The Dark Side of the Universe What is the Universe made of

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The Dark Side of the UniverseWhat is the
Universe made of?

• David G. Cerdeño
• Universidad Autónoma de Madrid
• Instituto de Física Teórica

CAB Madrid 20-03-07
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What is the Universe made of?

• Which is the main ingredient of the Universe?

• Light elements H, He, Li (made out of
  protons, neutrons, electrons) the atoms we are
  made of

these only account for a 4 of the
Universe (in terms of energy density)
The rest of the Universe is DARK
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Contents

• What is Dark Matter?
• An unidentified kind of non-luminous,
  non-relativistic, non-baryonic matter present in
  the Universe in large amounts
• Dark matter is a necessary ingredient in present
  models for the Universe but we have not
  identified it yet.
• Candidates for Dark Matter
• Can it be detected?
• Maybe in the near future!
• Dark Matter detection techniques experiments

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Why do we need Dark matter?
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Motivation for Dark Matter

• The motivation for dark matter arises from
  gravitational effects in astronomical
  observations at various scales. Luminous
  (visible) matter is insufficient to account for
  the observed effects.

At the galactic scale

• Rotation curves of spiral galaxies
• Gas temperature in elliptic galaxies

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Rotation curves in spiral galaxies
Centrifugal acceleration
Gravitational acceleration
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Rotation curves in spiral galaxies

• Rotation curves in spiral galaxies

Beyond the luminous disc (r5 kpc) one expects
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Rotation curves in spiral galaxies

• Rotation curves in spiral galaxies

Beyond the luminous disc (r5 kpc) one expects
However, observations seem to indicate
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Rotation curves in spiral galaxies
This gravitational effect may be caused by some
extra matter that we cannot see and which does
not emit or absorb light DARK MATTER
HIPOTHESIS DARK MATTER HALOES AROUND GALAXIES
Dark haloes contain around the 90 of the total
mass of the galaxy!
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Rotation curves in spiral galaxies

• Rotation curves in spiral galaxies

This gravitational effect may be caused by some
extra matter that we cannot see and which does
not emit or absorb light DARK MATTER

• An exotic alternative

Or it might be an indication of modifications in
Newtonian Dynamics
Only valid for small accelerations
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Motivation for Dark Matter

• The motivation for dark matter arises from
  gravitational effects in astronomical
  observations at various scales. Luminous
  (visible) matter is insufficient to account for
  the observed effects.

At the galactic scale

• Rotation curves of spiral galaxies
• Gas temperature in elliptic galaxies

Coma Cluster
Clusters of galaxies

• Peculiar velocities
• Gas temperature (X-ray measurements)
• Gravitational lensing

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Clusters of galaxies

• Peculiar motions of objects in clusters of
  galaxies constitute a probe of the gravitational
  potential

F. Zwicky 1933
Motions in the Coma cluster indicated the
existence of abundant non-luminous matter
Coma Cluster
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Weak Gravitational Lensing

• Light emitted by distant sources is bent by the
  gravitational field of massive intermediate
  objects (galaxies or galaxy clusters) according
  to Einsteins Theory of General Relativity

This can give rise to multiple images of the same
object
Gravitational lenses have been observed since
1979
Einsteins cross
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Weak Gravitational Lensing

• In weak lensing the images of distant galaxies
  are distorted when travelling across (dark)
  matter in the

Distant Galaxies
By measuring the distortion (shear), precious
information is gained on the gravitational field
between the emitter and the observer
Galaxy cluster
Observer (us)
Provides a measure of the amount (and
distribution) of dark matter, e.g., in clusters
of galaxies.
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Weak Gravitational Lensing

• In weak lensing the images of distant galaxies
  are distorted when travelling across (dark)
  matter in the

Abell 2667 cluster (HST)
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Weak Gravitational Lensing

• In weak lensing the images of distant galaxies
  are distorted when travelling across (dark)
  matter in the

Galaxy Cluster Abell 1689 (HST)
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Galaxy Collision

• Galaxy collision as observed by Chandra

Hot gas (luminous matter) observed by Chandra
Dark matter haloes (false colour, of course)
Chandra, 21 Ago. 2006
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Evidence from Cosmology

• First measured by Penzias Wilson in 1965, the
  Cosmic Microwave Background (CMB)

Discovered homogeneous electromagnetic radiation
with a thermal black body spectrum (T
3K) Evidence for the Hot Big Bang Model of the
Universe
Penzias Wilson (1965) Holmdel radiotelescope
The COBE satellite in 1992 measures temperature
anisotropies in the CMB. Their analysis
constitutes a primary tool to determine the
global properties of our Universe
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Evidence from Cosmology

• Observations of the Cosmic Microwave Background
  (CMB) are also consistent with the existence of
  large amounts of Dark Matter.

Recently, the WMAP satellite has provided high
precision data of the CMB anisotropies with which
cosmological parameters have been determined
http//map.gsfc.nasa.gov/
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Evidence from Cosmology

• Observations of the Cosmic Microwave Background
  (CMB) are also consistent with the existence of
  large amounts of Dark Matter.

