The Lure of Dark Matter

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The Lure of Dark Matter

• Richard E. Hughes
• Department of Physics

The most incomprehensible thing about the
universe is that it is comprehensible - Albert
Einstein
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Dark Matters..
This is luminous matter
This is dark matter
It is tempting to look at the universe, seeing
stars and galaxies, clusters of galaxies and come
to the conclusion that what you SEE is the
matter, and what you dont see is empty space.
But, you would be wrong! There is general
agreement that, in fact, MOST of the matter in
the universe is in a form that we cant SEE.
This matter is imaginatively referred to as Dark
Matter.
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Rotation Velocities in Our Solar System

• The falloff in speeds as the planets get further
  from the sun is called Keplerian decline
• It comes from Keplers 3rd Law
• The squares of the times to complete one orbit
  are proportional to the cubes of the semi-major
  axis of the ellipse.

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Newtons Generalization

• Isaac Newton generalized this with his 3 Laws
• The rotational speed of ANY object is only
  dependent on how much mass is INSIDE its orbit
• This applies to our solar system
• But ALSO applies to
• Rotation of stars around galactic centers
• Rotations of galaxies in clusters of galaxies

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The Milky Way Galaxy
Our sun is in the Milky way galaxy, about 28,000
light-years from its center. The speed of the
solar system relative to the galactic center is
approximately 220 km/s. At this speed, it takes
about 200 million years to make one complete
revolution. A galaxy is composed of stars and
other material which are held together by
gravity. The name Milky Way comes from the band
of light that can be seen during dark nights in
the summer. This band is actually an edge-on
view of the galaxy, and it is believed that when
viewed head on it is a spiral galaxy.
The Celestial River
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The Milky Way Galaxy

• The COBE satellite was designed to investigate a
  phenomenon called the Cosmic Microwave
  Background.
• COBE is sensitive to infra-red (IR) wavelengths
  of light.
• The Milky Way viewed in the visible, is obscured
  by dust.
• However, viewed in the IR, the Milky Way shows a
  clear central bulge overlaying a thin disk, as
  expected of an edge-on view of a spiral galaxy

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The Milky Way Galaxy

• The image shown is a rendition of what we believe
  the Milky Way galaxy looks like if it were viewed
  head on
• The radius is about 50,000 light-years
• The sun is about 28,000 light-years from the
  center
• Near the Orion arm
• Between the arms Perseus and Sagittarius

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The Rotation Curve For the Milky Way
The same sort of rotation curve can be made for
the Milky Way galaxy. Given that the Sun is on
the outer edge of the galaxy (about 2/3 out), we
expect that most of the mass is inside the
galactic radius of the Sun. So we should see a
decreasing rotation curve, like we do for the
solar system. But instead, it is FLAT (if not
increasing).
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What we expected, NOT!
These two curves are VERY different. Why? Our
solar system orbits the center of the milky way
galaxy just like the earth orbits the sun so we
expect Keplerian decline in the speeds of stars
as one moves from the center, but we dont see it.
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How about other galaxies?
NGC 6503 Galaxy in Constellation Draco
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Yet another galaxy
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Whatever it is, its DARK!
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Why are the rotation curves flat?

• Stars and gas in the galactic disks follow
  circular orbits whose velocity depends on the
  inner mass only
• A flat rotation curve means that the total M(ltr)
  increases linearly with r, while the total
  luminosity approaches a finite asymptotic limit
  as r increases. Clearly a large amount of
  invisible gravitating mass (more than 90 of the
  total mass in the case of the Milky Way and other
  examples) is needed to explain these flat
  rotation curves.
• This invisible mass is referred to as DARK MATTER
• Is there any other supporting evidence?

Dark Matter!
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Example of Gravitational Lensing
Foreground cluster of galaxies CL00241654
(constellation Pisces)
Blue galaxy behind the cluster
lensed copy of blue galaxy
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Example of Gravitational Lensing
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Gravitational Lensing
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What is causing the Lensing?
The majority of the dark matter is distributed
broadly and smoothly in the cluster, covering a
region on the sky more than 1.6 million
light-years across. The mass of the individual
cluster galaxies appears as pinnacles on this
mountain of dark matter mass. Overall, the dark
matter in the cluster outweighs all the stars in
the cluster's galaxies by 250 times!
From http//www.bell-labs.com/org/physicalsciences
/projects/darkmatter/darkmatter1.html
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What the Universe is Made Of
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What and where is the dark matter?
The dark matter cant be in the central disk of
galaxies. Why? Interstellar clouds would be
much thinner (due to gravitational forces of the
dark matter. So the dark matter must be in
halos of the galaxies. What the dark matter is
NOT 1) Stars even faint ones would radiate
some light. 2) Dust we would not be able to see
our own galaxy or others, since dust absorbs
and scatters light What some the dark matter
MIGHT be 1) Black holes 2) Dim, old white
dwarfs which are no longer bright 3) Proto-stars
in which fusion did not start What most of the
dark matter SEEMS to be Some new form of
elementary matter
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Super Particles?

