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.
"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."
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.
1. I was referring to the fact that the observable matter of the universe is not enough to generate the nearly flat and accelerating universe.
2. I am quoting from the article.
"In addition to the Chandra observation, the Hubble Space Telescope, the European Southern Observatory's Very Large Telescope and the Magellan optical telescopes were used to determine the location of the mass in the clusters. This was done by measuring the effect of gravitational lensing, where gravity from the clusters distorts light from background galaxies as predicted by Einstein's theory of general relativity."
Since they are calculating the mass by using General Theory of Relativity (indirect measurement, you have to trust the
theory for this), I don't see how this can be called direct detection.
'The most serious problem facing Milgrom's law is that it cannot completely eliminate the need for dark matter in all astrophysical systems: galaxy clusters show a residual mass discrepancy even when analysed using MOND.'
'Besides these observational issues, MOND and its generalisations are plagued by theoretical difficulties. Several ad-hoc and inelegant additions to general relativity are required to create a theory with a non-Newtonian non-relativistic limit, the plethora of different versions of the theory offer diverging predictions in simple physical situations and thus make it difficult to test the framework conclusively, and some formulations (most prominently those based on modified inertia) have long suffered from poor compatibility with cherished physical principles such as conservation laws.'
Basically, people have tried to fiddle with the laws as you suggested, but none of actually managed to fit the data better than GR + dark matter, plus the fiddling often just looked like curve-fitting without any good theoretical justification. Of course, the jury's still out.
It's the precession of Mercury's perihelion, not the eccentricity of its orbit: apart from the perihelion precession issue for Mercury and, to a lesser extent, Venus, the solar system is quite Newtonian. Also, the first lensing experiment measured the amount of light deflection by the sun, not galaxies.
> 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)".
While I agree with definition of dark matter that you provided, keep in mind that there are hypothesis that try and explain it as something that exists. For example: WIMPs.
People seem think think this allows our current theories to be wrong.....
Perhaps there might be small issues with Einstein theories, personally I doubt it, but, to me, I really don't think small fixes to the theory will explain this stuff away.
If you look at the equations, relativity looks like a refinement to Newtonian physics: at small masses and low velocities, relativity makes the same predictions as Newtonian physics.
However, it is not correct to call this a "small improvement." Relativity was a radical change to our understanding of how the universe behaves. In a Newtonian universe, information (including gravitation, electrical attraction, and light) travelled instantaneously. This does not happen in relativity, and nor does it happen in the universe we live in. Einstein did not merely improve upon Newton's work; Einstein proposed a theory which explained the ways in which we do not live in a Newtonian universe. Thomas Kuhn goes into this concept in detail in "The Structure of Scientific Revolutions."
There's nothing in Newtonian theory that requires light to travel instantaneously, and Ole Romer first demonstrated that light had a finite speed in the 1670s, roughly a decade before Newton published the "Principia".
It's not actually clear that there is any problem with current theory - it may just be that our understanding and application of it is incorrect. You can have a perfect mathematical model, but if just one of your premises is incorrect, the output can be nonsensical.
I am a physicist (well, I studied as one, before going technology entrepreneur), and have always sat in the "dark matter is bullshit" camp - so my opinion is likely tainted.
I think a far more likely explanation, as this paper could suggest, is that something far more mundane is happening.
For instance, we already understand tidal forces - that bodies can become tidally locked, that there is an inertial frame shear that drags bodies along around a rotating centre of mass, and exchanges energy between an orbiting body and the centre.... So why is it infeasible that galaxies are tidally locked, and the lack of fall-off in radial velocity is down to the gravitational shear from the galactic core being far more intense than we anticipate, causing the behaviour we see. It doesn't require a rewrite of relativity, "just" of galactic evolution.
Either that, or galaxies are sprites, and lazily rendered ones, at that.
> For instance, we already understand tidal forces - that bodies can become tidally locked, that there is an inertial frame shear that drags bodies along around a rotating centre of mass, and exchanges energy between an orbiting body and the centre.... So why is it infeasible that galaxies are tidally locked, and the lack of fall-off in radial velocity is down to the gravitational shear from the galactic core being far more intense than we anticipate, causing the behaviour we see. It doesn't require a rewrite of relativity, "just" of galactic evolution.
Because someone has indubitably already ran the numbers. The tidal force only has a non central, orbit changing influence on spatially extended bodies, and its effects even on objects as large as our solar system are completely negligible, as it doesn't seem to have a measurable effect on the orbits of the planets. The rotation curves are way off, which is a huge chunk of excess angular momentum that no tidal force can conjure. Where would it even come from, the rotational angular momentum of component stars?
Acknowledge and embracing the assumption that the speed of light is a universal constant -- disregard of reference frame? And as a consequence, allowing space and time to curve? That is not a small fix. That is a fundamental faith jump.
>It is also entirely possible that Dark Matter and Energy don't actually exist.
Exactly. The whole basis for belief in "dark matter" and "dark energy" is basically: "our theories on how the universe works don't compute, so there must be some invisible things (dark matter & energy) we cannot detect since our theories can't be wrong."
Except for the slight problem that observations of the bullet cluster show gravitational lensing inconsistent with the observed mass of physical matter. MOND has no explanation for this.
No our theories were wrong, that was obvious, dark matter and dark energy are (part of) the new theory
We're going with them because all the other new theories end up requiring fractional dimensions or some similarly bizarre space time and still don't end up explaining things as well as "I dunno guys maybe there's just some stuff we can't see". We already know there's a bunch of stuff we can't see, dark matter is just some other kind of stuff we can't see. Dark matter is on the correct side of the razor.
The shift here is that dark matter is detected only in aggregate. And massive aggregates at that. What are the properties of dark matter? Where does it come from? How does it interact with other matter, light or dark? No idea, because it's only detected as a correction term.
