This is why I think the Pioneer anomalies are very interesting. People can argue all they like about things on the galactic or universal scale, but we can never perform experiments, only guess at what's going on. If such effects are observable on the scale of the solar systme it will be possible to design experiments ourelves, which will give much more reliable results that are less open to interpretation.
Proof of Dark Matter
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ImaginalDisc
Penultimate Amazing
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Not at all, but perhaps it is your level of understanding that causes you to make such a statement.
Wikipedia entry on it says
Hint: incomplete understanding (what the article said) = inadequate theory (what I said).
From http://www.cnn.com/2006/TECH/space/08/21/dark.matter/index.html
Ie: if current theory predicts a lot more matter than what we observed, we'll not adjust the theory to fit the facts, but fit the facts to suit the theory by saying that therefore unobservable matter must be out there.
From the article
Argue with them.
Molehill = Mountain, according to Tai'Chi.
Current theory fails to describe the behavior of extremely large objects, such as galaxies. That does not threaten the credibility of science itself.
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TV's Frank
Critical Thinker
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Hey fellas!
I'm finally reading a thread when I'm fully rested, so I can actually respond to some of the comments here. So here are some thoughts from a practicing cosmologist (though merely a low-life grad student):
The evidence in favor of DM is, put simply, enormous. The ones I can think of off the top of my head are:
1) Galaxy rotation curves. The classic. Not enough visible matter in spiral galaxies to account for their velocities. Resolution: some other sort of matter, or modifications to F=ma at large scales (MOND). More on this later.
2) Cluster dynamics. Again, we count up all the visible stuff in a galaxy cluster, and it's not enough to account for their observed velocities. Some resolutions as 1), but MOND has difficulty matching observations near the centers.
3) CMB. CMB analysis shows that the universe is 25% matter. But when we go out and look for it, we only get around 4%. This number includes liberal estimates or stuff like brown dwarfs and black holes (i.e. regular matter that is dark).
4) Supernovae. These probes tell us that the universe is 75% "dark energy", and hence 25% matter, which agrees with measurements from the CMB. Again...where's all the mass? Resolutions: missing matter, of modified Einstein's equations at extrememly large scales. More on this later.
5) Cluster measurements. Using gravitational lensing and other techniques, we can estimate the mass of clusters without having to just count the visible stuff. What do we get? If you guessed "25%", you get a cookie. Again, the visible stuff only accounts or 4%.
6) Large-scale structure. This one is more circumstantial than the others, but it turns out the only way for inflation to work (i.e. for small quantum perturbations to generate the clusters we see today), is to have a type of matter that does not couple to light. Before the CMB was generated, good ol' everyday "baryonic" matter was still being tossed around by photons. Since the CMB, when photons and baryons don't really talk anymore, the baryons could start to form structure. But there wasn't enough time! We need a type of matter that doesn't talk to light at ALL, to form the seeds of structure early enough (i.e. before the CMB).
Okay, so there's the evidence that something fishy is going on. What are sopme candidates?
a) MOND. The explains galaxy rotation curves very well, but doesn't fit clusters so well. The original MOND was written in terms of accelerations of particles, which is a very odd way of describing the world post-Einstein. Just a few years ago, someone finally managed a relativistically-correct version of MOND. It is nasty and ugly, but there. With the relativistic version, you can actually start making predictions about, say, the CMB. The results? MOND is wrong. MOND gives predicitons for the CMB that are not observed. Too bad, so sad.
b) Modify General Relativity at large scales. This is possible, though extremely difficult. And, it has more to do with the dark energy, and still needs dark matter. Next.
c) Neutrinos. Hey, those guys have mass! Yeah! No. Even though there's a bunch out there, they only make up less than 1% of the mass needed. Also, using neutrinos as dark matter gives incorrect results for the CMB and growth of structure.
d) Something else? Say, a type of previously-unknown particle that doesn't interact with light, but has some mass? Do we have any motivation for this at all? Sure! High-energy theorists for years have been telling us about "supersymmetry", the way of uniying forces at high energies. But these theories predict particles that are massive and only interact weakly. So these high-energy guys have started building huge colliders to go look for then, like the LHC. You need lots of energy because these predicted particles are so massive. Wait...did you say that high-energy theorists predict a particle that is massive and only interacts weakly...like the kind of particle that would explain observations 1)-6)? And that in the next fews years we CAN MAKE OUR OWN? We should do lunch!
