic Tully-Fisher relation reasonably well over a wide range of galaxy sizes, from LSB galaxies to massive high-luminosity disks." https://arxiv.org/abs/1502.02949 https://arxiv.org/abs/1502.02949 It isn't that I think that must be the correct solution, but why was no one publishing the results of investigating that assumption until 2015?
- FrozenVoid 10y ago
  see https://arxiv.org/abs/astro-ph/0612434 https://arxiv.org/abs/astro-ph/0612434
  - nonbel 10y ago
    >"The stars in spiral galaxies are settled thin disks with an exponential surface mass distribution" Can you expand on that post? The paper you linked appears to assume the very thing being questioned here...
    - FrozenVoid 10y ago
      Its explained on the next page(it a simplification) In fact, by relaxing some of the assumptions made above, we have the more general relationship: L ∝ V 2/(+s+q−t) opt (here s, q and t can be band- dependent) that can be even more complex and non-linear when the scaling laws (2), (3), (4) are not just power laws. As a matter of fact, in several different large samples of galaxies it has been found that the TF has different slope and scatter in different bands: a I ≃ 10, s I ∼ 0.4mag, while a B ≃ 7.7, s B ∼ 0.5mag (Pierce & Tully 1992), (Salucci et al. 1993). Moreover, a non linearity in the TF is often found at low rotation velocities (Aaronson et al. 1982).
mgalka 10y ago
Unusual to see a new physics discovery that you don't need a PhD to understand
michaelrccurtis 10y ago
It always depresses me (as an astrophysicist) how poor the communication is on dark matter, and this leads people to unfair conclusions. We have known for some time about the limitations of our understanding of gravitational dynamics in extreme environments - the issue is not that "our theories must be right", it's that we have to square General Relativity (which is phenomenally well tested in other regimes) with the results we see. There are broadly 2 different ways of doing this: you can postulate that not all particles interact with electromagnetic radiation (after all, why should they?) or you can come how add extra terms into your gravitational laws. Both of these have been tried, but there are many reasons why the majority of astrophysicists prefer the dark matter approach. Firstly, there are observations which suggest large regions of mass that are not aligned with the visible mass (e.g. the bullet cluster). Secondly, dark matter can explain the statistical properties of the Cosmic Microwave Background radiation in a way that can not be achieved with ordinary matter alone. Thirdly, when we include dark matter in simulations, we get results that are a very good match for the universe as we observe it. etc. etc. If that weren't enough, we know that there are plenty of types of matter that are dark - e.g. neutrinos, early black holes etc. Whilst none of these have yet satisfied the conditions for dark matter as we understand it, it's perfectly possible that we have already identified some of the constituent parts, and there is a list of potential candidates as long as my arm that the particle physicists are working their way through. Finally, the biggest annoyance is the intimation that we are all just one big cabal. Let me put it like this: if you could provide indisputable evidence that dark matter was wrong, you would be a superstar in our field and everyone would be phenomenally excited, because it would demand the hunt for new physics. Scientists are a rational bunch, however, so any evidence must be weighed against the existing evidence already observed (which, as above, is substantial). Dark energy is slightly different. No one know's what this is, but models with a dark energy component do very well at modelling the universe. The question is, why? No one is pretending that this is a solved question, but what we do know is that whatever is driving the universe's accelerated expansion has properties that are very similar to what dark energy is. tldr: There are loads of great reasons for thinking dark matter exists, contrary to many posters here, and it is well accepted that dark energy is a mathematical term that represents something of unknown physical origin.
- cygx 10y ago
tjic 10y ago
I remember being in fifth grade science class (in the early 1980s) and hearing first about Kepler's law and that planets further out circle the sun less quickly than those closer in, and then next hearing that we had spiral galaxies. I raised my hand and asked "how do the galaxies maintain their spiral shape? Shouldn't the inner stars rotate much more quickly than the outer ones?" The teacher didn't understand my question (this was one of my first inklings that teachers didn't tend to be that bright). The last 10-15 years with headlines in the popular press about dark matter, MOND, etc. are quite satisfying - 10 year old me is thrilled that my question is mainstream now!
- wcoenen 10y ago
  It's a good question, but that is not the same issue as the one the article is about. The spiral arms maintain their shape not because of an anomaly in orbital velocities, but because they are density waves[1]. Kepler doesn't work because the arms themselves and the rest of the disk exerts significant gravity. [1] https://en.wikipedia.org/wiki/Density_wave_theory https://en.wikipedia.org/wiki/Density_wave_theory
- jessriedel 10y ago
  To emphasize what wcoenen said: the appearance of spiral arms persists even in the interior of galaxies where there is no discrepancy from Kepler's laws. Moreover, in order to explain fixed spiral structure using modified orbital mechanics, you would need to assert that outers stars have larger velocities than inner ones (in order that their angular speed is the same, to preserve disk structure), but in fact the rotation-curve conundrum is based on the observation that outer star have roughly constant velocity, and so reduced angular speed.
bawana 10y ago
Does the presence of dark matter and dark energy imply other 'dark' properties. Consider 'dark' intelligence - I think our presidential candidates qualify as vast repositories of this - you can't observe any intelligent speech but you know there must have been some - otherwise how could they get to where they are today?

