tag:blogger.com,1999:blog-7315236707728759521.post4826031309047992180..comments2024-03-27T22:28:06.861-06:00Comments on Dispatches From Turtle Island: Galactic Cluster Collision Observations Disfavor Heavy Particle Dark MatterAndrew Oh-Willekehttp://www.blogger.com/profile/02537151821869153861noreply@blogger.comBlogger6125tag:blogger.com,1999:blog-7315236707728759521.post-91538006128197486132015-05-23T04:24:00.001-06:002015-05-23T04:24:00.001-06:00Thanks for your patience, Andrew. Very nice read. ...Thanks for your patience, Andrew. Very nice read. <br /><br />"Ergo: (1) Cold dark matter cosmology is wrong, or (2) the simulations of cold dark matter cosmology are screwed up in some very material way".<br /><br />So no WIMPs, no MACHOs and no RAMBOs. The universe is much less machista than many physicists though?<br /><br />You also said that hot is impossible, so then only warm DM is possible with these results. Reading Wikipedia's entry on this issue (which is quite short as cold DM was the favored solution until now), I read that they mention sterile neutrinos and gravitinos as candidate particles, not the graviton as such. They also mentioned WIMPs but only if produced non-thermally (not sure of the implications). <br /><br />Gravitinos seem to pose some problems, because without some very specific solutions of SUSY they would decay and not be available for DM in sufficient numbers.<br /><br />I don't see any reference to the graviton as related to DM but your suggestion is interesting no doubt. However the existence of gravitons itself seems to be a confirmation of String Theory, what I know you hate. <br /><br />So I wonder why aren't you rather favoring sterile neutrinos instead. <br />Majuhttps://www.blogger.com/profile/12369840391933337204noreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-6942718079126040122015-05-22T18:31:30.544-06:002015-05-22T18:31:30.544-06:00Final bottom line re: El Gordo.
The El Gordo conc...Final bottom line re: El Gordo.<br /><br />The El Gordo conclusion is that you need some kind of dark matter that gives rise to many extremely large galactic clusters and allows them time to collide into each other very rapidly after the Big Bang.<br /><br />There is more than one possible dark matter theory that could produce this outcome (at least in theory) but it is hard to know which one is the best fit because good simulations exist only for a few of the many possibilities for dark matter. The researchers did not exhaustively consider the alternatives and only mentioned in passing that their result was at odds with the most popular dark matter cosmology in existence because that model is called the "Standard Model of Cosmology" and is the one to beat.andrewhttps://www.blogger.com/profile/08172964121659914379noreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-48324061908464565482015-05-22T17:21:27.712-06:002015-05-22T17:21:27.712-06:00To understand the El Gordo results, allow me to us...To understand the El Gordo results, allow me to use some fake numbers:<br /><br />El Gordo is about 10 billion year old light (which we known through something called "red shift" an issue too technical for now) (warning this is a fake, but order of magnitude right number).<br /><br />If you do simulations on computers of what the universe should look like when it is 3.5 billion years old, give or take, there is only a one in million chance (warning, this is another fake, but order of magnitude right number), that we would be able to see even one object as big or nearly as big as as El Gordo at that general age of the universe.<br /><br />But, in real life there are half a dozen or a dozen (warning, yet another fake but approximately right number) El Gordo scale objects that we can see in the sky at approximately the right age.<br /><br />Ergo: (1) Cold dark matter cosmology is wrong, or (2) the simulations of cold dark matter cosmology are screwed up in some very material way.andrewhttps://www.blogger.com/profile/08172964121659914379noreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-77025069066428368802015-05-22T17:11:50.096-06:002015-05-22T17:11:50.096-06:00One more addition to #2 above:
We also know that ...One more addition to #2 above:<br /><br />We also know that photons are not dark matter because it moves too fast. One of the few things we know about dark matter (if it exists) in addition to how heavy it is, and roughly where it is located, is that it moves quite slowly relative to the speed of light. If it moves really, really slowly, we call it cold dark matter, and if it moves a bit faster, we call it "warm dark matter".<br /><br />If dark matter moved at or near the speed of light, we would call it "hot dark matter" and one of the things we know for sure is that dark matter is not "hot" because we have observed its effects which are inconsistent with something moving close to the speed of light. If dark matter were "hot" then there would be no galaxies and there would be no galactic clusters, and the universe would be a lot less "lumpy" and instead would be far more of a homogeneous soup (imagine one huge nebula full of obscuring matter evenly spread out everywhere and you have an idea of what a hot dark matter universe would look like). An example of hot dark matter would be neutrinos, but we've measured how many of them there are too, and neutrinos contribute even less to the mass of the universe than photons do because they are each so light weight (and most neutrinos in space move at close to the speed of light).<br /><br />Now for the harder question:<br /><br />3. Mass v. mass-energy.<br /><br />Mass, as opposed to mass-energy, also known as rest mass, quantifies the amount of inertia that a particle (either a fundamental particle or a composite one) has. This is to say, rest mass tells you how much energy it takes to accelerate the particle to a greater velocity. At speeds much smaller than the speed of light, the formula is F=ma. <br /><br />At speeds approaching the speed of light, special relativity is required because it takes a bit more energy to shift a particle from v+1 to v+2 than it does to shift a particle from v to v+1, and the extra energy required to add the same amount of velocity increased infinitely as you approach the speed of light.<br /><br />Particles with zero rest mass, in contrast, always move at exactly the speed of light, without having any energy applied to them.<br /><br />But, both particle with rest mass and particles with zero mass are both equally affected by gravity according to E=mc^2.andrewhttps://www.blogger.com/profile/08172964121659914379noreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-40786413653206387672015-05-22T16:54:46.947-06:002015-05-22T16:54:46.947-06:00Easy questions first:
Yes, photons too.
Photon...Easy questions first:<br /><br />Yes, photons too. <br /><br />Photons are affected by gravity just like anything else (this is called gravitational lensing). And, photons give rise to gravitational fields proportional to their mass-energy at E=mc^2.<br /><br />The right hand side of the general relativity equation (Einstein's field equations) is 8pi*G*T(uv). 8, pi and Newton's constant (G) are numbers and in the case of Newton's constant physical units (General Relativity does not have "dimensionless constants" the way that the Standard Model does). T is a four by four number matrix called the <a href="http://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor" rel="nofollow">stress-energy tensor.</a><br /><br />The top-left element is energy density which is equal to matter-energy of both matter and photons at E=mc^2 at the relevant point, and the remaining elements describe four kinds of motion that different sources of gravity can have in particular directions (like pressure and energy flux and momentum).<br /><br />The left hand side of Einstein's field equations describe the curvature of space that is created by the stress-energy components on the right hand side (a.k.a. gravity).<br /><br />2. Photons are not dark matter candidates first of all because they aren't dark (of course), and more importantly because physicists, being clever, have measured the total weight of all the photons in the universe and in places where we observe dark matter effects (which they can do because it isn't dark), and there aren't nearly enough of them to fit the bill. Photons make up something on the order of 1% or less of the mass-energy in the universe.andrewhttps://www.blogger.com/profile/08172964121659914379noreply@blogger.comtag:blogger.com,1999:blog-7315236707728759521.post-60362236305697805582015-05-20T02:17:51.624-06:002015-05-20T02:17:51.624-06:00I'd wish I could understand or get a good &quo...I'd wish I could understand or get a good "explanation for dummies". I was reading the El Gordo press release yesterday (Basque research incidentally) but I really do not grasp well the implications, other than WIMPs seem to be discarded as Dark Matter because they are too massive, right?<br /><br />When you deal with the graviton issue, you got me even more perplex, because, yeah, by definition gravitons have zero mass. Yet you go on to discuss the "mass-energy" of the hypothetical particle. But wait: mass is a form of energy but energy is not the same as mass. Are you implying that every particle for the simple fact of having energy has some sort of implicit mass m=E/c²? Photons too? Why then nobody thinks of photons as dark matter candidates?Majuhttps://www.blogger.com/profile/12369840391933337204noreply@blogger.com