So here's the gist: cold particles that don't interact with light are our best candidate to explain observations 1)-6). We have motivation for this particle rom our high-energy friends (who came up with these particles on their own, by the way...my favorite is the "gravitino", which would make the DM the supersymmetric partner to the CMB...how sexy is that?). We have many observers trying to directly detect the DM particles passing through us right now (no signals yet, but we're pretty sure we're probing at too-low energy levels....but we're also ruling out a lot of candidates along the way). In fact, with these observations and LHC, we should get DM confirmation in the next 5 years or so.
Dark energy is a whole other can of worms. That, we basically don't know what the heck is going on.
Are these just epicycles? Trust me, we're all very aware of that possibility. If someone wants to jump in and elegantly explain it all, more power (and a free trip to Stockholm) to ya. It seems that we can't get away from DM, though. If we don't detect or observe it in the next few years...well, at least I'll still have a job trying to explain it!
Is it more complicated? Is there more than one DM particle? Does the DM and DE interact in some way (my current area of research)? These, any many questions, we just don't know. But we're working on it!
I'm finally reading a thread when I'm fully rested, so I can actually respond to some of the comments here. So here are some thoughts from a practicing cosmologist (though merely a low-life grad student):
The evidence in favor of DM is, put simply, enormous. The ones I can think of off the top of my head are:
1) Galaxy rotation curves. The classic. Not enough visible matter in spiral galaxies to account for their velocities. Resolution: some other sort of matter, or modifications to F=ma at large scales (MOND). More on this later.
2) Cluster dynamics. Again, we count up all the visible stuff in a galaxy cluster, and it's not enough to account for their observed velocities. Some resolutions as 1), but MOND has difficulty matching observations near the centers.
3) CMB. CMB analysis shows that the universe is 25% matter. But when we go out and look for it, we only get around 4%. This number includes liberal estimates or stuff like brown dwarfs and black holes (i.e. regular matter that is dark).
4) Supernovae. These probes tell us that the universe is 75% "dark energy", and hence 25% matter, which agrees with measurements from the CMB. Again...where's all the mass? Resolutions: missing matter, of modified Einstein's equations at extrememly large scales. More on this later.
5) Cluster measurements. Using gravitational lensing and other techniques, we can estimate the mass of clusters without having to just count the visible stuff. What do we get? If you guessed "25%", you get a cookie. Again, the visible stuff only accounts or 4%.
6) Large-scale structure. This one is more circumstantial than the others, but it turns out the only way for inflation to work (i.e. for small quantum perturbations to generate the clusters we see today), is to have a type of matter that does not couple to light. Before the CMB was generated, good ol' everyday "baryonic" matter was still being tossed around by photons. Since the CMB, when photons and baryons don't really talk anymore, the baryons could start to form structure. But there wasn't enough time! We need a type of matter that doesn't talk to light at ALL, to form the seeds of structure early enough (i.e. before the CMB).
Okay, so there's the evidence that something fishy is going on. What are sopme candidates?
a) MOND. The explains galaxy rotation curves very well, but doesn't fit clusters so well. The original MOND was written in terms of accelerations of particles, which is a very odd way of describing the world post-Einstein. Just a few years ago, someone finally managed a relativistically-correct version of MOND. It is nasty and ugly, but there. With the relativistic version, you can actually start making predictions about, say, the CMB. The results? MOND is wrong. MOND gives predicitons for the CMB that are not observed. Too bad, so sad.
b) Modify General Relativity at large scales. This is possible, though extremely difficult. And, it has more to do with the dark energy, and still needs dark matter. Next.
c) Neutrinos. Hey, those guys have mass! Yeah! No. Even though there's a bunch out there, they only make up less than 1% of the mass needed. Also, using neutrinos as dark matter gives incorrect results for the CMB and growth of structure.
d) Something else? Say, a type of previously-unknown particle that doesn't interact with light, but has some mass? Do we have any motivation for this at all? Sure! High-energy theorists for years have been telling us about "supersymmetry", the way of uniying forces at high energies. But these theories predict particles that are massive and only interact weakly. So these high-energy guys have started building huge colliders to go look for then, like the LHC. You need lots of energy because these predicted particles are so massive. Wait...did you say that high-energy theorists predict a particle that is massive and only interacts weakly...like the kind of particle that would explain observations 1)-6)? And that in the next fews years we CAN MAKE OUR OWN? We should do lunch!
