The Dark Side of the Universe:

A journey to the remote corners of the Universe and

back home in our Galaxy to finally land on the Earth,

Looking for tiny signals of existence of the mysterious

DARK MATTER

Flavio CavannaPhysics Dep.t University of L’Aquila

Dark Matter.. Just to start with…- I am reading about dark matter these days. Highly interesting ...- First question ... what is dark matter. Simple ... its matter thatʼs dark.Oops ... that sounded like a typical consultant ..., Well ... to make it simple ... Dark matter is

stuff which is out there in the universe which we canʼt see.- For some reason ... Donʼt ask me why. The brains (read scientists) havenʼt figured that out

yet, or at least thatʼs what i think.- Only thing i have been able to figure out yet is how they figured out its existence if they

canʼt see it. Well ... its simple.- We all know galaxies rotate around a centre. Now, the rotation speed of galaxies can be

calculated using the Doppler effect. So, if the galaxy were like a disk, then one end ofthe rotating disc would be coming at you (blue shifted), and the other end would be goingaway (red shifted). Based on the extent of the spectral shift, they could calculate therotational speed of the galaxy. Knowing this, the mass of the galaxy can be calculatedusing Newtonian mechanics.

- Now, the speed of rotation of galaxies is such that the stars along with their solar systemsshould probably get hurled out of the galaxy, given the calculated gravitational pull of themass calculated for the galaxy by the Mass-Luminosity equation.

- In other words, the calculated mass of the galaxy is not enough to keep these stars in placein the galaxy, and overcome the centrifugal force they would feel.

- Now, since the stars are actually in place in the galaxy, the only thing that can explain thisis that there is mass in the galaxy which we cant see. Ergo, Dark Matter ...

- According to what scientists say ... Mother Nature has hidden away 90% of the universefrom us.

- Question is ... why? I donʼt know! Maybe someday soon, we will find out.

…just like.. The Copernican Revolution• In 1543 Nicolas Copernicus published

his treatise De Revolutionibus OrbiumCoelestium (The Revolution of CelestialSpheres) where a new view of theworld is presented: the heliocentricmodel.

• It is hard to underestimate theimportance of this work: it challengedthe age long views of the way theuniverse worked and thepreponderance of the Earth and, byextension, of human beings.

• The realization that we, our planet, andindeed our solar system (and even ourgalaxy) are quite common in theheavens and reproduced by myriads ofplanetary systems provided a sobering(though unsettling) view of the universe.

• A new view of the world came intobeing.

Phenomena• A phenomena, as defined in

class, means “to show, beseen, or appear”. But thatfact alone isn’t what makesphenomena so interesting.

• Many things just appear inour lifetime, everyday in fact,and we are just accustomedto the how and why of thatparticular thing that we don’teven pay attention. Butwhen something appearsand we have no idea howthe thing works, that is whena phenomenon becomes,well, phenomenal.

• We could have everyunderstanding of whatmakes up something andhow it all flows together, andyet we still can’t explainthese phenomena that arejust there and seem asthough they shouldn’t be.

• The scientific communityseems to have aphenomenon which defyeverything that scientistshave established to behappening or what should behappening.

• This phenomenon is related to Dark Matter and our universe, a concept that wasestablished in the 1970’s.

• The concept of gravity is that anything with mass-produces a force on any thing around it, anattraction if you will. And the larger an object, the larger the gravitational forces are. Nowapply these basic principles to the universe, and everything makes sense. Asteroids aredrawn towards the gravity of moons, and moons are held in orbit by the gravity of planets,and all the planets in a solar system revolve around a much larger body, like a star.

• But when you apply these principles the entire universe, the whole thing should betheoretically be falling apart. Each galaxy revolves around its central point, and they shouldall be floating away from one another. And yet there is a central point that was found by VeraRubin in the late 1970s, a point that seems to keep all of the galaxy’s tied together somehow. We just don’t know why this is happening. And that is where the theory of DarkMatter has been established. It is a phenomenon that makes up 90 percent of theuniverse, and it has no concrete explanation.

