ÿþ<html> <body> <h1>Some very simple notes on Dark Matter (DM).</h1> <B>Version</B> : 0.7<br> <B>Date</B> : 04/07/2010<br> <B>By</B> : Albert van der Sel<br> <B>Type of doc</B> : It's just a few simple notes on DM.<br> <B>For who</B> : for anyone who likes a short orientation on the subject.<br> <hr/> <br> <B>This is a very short note, introducing some basics of "Dark Matter".</B><br> <br> So.., Dark matter, what is it?<br> Sorry to say this, but <I>we are still not quite sure to what it is!</I><br> <br> First of all, don't mix up "Dark Matter" with something that's called "Dark Energy".<br> <br> Dark Energy, is what many scientists think, some sort of field in all of space, exerting a "negative pressure",<br> and thus is driving the (accelerated) expansion of the Universe.<br> <br> Dark Matter, is supposed to be really "matter" of some sort. It's called "dark" because, unlike the matter in the Universe we are<br> all familiar with (like stars, dust, galaxies etc..), it does not expose it's presence directly.<br> For example, it does not seem to interact with electromagnetic radiation, and so far, we have not "seen" it in the usual way.<br> <br> So, Dark Matter appears not to interact via the electromagnetic force, and therefore neither emits nor reflects light.<br> To put it in simple words: <I>if you can "see" it, it's not Dark Matter.</I><br> <br> But scientist <I>do</I> have seen it <I>indirectly.</I>Or, we should probably say it <I>this way</I>: scientists have seen effects<br> in the Universe, which can be explained rather well by introducing "Dark Matter".<br> <br> Now, following some theoretical considerations and some observations, the "numbers" are downright astonishing!<br> The "normal" (baryonic) matter represents no more than about 4% of the total energy. "Dark Matter" is then supposed<br> to represent about 20%. And the rest is attributed to "Dark Energy".<br> So, if we focus on the "matter" components, Dark matter is then almost 5 times (or even more) as 'abundant' as normal matter!<br> <br> Note:<br> Personally I don't think we understand the Universe, or even come close to that, right now.<br> I certainly don't think the numbers are "speculative", but history showed us that "theories" get revised all the time,<br> and even sometimes gets completely replaced by a new one.<br> <br> Anyway, the clues that point to the true existence of "Dark Matter" (and "Dark Energy") are really strong enough.<br> And don't forget: the effort that is put on (theoretical and experimental) research, is enormous.<br> <br> <B> This small note will address the following:<br> <br> Section 1: Some important observations (that "point" to the existence of Dark Matter).<br> Section 2. Dark Matter: what can it be?<br> </B> <br> <br> <h2>Section 1. Some important observations.</h2> <B>1.1 High speeds of Galaxies in Clusters:</B><br> <br> The idea that something like dark matter could exists, is certainly not new. Already in the first half of the former century,<br> it was noted, that the individual velocties of galaxies within a cluster (especially at the rim) was too high to be accounted for<br> by the gravitational force due to the visible mass of the cluster. As you can imaging, it was quite puzzling. So, there was a presence<br> of mass that couldn't be observed directly, other then by the gravitational effects on the visible mass.<br> Anyway, it was one of the first clues that dark matter could exist.<br> <br> There are many other observations that lead to the assumption of additional mass (that cannot be "seen").<br> Here are two additional types of observations.<br> <br> <B>1.2 Orbital speed of stars in a spiral Galaxy:</B><br> <br> One very nice other example is the rotational speed of objects within a typical Spiral Galaxy (like our milkyway).<br> I'am not lying, but when I first heard of it, <B>it knocked me out of my socks !</B><br> <br> <img src="dm1.jpg" align="left"/> What is it then? If you take a look at some spiral Galaxy, you see those "arms" of stars and dust, in a "disk like" structure,<br> which rotates around a very bright centre. The centre contains a high concentration of stars, and likely a massive<br> black hole as well.<br> <br> Now, just as is the case with our local Solar system, where the inner planets move faster than the outer planets, you might<br> expect the same sort of behaviour of objects in the galactic disk. That is, stars located more at the rim would move slower,<br> compared to stars which are located more at the Centre of such a Galaxy.<br> In the Solar system, with our planets and a massive Sun "in the centre", all you need is Newtonian mechanics and the<br> laws of Kepler, to make clear why the inner planets move faster.<br> <br> In the '70 and '80 of the former century, a number of astronomers made the astonishing discovery that most stars<br> in spiral galaxies orbit at roughly the same speed !<br> With a massive Centre and stars and dust rotating around it, it would be very hard to explain.<br> <br> A good explanition for the effect would be, the "assumption" of (invisible) dark matter in the "halo" that<br> surrounds such a spiral galaxy. There are ofcourse mathematical considerations which supports such a view,<br> but "intuitively" we can can see that it is indeed a reasonable theory.