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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 Cavanna
Physics 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 of
the rotating disc would be coming at you (blue shifted), and the other end would be going
away (red shifted). Based on the extent of the spectral shift, they could calculate the
rotational speed of the galaxy. Knowing this, the mass of the galaxy can be calculated
using Newtonian mechanics.
- Now, the speed of rotation of galaxies is such that the stars along with their solar systems
should probably get hurled out of the galaxy, given the calculated gravitational pull of the
mass 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 place
in 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 this
is 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 universe
from us.
- Question is ... why? I donʼt know! Maybe someday soon, we will find out.
…just like.. The Copernican Revolution
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In 1543 Nicolas Copernicus published
his treatise De Revolutionibus Orbium
Coelestium (The Revolution of Celestial
Spheres) where a new view of the
world is presented: the heliocentric
model.
It is hard to underestimate the
importance of this work: it challenged
the age long views of the way the
universe worked and the
preponderance of the Earth and, by
extension, of human beings.
The realization that we, our planet, and
indeed our solar system (and even our
galaxy) are quite common in the
heavens and reproduced by myriads of
planetary systems provided a sobering
(though unsettling) view of the universe.
A new view of the world came into
being.
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Phenomena
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A phenomena, as defined in
class, means “to show, be
seen, or appear”. But that
fact alone isn’t what makes
phenomena so interesting.
Many things just appear in
our lifetime, everyday in fact,
and we are just accustomed
to the how and why of that
particular thing that we don’t
even pay attention. But
when something appears
and we have no idea how
the thing works, that is when
a phenomenon becomes,
well, phenomenal.
We could have every
understanding of what
makes up something and
how it all flows together, and
yet we still can’t explain
these phenomena that are
just there and seem as
though they shouldn’t be.
The scientific community
seems to have a
phenomenon which defy
everything that scientists
have established to be
happening or what should be
happening.
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This phenomenon is related to Dark Matter and our universe, a concept that was
established in the 1970’s.
The concept of gravity is that anything with mass-produces a force on any thing around it, an
attraction if you will. And the larger an object, the larger the gravitational forces are. Now
apply these basic principles to the universe, and everything makes sense. Asteroids are
drawn 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 be
theoretically be falling apart. Each galaxy revolves around its central point, and they should
all be floating away from one another. And yet there is a central point that was found by Vera
Rubin in the late 1970s, a point that seems to keep all of the galaxy’s tied together some
how. We just don’t know why this is happening. And that is where the theory of Dark
Matter has been established. It is a phenomenon that makes up 90 percent of the
universe, and it has no concrete explanation.
But scientists have long since figured out the principles that govern most everything else in
the universe. The universe is made up of things, all with quantified measurements. Lets look
at our Solar system, which is made up of planets, moons, the sun, asteroids, gases, and the
occasional meteor and comets.. Our solar system is a framework. Each solar system is a
framework of planets and moons that are centered around one or more stars. And the
universe 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 the
moon has a set pattern of orbit around the Earth. And the solar system’s clockwork is made
up 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
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When the same laws are applied to galaxies .. It doesn.t work… If the laws are
correct, as it is for the solar system(s) we are lead to the conclusion that about
90% 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 a
position to explore the dark matter in large areas of the Universe through a
technique called gravitational lensing.
As the light from a distant source passes by a mass concentration its ray path is
bent, causing the distant source to appear at an altered place on the sky and
resulting in a tell-tale distortion of its shape. This gravitational lensing effect
provides the first, and currently only, way to directly "weigh" cosmic mass.
Strong gravitational lensing happens when there is so much mass
contrast in the lens that the light rays from a distant source bend
around both sides of the lens and cross near Earth. Then multiple
images of the source may be seen. This was first seen in a quasar (a
very heavy astronomical object) lensed by a galaxy in 1979.
The huge dark matter concentrations in clusters of galaxies create
typical bending angles of 30 arcseconds, and multiple highly distorted
images of a source galaxy.
evolution of the distortion as the clusters move against the
background over half billion years
Dark Matter: gravitational lensing
• So it seems that we can explain (ALSO) how the universe
works, according to the applied principles of gravity that
govern each solar system.