• But scientists have long since figured out the principles that govern most everything else inthe universe. The universe is made up of things, all with quantified measurements. Lets lookat our Solar system, which is made up of planets, moons, the sun, asteroids, gases, and theoccasional meteor and comets.. Our solar system is a framework. Each solar system is aframework of planets and moons that are centered around one or more stars. And theuniverse itself is a framework, being made up of the smaller frameworks that are galaxies.There are also clockworks involved. Our planet has a clockwork with the moon, and themoon has a set pattern of orbit around the Earth. And the solar system’s clockwork is madeup of the forces of gravity, which keep all of the planets in a set rotation around the sun,which then creates our planet’s clockwork of the seasons in a year.

Dark Matter: gravitational lensing

• When the same laws are applied to galaxies .. It doesn.t work… If the laws arecorrect, as it is for the solar system(s) we are lead to the conclusion that about90% of the Universe is dark. We can't see it except through its gravitational pull.Although this was suspected more than 60 years ago, we are just now in aposition to explore the dark matter in large areas of the Universe through atechnique called gravitational lensing.

• As the light from a distant source passes by a mass concentration its ray path isbent, causing the distant source to appear at an altered place on the sky andresulting in a tell-tale distortion of its shape. This gravitational lensing effectprovides the first, and currently only, way to directly "weigh" cosmic mass.

• Strong gravitational lensing happens when there is so much masscontrast in the lens that the light rays from a distant source bendaround both sides of the lens and cross near Earth. Then multipleimages of the source may be seen. This was first seen in a quasar (avery heavy astronomical object) lensed by a galaxy in 1979.

• The huge dark matter concentrations in clusters of galaxies createtypical bending angles of 30 arcseconds, and multiple highly distortedimages of a source galaxy.

evolution of the distortion as the clusters move against thebackground over half billion years

Dark Matter: gravitational lensing

• So it seems that we can explain (ALSO) how the universeworks, according to the applied principles of gravity thatgovern each solar system.

• But that isn’t what governs the universe; it is governed bysome unexplainable phenomenon, which we are justcalling Dark Matter.

• So we can explain, measure and identify all of the things,frameworks, and clockworks that make up our universe(according to the theory of gravity), ….yet the 90 percent of the mass around us is unexplainable,which is one magnificent phenomenon !!!

The largest structures in the Universe• Let’s start (again) from our beloved

solar system:• We all know that the solar system

is organized into planets (includingthe Earth) orbiting around a star:the Sun.

• We also know that stars collectthemselves into galaxies, on ascale much larger than the solarsystem (about 100 million timeslarger!). Our Sun is an average starin an average galaxy called theMilky Way. The Milky Waycontains about 100 billion stars.Yes, that's 100,000,000,000 stars!

• We just learned that on still largerscales, individual galaxies areconcentrated into groups, or whatastronomers call clusters ofgalaxies.

Dark Matter: gas temperature

• These very large astronomical objects called clusters thus includes the galaxiesand also any material which is in the space between the galaxies.

• The force, or glue, that holds the cluster together is gravity -- the mutualattraction of everything in the Universe for everything else. The space betweengalaxies in clusters is filled with a hot gas. In fact, the gas is so hot (tens ofmillions of degrees!) that it shines (in X-rays instead of visible light). In the imageabove, the hot X-ray gas (shown in pink) lying between the galaxies issuperimposed on an an optical picture of the cluster of galaxies.

• By studying the distribution and temperature of the hot gas we can measure howmuch it is being squeezed by the force of gravity from all the material in thecluster. This allows scientists to determine how much total material (matter)there is in that part of space.

• Remarkably, it turns out there is five times more material in clusters ofgalaxies than we would expect from the galaxies and hot gas we can see.Most of the stuff in clusters of galaxies is invisible and, since these are thelargest structures in the Universe held together by gravity, scientists thenconclude that most of the matter in the entire Universe is invisible.

• This invisible stuff is (again) identified as 'dark matter'.• There is currently much ongoing research by scientists attempting to discover

exactly what this dark matter is, how much there is, and what effect it may haveon the future of the Universe as a whole.