<br> If it's true, Dark Matter would be very "transparant" to electromagnetic radiation (like light), because usually we can<br> see remote objects right through it (like other remote galaxies etc..)<br> <br> Even from those two examples discussed above, although we can't see it, dark matter has a strong<br> gravitational influence.<br> We still haven't said anything about the <I>nature</I> of Dark Matter. As we shall see in section 2,<br> there exists a whole array of possible candidates.<br> <br> <B>1.3 Gravitational lensing:</B><br> <br> This phenomenon, predicted by Einstein, is caused by the deflection of light rays by massive objects (or regions).<br> <br> <img src="dm2.jpg" align="left"/> A massive object deforms the nearby geometry of space, so that instead of flat, it becomes curved.<br> Now, in some special cases, for an observer on earth. it really looks like he or she sees two very <br> "identical" objects, like quasars, who then "incredably" looks the same.<br> <br> Or, in case of a cluster or group of galaxies, we might see even see the same galaxy, multiple times.<br> In the figure on the left, this phenomenon is illustrated.<br> In this figure, the "blue coloured" galaxies are actually multiple images of the same one galaxy.<br> <br> If the galaxies are precisely aligned, the appearance is that of a near perfect ring or a symmetric cross.<br> <br> In principle, any massive object or region (in "front" of the distorted objects), could be responsible for the effect.<br> Some attribute the effect to "cosmic strings", or an unseen black hole who happened to be in "the line of sight".<br> These candidates are not so very likely, when compared to Dark matter. Since Dark matter is very transparant to light,<br> and exerts strong gravitational effects, and given the fact that we "don't see" anything else as the distorting factor,<br> then Dark matter is actually a very good candidate for quite some of those observed lensing phenomenea.<br> <br> So, above we have seen three different "sorts" of observations that supports the existence of "Dark Matter".<br> <br> Now it's time to take a look at what scientists think it is made of.<br> <br> <h2>Section 2. Dark Matter: what could it be?</h2> <br> <h3>2.1 Main candidates</h3> There are two "mainstreams" here:<br> <br> <ol> <li> Most cosmologist bet on an <B>yet unknown particle</B> with special properties, like "wimps"<br> Those particles should be massive, unable to interact via the electromagnetic force,<br> and they should be non-baryonic.<br> These hypothetical particles, of which multiple models exist, are collectively called "wimps", or<br> Weakly Interacting Massive Particles.<br> "Non baryonic" matter, more or less means, that it contains no atoms (protons, neutrons) and does not interact<br> with ordinary matter via the electromagnetic force in the usual way. See the note below.<br> <br> Many "candidates" exists. From considerations of supersymmetry, and extensions of the Standard Model, particles called "axions"<br> are suggested. Other considerations from supergravity, suggest the "gravitino" as a candidate.<br> Whenever supersymmetry is broken in supergravity theories, the gravitino acquires a mass which could account for the<br> "missing mass".<br> </li> <br> <li> A few still like to hold on to more mundane solutions, like the MACHO's (Massive Astrophysical Compact Halo Object).<br> MACHO's could be "brown dwarfs", or even "neutron stars". Especially with the example in section 1.2, where "Dark matter"<br> is believed to exist in the Halo of a spiral Galaxy, those cosmologist give (or gave) the MACHO solution a reasonable chance.<br> Brown Dwarfs are more like large versions of a Jupiter like planet. They are stars, but too light to sustain the<br> fusion processes like in the (regular) heavier stars.<br> Note that the "brown dwarfs" is just baryonic matter.<br> The objects are just too dim to be observed.<br> </li> </ol> <br> Recent observational studies however, made the MACHO solution not very likely as the extra mass in the Galactic Halo.<br> <br> But, some scientists argue, that the "white dwarf" might be a <I>very abundant</I> type of star. Ofcourse, they are very "dim" and<br> difficult to detect. Don't forget, the stars that we can see, <I>are the "succesfull" ones</I>. It certainly cannot be ruled out that many of such<br> mini-stars <I>"are out there"</I>. But, as it seems today, most cosmologist favor the WIMP solution for Dark Matter.<br> There is another good reason for not accounting MACHO's for Dark Matter. Dark Matter is probably everywhere, also in<br> inter-Galactic voids, and probably even "here" as well.<br> <br> <hr/> <h4> Notes:<br> <br> - about "baryonic" and "non-baryonic" matter:<br> <br> If you would <B>insist</B> on the "strict" classification in naming elementary particles (as you would do in particle physics),<br> You would have a harder job here.<br> <br> If people talk about "baryonic" matter, they largely mean protons & neutrons (the quark stuff). This constitutes the "regular"<br> building blocks of ordinary matter. It will interact will all know forces, like the electromagnetic force etc..<br> It's also nice that electrons (which are "leptons" and not baryons) like to "hang out" with<br> the protons and neutrons anyway.