• But that isn’t what governs the universe; it is governed by
some unexplainable phenomenon, which we are just
calling 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
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Let’s start (again) from our beloved
solar system:
We all know that the solar system
is organized into planets (including
the Earth) orbiting around a star:
the Sun.
We also know that stars collect
themselves into galaxies, on a
scale much larger than the solar
system (about 100 million times
larger!). Our Sun is an average star
in an average galaxy called the
Milky Way. The Milky Way
contains about 100 billion stars.
Yes, that's 100,000,000,000 stars!
We just learned that on still larger
scales, individual galaxies are
concentrated into groups, or what
astronomers call clusters of
galaxies.
Dark Matter: gas temperature
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These very large astronomical objects called clusters thus includes the galaxies
and also any material which is in the space between the galaxies.
The force, or glue, that holds the cluster together is gravity -- the mutual
attraction of everything in the Universe for everything else. The space between
galaxies in clusters is filled with a hot gas. In fact, the gas is so hot (tens of
millions of degrees!) that it shines (in X-rays instead of visible light). In the image
above, the hot X-ray gas (shown in pink) lying between the galaxies is
superimposed on an an optical picture of the cluster of galaxies.
By studying the distribution and temperature of the hot gas we can measure how
much it is being squeezed by the force of gravity from all the material in the
cluster. 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 of
galaxies 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 the
largest structures in the Universe held together by gravity, scientists then
conclude 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 have
on the future of the Universe as a whole.
Crash on a Dark Matter wall…(?)
• DM is an invisible substance composed of
particles that are far different from those that
make up the universe’s normal matter, such
as stars and galaxies.
• In fact, if you drove into a wall made of dark
matter, you wouldn’t crack a headlight or
inflate an airbag. You wouldn’t even know it
happened. But what happens to dark matter
during a collision?
Crash on a Dark Matter wall…(?)
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Astronomers using NASA’s Hubble Space Telescope got a first-hand view
of how dark matter behaves during a titanic
galaxy clusters.
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collision between two
The wreck created a “ripple of dark matter”, which is somewhat similar to a
ripple formed in a pond when a rock hits the water.
• The picture of the “big crash” shows a ring all around it..
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The ring's discovery is among the strongest evidence yet that dark
matter exists !!
A spectacular
observation
This NASA Hubble Space
Telescope composite image
shows a ghostly "ring" of
dark matter in the galaxy
cluster Cl 0024+17.
The ring-like structure is
evident in the blue map of
the cluster's dark matter
distribution.
The map is superimposed on
a Hubble image of the
cluster.
The ring is one of the
strongest pieces of
evidence to date for the
existence of Dark
Matter,
(the unknown substance that
pervades the universe).
Although astronomers cannot
see dark matter, indeed they can
further infer its existence in
galaxy clusters by observing how
its gravity bends the light of more
distant background galaxies (the
powerful effect called
gravitational lensing,
discussed before..).
The blue streaks near the center
of another Hubble image of the
same cluster are the distorted
shapes of more distant galaxies,
whose light was bent and
magnified by the powerful gravity
of Cl 0024+17.
the shapes of the background galaxies,
it's like looking at the pebbles on the
bottom of a pond with ripples on the
surface. The pebbles' shapes appear
to change as the ripples pass over
them. So, too, the background galaxies
behind the ring show coherent changes
in their shapes due to the presence of
the dense ring.
Light on mysterious 'dark matter'
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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 the
atoms 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 the
scientific research.
The recent scientific breakthroughs (in part just shown before) have shown that most of
the matter in the universe -- about four-fifths -- is not made up these “standard
particles” , but of something else, called dark matter. The evidence for dark matter
is now overwhelming, and the mandatory amount of dark matter is becoming precisely
known.
•Dark Matter is believed to exist in the
form of tiny particles that do not interact
with light (the size of these being about
the one of a heavy atomic Nuclei, e.g. Pb).
•Because they don’t emit or reflect
electromagnetic radiation the way atomic,
(or “baryonic”) matter does, these dark
matter particles haven’t been directly
observed.
•At present, through a series of indirect
observations, we can tell that the universe
is full of some sort of stuff that we just
cant see.........
