# An “Elemental” Explanation of Dark Matter

by Jason Major
on June 28, 2013

Atoms, string theory, dark matter, dark energy… there’s an awful lot about the Universe that might make sense on paper (to physicists, anyway) but is extremely difficult to detect and measure, at least with the technology available today. But at the core of science is observation, and what’s been observed of the Universe so far strongly indicates an overwhelming amount of… *stuff*… that cannot be observed. But just because it can’t be seen doesn’t mean it’s not there; on the contrary, it’s what we *can’t* see that actually makes up the majority of the Universe.

If this doesn’t make sense, that’s okay — they’re all pretty complex concepts. So in order to help non-scientists (which, like dark energy, most of the population is comprised of) get a better grasp as to what all this “dark” stuff is about, CERN scientist and spokesperson James Gillies has teamed up with TED-Ed animators to visually explain some of the Universe’s darkest secrets. Check it out above (and see more space science lessons from TED-Ed here.)

');
}
//-->
Because everything’s easier to understand with animation!

*Lesson by James Gillies, animation by TED-Ed.*

A graphic designer in Rhode Island, Jason writes about space exploration on his blog Lights In The Dark, Discovery News, and, of course, here on Universe Today. Ad astra!

*Related*

Comments on this entry are closed.

Dark matter and energy we just cannot see. We know they are both there. It is like the air or the paranormal spirit like. How exactly to they work. I wonder how they individually act and react to and why. Both are eery like. If seen we’d solve so many mysteries and theories. Stephen Hawking and others would have a true monumental field day. The implications would be astounding! – IF ONLY – 😉

Why doesn’t “Universe Today” show the names of the “who” ‘vote-downs”? They do with “vote-ups”. Takes all kinds to make the world go around.

Good one Jason! Expect a MAJOR outpouring of comments! (Wink-wink ~@; )

Always befuddling and counter intuitive, the ideas behind Dark Matter and Dark Energy are so very controversial. Based of course upon an accumulation of scientifically accepted theorem, these ideas must yet be taken as a ‘best reckoning’ to answer profound astronomical observations. What always bothers me, is that if any of the tenets used by these theories to explain this gravitational conundrum should prove to be wrong or even ‘off’ fractionally, then the whole ‘deck of cards’ comes tumbling down… BUT there is hope!

The Large Hadron Collider is currently (Oh no… another pun?) undergoing an upgrade which should allow it to generate even higher energies in the search for WIMPS (Weakly Interacting Massive Particles). The confirmation of the existence of WIMPS will go a long way into confirming the existence of Dark Matter. The ‘discovery’ of dark matter seems almost imminent if the LHC proves successful.

Meanwhile… I’ll continue to play ‘solitaire’ with my deck of MOND cardszuh….

What “tenets” are you talking about?

As I mention and link to above, DM & DE are not controversial today and as simple an observation as big bang due to the CMB. There are no “tenets” used in the observational constraints except known physics of hydrogen gas.

If you by the “ideas behind” mean theories that can predict DM & DE, they are still open areas. But it isn’t much of a controversy to have DE as a measure of vacuum energy or DM as the heavy components of remaining particle sectors.

Without bias I always think of epicycles and how many centuries their model was uncontroversial and based on the unassailable assumption that planets must move in circles.Science improves but the nature of human thought remains the same.

Good movie, good animations and very easily followed.

I thought Democritus was the originator of atomic theory. The narrator says it was somebody else (but I could not catch the name).

Regards,

TB

DA, DM, WA(White Atom-‘Atom’), WM(White Matter-‘Baryon’)-shows SUPPER-SYMMETRY[obviously-not by symmetrical style of writing]. DE is the key factor in explaining the Basics of Science. Our present concept is based on 4-5% knowledge. 76-75% play a very important role in explaining the science.

Is the number of element in Universe are only that- we know? Is F depends only on GMm/R2? Is our solar system formed exactly as we are explaining? Is only gases are displaced when water enter the so called empty bottle? ……..

Our “BASICS” are other-ways round. Are we ready to understand the universe from another angle? If yes, ‘The theory of UNIFIED …..’ can change the Science. The 5th Force is the FINAL FORCE that govern the universe.

Neither DA, DM & DE are not too away to understand nor their interaction with White Matter, atom & energy are too complicated to Understand.

Please read:

http://swarajgroups.blogspot.in/2013/06/out-of-box-thinking-is-essential-in.html

— Explain About Dooms Day-21.12.12 Situation, —

Doomsday 2012???? Facepalm.

