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What If Dark Matter Really Doesn't Exist?
sonar67 writes "According to The Economist: 'It was beautiful, complex and wrong. In 150AD, Ptolemy of Alexandria published his theory of epicycles--the idea that the moon, the sun and the planets moved in circles which were moving in circles which were moving in circles around the Earth. This theory explained the motion of celestial objects to an astonishing degree of precision. It was, however, what computer programmers call a kludge: a dirty, inelegant solution. Some 1,500 years later, Johannes Kepler, a German astronomer, replaced the whole complex edifice with three simple laws. Some people think modern astronomy is based on a kludge similar to Ptolemy's. At the moment, the received wisdom is that the obvious stuff in the universe--stars, planets, gas clouds and so on--is actually only 4% of its total content. About another quarter is so-called cold, dark matter, which is made of different particles from the familiar sort of matter, and can interact with the latter only via gravity. The remaining 70% is even stranger. It is known as dark energy, and acts to push the universe apart. However, the existence of cold, dark matter and dark energy has to be inferred from their effects on the visible, familiar stuff. If something else is actually causing those effects, the whole theoretical edifice would come crashing down.'"
It's more than that. If Dark Matter doesn't exist, we will be forced to re-examine more than just our current picture of the universe. Galactic Rotation curves, velocity dispersions of galaxy clusters, the flatness of the universe implied by the CMB, type Ia supernovae data, as well as other distance indicators, all imply that the parameter "Omega_mass" (the mass density of the universe divided by the critical density) is about 0.3. If there is no "dark matter", we don't know how to explain this number. Baryons, i.e. stars, planets, gas, etc., make up only an "Omega" of 0.044 +/- 0.009. This constraint is from Big Bang Nucleosynthesis and is very strong. Although there are plenty of open questions about dark matter, it seems to me (just an astrophysics grad student) that there is an overwhelming amount of evidence for not only dark matter, but the model of "cold" dark matter as well. None of the alternatives can explain even half of what Dark Matter can, including modifying gravity. Plus, Dark Matter is consistent with GR, the big bang, and everything else we hold dear about physics and astronomy, whereas other theories don't. Just my two cents... Ethan
If the data is there and convincing, the views will change. But any alternative theory is going to have to explain all the observables, not just the two mentioned in the artice.
(E.g., the convincing data on dark energy comes from two independent groups studying supernovae.)
You keep telling yourself that... God boy. However only 7% of scientists believe in a personal god.
Actually, most Physicists don't believe in God.
--Keeping the flame wars alive, one post at a time
This question we may actually know the answer to in a decade or so when the LHC comes online and is producing results.
Dark energy is much weirder.
At the risk of feeding the trolls...
No, Relativity (neither the Special nor General theory) says that "everything is relative". Special Relativity says that inertial motion is relative in flat spacetime (i.e. in the absence of gravity). This is another way of saying that all inertial coordinate reference frames are equivalent. (Special Relativity says more than that, namely that light propagates at the constant speed 'c' independent of the motion of its source. This is what separates Special Relativity from Galilean Relativity.) General Relativity says that *locally*, accelerated motion is equivalent to inertial motion in a gravitational field. (The "locally" part accounts for the fact that the gravitational field lines are not parallel, but converge on the gravitational source.)
What this boils down to is that circular motion is accelerated motion, not inertial motion, and is not simply relative, and spacetime is not flat surrounding bodies that planets orbit. So no, Relativity does not validate the epicycles theory.
It's so easy too illustrate: Imagine holding two small magnets next to eachother. You can feel them pulling on each other and if you get them too close, they snap together.... so that's electromagnetism Now hold two unmagnetized chunks of the same weight/material. Feel the force of gravity pulling them together? What you can't feel it? It is there, it's just THAT weak... compared to electromagnetism.