145 points by altstar 10y ago

The observation that the visible galaxies all behave similarly, as if they have a dark matter halo, clashes with the observation that our galaxy doesn't have any detectable dark matter in the neighborhood.[1] [1] http://www.space.com/15333-dark-matter-missing-sun.html http://www.space.com/15333-dark-matter-missing-sun.html

vorg 10y ago
According to that article (2012), you'd better clarify that as "...our galaxy doesn't have any detectable dark matter in the neighborhood of our sun." Perhaps the phenomenon is the result of new relativistic dynamical laws instead of dark matter, e.g. "All large-scale rotating structures appear to be indistinguishable from a single rotating object when viewed from any observer's frame of reference" for some definition of "large-scale".

conistonwater 10y ago

https://arxiv.org/abs/1609.05917 https://arxiv.org/abs/1609.05917

josephagoss 10y ago

I thought that Dark Matter and Dark Energy are simply placeholders for our lack of explanation of observed phenomenon? In which case these types of discoveries are to be expected as we fill in the missing blanks. If so then I am happy to see progress being made. If I am wrong about my assumptions about Dark Matter and Dark Energy I wouldn't mind some starters :) Thanks!

conistonwater 10y ago
My understanding is that the difference between observed and predicted galaxy rotation rates is real, and the thing that is thought to cause it is given the name of dark matter. But that doesn't make it a placeholder! Any more, for example, than when people detected the effects of Mercury or Neptune on the orbits of other planets in the solar system, before they could see those planets directly, they could give them a name without being able to see them: yet those weren't placeholders, rather their effects on orbits was evidence of their existence. OTOH, maybe everything in science is a kind of a placeholder, but that sort of thing can get unpleasantly philosophical.
- hanoz 10y ago
  It seems to me there's a big difference between an explanation of an expected/obsevered mismatch which says there must be another massive object involved - and here's where to point your telescopes to look for it, and one which says well there must be some other mass of some form somewhere.
  - conistonwater 10y ago
    For a pretty long time, for both those planets, they didn't know where to point their telescopes. This was also true for the outer planets, and Pluto too. A similar sort of thing, to a different/lesser extent, used to be the case with gravitational waves not having been directly observed despite other evidence. There were similar problems with the early discussions of atomic nuclei, electrons, neutrinos, neutrons, where all kinds of things were posited based on their effects only. I know it's quite a bit more difficult, and less direct, with dark matter, but still. (Really, I was objecting to the use of the word placeholder, because I found it unfair.)
- samatman 10y ago
  Mercury is visible to the naked eye. Consequently, its existence has been known since prehistory.
  - conistonwater 10y ago
    You're right. I think I must have misremembered something I read about https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) But I think I got the example of Neptune right: https://en.wikipedia.org/wiki/Discovery_of_Neptune#Irregularities_in_Uranus.27s_orbit https://en.wikipedia.org/wiki/Discovery_of_Neptune#Irregular...
Avshalom 10y ago
Sort of, "dark matter" would just be matter that doesn't interact with photons. We're pretty dang sure dark matter isn't neutrinos but for reference neutrinos don't interact with photons either, so it's not like dark matter is actually all that exotic. We just can't constrain every thing about it. What it comes down to, and this paper doesn't change this at all, is that either there's a bunch of matter we can't see or force carrying particles don't obey basic geometry (or the universe has much weirder geometry than we think). Most of us are a lot more comfortable with basically invisible matter. https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/MACS_J0025.4-1222 https://en.wikipedia.org/wiki/MACS_J0025.4-1222 https://en.wikipedia.org/wiki/Dragonfly_44 https://en.wikipedia.org/wiki/Dragonfly_44
- lisivka 10y ago
  It was called "Luminiferous aether". Sagnac effect shows that speed of light is not constant: http://www.conspiracyoflight.com/Sagnac/SagnacEarth.html http://www.conspiracyoflight.com/Sagnac/SagnacEarth.html . My understanding is that our Universe is endless in all 4 dimensions and in scale. Every point of Universe contains infinite number of small, even infinite small, particles. These particles are forming large objects, even infinite large objects. And these objects are oscillating endlessly: condensing and evaporating, so whole Universe cannot be sink into singularity, even in endless time. IMHO, we are in large bulb of plank-scale particles, which are called "physical vacuum", which are forming medium for all other particles and forces to travel. This medium is not regular: it denser in center (so it causes red shift), it denser around massive objects.
  - ifdefdebug 10y ago
    IMHO it's just turtles all the way down
    - Joof 10y ago
      At least that means it converges and we can estimate it to some useful degree.