There are other possibilities than just a just what is accepted by the science communities. This cosmos is unfathomable to the human mind. We are in the infancy of space exploration and knowledge of it. Period.

Wait… What? Had a really hard time reading / understanding that

Could you please explain the evidence for a “5th force” and explain in what way it is “final”?

Sigh! You can’t make observations out to be “4-5 % knowledge” as they are 100 % knowledge (albeit with some uncertainty – in this case very low, see my response to A4u).

Dear all pl. read the link DATED 4TH,JULY’13

http://www.dailygalaxy.com/my_weblog/2013/07/is-an-unknown-force-of-the-universe-acting-on-dark-matter.html

I am not writing much because it is not the right platform. [pl. don’t take anything new as garbage-this is called ‘Mental Blockage’]

Leucippus was the mentor to Democritus, and the origin of atomic theory comes from these two. Since Leucippus was the elder or teacher some credit him with the idea more than Democritus.

Dark energy is in a way easy to understand. One can understand the role of dark energy with Newton’s laws. Newton’s law of gravity is that a force F between two masses M and m is proportional to a constant G times Mm divided by the square of the distance between them.

F = -GMm/r^2.

Here the minus sign indicates the force is attractive. Newton’s second law tells us that F = ma, for a the acceleration of the smaller test mass m. So we have a = -GM/r^2. A rule is that potential energy ? gives a force F = -??, or that this gradient is just a derivative with respect to r, so F = -d?/dr and the potential energy is ? = ?Fdr. For those who remember their calculus course this means the potential energy per mass is

? = -GM/r.

Now for a bit of cosmology. Let d be the distance between two galaxies. This distance changes with time, so there is a scale factor a that adjusts r with time. So if we let d_0 be a fiducial or initial distance then r = ad_0. The relative velocity of these galaxies is v = dr/dt = (da/dt)d_0. I am going to set d_0 to one, which is just to indicate it is just a define ruler unit. Kinetic energy of a moving particle is K = (1/2)mv^2 and the total energy per unit mass (m) is then E = K/m + ?, which is then

E = (1/2)(da/dt)^2 – GM/a.

I will write da/dt = a’ from now on. The mass M is all the masses of galaxies or fields etc in a region with a volume V = (4?/3)r^3 which induces the acceleration of the test mass m, m is of course factored out of our total energy so E is really total energy per mass. The mass M is then a mass-energy (remember mass and energy are equivalent by E = mc^2) density ? so M = ?V. Now we put this into our total energy equation to get

E = (1/2)a’^2 – (4?/3)G?a^2

Now divide through by a^2 and we can set the total energy = 0 since physics really is concerned with differences in energy and not an absolute energy and we get the little equation

(a’/a)^2 = 8?G?/3.

The Hubble constant, which is really a constant in space but not time, is H = (a’/a)^2. Now let me assume that the density of mass-energy in this cosmology is constant. It is a fluid that permeates space, which is further interpreted as the vacuum energy due to quantum mechanics. This gives the differential equation

a’ = sqrt{8?G?/3}a.

which for those who recall their course in elementary or ordinary differential equations this has the solution

a(t) = a_0 exp(sqrt{8?G?/3}t).

This predicts that the test masses m then exponentially separate away in an accelerated motion. This is the sort of cosmology we by observation appear to exist within. I can later show how one can work similar arguments for the density ? pertaining to radiation and matter and what this looks like if you put these together with the mass density for this constant fluid or vacuum energy in the universe.

What makes this fluid, or vacuum energy etc dark energy is that it can’t be readily accounted for with quantum field theory calculations. A straight forwards calculation gives a far larger vacuum energy density and by consequence a much larger cosmological constant ? = 8?G?. However, it does appear that the total information in the universe that can be accessed by an observer is given not by a volume but a bounding area. If that area is given by the cosmological horizon R_h = sqrt{3/?}, around 10^{10} light years out the answer is more natural. This gets into frontier research with the holographic principle and quantum gravity etc, I will leave things off at this point.

I will later give a similar argument using Newton’s laws for why dark matter should exist.

LC

LC maybe you can explain what “holographic” means, in this explanation?

I think that a lot of readers associate “holographic” with some kind of thing that has some magical properties and only occurring in SF movies..

However this clip brought me back to reality that holographic is something quit simple. http://www.youtube.com/watch?v=XUy8lELWhJg

There exist another clip of that guy that shows how he creates these holograms but I can’t find it. But it is basically creating curved scratches and it is the point where the sunlight reflects 2 crossing scratches in the right direction of your eye that creates the image. bending the metal sheet means that a different cross section will reflect light towards your eyes.