The key word here is inertial. An orbiting body is accelerating towards the center, and is therefore not an inertial frame of reference by definition. As far as calculations on the surface of the Earth go, non-inertial effects (also present because of Earth's rotation) can generally be ignored for comparatively small times and distances without significant loss of accuracy, but on the scale of the Solar System and other systems where celestial mechanics are employed, they cannot be ignored. Epicycles introduce a new non-inertial component, and therefore can't be regarded as merely relative.
And the brethren went away edified.
Ok, Dark Matter in a nutshell.
When scientists look at the way that galaxies move through space, they see that many of them move a great deal faster (about a factor of 10) than theory predicts. Assuming that current theory is correct, the most likely explanation of these observations is that there is a great deal more matter in the universe than we can currently detect. If we can't detect it then it must be pretty much invisible across the EM spectrum, so scientists have christened it dark matter. Much effort has gone into trying to prove its existance but as as far as i'm aware there has not been too much sucess.
As I remember from my astrophysics class (and this was some years ago so feel free to correct me) there are two main candidates for dark matter, both of which have been tediously acronymed.
MAssive Comapact Halo Objects (or MACHOs) are basically chunks of ordinary matter, floating around in space that give off no radiation. Think brown dwarfs (stars without the necessary mass to initiate fusion). As I remember, most scientists are very sceptical that a significant amount of dark matter could be contained in MACHos.
Weakly Interacting Massive Particles (WIMPs- gotta love that scientist humour) are the other candidates and are hypothetical particles, heavier than neutrons, that were formed in the Big Bang and have been travelling through space ever since. As their name inplies they would have almost no interactions with normal matter and so by definition would be almost impossible to detect. Again there have been attempts to prove the existance of these particles, mainly involving mine shafts and a lot of water, and again there have been no conclusive results.
Now the significance of all this is that as you may or may not know, the universe is presently expanding and will continue to expand for some time. What will happen after that, however, is a matter of some confusion. One theory says that it will continue expanding forever (open universe) , while another says that the gravitational force of the matter in the universe will cause the expansion to stop and then a period of contraction to start, ending up with all the matter coming together in a 'big crunch'. This second theory creates what is known as a closed universe and people have postulated that the 'big crunch' is analagous to the 'big bang' that started the universe in the first place. In this way we get an infinite cycle of universes, each starting with a bang and ending with a crunch
How many scientists believe in God?
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Just to show another angle (as opposed to the highly doubtful statement that only 7% believe in God), I Googled and found:
In the US, according to a survey published in Nature in 1997, four out of 10 scientists believe in God. Just over 45% said they did not believe, and 14.5% described themselves as doubters or agnostics. This ratio of believers to non-believers had not changed in 80 years. Should anybody be surprised? (from http://www.guardian.co.uk/life/feature/story/0,13
So no, it's not most scientists, as it's mostly half and half (according to Nature/Guardian).
Would someone please translate this and tell me what the hell is being said?
;-) (Perhaps Pi_0's don't shower would like to confirm/deny this translation?)
"Nobody knows for sure whether Dark Matter exists or not. But there are a whole lot of independent reasons to believe it exists, all of which result in very, very similar numbers for how much Dark Matter there is. If the Dark Matter theory is wrong, it's very hard to imagine what else could possibly explain all those numbers, all at once. (It's easy enough to explain one or two of those, but that's not an improvement over current theories, that's a step back.) Dark Matter hasn't made it to 100% certainty, but other theories have a lot to explain if Dark Matter doesn't explain what we're seeing."
I think that's about right. I Am Not A Physicist, but I follow this and generally understand math, so I think I'm at least competent to translate...
(On my own, I'd note that giving how flexible geometry can be, I can easily imagine someone constructing a geometry of the universe that doesn't need dark matter that turns out to be mathematically equivalent to a universe that does have dark matter. I'd give some of the geometry-based theories some time to be vetted by real astrophysicists before assuming they provide a real alternative; they may well just encode the Dark Matter into the structure of the Universe itself, which really isn't an alternate theory, just a restatement of the original in a different form. Until someone produces some Dark Matter, or the Universe is explored to our satisfaction to determine no such matter exists, this may remain unresolvable.)