peter303 10y ago

Dark matter is used to explain other observations - the accelerated movement of galaxy clusters, lensing of objects behind galaxies, the spatial frequency off the background radiation.

Avshalom 10y ago
The authors have for some reason decided to leave "the amount of visible matter [is] directly related to the amount of dark matter" out of their list of possible conclusions but I suspect that if their data/methods are legitimate than that is exactly the conclusion we're going to end up with.
- nshepperd 10y ago
  That isn't on the list because two of them items on their list are possible explanations for why the visible matter would be directly related to the dark matter. The fact that visible and dark matter are related is not much of a conclusion by itself.

infectoid 10y ago

I'm a bit simple, but is this saying that dark matter (at least at a galactic scale) is in some way proportional to visible matter and that this is something that had not been tested for until now?

Avshalom 10y ago
Yeah. There's no obvious reason it should be but this data says it is. The actual paper also says that but then ignores the fact that they just said it so they can suggest MOND.
Joof 10y ago
As far as I can tell it has only been observed in stars. Whether it is proportional in the same way to other objects remains to be seen.

joe_the_user 10y ago

I am not sure why this isn't MOND, Modified Newtonian Dynamics in different garb [1] - instead of modifying Newton's equations, just modify the predictions of those equation - ie, the laws of planetary motion. And MOND is an interesting theory that's been explored but hasn't gone anywhere. [1] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

hzhou321 10y ago

This is good. Dark matter has always in my eye so similar to the God hypothesis that we conjure something that is mysterious with undefined but whatever necessary to explain away the discrepancy or mystery. Acknowledging that we don't know -- not even assume it is matter that fits our current understanding -- is the start of understanding. Now it appears we do know it fits some mathematical equation ... PS: the God hypothesis could well be correct, but that should be the last resort -- It is not really an explanation, so it is not necessary until the explanation is needed for reasons of other than explaining.

pharrington 10y ago
If that's what bugs you, then just call dark matter "Fred" instead. Its functionally equivalent. e: I'm agreeing with you that putting "matter" in its name is fairly misleading. Fortunately, that doesn't confuse physicists.
- ab5tract 10y ago
  I'm sorry, but scientists rarely talk about dark anything as if it is an unknown. They use it all of the time as a central component of their explanations. It's as baseless as the ether of the 19th century and as ubiquitous in its invocation. But mention this and get downvoted.
  - ifdefdebug 10y ago
    That's not entirely fair, because treating it as "matter" gives a quite good mathematical fit for current observations so far, while the first experimental observation of light speed ever made immediately falsified the ether hypothesis.
    - hzhou321 10y ago
      It took a while even after the experiment was repeated a couple times. And even after the experiment only very few could abandon the idea of ether, and only after abandon the idea of ether, they see the new physics. Now with dark matter, we have no experimental idea and it is out-of-reach. It is there to protect the consistency (cover up the inconsistency) of our current understanding -- familiar?

