There are more mistakes in the/. gist than in the NYTimes article -- which incidentally is a good summary for the LHC. Well, the writer was at CERN about a month ago, so I am assuming it took about that long to write it.
It is called the LHC -- Large Hadron Collider. Not the Hadron SuperCollider. The SuperCollider is dead. It was called the SSC. But it has passed on. It has ceased to be! It has expired and gone to meet its maker! Its a stiff! Bereft of line and rests in peaces in TX! It's kicked the bucket and shuffled off its mortal coil! (Gee. I wish I could write this about the M$! Grrr!!)
The energy consumption is 14 trillion electron volts?! Wt..? Last time, I checked the LHC could not run on days where the electricity prices were high. Actually, it can not run during winter for that reason. It and the detectors consume as much energy as you get out from a medium-sized nuclear reactor -- and that's why it sits partially in France and not fully in Switzerland. (France produces a whole lot more power than Switzerland.)
"The piece also goes into some detail on the expected experiments. " Huh? What expected experiments? The experiments have been in construction now for seven years. You mean expected results?!
Honestly, how many mistakes can you make in one paragraph??
Sorry about the rant, but I am so annoyed with the latest reports about M$'s threats, that I had to vent. I feel better now. Slightly.
Why this generates so much discussion on/. puzzles me greatly! Is this a slow news week?
To me, it is pretty obvious that Ubuntu, which has Bug #1 as top priority, can not cast itself to the public eye as some piece of free software which seems or tries to emulate the Windows system with which its stars are crossed, by design. My hat is off to Mark Shuttleworth, who once again, shows great leadership.
But where I see the problem is that following: There are no Feynmans writing on Wikipedia. I know a few people who are brillliant in explaining physics at a good level, but they have never heard of Wikipedia and would never consider writing for Wikipedia. I have tried to get them involved and failed! Why? Because they are short on time. They have students and research projects and have more concrete problems on their hand with a shorter timescale than Wikipedia. I have contributed tiny little bit -- and that's because I do not even know where to start generally!
But then, I know some people who have contributed to Wikipedia. And quite a bit! They are generally non-experts, who have a physics as a hobby but read quite a bit and write quite a bit. I have quite often felt that it is a matter of "honor" for them, as it is their contribution to physics. But then it makes the articles bloated as they try to "show off" their knowledge. Equations are aglore and sense is minimal in such articles.
Even worse are the physics theory/phenomenology articles! They often end up being awfully slanted as the follower of some "theory cult" can edit the article away from the mainstream physics understanding so much that I sometimes fail to recognize what it is about completely! I have no clue on how to solve this problem...
Parent says: "We predict dark matter exists, then we show it exists."
Err. No. We did *not* predict dark matter. We were not expecting dark matter or anything like it when the Zwicky first saw that there had to be some "more" matter in the galaxies to explain the observed rotational curves. He probably first said: "Gee, well, that looks funny!" Zwicky probably said something a lot better actually, as he was known for his, often rude, mannerisms.
The astonishing discoveries in science come when humans really have no clue what is coming next! Case in point: The November Revolution in Physics . That was the last time that the whole paradigm of understanding of particle physics shifted! That was back in 1974 and hasn't changed since! One new totally-unexpected particle, called the J/psi, was found and boom... the consequences were huge, for now, you *knew* that there had to more particles, namely the top and the bottom and that the W and the Z were predicted as well. Only after the discovery of all these predicted particles did the public came to accept the Standard Model and particle physics became a mature field. But, back in 1974, there were those who could see ahead in the light of this new discovery.
A large shift in the understanding of the universe happened already in astrophysics with the CMB(Cosmic Microwave Background) measurements. I liken it very much to the November Revolution. The CMB observations, first from COBE and later from WMAP and various other ground based observations, show with high statistics that there is something missing if we assume that the universe is all baryonic matter. Imagine a puzzle where there is a missing piece and now, you think of a piece that fits in this place. Well, dark matter fits the bill very well and other observations, also back it up. So somehow, dark matter is required by experimental results... Now, those who can see ahead make predictions on what we will --hopefully -- discover next: a dark matter candidate particle at the LHC, annihilation products of dark matter in space, a signal in gamma rays from annihilation, plenty of lensing examples in galaxies,.. This is called phenomenology for a reason. You get an idea inspired by experimental results from an experiment and look at what other phenomena you would observe in the light of your idea/theory.
End of my rant.
