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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.'"
If something else is actually causing those effects, the whole theoretical edifice would come crashing down.
As it should.
-Colin
Much like a dog staring at a shiny object, I'm fascinated by this but I don't understand it.
Jedi don't stand a chance.
HAD
It will be interesting to see how scientists who have staked their entire careers upon the existence of dark matter would react to the discovery that it does not in fact exist. Ideally an invalid theory is dropped, and a new, more "correct" theory is created. However, I have a feeling that a lot of people have invested too much time and effort into dark matter to let it go without some serious evidence.
My patience is infinite, my time is not.
We don't understand something fully? Wow... that's about as brilliant as deciding to cut my sandwich in triangles instead of in squares.
The truth is this. We have such a little understanding of actually governing laws that we can't begin to fathom it. However, that doesn't stop us in progression to learning. Just because this theory might not be right (and probably isn't) doesn't mean we are any less an idiotic species. We've been working on these theories for many millenia. One of them turning out to be wrong won't be a surprise... it's a probability. Without the wrong hypthosesis, we can never stumble onto the correct ones. Its Edison's, "Every time I fail, I know one more way how to NOT build it" idea.
...then 99.9999999% of the world won't notice. But it will be on CNN anyways.
There seem to be growing "hints" that something is wrong with current theories about the very nature and behavior of gravity. This includes alleged dark matter that cannot be identitied, planetary space probes with slight deviations from expected sun "pull" [1], and the fact that there is no identifiable "negative" gravity while the other forces do have negative values or particles.
[1] It was originally thought that heat generated from nuclear fuel cells was "pushing" the probes, but this was mostly ruled out because the heat lessens over time, but the pull was constant.
Table-ized A.I.
Sure, in the long run it doesn't matter.
That is, of course, if we keep testing it and trying to see if it is true. (Or the closest approximation of 'true' we have been able to come up with.)
It matters now if it is not true because then we know we need a better theory. And that means we either didn't understand something we thought we understood, or that we hadn't explored our understanding fully. Either way, there is likely something else that will be affected...
'Sensible' is a curse word.
Dark matter had better exist- otherwise, I've wasted a hell of a lot of money on that dark matter damage insurance I bought a couple years back...
Gentlemen, you can't fight in here! This is the War Room!
Well actually knowledge of its existence and how much of it exists will determine whether or not the Universe eventually implodes on itself in the "Big Crunch" or whether the universe will keep expanding at the speed of light forever. So technically speaking, "in the long run" it will matter quite a bit :)
Regards,
Steve
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
That's what I love about physics: it's so out there that you'd think the person who just said something like that was smoking crack, if he didn't have a PhD.
Gravity bleeding between universes...
Who needs science fiction?
Meep.
You keep telling yourself that... God boy. However only 7% of scientists believe in a personal god.
So what if it doesn't really exist?
You can write a "hello world" program in most programming languages in under ten lines of code.
You could also write a program to synthesize speech to say "hello world" in an MP3, rip the MP3 to a wav file, and then write a speech-to-text engine to finally dump "hello world" to the screen.
Same idea here. Kepler's laws reduced a nightmarish tangle of mathematics to a three line "program", if you will. Out current model of how various things in our universe interact requires a degree in cosmology to fully grasp, and a PhD to do any meaningful work in. Imagine reducing that to one chapter of a freshman-level physics or astronomy course.
So, it matters for that reason. Unneccessary complexity slows down work in the field, and in the long run can actually prove counterproductive to the field as a whole (think about it - 1500 years wasted trying to make epicycles work).
well, let's see here. 4% of postulated matter in the universe is known to exist. 96% of postulated matter in the universe is NOT known to exist. that's a fine fudge factor to have in a test, and might explain where budget figures come from in the government :-D
:-D
it certainly explains where a lot of my assignments come from at work, lol
if this is supposed to be a new economy, how come they still want my old fashioned money?
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.
>> think about it - 1500 years wasted trying to make epicycles work.
Dang, their billable hours must have krunked the project.
Maybe if we wait another year the program will halt...?
Too bad they were so bent on epicycles, TRON has a much cooler cycle game and it works!