Crash on a Dark Matter wall…(?)

• DM is an invisible substance composed ofparticles that are far different from those thatmake up the universe’s normal matter, suchas stars and galaxies.

• In fact, if you drove into a wall made of darkmatter, you wouldn’t crack a headlight orinflate an airbag. You wouldn’t even know ithappened. But what happens to dark matterduring a collision?

Crash on a Dark Matter wall…(?)• Astronomers using NASA’s Hubble Space Telescope got a first-hand view

of how dark matter behaves during a titanic collision between twogalaxy clusters.

• The wreck created a “ripple of dark matter”, which is somewhat similar to aripple formed in a pond when a rock hits the water.

• The picture of the “big crash” shows a ring all around it..

• The ring's discovery is among the strongest evidence yet that darkmatter exists !!

This NASA Hubble SpaceTelescope composite imageshows a ghostly "ring" ofdark matter in the galaxycluster Cl 0024+17.

The ring-like structure isevident in the blue map ofthe cluster's dark matterdistribution.The map is superimposed ona Hubble image of thecluster.

The ring is one of theThe ring is one of thestrongest pieces ofstrongest pieces ofevidence to date for theevidence to date for theexistence of Darkexistence of DarkMatterMatter,,(the unknown substance thatpervades the universe).

A spectacular observation

Although astronomers cannotsee dark matter, indeed they can

further infer its existence ingalaxy clusters by observing howits gravity bends the light of moredistant background galaxies (the

powerful effect calledgravitational lensing,

discussed before..).The blue streaks near the centerof another Hubble image of thesame cluster are the distorted

shapes of more distant galaxies,whose light was bent and

magnified by the powerful gravityof Cl 0024+17.

the shapes of the background galaxies,it's like looking at the pebbles on thebottom of a pond with ripples on thesurface. The pebbles' shapes appearto change as the ripples pass overthem. So, too, the background galaxiesbehind the ring show coherent changesin their shapes due to the presence ofthe dense ring.

Light on mysterious 'dark matter'• We’ve all been taught that our bodies, the Earth, the stars in the sky and in fact all

matter in the universe is composed of tiny building blocks called atoms, and inside theatoms electrons and Nuclei made by even smaller blocks: neutrons and protons.Now imagine if this weren’t the case. This mind-bending concept is at the core of thescientific research.

• The recent scientific breakthroughs (in part just shown before) have shown that most ofthe matter in the universe -- about four-fifths -- is not made up these “standardparticles” , but of something else, called dark matter. The evidence for dark matteris now overwhelming, and the mandatory amount of dark matter is becoming preciselyknown.

•Dark Matter is believed to exist in theform of tiny particles that do not interactwith light (the size of these being aboutthe one of a heavy atomic Nuclei, e.g. Pb).•Because they don’t emit or reflectelectromagnetic radiation the way atomic,(or “baryonic”) matter does, these darkmatter particles haven’t been directlyobserved.•At present, through a series of indirectobservations, we can tell that the universeis full of some sort of stuff that we justcant see.........

WMAP Reveals Neutrinos, End of DarkAges, First Second of Universe

• WMAP measures a remnant of the earlyuniverse - its oldest light. The conditions of

the early times are imprinted on this light. It isthe result of what happened earlier, and abacklight for the later development of the

universe.• This light lost energy as the universeexpanded over 13.7 billion years, so WMAP

now sees the light as microwaves. By makingaccurate measurements of microwavepatterns, WMAP has answered many

longstanding questions about the universe'sage, composition and development.

• The universe is awash in a sea of cosmicneutrinos. These almost weightless sub-atomic particles zip around at nearly the

speed of light. Millions of cosmic neutrinospass through you every second. "A block of

lead the size of our entire solar systemwouldn’t even come close to stopping a

cosmic neutrino”.

Dark Matter Halo Around theMilky Way

..back home… from the far corners of the Universe

We believe our Galaxy, the Milky Way,sitting inside a huge cloud of DarkMatter.

This cloud forms a vast halo all around.