To put your "inexorableness" theory in perspective. There are more humans living on the planet right now, then has ever lived in total in the history of earth. So take humans and divide them into two groups: Group1: from the beginning of human evolution to 1920 and Group2: from 1920 to today. Group2 is significantly larger in population. Do you think Group2 achieved more? Really?!! I dont think so! I see most of the population watching TV and going to work where they try to minimize thinking! Group1 had to struggle more for survival and had to be more inventive to survive. The pressure is off on Group2. Laziness is settling in fast.
First of all, I do not think there is anything wrong with being a dark-matter "agnostic". I applaud you for not believing in "highly theoretical proofs". I would not either. And I am not aware that physics can be understood through them. Physics is what we observe and the method of understanding that observation.
The problem I am still having problem conveying to you, I think is that 'dark matter' is not exactly a theory. It is rather a "solution" or "possible phenomena" that might be used to explain the observed results which have a high-statistics deviance from what we expect from the standard model of particle and astro physics. In some way, if you do not "believe" that the reason for such large deviations of observation from theory is not dark-*matter*-related but rather related to something else, such as *force* related, that's fine by me, I suppose. The problem with "messing" with forces rather than particles is that, well, forces effect quite a few things and so, you fix a problem but generall end up making another on the other side of the theoretical/observation universe.
My problem with the general public conception of dark matter or for that matter, most physical concepts, is that there is very little awareness of what is "observed" versus what is theorized. There are lots of astrophysical data that shows large deviations from what is expected. If you add in dark-matter, well, these deviations become trivial to explain. (No, the math there is really not *that* bad! Dont let people scare you -- if you are interested, try reading some papers.) The problem is that Nature is asking/forcing us to add something to the mixture. Dark matter, unlike Dark energy, was not cooked up by theorists, but rather forced on them, by experimental evidence. Now. maybe, just maybe, although very unlikely, there is another way to explain away the observed effects by changing how "forces" in nature act in some smart way. Again, I can not imagine how -- and yes, such theories require serious amounts of maths and ridiculous amounts of hand-waving to get around the problems they create while solving others!
The reason that so many people these days think that there is dark matter is that dark matter fits perfectly to the data from several astronomical epochs and from very different observational methods. The largest evidence comes from the CMB (Cosmic Microwave Background) which was when the universe was at about 300,000K so pretty young. Now, the supernova evidence which is higher by the day, spans through about half of the life of the universe. Observations are pushing back towards earlier and earlier times in the universe. The observed galactic rotational curves are from (approximately) current time of the universe. Thinkers of dark matter see that these nearly cover the whole 14 billion years of evolution of the universe and leave very little wiggle room for other theories.
Maybe you could say that dark matter has been hyped to give particle physicists something to discover at the LHC, the most expensive physics experiment to date. Certainly, if observed phenomena is due to dark matter, well, then LHC should discover the naughty particle which is at the heart of this. That would be a direct detection, but would than need further confirmation that this naughty particle is what accounts for a large fraction of the universe. Any experimental booklet will tell you that the first goal of the LHC is to find the Higgs and the next one is to find a dark matter candidate. If the LHC can find this particle, other observational particle physics experiments will have the further chance to change or tune their experiments to confirm that this is indeed true. So stay an agnostic! Hopefully, you will be convinced when and if the LHC discovers a particle which fits the bill... I am an experimental particle physicist working on an LHC experiment and I spend most of my time, thinking of how to find such a particle...
(and sorry about my English which sometimes fails me after a long and tiring day at work.)
Sorry, Alaren. You got it mostly right, but forgot one thing. The weak interactions. Practically, our only hope to detect directly or through their annihilation products, these elusive dark matter particles.
You raise a valid point about why we haven't seen any on earth. Well, they *are* all around us. They do not "cluster" as much as normal matter gravitationally so their concentration is not dense. We can not detect them directly through their gravitational influence, as gravity is too weak. Most experiments try to detect them through their weak interactions and hopefully soon will. There is an incredible amount of effort in this experimental effort right now.
True. But they dont try to detect it through its gravitational effect, but rather through its proposed weak interactions, as opposed to gravity as the parent would suggest. To detect the effect of one dark matter particle through its gravitational influence is probably next to impossible... but then again, who knows...
Yey! This was actually suggested as a cause sometime ago, in some paper, but I can't find it for the life of me.