Gravity is weak in the short-range when compared to the three other known forces of nature. The four forces are gravity, electromagnetic, strong nuclear, and weak nuclear. Gravity is an attractive force between all matter and energy, electromagnetic is an attractive and repulsive force between charged particles and magnetic fields, the strong nuclear force is what holds together protons and neutrons in an atom's nucleus and the weak nuclear force is what mediates electron and positron decay.
If you set the strength of gravity equal to 1 then the other forces have the following approximate strengths relative to gravity:
Strong nuclear force: 10^40
Electromagnetic force: 10^38
Weak nuclear force: 10^15
Gravity: 10^0
So why is gravity so important if it is so weak? The thing about gravity is that it falls off slowly with distance and it can't be negated or blocked (as far as we know). Other forces are either extremely short-range or, in the case of the electromagnetic force, have both an attractive and repulsive component that tends to cancel out in the long-run.
Sapere aude!
Lots of things were done this way. Specifically, Planck's attempt to correct the ultra-violet catasctrophe of black-body radiation theory by quantizing the radiation was a total kludge. The theory matched the data fairly well, which led to a flood of new inquiries, leading to Einstein's description of the 'photoelectric effect' and the birth of quantum mechanics.
The concept of the gyromagnetic ratio, or Lande g factor, for particles was another kludge that can be adequately explained using sufficient detail of Quantum Field Theory.
Even more macroscopic phenomenological theories, like Landau's theory of 2nd-order phase transitions expands the free energy of a physical system in terms of one or more order parameters. That's a kludge and a half, but in many cases adequately describes physical systems close to phase transition points that formal Hamiltonian interaction methods cannot get to.
Extending on this is the Ginzberg-Landau theory using a complex order parameter for superconductors. (Remember Ginzberg just won the Nobel Prize for Physics a few months ago. Landau won it decades ago and would have won it again if he was alive). It was shown by Gor'kov that the BCS theory of superconductivity (ie, formally-applied theory involving Cooper pairs of electrons and superconducting gap) approaches the Ginzberg-Landau expansion at the critical point.
So yes, Kludges are really used all the time in physics, and they're no black eye at all. There's two reasons we need to use these. Firstly - macroscopic systems are just so damn complex one cannot solve a 10^23 dimensional Hamiltonian, that's ridiculous. So even from basic principles complicated order can emerge.
The second reason is that it is quite likely we don't fully know the ultimate physics basic building blocks, just a very good approximation of them. Complicated systems can reveal small perturbations from the standard model that's accepted.
make world, not war
Dark matter is simply a theory. If Newtonian mechanics is correct (we don't even need to worry about relativistic corrections here), and the laws of physics are the same everywhere (a fundamental principle of science), then there is a lot more matter than we can see (i.e., that is glowing). We can tell this by looking at the rotation curves of galaxies, and even the behavior of clusters of galaxies. There must be a lot of matter there that we can't see, if Newtonian mechanics is a reasonable approximation. It's called dark matter.
Dark matter in and of itself is really not a revolutionary concept. In most wavelengths of light, for instance, you qualify as dark matter (you emit no visible light, although you do emit infrared radiation, so you're not completely dark matter). Look around your room or office. How many things emit electromagnetic radiation. Your computer and your monitor, sure. Your light fixtures and other electronic equipment either emit light or heat. But most of the stuff around you emits internal radiation. A pen is dark matter. A cup of dark matter (once its reached thermal equilibrium, of course). That book is dark matter. The concept of dark matter is not only not revolutionary and mind-blowing, it's downright mundane. Given the survey of stuff in your office/room, is it any surprise that most of the junk in the Universe doesn't emit radiation on its own?
When we start getting into the weird realms of dark matter is when we start applying the Standard Model and find out that it doesn't seem like all that dark matter can be explained by baryonic matter (basically, protons and neutrons -- what we would normally consider matter). That's where things start getting sketchy and speculative, although we have some theories about what might be responsible. But dark matter in and of itself is simply a consequence of the mediocrity principle (that is, the laws of physics operate elsewhere just the same as they do here) and Newtonian gravitation.