gh1 10y ago

First we had Newton's laws of gravity. It worked quite well in the solar system scale and everyone was happy. Then Einstein formulated the special theory of relativity and Newtonian gravity was found to be incompatible with that. So Einstein formulated the General Theory of Relativity (Einsteinian Gravity) which solves that problem. As a bonus, his theory could also explain the eccentric behavior of Mercury's orbits, which Newtonian Gravity couldn't. It also predicted that galaxies should be able to bend light (lensing) and this was confirmed by Arthur Eddington. However, for many galaxies, the amount of lensing is way off from what the Einsteinian theory predicts. Similarly, rotational velocities of stars in a galaxy are also way off from what is predicted by both Einsteinian and Newtonian theory. In addition, when you apply the Einsteinian theory to the entire universe, you can't explain the observable geometry of the universe. That is also way off. Dark matter and Dark Energy are just a very clever way of saying "we have no idea what's going on with gravity in the large scale (beyond solar system scales)". No one has ever observed any dark matter or dark energy so far. It is entirely possible that Einsteinian theory of Gravity is just an approximation that works well for small scales and that we need a new theory of gravity at the galactic scale or the scale of the universe. It is also entirely possible that Dark Matter and Energy don't actually exist.

danharaj 10y ago
> In addition, when you apply the Einsteinian theory to the entire universe, you can't explain the observable geometry of the universe. https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtr... "The Friedmann–Lemaître–Robertson–Walker (FLRW) metric is an exact solution of Einstein's field equations of general relativity; it describes a homogeneous, isotropic expanding or contracting universe that is path connected, but not necessarily simply connected." > No one has ever observed any dark matter http://chandra.harvard.edu/press/06_releases/press_082106.html http://chandra.harvard.edu/press/06_releases/press_082106.ht... "NASA finds direct proof of Dark Matter" > or dark energy so far. This is probably the only part of your post that actually came close to the truth. Dark energy is far more of a placeholder than dark matter at this point.
- 10y ago

slicktux 10y ago

http://blogs.scientificamerican.com/critical-opalescence/is-dark-matter-a-glimpse-of-a-deeper-level-of-reality/ http://blogs.scientificamerican.com/critical-opalescence/is-...

nonbel 10y ago

One thing that bugs me about dark matter is that all the assumptions being made do not seem to have been fully investigated before "accepting" the idea as mainstream: "The Duhem–Quine thesis, also called the Duhem–Quine problem, after Pierre Duhem and Willard Van Orman Quine, is that it is impossible to test a scientific hypothesis in isolation, because an empirical test of the hypothesis requires one or more background assumptions (also called auxiliary assumptions or auxiliary hypotheses). In recent decades the set of associated assumptions supporting a thesis sometimes is called a bundle of hypotheses." https://en.wikipedia.org/wiki/Duhem%E2%80%93Quine_thesis https://en.wikipedia.org/wiki/Duhem%E2%80%93Quine_thesis For example, when predicting the rotation curves you can instead assume the mass is log-normally distributed and get the flat curves: "The generation of the 37 galaxy velocity profiles presented in this paper assumed that most of the galactic mass is in the disk, and gravity is Newtonian. A best-fit algorithm was used to generate the curves of Figures 1 and 3 – 38 using a truncated log-normal surface density distribution function. The log-normal models closely matched the shape of observational rotation velocities, and the predicted masses for these curves fitted a baryonic Tully-Fisher relation reasonably well over a wide range of galaxy sizes, from LSB galaxies to massive high-luminosity disks." https://arxiv.org/abs/1502.02949 https://arxiv.org/abs/1502.02949 It isn't that I think that must be the correct solution, but why was no one publishing the results of investigating that assumption until 2015?