<br> So, to make a long story short: here, we mean that "baryonic" matter, is the stuff we all know about.<br> That is, atoms (protons+neutrons+electrons), the planets, the stars, the gas, the dust etc..<br> As we know, the baryons does interact with the Electromagnetic (EM) force (it can interact with light through EM)<br> As is observed, Dark Matter does not interact (so it seems) with the EM force.<br> <br> - about "fermions" and "bosons":<br> <br> Sometimes in the DM discussions, you might encounter the terms "fermions" and "bosons".<br> Well known fermions are the quarks (e.g. as the constituents of the proton) and the leptons (like an electron).<br> Technically, the criteria is that the fermions should have a half-integer "spin", as opposite to "bosons",<br> which have integer spin.<br> Practically speaking: the fermions is our normal "matter", while bosons are often considered to be the force carriers.<br> <br> </h4> <h5>Remark: One special type of lepton, the "neutrino", is "all around", and "everywhere". Indeed, there is an awfull lot<br> of them. For a long time, it was thought the they have no mass, but experiments have shown they <B>do</B> have<br> mass (albeit very low). If you would ask what their role is in the equation, I would say that it is a very good question.<br> The family of "sorts of" neutrino's is rather large, exibiting different properties, like color, mass and others.<br> </h5> <hr/> <br> <h3>2.2 What are the WIMPS ?</h3> It's non-baryonic matter, and probably of a stable form. It shoud not interact via the Electromagnetic force since we can't see<br> Dark Matter, and since we can observe remote objects right through it.<br> Ofcourse, it should interact with gravity, and it is suggested that it does too with the weak nuclear force.<br> Actually, there are multiple streams of thought here. The most important ones are:<br> <br> - It could be "hot dark matter (HDM)", like relativistic "neutrino-like" particles, but still with a considerable mass.<br> - Or it could be "cold dark matter (CDM)", like heavier particles, which move much slower than the "hot" counterparts.<br> <br> A nice feature of CDM could be, that it also could exists in more clumpsy, cloud like structures, in the voids<br> between galaxies, and in the Halo's of Galaxies. At least, that would be a nice "picture" to visualize.<br> And don't forget: we do not see the "gravitational lenses" (as was touched in section 1.3) <I>everywhere</I>.<br> Nowadays it seems quite accepted that dark matter tends to clump together.<br> <br> <br> <h3>2.3 Other idea's:</h3> <br> <B>Micro Black Holes:</B><br> <br> On this subject, many speculations circulate. First, they <I>could</I> be "left-overs" from the first stages of the Big Bang,<br> where extremely high densities occurred.<br> One known problem with micro black holes is, the socalled "Hawking" radiation, which means that they should evaporate<br> very fast. But, some collaries of some theories suggest that they could be stable, and thus a few scientist see them as<br> a possible candidate for Dark Matter. But that's certainly not the mainstream view.<br> <br> As an interesting side note, some string theories (using Large extra dimensions), suggest they might be created at energies<br> that an accelerator like the LHC can produce (in the TeV range). So, if a certain particle "shower" is detected<br> with the LHC experiments, it might even turn out that the effect get's attributed to the decay of a micro Black Hole.<br> That <I>indeed</I> would be some discovery !<br> <br> As another interesting side note, it could indeed be true that the more massive micro black holes, "live" longer.<br> In principal, an encounter with the earth, or other object (e.g.: in the Solar system), is possible (or even likely!).<br> What would be the effect? All theories suggest that the Schwarzschild radius will be of subatomic dimensions anyway,<br> so, most physicists predict it goes right through earth without us even noticing that event.<br> Again, a few argue, that if we are <I>very lucky</I>, a "tube" of particle creations might be visible in the detectors.<br> Well, who knows? Never say never!<br> <br> <B>neutrino's:</B><br> <br> Some scientists just held the neutrino's accountable for the "missing mass". But it is difficult to reconsile that<br> with certain observations, like the "gravitational lens". It's hard to see how neutrino's could imply that effect,<br> and then only at certain observations.<br> <br> <B>Cosmic Microwave Background:</B><br> <br> Recent observations showed tiny variations in the temperature "distribution" of the 3K background radiation.<br> The cosmic microwave background could show, as some scientists say, the early effects of dark matter clumping,<br> and these clumps grew under gravitational attraction. This then, have triggered (or "helped") the "normal" matter<br> clump as well, ultimately forming Galaxies.<br> Although the upper remark might be true, it says nothing about the nature of Dark Matter.<br> <br> <br> So, it's quite fair to say that Dark Matter is still elusive to us. We are still not sure to what it is.<br> <br> For now, this concludes this small note. Hope you have liked it!<br> <br> <br> <br> </body> </html>