All the popular media's fascination with dark matter is only so much hoopla.
There is no decent explaination... emergent intelligence...
it may explain why things can be completely random at a quantum mechanical level, but balance out in larger systems...
It's called the Central Limit Theorem and Superposition. You've got billions of identical particles (low variance), and a huge sample size at macroscopic scales, thus your mean (likelyhood of "expected" things, the precision, and thus "intelligence" in systems) will be pointy as a pin.
I am 100% dead serious.
Perhaps maybe your REAL question is "why are quarks so damned sticky, protons so stable, and h_bar conveniently small?" because that encapsulates the huge gap between the quantum world and the stable world we live in.
THIS THING CAN TURN ON A DIME, MACROSSZERO STYLE ALSO FUCK BETA, ~NYORON
You're absolutely correct. If we accepted theory as fact without any repeatable testing it would be religion, not science.
We may never fully understand the nature of our universe, and almost certainly will never understand it in our lifetimes. But the question raised in the topic is actually a fundamental one that spans far beyond dark matter to all forms of theoritical science. Many theories are based heavily upon other theories. The "root" theories (with any luck) will eventually be proven or disproven, affecting all research and theories which follow that "root".
What is important is for scientists to fully understand the theories that they base their work upon, and knowing the risks involved. Not doing so is irresponsible, and can lead to misinformation and confusion.
With the above in mind, it's also important to note that many theories have been disproven throughout and entire scientific disciplines have crumbled around the fall of these theories. However, from those ashes, new disciplines have arisen (the first that comes to mind is chemistry rising from the "ashes" of alchemy). I'm sure that in 100 years, many if our current ideas will be laughable, but this failure has proven fundamental to our growth (how's that for rhetoric!?)
-Turkey
A few minor quibbles...
(I am an astrophysicist. I am not a cosmologist, but I do galaxy evolution... we hang out with cosmologists)
There are quite a few pieces of evidence for dark matter:
- internal dynamics of galaxies: when you look at how fast the outer parts of galaxies move around the central parts, you find that the amount of mass necessary is much more than what you see
- dynamics of galaxies in clusters: when you look at how fast galaxies move around in galaxy clusters, you find the amount of mass necessary is much more than what you see
- non-linear growth of primordial perturbations (sounds complicated, isn't really): the universe used to be almost completely smooth. now it's filled with clumps of matter like galaxies and clusters and big voids without much matter. the structures collapsed because of their mass. if there were only as much mass as you can see, there hasn't been enough time for galaxies to have collapsed
The amazing thing about all of these measurements is that they all give you the same answer for how much mass is really out there.
[TMB]
Dark matter is implied by several things:
- Galactic rotation curves
- The velocities of galaxies in clusters
- Anisotropy of the Cosmic Microwave Background
I'm leaving out Type Ia supernovae because I don't think they really imply dark matter *by themselves*.Galactic rotation curves: If you have an object that rotates, and you know the velocity as a function of radius, you should be able to get the density as a function of radius. This is obvious, because the velocity is coming from gravity.
The problem: you can also get the density by assuming that light-emitting material carries the majority of the matter (stars - pretty good approximation) and then looking at the luminosity as a function of radius (how bright it is). So, in a perfect world, these two profiles would match.
They don't. Therefore either
- Not all of the matter is light emitting
- Gravity doesn't work.
Option 1 there breaks the least physics, so it's preferred.Velocity dispersion in clusters : See above - just with galactic clusters, rather than galaxies. Note that fixing one of these problems would probably fix the other!
Anisotropy of the CMB : This one's tougher to explain easily. 100,000 years after the Big Bang, the Universe was an extraordinarily uniform big fireball. Extremely uniform - because electrons hadn't cooled enough to form hydrogen yet, so it was one big hot plasma.
When hydrogen cooled, the photons in the Universe suddenly found themselves free to move, because hydrogen can only absorb certain wavelengths, and free electrons absorb continuously. Those photons are the Cosmic Microwave Background. Their uniformity is a very good indicator that the Big Bang theory is real - at least, from 100,000 years after the Big Bang to now.