FrozenVoid 10y ago
see https://arxiv.org/abs/astro-ph/0612434 https://arxiv.org/abs/astro-ph/0612434
- nonbel 10y ago
  >"The stars in spiral galaxies are settled thin disks with an exponential surface mass distribution" Can you expand on that post? The paper you linked appears to assume the very thing being questioned here...
  - FrozenVoid 10y ago
    Its explained on the next page(it a simplification) In fact, by relaxing some of the assumptions made above, we have the more general relationship: L ∝ V 2/(+s+q−t) opt (here s, q and t can be band- dependent) that can be even more complex and non-linear when the scaling laws (2), (3), (4) are not just power laws. As a matter of fact, in several different large samples of galaxies it has been found that the TF has different slope and scatter in different bands: a I ≃ 10, s I ∼ 0.4mag, while a B ≃ 7.7, s B ∼ 0.5mag (Pierce & Tully 1992), (Salucci et al. 1993). Moreover, a non linearity in the TF is often found at low rotation velocities (Aaronson et al. 1982).

mgalka 10y ago

Unusual to see a new physics discovery that you don't need a PhD to understand

michaelrccurtis 10y ago

It always depresses me (as an astrophysicist) how poor the communication is on dark matter, and this leads people to unfair conclusions. We have known for some time about the limitations of our understanding of gravitational dynamics in extreme environments - the issue is not that "our theories must be right", it's that we have to square General Relativity (which is phenomenally well tested in other regimes) with the results we see. There are broadly 2 different ways of doing this: you can postulate that not all particles interact with electromagnetic radiation (after all, why should they?) or you can come how add extra terms into your gravitational laws. Both of these have been tried, but there are many reasons why the majority of astrophysicists prefer the dark matter approach. Firstly, there are observations which suggest large regions of mass that are not aligned with the visible mass (e.g. the bullet cluster). Secondly, dark matter can explain the statistical properties of the Cosmic Microwave Background radiation in a way that can not be achieved with ordinary matter alone. Thirdly, when we include dark matter in simulations, we get results that are a very good match for the universe as we observe it. etc. etc. If that weren't enough, we know that there are plenty of types of matter that are dark - e.g. neutrinos, early black holes etc. Whilst none of these have yet satisfied the conditions for dark matter as we understand it, it's perfectly possible that we have already identified some of the constituent parts, and there is a list of potential candidates as long as my arm that the particle physicists are working their way through. Finally, the biggest annoyance is the intimation that we are all just one big cabal. Let me put it like this: if you could provide indisputable evidence that dark matter was wrong, you would be a superstar in our field and everyone would be phenomenally excited, because it would demand the hunt for new physics. Scientists are a rational bunch, however, so any evidence must be weighed against the existing evidence already observed (which, as above, is substantial). Dark energy is slightly different. No one know's what this is, but models with a dark energy component do very well at modelling the universe. The question is, why? No one is pretending that this is a solved question, but what we do know is that whatever is driving the universe's accelerated expansion has properties that are very similar to what dark energy is. tldr: There are loads of great reasons for thinking dark matter exists, contrary to many posters here, and it is well accepted that dark energy is a mathematical term that represents something of unknown physical origin.

cygx 10y ago

tjic 10y ago

I remember being in fifth grade science class (in the early 1980s) and hearing first about Kepler's law and that planets further out circle the sun less quickly than those closer in, and then next hearing that we had spiral galaxies. I raised my hand and asked "how do the galaxies maintain their spiral shape? Shouldn't the inner stars rotate much more quickly than the outer ones?" The teacher didn't understand my question (this was one of my first inklings that teachers didn't tend to be that bright). The last 10-15 years with headlines in the popular press about dark matter, MOND, etc. are quite satisfying - 10 year old me is thrilled that my question is mainstream now!

wcoenen 10y ago
It's a good question, but that is not the same issue as the one the article is about. The spiral arms maintain their shape not because of an anomaly in orbital velocities, but because they are density waves[1]. Kepler doesn't work because the arms themselves and the rest of the disk exerts significant gravity. [1] https://en.wikipedia.org/wiki/Density_wave_theory https://en.wikipedia.org/wiki/Density_wave_theory
jessriedel 10y ago
To emphasize what wcoenen said: the appearance of spiral arms persists even in the interior of galaxies where there is no discrepancy from Kepler's laws. Moreover, in order to explain fixed spiral structure using modified orbital mechanics, you would need to assert that outers stars have larger velocities than inner ones (in order that their angular speed is the same, to preserve disk structure), but in fact the rotation-curve conundrum is based on the observation that outer star have roughly constant velocity, and so reduced angular speed.

bawana 10y ago

Does the presence of dark matter and dark energy imply other 'dark' properties. Consider 'dark' intelligence - I think our presidential candidates qualify as vast repositories of this - you can't observe any intelligent speech but you know there must have been some - otherwise how could they get to where they are today?