Actually, this parallel universe theory is mostly used explaining why gravity is so weak as opposed to the other 3 physical forces. If gravity acts in many dimensions (more than 3) it could be that it is weaker cause it has to divide up its force among so many dimensions. Lisa Randall is the main propogator of this theory, so look up her papers if you are interested in this one.
The purpose of dark matter is not just to explain spiral galaxy rotational curves. The bigger problem is the energy budget of the universe.
If you ONLY read the NASA press releases with colorized images, that is, I am afraid your problem. There is over-whelming evidence for the existance of dark matter and what are scientists to do if the only thing that puts dark matter on a \.er's mind is just a pretty picture. If you are interested, go to arxiv.org and search for results from dark matter experiments and read the papers. Yes, they are technical. Yes, it produced many PhDs -- not a bad thing, last time I checked. And yes, the experimental evidence is overwhelming.
The number of people working on dark matter experiments, greatly exceeds the number of dark matter theorists, probably by an order of magnitude actually. This is *the* one field in astro-particle physics, where there is great wealth of data and that data is driving the evolution of the field. In particle physics, this is not the case! There is no data on particles which might form dark matter! There are too many theories! Hopefully, the tables will turn when the LHC at CERN turns on next year!
I should also point out that this is one of the "nicest" sort of theoretical astrophysics papers there is. It suggests a possible phenomena that produces an experimental signature in space experiments like or AMS.
Yes, dark matter interacts with gravity but not with the electro-magnetic forces.
No, this is not an anti-matter star. Anti-matter is the "opposite" of particles that we are accustomed to, but still have the same interactions as the normal particles around us. So yes, they interact electro-magnetically. Say, you were a human being made of anti-matter on an anti-matter earth, in the part of the galaxy dominated by anti-matter, all visible physics laws would look the same. (Yes, there are one or two very weird experiments that would yield the opposite results. Feynman discusses this in his books, if you are interested.)
There are 4 forces, as we know it, in the universe. Gravitational, electro-magnetic, strong and weak. All these forces treat anti-matter pretty much the same way that they treat matter. Dark matter is something completely different. The reason is, it does not interact electro-magnetically. We know this cause we can "see" that it does not interact with photons( light )-- the force carrier of the electro-magnetic force. All observations agree that it does interact gravitationally. And whether or not it interacts weakly or not is under contention.
The significance of the results of dark matter experiments is very high. So, we pretty much, by now, know that dark matter exists and it does account for a large fraction of the energy budget of our universe, about ~22%. Good old normal matter accounts for about 4% of the energy budget.
Dark matter particles are probably flowing through you read this. They are around us. They interact only very very weakly and so we dont "feel" them. As their concentration is not very high, they do not contribute to our weight either. But they are around us, that's pretty clear.
If you think all this dark matter stuff sounds crazy, well, then a little factoid. About a billion neutrinos are passing through your eye-ball per second! They are mostly coming from the sun! And they flow through us, with extremely low probability of interacting, and with no real effect on our daily lives. Actually, the probability that ONE neutrino has interacted in the body of an 80-old person in his life time is about 50%. So that's a pretty rare event. The interaction of dark matter particles are on the same order of magnitude. For sometime, people thought that neutrinos could be dark matter candidates, until experiments showed that neutrinos are not heavy enough to account for ~22% of the energy budget of the universe.
So what does this whole thing mean? Dark matter particles are heavy particles, which do not like interacting with normal matter particles and mostly go about their own way, but still make their presence be felt, through gravity by structuring the universe through their overwhelming-numbers.
In this case, dark matter particles would annihilate with each other. Just like photons can annihilate with each other -- if they have the right helicity/spin. Dark matter particles are neutral and yes, could, annihilate with each other under certain conditions.
Note that dark matter is *not* regular matter. It is matter which does not interact through the electro-magnetic forces. It does not interact "with charged particles" nor with light! Hence, the name "dark." If light can not scatter from it, then that makes it "dark."