However, matter that was in that fireball DID distort the radiation slightly - through gravity. And so we see anisotropy (nonuniformity) in the microwave background, and it looks very much like standing waves in the sky. The ratios of the strengths of certain frequencies tell us the ratio of dark energy ("lambda", the cosmological constant) to matter, AND also tell us how "flat" - i.e., how much total energy - the Universe has. It's flat. Exactly. Really really flat. It has exactly as much energy as would be needed to reverse the initial Big Bang (if it were all in matter, which it isn't). And it also tells us that dark energy is 70% of the energy content of the universe, and matter is 30%.
Big bang nucleosynthesis . BBN basically says "you can only get this much normal matter from a big bang explosion cooling to form atoms". It's amazingly accurate so far - it gives great answers for the ratio of certain elements, for instance. But it also puts a stringent limit on the amount of normal matter, of about 5%. The CMB *also* gives this same measurement - and, amazingly! - they agree! There are in fact even OTHER measurements which give values consistent with this number - 5% - so it's hard to imagine how measurements coming from completely different areas of physics (one is standing waves in the early Universe, one is nuclear physics) could give the same answers, and both be wrong. (But Nature can be perverse...)
So, Omega_m has to be about 30%, and Omega_b is about 5%. Plus there has to be something making stars and galaxies rotate too fast. Physicists, wanting elegance, say "two problems, one solution is a great theory."
Basically: If dark matter doesn't exist, we've got a lot of work to do to come up with other models, and a huge amount of it would affect gravity, which we thought we were beginning to understand!
It's very hard to imagine a form of gravity which could answer all of these problems, AND still be consistent with what we observe today.
I'm reading the presentation at the moment, but my math's a bit rusty (20 years rusty, if truth be told), so I can't comment on it.
oh brave new world, that has such people in it!
No one has bothered to even look to see if the rules by which our universe exists today are the same as a few million years ago, or a few billion years ago. How would you be able to tell that, say, the gravitational constant of the universe has been constant all along?
You are mistaken. There have been a number of studies done to try to determine if fundamental "constants" such as the speed of light are in fact constant.
It is, of course, very difficult to devise experiments to test such theories, but a number have been designed and performed. The phrase "no one has bothered to even look" comes up in other fields, such as paranormal research, and it is just as untrue there. Scientists would love to find evidence of (say) the gravitational constant changing, extraterrestrial organisms, or psychic power, and to suggest that they haven't even bothered to look is an insult to the field.
ZFS: because love is never having to say fsck
Not laughable, and still usefull, even if it is 'wrong.' It is still 'right' in most circumstances, and we know what those circumstances are. We know when to use it as a good enough approximation, and when we need to use more accurate theories.
Theories can never be proved. We will never fully understand the universe. We may develop theories that accurately predict every phenomenon, then the next day, something new could come along and show us we were wrong.
What is important is for scientists to fully understand that theories are always merely theories, not facts.
- None can love freedom heartily, but good men; the rest love not freedom, but license. -- John Milton
They are all made-up, most likely by someone with Schizotypal syndrome.
Help fight continental drift.
Five years ago, every cosmologist "knew" that the universe was flat and matter supplied the critical density (in other words, no dark energy, that 70%). Conventional wisdom has completely changed with the discovery of the accelerating universe.
No they didn't. I hung out with cosmologists when doing my astrophysics PhD over 10 years ago, and they were considering various mixes of hot and cold dark matter, dark energy, open and closed universes. Flat universe and no dark energy was merely the provisionally accepted most likely solution.
The implied existence of dark energy is revolutionary to cosmology, but it didn't catch people by surprise - they were actively looking for it.
Quattuor res in hoc mundo sanctae sunt: libri, liberi, libertas et liberalitas.
f = G.m1.m2/d^2
actually that's newtonian. They use Einstein's theories for gravity now.
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Go canucks, habs, and sens!