So here's the gist: cold particles that don't interact with light are our best candidate to explain observations 1)-6). We have motivation for this particle rom our high-energy friends (who came up with these particles on their own, by the way...my favorite is the "gravitino", which would make the DM the supersymmetric partner to the CMB...how sexy is that?). We have many observers trying to directly detect the DM particles passing through us right now (no signals yet, but we're pretty sure we're probing at too-low energy levels....but we're also ruling out a lot of candidates along the way). In fact, with these observations and LHC, we should get DM confirmation in the next 5 years or so.
Dark energy is a whole other can of worms. That, we basically don't know what the heck is going on.
Are these just epicycles? Trust me, we're all very aware of that possibility. If someone wants to jump in and elegantly explain it all, more power (and a free trip to Stockholm) to ya. It seems that we can't get away from DM, though. If we don't detect or observe it in the next few years...well, at least I'll still have a job trying to explain it!
Is it more complicated? Is there more than one DM particle? Does the DM and DE interact in some way (my current area of research)? These, any many questions, we just don't know. But we're working on it!
Great post!
You seem to be very optimistic about the LHC, an attitude not very common in theoretical physicists nowadays. Most high energy guys I know are afraid it will only confirm our current theories (standard model, Higgs boson,...) which would basically kill particle physics with its own success...
You seem to be very optimistic about the LHC, an attitude not very common in theoretical physicists nowadays. Most high energy guys I know are afraid it will only confirm our current theories (standard model, Higgs boson,...) which would basically kill particle physics with its own success...
TV's Frank
Critical Thinker
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Yeah, well they're probably selling it short on purpose, playing it down...and then they turn on the LHC, and BAM! a dozen Nobel prizes!
Even if we can't make our own, we can at least use the LHC to rule out some candidates. Also, the DM detector guys should get a confirmed signal in the next few years.
Who on Earth thinks that? We know the standard model is wrong. It says neutrinos have zero mass and has several other problems, especially with the Higgs Boson and gravity. At best (or worst) the LHC can confirm that the standard model is close to reality, no-one believes it is the real answer.
What the LHC can do is test predictions of supersymmetry and superstring theories, many of which make predictions close to the standard model, but also account for things such as neutrino mass.
As far as I know, no-one even thinks the LHC will give anything conclusive since it collides hadrons which give very messy data to deal with. The ILC currently in planning will be a lepton collider that should be able to provide much more acurate data on any new particles found at the LHC.
Coincedently this article was published just today that explains quite a bit aabout what these two colliders will do. http://www.newscientist.com/channel...0-upcoming-colliders-physics-on-the-edge.html
Hellbound
Merchant of Doom
Dammit, I wanna be a physicist.
I wish I'da stayed in college the first time, with my physics/comp sci double major.
I love reading about this stuff.
I wish I'da stayed in college the first time, with my physics/comp sci double major.
I love reading about this stuff.
That's what I meant. That it will not be enough to falsify the standard model. Of course this model must be wrong, I'm just saying that many people think the LHC will not provide much data in the region where it fails. What my high energy friends fear is that the LHC will discover little more than the Higgs boson, as predicted by the SM. In this scenario, we would lack any serious quantitative data to advance our theories beyond the standard model.
This agrees with what I am saying. If the LHC is not enough to give anything conclusive, then the SM would be succesful even at those energy scales. If it is succesful at those scales, we will not have experiments where it fails, so research in particle physics will remain somewhat stuck. That's what I meant with 'die of its own success', the advances in that area during the 20th century have been incredible. Many people think that the theory has advanced so much that we have gone beyond our experimental capabilities. Think about the common remarks about humanity having found string theory '100 years too soon', or the incredibly complicated setups proposed in other areas (such as LISA for gravitational waves).