The authors say: "The nature of the cold dark matter in the universe is as yet unknown. Weakly Interacting Massive Particles (WIMPs) are possibly the strongest candidates, as
WIMPs that were in thermodynamic equilibrium in the
early universe automatically provide the appropriate relic
abundance to give the observed matter density. More-
over, WIMPs have a natural origin in particle physics,
e.g. neutralinos in supersymmetric models are excellent
DM candidates. [..]T he details of the interactions
and masses of the neutralinos depend on a large number of model parameters. In the minimal supergravity
model, experimental and observational bounds restrict
the neutralino mass m to 50 GeV-2 TeV, while the annihilation cross section v lies within an order of magnitude of h vi = 3 × 10^-26cm3/sec (except at the low
end of the mass range where it could be several orders
of magnitude smaller). "
So the authors make it clear that they are working under a set of assumptions, which are now fairly well accepted in the astrophysics community. Yes, maybe, these set of assumptions are wrong and if they are, their nice constructed dark stars would not exist.... If the annihilation cross section was very very high, then all dark matter would have self-annihilated by now. So there are bounds on that. Yes, it is theoretically possible still, I suppose, that dark matter may not self-annihilate! That makes it harder to detect! Most favored particle physics phenomenology would suggest that there should be some annihilation cross-section, on the order of magnitude suggested by the measured strength of the weak-forces. It turns out that this annihilation cross section is low enough that most dark matter would have survived to this day after the ~14billion history of the universe.
As a matter of fact, there are several experiments looking for dark matter from the sun. Yes, there could be some dark matter loosely bound to the sun's gravitational potential. I can not give a comprehensive list here but a good example is CAST . There are other dark matter experiments which may be sensitive to a signal from the sun such as CRESST and CDMS.
What makes normal matter collapse is the "friction" or "interaction" between the charged particles. Dark matter is neutral as far as we know and it does not interact through the EM-forces. Hence the name "dark," meaning it does not interact with light either. It is hard to form models where dark matter "collapses". The reason is that the dark matter particles do not exchange energy/momentum easily, as they interact through the "weak" forces only.
On \. OriginalArlen reports the news. I look at the linked website, called Universe Today and I see that there is one "publisher" by the name of Fraser Cain. Following the link there, finally, I get to the article on the arxiv, the definitive source of new physics papers. So to get to the source, it takes three jumps. So what has Fraser Cain done for us? Watered down the content? Couldn't OriginalArlen read the article and write a gist himself/herself? Or is Fraser Cain the same person as OriginalArlen?
Reading the original article, I find "some" correlation on what ends up on \. and what is in the article. Or is this not the point?
If I had to write a review for this article, I would have said that the last sentence of the abstract is what is most important: "A..star.. detectable via annihilation products (gamma-rays, neutrinos, anti-matter) possibly in combination with hydrogen lines."
The brilliant thing about this article is that these theorists are cooking up something that is actually detectable! Something that can be tested and hopefully will! *Finally* congrats to Douglas Spolyar, Katherine Freese and Paolo Gondolo, who *wrote* the article. (No, I dont know any of them. But isn't it time we cited those whose ideas we regurgitate?)
I submitted this story to Slashdot, but sadly, I see that my original wording has been altered by kdawson. Unfortunately, I do not have a copy of my original post, but I would like to clarify what I *meant*.
First of all, I do not consider this "good news" -- but "good results." The MiniBooNE team clearly worked very hard to get here so a big "Congrats" goes out to them. You could not rule out the LSND result, just because "we did not expect it" and "found it fishy." The unexpected results are sometimes the best ones and in science, remember: one scientist's junk is another scientist's signal. The CMB discovery story is the best example to this.
Secondly, the neutrino mass indeed does not belong in the standard model, which already several people have pointed out. What belongs in the standard model is the number of lepton families. It is good to see it confirmed that no "sterile" neutrino is needed to explain the results. Yes, cosmologists have had some say in the subject matter already, but it is good to see it confirmed. This is, afterall, how physics is done. "I told you so" is never a good thing to say in physics. You never know what comes out next afterall. I do not believe that Standard Model has been salvaged by this result nor do I want to live with the Standard Model for the rest of my life. There is already plenty of evidence that the Standard Model is not a sufficient model for explaining all the physical phenomena we observe and soon, I hope soon we will have evidence what that new "something" might be.
At this point, I would also like to take this chance, as a physicist who works at CERN, to reply to the highly excited conspiracy theorists: Calm down! CERN, Fermilab and other physics labs are not part of corporate America! Yes, of course, I want CERN (and my experiment, in specific) to be the one who finds the Higgs, but I am willing to bet all my fortune, little as that may be, on that Fermilab's calculation mistake was not intentional. Yes, we, physicists are a funny bunch, with lots of things to argue and get excited about. But, we do have a common goal in life, to dig deeper into the mystery of the universe. And a common understanding -- that the truth *will* reveal itself and you can not determine when it does.
