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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
You keep telling yourself that... God boy. However only 7% of scientists believe in a personal god.
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.
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
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.
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.
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