• Particles making up normal matter stars,
  planets, people, etc

Shadow particles NONE have been observed yet
but one of these predicted Particles could be
the source of dark matter it is called the
NEUTRALINO.
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The Neutralino

• Predicted to exist for reasons that have NOTHING
  to do with dark matter BUT has properties that
  would make it a very good candidate
• There might be enough dark matter particles in
  the halo of galaxies that the dark matter
  particles will collide from time to time
• Since the dark matter particle is its OWN
  anti-particle, when the particles collide, they
  will ANNIHILATE

High energy photons from dark matter
annihilation
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Seeing dark matter
Unfortunately, the atmosphere is a shield to high
energy gamma rays. To see them, we need to go
above the atmosphere - we need a satellite!
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Gamma-ray Large Area Space Telescope
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Viewing the universe in many different wavelengths
See http//www.ipac.caltech.edu/Outreach/Multiwave
/gallery3.html for image citations.
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GLAST in Action!
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Launch of Satellite

• GLAST will launch in 2007
• First data in 2008
• Will we see dark matter?
• We will be looking for ANOMALOUS sources of gamma
  rays
• If they have the right properties, this could be
  the signature of dark matter!
• Stay Tuned!

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GLAST Mission

• GLAST measures the direction, energy arrival
  time of celestial gamma rays
• GLAST is two instruments
• - Large Area Telescope(LAT) measures gamma-rays
  in the energy range 20 MeV - gt300 GeV
• - Gamma-ray Burst Monitor(GBM) provides
  correlative observations of transient events in
  the energy range 20 keV 20 MeV

Launch Feb 2007 Orbit 550 km,28.5o
inclination Lifetime 5 years (minimum)
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GLAST LAT Overview Design
Si Tracker pitch 228 µm 8.8 105 channels 12
layers 3 X0 4 layers 18 X0 2 layers
Grid ( Thermal Radiators)
3000 kg, 650 W (allocation) 1.8 m ? 1.8 m ? 1.0
m 20 MeV 300 GeV
CsI Calorimeter Hodoscopic array 8.4 X0 8
12 bars 2.0 2.7 33.6 cm
Flight Hardware Spares 16 Tracker Flight
Modules 2 spares 16 Calorimeter Modules 2
spares 1 Flight Anticoincidence Detector Data
Acquisition Electronics Flight Software

• cosmic-ray rejection
• shower leakage
• correction

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Gamma Ray Bursts
BATSE map of its 2704 detected GRBs

• Gamma Ray Bursts are intense flashes of gamma
  rays lasting from fractions of a second to hours,
  some with fading afterglows visible for months.
  What are they?
• collisions between highly dense neutron stars or
  black holes?
• signatures of the birth of a black hole?
• Example GRB 990123Distance 10 billion
  light-yearsSize emitting region is
  light-seconds acrossPower at maximum up to
  1,000,000,000,000,000,000 (quintillion) times the
  Sun's power or roughly equal to the energy
  released by 100 billion Suns in a year's time
• GLAST should observe more than a 200 bursts per
  year

Artists conception of a GRB
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Active Galactic Nuclei (AGN)
Hubble Heritage image of M87

• AGN are a special class of glaxies that are the
  source of tremendous energy, shining with power
  equivalent to trillions of suns. It is believed
  that at the center of these objects there lies a
  supermassive black hole, which ejects jets of
  matter in opposite directions at nearly the speed
  of light.
• If one of the jets is directed toward us the AGN
  is referred to as a Blazar
• GLAST will detect thousands of blazars and will
  try to answer questions like
• How are the jets formed?
• How is the matter in the jets accelerated to such
  fantastic speeds?
• Is a billion-solar-mass black hole really the
  central power source?

Schematic diagram of an AGN
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GLAST is an International Mission

• NASA - DoE Partnership on LAT
• LAT is being built by an international team
• Si Tracker Stanford, UCSC, Japan, Italy
• CsI Calorimeter NRL, France, Sweden
• Anticoincidence GSFC
• Data Acquisition System Stanford, NRL, Ohio
  State
• GBM is being built by US and Germany
• Detectors MPE

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Why study g-rays ?

• Gamma-rays carry a wealth of information
• g-rays offer a direct view into Natures largest
  accelerators
• the Universe is mainly transparent to g-rays can
  probe cosmological volumes.
• g-rays readily interact in detectors, with a
  clear signature.
• g-rays are neutral no complications due to
  magnetic fields point directly back to sources,
  etc.

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Searching for Dark Matter

• If we believe that Dark Matter really does exist,
  how do we look for it?
• Well, we need a model. And one which is pretty
  handy is the Standard Model!
• Well, actually not the Standard Model, but a
  close relative, which involves something called
  SuperSymmetry
• A particle predicted by the SuperSymmetry theory
  is called the Neutralino
• This particle is predicted for reasons having
  NOTHING to do with dark matter, but in a happy
  coincidence it COULD BE that the neutralino is
  the mysterious source of Dark Matter.
• Once the neutralino is made, it cant decay into
  something else
• UNLESS it meets its antiparticle.