(-1, troll)
Actually it will make a huge difference. Just look at how Bohr's model of the atom changed chemistry and particle physics. Or how Plank's quantum theory caused a revolution in the physics community. And one of the most famous examples of an upset in scientific theory is Einstein's theory of Relativity verses the Newtonian theories most commonly held at the time.
Each of these theories caused an almost immediate revolution in their respective fields which spread out to similar disciplines. Fast forward 20, 30, 50 years or more and a number of innovations and inventions appear which stem from these theories. If these theories had not been introduced then we would most likely not have had such an explosion in technology.
Just because we wave our hands and say something is out there doesn't mean that we understand it or can use it. If we know the true mechanism behind dark matter and wether or not it is just "hand waving" then we can apply that knowledge to useful applications. For example, it is assumed that this dark energy exhibits a repulsive force similar to gravity but opposite to it in direction. If we truly understand how this works then we might be able to apply that knowledge toward "anti-gravity" spacecraft, etc. On the other hand if there is some other cause for the repulsion then we would need to know IT'S mechanism in order to utilize it.
In the end, science is the quest for truth, not convenience. Just knowing that there is a certain effect is not enough. Scientists are not looking to solve the question of "what is that" but rather "why does that exist and how does it work". That is why it is important to seek out the true reasons behind the dark matter observations.
Sapere aude!
No its not, it just has a lot of "dark content" that you can't see but which weighs heavily against its moderation. ... really sorry, I couldn't resist.
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
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.
However in this case your comment is only slightly wrong and therefore I have some hope that my reply might be a useful contribution.
You are correct that mathematical proofs are based on axioms. However there is still a crucial difference between a mathematical proof and a scientific theory. A mathematical proof is an absolute certainty. Note that I am not claiming that the underlying axioms are certain. I am only claiming that the proof itself is certain.
To put it another way, mathematicians are never certain about their underlying axioms but they are absolutely certain that if those axioms hold then the result stated in the proof also holds. It's kind of like a building with indestructible walls but no foundation.
Scientific theory is a whole different kettle of fish. You cannot prove a scientific theory with absolute certainty. In fact it is not even clear to me how one can define certainty within the framework of the scientific method. You never have any guarantee in science that future observations will be consistent with past observations.
In science you can prove a theory in the sense of preponderance of the evidence. You can even sometimes prove a theory beyond all reasonable doubt. But there is no way to eliminate the unreasonable doubts. Any endeavour based on empirical observation suffers from the fundamental limitation that you can never be sure of the next observation.
Finally, regarding 1+1=2, the foundational proof of this fact using the standard propositional axioms of mathematics really does require 362 pages. You can see the 362nd page on the bottom half of this Russell's paradox site.
I always thought this was an interesting an odd part of modern science and cosmology. Why should we assume occam's razor, that simpler explanations are better? Why should the universe be simple and elegant?
You have misunderstood Occam's razor. It doesn't say that at all.
Occam's razor, in its original form, translates to "Do not multiply entities unnecessarily". That has been modernized to "The simplest explanation is usually correct", which is close, but not exactly the same.
What Occam's razor really means is: given two (or more) possible explanations of a phenomenon, with no evidence favoring one over the other, assume that the simplest one is correct.
For instance, if I find a pinecone lying on the ground under a pine tree, the simplest explanation is that it fell off of the pine tree. Sure, it might have been planted there by invisible space aliens in conjunction with the Illuminati acting in strict accordance with the Masonic doctrine of the Coming of the Pine Cone King, but since there is no evidence to favor one explanation over the other, I should assume that it fell off of the pine tree.
That doesn't mean that it did fall off the pine tree, and it doesn't mean that I might not change my mind as more evidence is found. It also doesn't mean that I shouldn't look for more evidence and try to determine the origin of the pine cone with greater accuracy. That isn't what it says at all. It just means that until such evidence arises which would cause me to revise my view of things, I should assume the simplest explanation that fits the facts. The explanation should only change when the known facts do, or a better explanation is found.
ZFS: because love is never having to say fsck
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
This thread misses an important point. Even though Ptolemy's theory was wrong, it was a lot closer to the truth than previous ideas like "the lights in the sky are gods with flashlights." The point is that even theories that are wrong add to our knowledge by providing a starting place for deeper inquiries.