A new CERN press release today reports that the first sector (1/8 of the LHC or 3.3km of it) has now reached the operational temperature of 1.9K. Afterall, it is the world's largest cryogenic system and contains 800,000 litres of helium. 12,000,000 litres of liquid nitrogen will have been vaporized to achieve the full cool-down of 31,000 tons of material.
Cheers!
Slashdot Mirror
It is called the LHC -- Large Hadron Collider. Not the Hadron SuperCollider. The SuperCollider is dead. It was called the SSC. But it has passed on. It has ceased to be! It has expired and gone to meet its maker! Its a stiff! Bereft of line and rests in peaces in TX! It's kicked the bucket and shuffled off its mortal coil! (Gee. I wish I could write this about the M$! Grrr!!)
The energy consumption is 14 trillion electron volts?! Wt..? Last time, I checked the LHC could not run on days where the electricity prices were high. Actually, it can not run during winter for that reason. It and the detectors consume as much energy as you get out from a medium-sized nuclear reactor -- and that's why it sits partially in France and not fully in Switzerland. (France produces a whole lot more power than Switzerland.)
"The piece also goes into some detail on the expected experiments. " Huh? What expected experiments? The experiments have been in construction now for seven years. You mean expected results?!
Honestly, how many mistakes can you make in one paragraph??
Sorry about the rant, but I am so annoyed with the latest reports about M$'s threats, that I had to vent. I feel better now. Slightly.
To me, it is pretty obvious that Ubuntu, which has Bug #1 as top priority, can not cast itself to the public eye as some piece of free software which seems or tries to emulate the Windows system with which its stars are crossed, by design. My hat is off to Mark Shuttleworth, who once again, shows great leadership.
But where I see the problem is that following: There are no Feynmans writing on Wikipedia. I know a few people who are brillliant in explaining physics at a good level, but they have never heard of Wikipedia and would never consider writing for Wikipedia. I have tried to get them involved and failed! Why? Because they are short on time. They have students and research projects and have more concrete problems on their hand with a shorter timescale than Wikipedia. I have contributed tiny little bit -- and that's because I do not even know where to start generally!
But then, I know some people who have contributed to Wikipedia. And quite a bit! They are generally non-experts, who have a physics as a hobby but read quite a bit and write quite a bit. I have quite often felt that it is a matter of "honor" for them, as it is their contribution to physics. But then it makes the articles bloated as they try to "show off" their knowledge. Equations are aglore and sense is minimal in such articles.
Even worse are the physics theory/phenomenology articles! They often end up being awfully slanted as the follower of some "theory cult" can edit the article away from the mainstream physics understanding so much that I sometimes fail to recognize what it is about completely! I have no clue on how to solve this problem...
(Yes, I am a physicist.)
Err. No. We did *not* predict dark matter. We were not expecting dark matter or anything like it when the Zwicky first saw that there had to be some "more" matter in the galaxies to explain the observed rotational curves. He probably first said: "Gee, well, that looks funny!" Zwicky probably said something a lot better actually, as he was known for his, often rude, mannerisms.
The astonishing discoveries in science come when humans really have no clue what is coming next! Case in point: The November Revolution in Physics . That was the last time that the whole paradigm of understanding of particle physics shifted! That was back in 1974 and hasn't changed since! One new totally-unexpected particle, called the J/psi, was found and boom... the consequences were huge, for now, you *knew* that there had to more particles, namely the top and the bottom and that the W and the Z were predicted as well. Only after the discovery of all these predicted particles did the public came to accept the Standard Model and particle physics became a mature field. But, back in 1974, there were those who could see ahead in the light of this new discovery.
A large shift in the understanding of the universe happened already in astrophysics with the CMB(Cosmic Microwave Background) measurements. I liken it very much to the November Revolution. The CMB observations, first from COBE and later from WMAP and various other ground based observations, show with high statistics that there is something missing if we assume that the universe is all baryonic matter. Imagine a puzzle where there is a missing piece and now, you think of a piece that fits in this place. Well, dark matter fits the bill very well and other observations, also back it up. So somehow, dark matter is required by experimental results... Now, those who can see ahead make predictions on what we will --hopefully -- discover next: a dark matter candidate particle at the LHC, annihilation products of dark matter in space, a signal in gamma rays from annihilation, plenty of lensing examples in galaxies,.. This is called phenomenology for a reason. You get an idea inspired by experimental results from an experiment and look at what other phenomena you would observe in the light of your idea/theory.