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.
Personally, I'm interested to see what happens to conformal gravity. The theory is able to fit galactic rotation curves without a need for dark matter[1], but supposedly leads to inconsistent results when trying to do the same for gravitational lensing[2]. However, that might have been an artifact of incorrect approximations and the calculations should be repeated using exact solutions[3].
I keep seeing a couple problems in this whole discussion.
Number one is the continued references to Keplers laws. Kepler only applies to 2-body systems where the mass of one is dramatically larger than the other. This works well for the solar system, where the sun is huge compared to the planets and the planets have little effect on each other. But do note that some planets were predicted to exist based on their effect on the orbits of other planets, so even in this sparse system the effects do make a difference. My point is that Kepler has no place in galactic dynamics but is often referenced - if that were the model that doesn't match observation then DUH! But the physicists are smarter than that.
Second, I occasionally see mis-application of the divergence theorem and/or gauss's law. Some folks seem to treat a portion of a galaxy out to a radius R as a point mass by using this theorem. The problem of course is that to make that leap you must have a uniform spherical distribution of matter inside the spherical boundary. In other words, a disk-like distribution of matter does not have a uniform gravitational pull at all points of distance R from its center. You'll find it is stronger near the edge of the disk than at the same distance perpendicular to it.
Third, this particular article keeps making reference to the "radial velocity" or speed of stars where I believe they are actually talking about the orbital or tangential velocity.
And lastly, if the dark matter obeys the same laws of gravitation, there must be a reason for its distribution to be different than the other matter. At that point, I don't see any need for "strange dark matter" or new particles or physics. You really can't have it both ways - it's fundamentally different, yet it obeys the same rules. Having said that, I could see how exploding stars would spread non-visible matter (remnant material) in a way that might be very different from the stars themselves.
In other words, a disk-like distribution of matter does not have a uniform gravitational pull at all points of distance R from its center. You'll find it is stronger near the edge of the disk than at the same distance perpendicular to it.
But stars (and gas clouds) orbiting in a disk are essentially always "near the edge" of the disk (that is, the disk defined as matter interior to the orbit in question), so the approximation is usually pretty good.
reference to the "radial velocity" or speed of stars where I believe they are actually talking about the orbital or tangential velocity.
The (understandable!) confusion originates because "radial" and "tangential" refer, in an observational sense, to velocities along or perpendicular to the line-of-sight vector. To simplify a bit: for observations of astronomical objects, velocities along the line of sight from you to the object (detectable from Doppler shifts in its light) are "radial", while velocities perpendicular to the line of sight (which require that you wait long enough to see things change position on the sky) are "tangential". For edge-on spiral galaxies, the orbital motions that we can observe are those producing Doppler shifts along the line of sight (which are, in the frame of the galaxy, "orbital or tangential", as you suggest).
And lastly, if the dark matter obeys the same laws of gravitation, there must be a reason for its distribution to be different than the other matter.
The usual argument is that dark matter is "non-dissipative" -- that is, it can't lose orbital energy via things like radiation, the way ordinary matter in the form of gas can. (And, unlike gas, it doesn't feel pressure forces.) Stars are also non-dissipative, but stars form out of gas, so their distribution reflects the effects of dissipation in ways that dark matter can't.
>> But stars (and gas clouds) orbiting in a disk are essentially always "near the edge" of the disk (that is, the disk defined as matter interior to the orbit in question), so the approximation is usually pretty good.
Not sure we're on the same page. For a star orbiting at radius r on a disk of radius R, some folks will lump all mass inside r into a point and apply keplers laws. They will reject all the mass between r and R because of the same surface integral theorems. The problem is that neither of those assumptions are anywhere near correct. The pull from the interior <r mass is not the same as though it were a point, and the pull from the outer mass >r is very much non-zero. For an example using electric charge instead of gravity:
Ah, I thought you were making some special argument about stars orbiting with very tilted orbits or something like that.
Yes, the orbits within the inner part of a galaxy don't follow a Keplerian curve. However, at large radii most of the visible matter is well inside the orbit and very little is outside (galaxies are quite centrally concentrated), so the rotation curve should approach the Keplerian limit as you go to larger and larger radii. The fact that it doesn't is one of the primary pieces of evidence for dark matter in galaxies (though not the only one).
(I'm not sure why you linked to that slide, since it's not calculating the field in the plane of the ring, nor is it dealing with a continuous charge distribution. Regardless, you'll note that E_x goes to 1/x^2 for x >> a, which is a "Keplerian" limit.)
>> so the rotation curve should approach the Keplerian limit...
I don't agree. If we treat the galaxy as a bunch of concentric rings, many of them are nowhere near far enough away to reach that "Keplerian" limit.
As an experiment I modeled a flat disk of uniformly distributed stars. Without doing a dynamic simulation one can do the n-body calculation to determine the total pull on each star and determine what its orbital velocity must be to go in a circular orbit. You find that the "galactic rotation curve" will have a velocity that actually increases all the way to the edge. Of course a uniform distribution is not what a real galaxy looks like, but I think we can all agree reality is somewhere between the uniform flat disk and the highly concentrated central mass. And the actual rotation curves are somewhere between Keplers and mine. I'd expect people to work backward from the rotation curves to determine the mass distribution and then try to understand why that isn't what is observed. I suppose that's just a different way to get at the same mystery.
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!
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.
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.
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.
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.
Every particle is composed from smaller particles, which are floating in a medium (field) formed by much much smaller particles. And this pattern repeats endlessly at any scale.
So there no particles, only fields OR there is no fields, only particles.
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.
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.
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.)
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.
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?
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.
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.
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.
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]
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".
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?
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
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
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?
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).
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.
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.
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.
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?