The DM halo outweighs the galaxy'snormal matter by a factor of 20 !!

• The Milky Way's bright ordinary matter is composed of a spiraldisk and outer ring, which in turn is embedded in a largerluminous halo that is only the visible part of an even larger haloof dark matter (more).The size of the Dark Matter halo..

In fact, the motion of objects in the Milky Way is not consistent with theamount of luminous matter, which is not enough to confine these objects

inside the Milky Way boundary. The problem (as for other far astronomicalobjects in the universe) can be reconciled if a lot of dark matter still

remains in the halo, while the cooling of the hydrogen allows ordinary matter tocontract, and settled into the visible matter disk.

• This cartoon suggests the immense halo of mysterious dark matter thatastrophysicists believe surrounds our galaxy. The visible stars and gases of theMilky Way contribute about five percent of its total mass.

• The invisible halo of Dark Matter at itscenter is much larger and more sphericalthan the luminous galaxy.

• Recent computer simulations have shownthat the halo is surprisingly clumpy, withrelatively dense concentrations of darkmatter in gravitationally bound 'subhalos'within the halo.

… more about the Dark Matter Halo ..

Dark matter halo warpsMilky Way's fringe

• Astronomers surveying hydrogen gas discovered in 1957 that the MilkyWay is not flat but warped near its edges like a fedora hat, with oneside of its disc curving as much as 20,000 light years above the maingalactic plane and the other dipping a little less below it.

• Some researchers suspected the warp was caused by two satellitegalaxies, called the Magellanic Clouds, that orbit the Milky Way every1.5 billion years. But calculations showed they alone were not massiveenough to produce the effect.

• Now, researchers have shown the Magellanic Clouds can account forthe warp - but only because their motion creates a powerful wake in themassive "halo" of dark matter that is thought to cocoon the Milky Way.

• This allows to calculate that the halo outweighs the galaxy'snormal matter by a factor of 20 - massive enough to warp thedisc.

.. and now (finally) we land on the Earth:the role of particle physicsin the Dark Matter puzzle

All the hints of existence of DM are “indirect proofs”,based on astronomical observations.

The direct proof of existence may only come fromexperiments in our laboratories where signals of interactionsof DM particles with ordinary matter (the “active part” of our Detectors) might be recorded in a irrefutable way.

This kind of experiments represents the ultimate frontier ofthe present research programmes in (astro)particle Physics.

…. Obviously, this is far to be an easy game…

The DAMA Project at LNGS• Claim for detected signals from Dark Matter particles in the galactic halo

as been proposed by the DAMA experiment at the Gran Sasso Laboratory

•As a consequence of itsannual revolution aroundthe Sun, the Earth shouldbe crossed by a largerflux of Dark Matterparticles in summer(when its rotationalvelocity is summed to theone of the solar systemwith respect to theGalaxy) and by a smallerone in winter (when thetwo velocities aresubtracted).

DAMA/NaI results on particle Dark Matterinvestigation

Annual modulation of recorded signal with the DAMA experiment

at LNGS

WIMP’s are expected to weakly interact with nuclei of ordinary matter

No signal was found in the region where it was expected.

The experiment goes on and look forward a handful of“events” to finally confirm the existence of Dark Matter

Could dark matter be gravityfrom other dimensions?

• The Question:Since string theory implies up to 11 dimensions, could dark matter be gravitons leaking fromother dimensions into ours?

• The AnswerActually, that's exactly one possibility that's being explored by brane-world theorists. Ofcourse, the jury is still out on brane-world theories....

"In brane-world theory, the ends of strings are anchored in our brane, sothe particles we see can only move within the brane. But the particlesthat carry the gravitational force, known as gravitons, are closed strings-- little Cheerios -- and can "leak" out of the brane. This explains whygravity is much weaker than the electromagnetic force and the strongand weak nuclear forces. It also offers a possible explanation for the"dark matter" that astronomers need to explain why the mass of theuniverse doesn't agree with the observed objects. Dark matter could bein an adjacent brane, with its gravitons leaking into ours."