End of my rant.
To put your "inexorableness" theory in perspective. There are more humans living on the planet right now, then has ever lived in total in the history of earth. So take humans and divide them into two groups: Group1: from the beginning of human evolution to 1920 and Group2: from 1920 to today. Group2 is significantly larger in population. Do you think Group2 achieved more? Really?!! I dont think so! I see most of the population watching TV and going to work where they try to minimize thinking! Group1 had to struggle more for survival and had to be more inventive to survive. The pressure is off on Group2. Laziness is settling in fast.
First of all, I do not think there is anything wrong with being a dark-matter "agnostic". I applaud you for not believing in "highly theoretical proofs". I would not either. And I am not aware that physics can be understood through them. Physics is what we observe and the method of understanding that observation.
The problem I am still having problem conveying to you, I think is that 'dark matter' is not exactly a theory. It is rather a "solution" or "possible phenomena" that might be used to explain the observed results which have a high-statistics deviance from what we expect from the standard model of particle and astro physics. In some way, if you do not "believe" that the reason for such large deviations of observation from theory is not dark-*matter*-related but rather related to something else, such as *force* related, that's fine by me, I suppose. The problem with "messing" with forces rather than particles is that, well, forces effect quite a few things and so, you fix a problem but generall end up making another on the other side of the theoretical/observation universe.
My problem with the general public conception of dark matter or for that matter, most physical concepts, is that there is very little awareness of what is "observed" versus what is theorized. There are lots of astrophysical data that shows large deviations from what is expected. If you add in dark-matter, well, these deviations become trivial to explain. (No, the math there is really not *that* bad! Dont let people scare you -- if you are interested, try reading some papers.) The problem is that Nature is asking/forcing us to add something to the mixture. Dark matter, unlike Dark energy, was not cooked up by theorists, but rather forced on them, by experimental evidence. Now. maybe, just maybe, although very unlikely, there is another way to explain away the observed effects by changing how "forces" in nature act in some smart way. Again, I can not imagine how -- and yes, such theories require serious amounts of maths and ridiculous amounts of hand-waving to get around the problems they create while solving others!
The reason that so many people these days think that there is dark matter is that dark matter fits perfectly to the data from several astronomical epochs and from very different observational methods. The largest evidence comes from the CMB (Cosmic Microwave Background) which was when the universe was at about 300,000K so pretty young. Now, the supernova evidence which is higher by the day, spans through about half of the life of the universe. Observations are pushing back towards earlier and earlier times in the universe. The observed galactic rotational curves are from (approximately) current time of the universe. Thinkers of dark matter see that these nearly cover the whole 14 billion years of evolution of the universe and leave very little wiggle room for other theories.
Maybe you could say that dark matter has been hyped to give particle physicists something to discover at the LHC, the most expensive physics experiment to date. Certainly, if observed phenomena is due to dark matter, well, then LHC should discover the naughty particle which is at the heart of this. That would be a direct detection, but would than need further confirmation that this naughty particle is what accounts for a large fraction of the universe. Any experimental booklet will tell you that the first goal of the LHC is to find the Higgs and the next one is to find a dark matter candidate. If the LHC can find this particle, other observational particle physics experiments will have the further chance to change or tune their experiments to confirm that this is indeed true. So stay an agnostic! Hopefully, you will be convinced when and if the LHC discovers a particle which fits the bill... I am an experimental particle physicist working on an LHC experiment and I spend most of my time, thinking of how to find such a particle...
(and sorry about my English which sometimes fails me after a long and tiring day at work.)
You raise a valid point about why we haven't seen any on earth. Well, they *are* all around us. They do not "cluster" as much as normal matter gravitationally so their concentration is not dense. We can not detect them directly through their gravitational influence, as gravity is too weak. Most experiments try to detect them through their weak interactions and hopefully soon will. There is an incredible amount of effort in this experimental effort right now.
True. But they dont try to detect it through its gravitational effect, but rather through its proposed weak interactions, as opposed to gravity as the parent would suggest. To detect the effect of one dark matter particle through its gravitational influence is probably next to impossible... but then again, who knows...
Actually, this parallel universe theory is mostly used explaining why gravity is so weak as opposed to the other 3 physical forces. If gravity acts in many dimensions (more than 3) it could be that it is weaker cause it has to divide up its force among so many dimensions. Lisa Randall is the main propogator of this theory, so look up her papers if you are interested in this one.