It is a fact that many theoretical particle physicists have emigrated either to String theory or to statiscal field theory, because their original area is quite stagnant at the moment. I didn't mean the comment as a criticism of high energy physics, my area of interest (gravitation) suffers very similar problems.
Check for instance this blog post about LHC predictions:
Other equally famous physicists are more optimistic. Witten, however, also thinks that the LHC will only tell us the structure of the electroweak symmetry breaking sector. Warning: I am linking this blog because it contains quotes by many famous physicists, the author of the post is a declared enemy of string theory, so his own opinions at the end may be biased.
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ID, DM was proposed originally because of the current theory being inadequate. That is, matter was proposed to exist, sans evidence, because it must be there, of course.
Hellbound
Merchant of Doom
Yes. Much like antimatter was proposed to exist, sans evidence, because it must be there. And it was.
Just like te various force particles were proposed to exist sans evidence, because they must be there. And they were!
Just like neutrinos were supposed to exist, sans evidence, because they must be there...wow, found those too!
Just like muons were proposed to exist, sans evidence, because they must be there. Man, another one!
Just like Missing elements on the period table, when it was first being developed, were proposed to exist sans evidence, because they must be there. Dang, got those too!
Should I go on?
ETA:
You see, this is how "science" works. I know you aren't familiar with it, so I'll break it down for you.
An observation is made. In this case, the rotational speeds of various galaxies. Occam's Razor is applied. It is first attempted to be explained in terms of the known. However, the known provides no explanation that covers everything. GR doesn't fit the observation, and neither does Newtonian Mechanics.
Now, we have something new. At this stage, we create what's called a "hypothesis". A hypothesis is an educated guess at an explanation. In this particular case, we had two hypotheses: MOND and Dark Matter.
Then comes time to test the hypotheses against the observations. MOND worked well to predict the spiral galaxy behavior, but not that of clusters. Dark Matter worked well to describe both.
Now comes the time for testing. This is called the "experiment" stage. IN this case, the dark matter theory predicted the exiostence of certain characteristic gravity phenomena. They looked, and such phenomena was found, confirming the hypothesis. This should promote it to the level of theory, although more support for the theory is desired.
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Tez
Graduate Poster
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AFAIK there are no particular problems with adding massive neutrinos to the standard model - in fact I think "what modifications would the SM require if neutrinos had mass" was a question on an assignment I once did ( before it was confirmed they have mass!) and the answer was "very little, and nothing interesting". Although if youre looking for a prediction of what their mass is then that of course that is different - but one can ask the same for most of the fundamental particles.
A point worth emphasizing is that the LHC must find something beyond the SM (even with massive nuetrinos) because the SM is non-unitary at the energies LHC will reach (which means it makes predictions that certain things will happen with probability greater than 1!). Now some people would be disappointed if "all" we found was some kind of Higgs. However I think that the mysteries of mass are very interesting (and there are alternatives to Higgs - such as technicolor - which need to be ruled in or out), and the mysteries of if, how, why and when symmetry breaking occurs in quantum field theory are also very deep. Thus IMO its unlikely to all be a wash...
bob_kark
Person of Hench
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Its never too late. They do have night classes you know.
Hellbound
Merchant of Doom
I know. ALready thinking about that
Sadly, I have too many other bills (and my last student loan) to pay off first, but maybe in a few years
bob_kark
Person of Hench
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Well, you can defer your student loans too...I know. ALready thinking about that
Sadly, I have too many other bills (and my last student loan) to pay off first, but maybe in a few years
Hellbound
Merchant of Doom
Well, you can defer your student loans too...
Yeah, I know. But the payments are already high, and that would just make them higher. I've got about $25k to pay off right now, so I'd rather get that done (at least somewhat). Good news, though, I can get the Government to start paying on them soon (SLRP through the reserves). THey should pay $15k of em, so that doesn't leave me too much to deal with.