WMAP Reveals Neutrinos, End of Dark
Ages, First Second of Universe
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WMAP measures a remnant of the early
universe - its oldest light. The conditions of
the early times are imprinted on this light. It is
the result of what happened earlier, and a
backlight for the later development of the
universe.
• This light lost energy as the universe
expanded over 13.7 billion years, so WMAP
now sees the light as microwaves. By making
accurate measurements of microwave
patterns, WMAP has answered many
longstanding questions about the universe's
age, composition and development.
• The universe is awash in a sea of cosmic
neutrinos. These almost weightless subatomic particles zip around at nearly the
speed of light. Millions of cosmic neutrinos
pass through you every second. "A block of
lead the size of our entire solar system
wouldn’t even come close to stopping a
cosmic neutrino”.
..back home… from the far corners of the Universe
Dark Matter Halo Around the
Milky Way
We believe our Galaxy, the Milky Way,
sitting inside a huge cloud of Dark
Matter.
This cloud forms a vast halo all around.
The DM halo outweighs the galaxy's
normal matter by a factor of 20 !!
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The Milky Way's bright ordinary matter is composed of a spiral
disk and outer ring, which in turn is embedded in a larger
luminous halo that is only the visible part of an even larger halo
of dark matter (more).
The size of the Dark Matter halo..
In fact, the motion of objects in the Milky Way is not consistent with the
amount of luminous matter, which is not enough to confine these objects
inside the Milky Way boundary. The problem (as for other far astronomical
objects 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 to
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contract, and settled into the visible matter disk.
This cartoon suggests the immense halo of mysterious dark matter that
astrophysicists believe surrounds our galaxy. The visible stars and gases of the
Milky Way contribute about five percent of its total mass.
… more about the Dark Matter Halo ..
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The invisible halo of Dark Matter at its
center is much larger and more spherical
than the luminous galaxy.
Recent computer simulations have shown
that the halo is surprisingly clumpy, with
relatively dense concentrations of dark
matter in gravitationally bound 'subhalos'
within the halo.
Dark matter halo warps
Milky Way's fringe
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Astronomers surveying hydrogen gas discovered in 1957 that the Milky
Way is not flat but warped near its edges like a fedora hat, with one
side of its disc curving as much as 20,000 light years above the main
galactic plane and the other dipping a little less below it.
Some researchers suspected the warp was caused by two satellite
galaxies, called the Magellanic Clouds, that orbit the Milky Way every
1.5 billion years. But calculations showed they alone were not massive
enough to produce the effect.
Now, researchers have shown the Magellanic Clouds can account for
the warp - but only because their motion creates a powerful wake in the
massive "halo" of dark matter that is thought to cocoon the Milky Way.
This allows to calculate that the halo outweighs the galaxy's
normal matter by a factor of 20 - massive enough to warp the
disc.
.. and now (finally) we land on the Earth:
the role of particle physics
in 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 from
experiments in our laboratories where signals of interactions
of 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 of
the present research programmes in (astro)particle Physics.
…. Obviously, this is far to be an easy game…
The DAMA Project at LNGS
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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 its
annual revolution around
the Sun, the Earth should
be crossed by a larger
flux of Dark Matter
particles in summer
(when its rotational
velocity is summed to the
one of the solar system
with respect to the
Galaxy) and by a smaller
one in winter (when the
two velocities are
subtracted).
DAMA/NaI results on particle Dark Matter
investigation
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 gravity
from other dimensions?
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The Question:
Since string theory implies up to 11 dimensions, could dark matter be gravitons leaking from
other dimensions into ours?
The Answer
Actually, that's exactly one possibility that's being explored by brane-world theorists. Of
course, the jury is still out on brane-world theories....
"In brane-world theory, the ends of strings are anchored in our brane, so
the particles we see can only move within the brane. But the particles
that carry the gravitational force, known as gravitons, are closed strings
-- little Cheerios -- and can "leak" out of the brane. This explains why
gravity is much weaker than the electromagnetic force and the strong
and weak nuclear forces. It also offers a possible explanation for the
"dark matter" that astronomers need to explain why the mass of the
universe doesn't agree with the observed objects. Dark matter could be
in an adjacent brane, with its gravitons leaking into ours."