The purpose of dark matter is not just to explain spiral galaxy rotational curves. The bigger problem is the energy budget of the universe.
If you ONLY read the NASA press releases with colorized images, that is, I am afraid your problem. There is over-whelming evidence for the existance of dark matter and what are scientists to do if the only thing that puts dark matter on a \.er's mind is just a pretty picture. If you are interested, go to arxiv.org and search for results from dark matter experiments and read the papers. Yes, they are technical. Yes, it produced many PhDs -- not a bad thing, last time I checked. And yes, the experimental evidence is overwhelming.
The number of people working on dark matter experiments, greatly exceeds the number of dark matter theorists, probably by an order of magnitude actually. This is *the* one field in astro-particle physics, where there is great wealth of data and that data is driving the evolution of the field. In particle physics, this is not the case! There is no data on particles which might form dark matter! There are too many theories! Hopefully, the tables will turn when the LHC at CERN turns on next year!
I should also point out that this is one of the "nicest" sort of theoretical astrophysics papers there is. It suggests a possible phenomena that produces an experimental signature in space experiments like or AMS.
No, this is not an anti-matter star. Anti-matter is the "opposite" of particles that we are accustomed to, but still have the same interactions as the normal particles around us. So yes, they interact electro-magnetically. Say, you were a human being made of anti-matter on an anti-matter earth, in the part of the galaxy dominated by anti-matter, all visible physics laws would look the same. (Yes, there are one or two very weird experiments that would yield the opposite results. Feynman discusses this in his books, if you are interested.)
There are 4 forces, as we know it, in the universe. Gravitational, electro-magnetic, strong and weak. All these forces treat anti-matter pretty much the same way that they treat matter. Dark matter is something completely different. The reason is, it does not interact electro-magnetically. We know this cause we can "see" that it does not interact with photons( light )-- the force carrier of the electro-magnetic force. All observations agree that it does interact gravitationally. And whether or not it interacts weakly or not is under contention.
The significance of the results of dark matter experiments is very high. So, we pretty much, by now, know that dark matter exists and it does account for a large fraction of the energy budget of our universe, about ~22%. Good old normal matter accounts for about 4% of the energy budget.
Dark matter particles are probably flowing through you read this. They are around us. They interact only very very weakly and so we dont "feel" them. As their concentration is not very high, they do not contribute to our weight either. But they are around us, that's pretty clear.
If you think all this dark matter stuff sounds crazy, well, then a little factoid. About a billion neutrinos are passing through your eye-ball per second! They are mostly coming from the sun! And they flow through us, with extremely low probability of interacting, and with no real effect on our daily lives. Actually, the probability that ONE neutrino has interacted in the body of an 80-old person in his life time is about 50%. So that's a pretty rare event. The interaction of dark matter particles are on the same order of magnitude. For sometime, people thought that neutrinos could be dark matter candidates, until experiments showed that neutrinos are not heavy enough to account for ~22% of the energy budget of the universe.
So what does this whole thing mean? Dark matter particles are heavy particles, which do not like interacting with normal matter particles and mostly go about their own way, but still make their presence be felt, through gravity by structuring the universe through their overwhelming-numbers.
Note that dark matter is *not* regular matter. It is matter which does not interact through the electro-magnetic forces. It does not interact "with charged particles" nor with light! Hence, the name "dark." If light can not scatter from it, then that makes it "dark."
Ok, so say you are not a physicist, you can still read the article. It may have equations, but it is still English: http://arxiv.org/PS_cache/arxiv/pdf/0705/0705.0521 v1.pdf
The authors say: "The nature of the cold dark matter in the universe is as yet unknown. Weakly Interacting Massive Particles (WIMPs) are possibly the strongest candidates, as WIMPs that were in thermodynamic equilibrium in the early universe automatically provide the appropriate relic abundance to give the observed matter density. More- over, WIMPs have a natural origin in particle physics, e.g. neutralinos in supersymmetric models are excellent DM candidates. [..]T he details of the interactions and masses of the neutralinos depend on a large number of model parameters. In the minimal supergravity model, experimental and observational bounds restrict the neutralino mass m to 50 GeV-2 TeV, while the annihilation cross section v lies within an order of magnitude of h vi = 3 × 10^-26cm3/sec (except at the low end of the mass range where it could be several orders of magnitude smaller). "
So the authors make it clear that they are working under a set of assumptions, which are now fairly well accepted in the astrophysics community. Yes, maybe, these set of assumptions are wrong and if they are, their nice constructed dark stars would not exist.... If the annihilation cross section was very very high, then all dark matter would have self-annihilated by now. So there are bounds on that. Yes, it is theoretically possible still, I suppose, that dark matter may not self-annihilate! That makes it harder to detect! Most favored particle physics phenomenology would suggest that there should be some annihilation cross-section, on the order of magnitude suggested by the measured strength of the weak-forces. It turns out that this annihilation cross section is low enough that most dark matter would have survived to this day after the ~14billion history of the universe.