Holography comes about from a rather elementary observation about relativity. If you Lorentz boost a system to a velocity very close to the speed of light two things appear to happen. The length of the system along the direction of motion contracts to near zero. The other thing that happens is the time rate one observes of that system slows down incredibly. The result is that if there are motions within that system that are much slower than the velocity of the boost direction u << v appears classical due to time dilation and all dynamics along the boost direction or the longitudinal modes vanish.

If one looked at a system approaching a black hole it approaches the speed of light as it also approaches the event horizon. Further due to the time dilation effect you never see the system reach the horizon. This is much the same as the above physics with special relativity. The system approaching the black hole becomes then frozen above the black hole, as does everything which falls into the black hole. This means the black hole consists of all these physical systems squashed down by one dimension and which form oscillators on this surface above the horizon, called a stretched horizon, and which determine everything about these systems in one dimension larger outside the black hole.

This is the origin of the term holography, for field oscillators or strings on this stretched horizon determine the thermodynamic properties of the black hole and the field properties outside. This is an amazing result, for the entropy of a box of particles is by extension not determined by all the particle data in the volume, but on the box walls or boundary. A hologram reconstructs a 3-d image from a 2-d plate, and this is the origin of the term holographic principle.

A more general approach to the holographic principle involves a correspondence between an anti-de Sitter (AdS) spacetime and a conformal field theory of one dimension lower on the boundary (conformal boundary) of the AdS. I will only say that an AdS spacetime is a strange hyperbolic space, which in two dimensions is the Poincare half plane or disk. The drawing “circle limit” by Escher is a good depiction. This boundary is a conformally flat spacetime, which requires some idea of conformal transformations are, but the de Sitter spacetime of the universe in four dimensions is a time dependent conformal spacetime sitting on the boundary of an AdS in 5 dimenions. This AdS_5 is a system of symmetries on a D-brane.

Yeah, it runs deep. LC

Space time is so unfathomable to the human brain. Our brain can only conform to this space-time realm or dimension. The possibilities are endless. The truth is there.

Deep? That’s an understatement.

like Walter explained I’m stunned that a stay at home mom able to earn $5080 in a few weeks on the computer. did you see this web link www.KEP2.com

You mean, Walter Cronkite from from the 1970-BC? …lol. Well, good for you sweetheart. Hope you make millions! ;-). Sounds like a sales pitch to me. I guess I will start selling land on a planet very near Alpha CMo-(Sirius). 5$ an acre? bye bye 😉

ps- Its all good Isabel ;-). I respect your intentions. I was just having a lil’fun with you. I have seen other sites like the one your working with and for.Personally, I would stay way from job(s) you must pay to acquire. 😉

“Holography” means whole image reconstruction [ http://en.wikipedia.org/wiki/Holography ], which is analogous to what the idea of a “holographic principle” means. That holography in practice often involves reconstructing 3D images out of 2D image templates may or may not be what the physicists wanted to refer to.

One embodiment of the holographic principle is differential equations, where the know local law (the diff eq) and boundary constraints can reconstruct a solution within a bounded area (the integrated diff eq).

Molecular Hydrogen is dark and a survey by Heschel suggests the amount in the Galaxy is hugely underestimated so Dark Matter is probably just that.

No. It is basic cosmology, and a simple observation, that DM is not baryon matter such as in atoms. See the link in my first comment how we know this, and take the 1 s needed to see it for yourself out of the CMB spectra.

Thanks for replying to this; I had wanted to do so myself, but was unsure I could be comprehensive enough. One question, though: don’t we have additional evidence for the non-baryonic nature of DM from observations that set a very low upper limit to its electromagnetic cross section?

I promise you I will, and continue to do so, but it will take more than 1s

As I indicated I would discuss dark matter from an elementary physics perspective. This is based on just Newton’s law and Gauss’ integral laws.

Newton’s law of gravity is that a force F between two masses M and m is proportional to a constant G times Mm divided by the square of the distance between them.

F = -GMm/r^2.