As a matter of fact, there are several experiments looking for dark matter from the sun. Yes, there could be some dark matter loosely bound to the sun's gravitational potential. I can not give a comprehensive list here but a good example is CAST . There are other dark matter experiments which may be sensitive to a signal from the sun such as CRESST and CDMS.
What makes normal matter collapse is the "friction" or "interaction" between the charged particles. Dark matter is neutral as far as we know and it does not interact through the EM-forces. Hence the name "dark," meaning it does not interact with light either. It is hard to form models where dark matter "collapses". The reason is that the dark matter particles do not exchange energy/momentum easily, as they interact through the "weak" forces only.
On \. OriginalArlen reports the news. I look at the linked website, called Universe Today and I see that there is one "publisher" by the name of Fraser Cain. Following the link there, finally, I get to the article on the arxiv, the definitive source of new physics papers. So to get to the source, it takes three jumps. So what has Fraser Cain done for us? Watered down the content? Couldn't OriginalArlen read the article and write a gist himself/herself? Or is Fraser Cain the same person as OriginalArlen? Reading the original article, I find "some" correlation on what ends up on \. and what is in the article. Or is this not the point? If I had to write a review for this article, I would have said that the last sentence of the abstract is what is most important: "A ..star .. detectable via annihilation products (gamma-rays, neutrinos, anti-matter) possibly in combination with hydrogen lines."
The brilliant thing about this article is that these theorists are cooking up something that is actually detectable! Something that can be tested and hopefully will! *Finally* congrats to Douglas Spolyar, Katherine Freese and Paolo Gondolo, who *wrote* the article. (No, I dont know any of them. But isn't it time we cited those whose ideas we regurgitate?)
I submitted this story to Slashdot, but sadly, I see that my original wording has been altered by kdawson. Unfortunately, I do not have a copy of my original post, but I would like to clarify what I *meant*. First of all, I do not consider this "good news" -- but "good results." The MiniBooNE team clearly worked very hard to get here so a big "Congrats" goes out to them. You could not rule out the LSND result, just because "we did not expect it" and "found it fishy." The unexpected results are sometimes the best ones and in science, remember: one scientist's junk is another scientist's signal. The CMB discovery story is the best example to this. Secondly, the neutrino mass indeed does not belong in the standard model, which already several people have pointed out. What belongs in the standard model is the number of lepton families. It is good to see it confirmed that no "sterile" neutrino is needed to explain the results. Yes, cosmologists have had some say in the subject matter already, but it is good to see it confirmed. This is, afterall, how physics is done. "I told you so" is never a good thing to say in physics. You never know what comes out next afterall. I do not believe that Standard Model has been salvaged by this result nor do I want to live with the Standard Model for the rest of my life. There is already plenty of evidence that the Standard Model is not a sufficient model for explaining all the physical phenomena we observe and soon, I hope soon we will have evidence what that new "something" might be. At this point, I would also like to take this chance, as a physicist who works at CERN, to reply to the highly excited conspiracy theorists: Calm down! CERN, Fermilab and other physics labs are not part of corporate America! Yes, of course, I want CERN (and my experiment, in specific) to be the one who finds the Higgs, but I am willing to bet all my fortune, little as that may be, on that Fermilab's calculation mistake was not intentional. Yes, we, physicists are a funny bunch, with lots of things to argue and get excited about. But, we do have a common goal in life, to dig deeper into the mystery of the universe. And a common understanding -- that the truth *will* reveal itself and you can not determine when it does.
A new CERN press release today reports that the first sector (1/8 of the LHC or 3.3km of it) has now reached the operational temperature of 1.9K. Afterall, it is the world's largest cryogenic system and contains 800,000 litres of helium. 12,000,000 litres of liquid nitrogen will have been vaporized to achieve the full cool-down of 31,000 tons of material. Cheers!