Here the minus sign indicates the force is attractive. Newton’s second law tells us that F = ma, for a the acceleration of the smaller test mass m. So we have a = -GM/r^2. A rule is that potential energy ? gives a force F = -??, or that this gradient is just a derivative with respect to r, so F = -d?/dr and the potential energy is ? = ?Fdr. For those who remember their calculus course this means the potential energy per mass is

? = -GM/r. enclosed by that surface. ?F*da = ??*Fdv The mass M is also evaluated as the volume integration of the mass density M = ??dv. Now we can equate these and peel off the integral and we are just left with

?*F = -4?G?

and if I use F = -?? I get Poisson’s equation

?^2? = 4?G?.

This is straight early to mid 19th century stuff. It is nothing new.

This equation tells us something about the gravity force in the presence of matter. This is where dark matter enters into the picture. The divergence is taken to be radial dependent derivative ?*F = dF/dr. If we write the density for a uniform distribution as ? = M/(4?/3)r^3, where I assume the mass M is fixed. This is then expressed in the equation,

Fm = (GM/3)?dr/r^3 = -GM/r^2,

where I have recovered Newton’s gravity force law or the acceleration a = Fm. Now let me assume that the mass depends on the volume M ~ r^3, it increases with the volume, or equivalently there is a constant density ? in the space or volume of interest. In this case

a = -4?G??dr = -4?G?r.

What this means that if we are looking at the motion of a particle in a distribution of ordinary matter that obeys Newton’s laws that the force becomes greater with the distance of a mass from the center of this region. This is the motion of a harmonic oscillator, which is significantly different from the motion in a standard Newtonian gravity field.

Newton’s second law F = ma for circular motion recovers Kepler’s laws of planetary motion with a frequency squared proportional to the reciprocal of the cube of the distance. Using the centripetal acceleration a = ?^2r, ? = angular frequency, and Newton’s law of gravity I leave this as an exercise. For the harmonic oscillator case this differs for the acceleration a = d^2r/dt^2 gives an elementary differential equation with frequency ? = sqrt{4?G?}. The solutions are sines or cosines that are oscillatory. The period is the same for all radial distances. The velocity of these orbits v = ?r increases with radius as well.

What is found in galaxies is a sort of mixture between the straight up Newton gravity result and Kepler’s laws. The spring force law would say the velocity of stars in a galaxy would increase with radius. The Kepler law says they should decrease. The actual profile has stars moving with near constant velocity in the spiral arms. There is enough ordinary matter at the center to contribute higher velocity near the center and this competes with the spring force law.

LC

Dark matter is a lame attempt by cosmologists to explain how they don’t know everything.

Your comment is a lame attempt to show us you know something.

But the attempt fails, see my 1st comment and how DM is an observation out of the cosmic microwave background, And a very simple observation, such that you can do it yourself.

There are so many New-Age spiritualist arguments, claiming that the arrogant yet feeble astrophysicists are not capable of comprehending the true nature of the universe, and it is a waste of time trying to explain it to them. Please take this as a follow-up comment to any and all…

Okay, we don’t know what ‘dark matter is’ but it was known since the 1930’s that something was needed to balance galaxies, and we have recently been able to measure the different densities of dark matter in different parts of the universe. This is important, because if dark matter was something that existed everywhere with equal density, then it might be some cosmological constant, rather than a distribution of actual stuff.

Dark energy we know even less about. However, if we need dark matter to balance the galaxies, then we need dark energy on an even greater scale to explain the accelerating expansion. If ‘dark energy’ seems uniformly spread about the universe, then that might be another cosmological constant of space rather than an actual new sort of stuff.

You are lucky: you are seeing science as it is happening. We name stuff we don’t understand yet, and we look for its properties. We set limits on what it is and what it does, and then we have to postulate other stuff for the new bits we cannot explain. Newton had to coin the word ‘gravity’, and the idea that mass measured by weight, and mass measured by resistance to acceleration might be different things. And new things are being found, and new theories try to cover them, and much of this will be wrong, but some of it will be right for a while at least, and when we don’t know, we say we don’t know. I think that is so much more exciting than that everything was written in the Da Vinci code, or predicted by Nostradamus, and depends on a fifth element that was known to the citizens of Atlantis, and hidden by the ancient Greeks, the Vatican, and the CIA .

Not convinced? Well, science works, whether you believe in it or not. You aren’t hurting us. But you are missing all the fun.

The video messed up a little in not mentioning that the cosmic microwave background (CMB) gives the best evidence for _all_of the dark universe, both dark energy and dark matter. In fact, it makes it as simple an observation as it makes big bang an observation.

Here is an article that describes how simple it is to make those observations out of the CMB: http://galileospendulum.org/2012/02/17/the-genome-of-the-universe/ .

You’re not the only one……..sigh!