Recently, the WMAP satellite has provided high
precision data of the CMB anisotropies with which
cosmological parameters have been determined
Combining WMAP with other experiments the best
fit is obtained for
We live in a Flat Universe
Dominated by some sort of Dark Energy
27 of matter
ONLY A 4 is baryonic (ordinary) matter
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Evidence from Cosmology

• Observations of the Cosmic Microwave Background
  (CMB) are also consistent with the existence of
  large amounts of Dark Matter.

Recently, the WMAP satellite has provided high
precision data of the CMB anisotropies with which
cosmological parameters have been determined
Combining WMAP with other experiments the best
fit is obtained for
From where a bound on the abundance of Cold Dark
Matter can be extracted
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So WHAT is the Dark Matter?
WE DO NOT KNOW YET

• Ordinary (baryonic) matter was proposed
  originally
• Cold gas in the Intergalactic Medium
• Massive Compact Halo Objects (MACHOs)
• White dwarves, Jupiter-like objects, black holes,
  

They can only account for a SMALL PART of the
total Dark Matter Large amounts of baryonic
matter are inconsistent with Big Bang
Nucleosynthesis.

• Non-baryonic STABLE matter
• Neutrinos are good candidates for two reasons
• 1.- They are known to exist
• 2.- They are massive and populate the Universe in
  large amounts

NEUTRINOS! Elementary particles, neutral and
extremely light, postulated in 1930 to explain
beta decays of neutrons into protons.
(Discovered in 1956)
However, neutrinos are RELATIVISTIC (HOT dark
matter). Large amounts of these would give rise
to a different formation of large scale
structures in the Universe than observed.
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So WHAT is the Dark Matter?
THE BIG PICTURE
MOST OF THE MATTER IN THE UNIVERSE IS DARK
Stars
1
Visible Baryonic Matter
Visible gas
Gas in the Intergalactic Medium,
Non-baryonic dark matter
DARK MATTER
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So WHAT is the Dark Matter?
THE BIG PICTURE
MOST OF THE MATTER IN THE UNIVERSE IS DARK
Stars
1
Visible Baryonic Matter
Visible gas
Gas in the Intergalactic Medium,
Non-baryonic dark matter
DARK MATTER
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So WHAT is the Dark Matter?
WHAT DO WE KNOW

• We have a good idea of what we are looking for

• However, the number of suspects is large, all
  postulated in modern Particle Physics

Axions with a small mass ma?10-5 eV
Weakly Interacting Massive Particles (WIMPs)
Lightest Supersymmetric Particle
Lightest Kaluza-Klein Particle
SIMPs, CHAMPs, SIDM, WIMPzillas, Scalar DM, Light
DM
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How is Particle Dark Matter produced?
time
Today t13.6 Gyr
T3K
Temperature
Due to the expansion of the Universe DM particles
fall out of equilibrium and cannot annihilate any
more.
Thermal equilibrium
A Relic Density of DM is obtained which remains
constant.
The number density, n, of DM decreases with T.
time
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How is Particle Dark Matter produced?
time
Today t13.6 Gyr
T3K
Temperature
Due to the expansion of the Universe DM particles
fall out of equilibrium and cannot annihilate any
more.
Thermal equilibrium
A Relic Density of DM is obtained which remains
constant.
How much dark matter remains depends on its
interaction rate
The number density, n, of DM decreases with T.
A particle with stronger interactions keeps in
equilibrium for longer
and is more diluted
time
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How is Particle Dark Matter produced?
time
Today t13.6 Gyr
T3K
Temperature
Due to the expansion of the Universe DM particles
fall out of equilibrium and cannot annihilate any
more.
Thermal equilibrium
A Relic Density of DM is obtained which remains
constant.
How much dark matter remains depends on its
interaction rate
The number density, n, of DM decreases with T.
Particles with very weak interactions decouple
earlier, having a larger relic density
time
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OK we dont know what dark matter is but can
we detect it?
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Dark matter direct detection

• Our galaxy is surrounded by a Halo of Dark
  Matter DM particles are constantly crossing the
  Earth. Can we detect them?

By measuring the rotation curve of the Milky Way
we extract the Dark Matter density at the Earths
position in the galaxy
?Dark Matter 5 ? 10-24 g cm-3 0.3 GeV cm-3
A privileged position for detecting Dark Matter?
For a typical Dark Matter candidate, this
implies a number density
nDark Matter 0.003 cm-3
From their velocity in the galactic halo of Dark
Matter, v 300 km s-1, their flux through the
Earth is
JDark Matter nDark Matter ? v 105 cm-2 s-1
10 000 000 000 dark matter particles have crossed
this laptop during this talk
The flux is enormous! But remember their
interaction rate is extremely small
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Dark matter direct detection

• Our galaxy is surrounded by a Halo of Dark
  Matter DM particles are constantly crossing the
  Earth. Can we detect them?

The direct detection of Dark Matter can take
place through their interaction with nuclei
inside a detector

• The nuclear recoiling energy is measured
• Ionization on solids
• Ionization in scintillators (measured by the
  emmited photons)
• Temperature increase (measured by the released
  phonons)

• Problems
• Small interaction rate
• Large backgrounds (experiments must be deep
  underground)
• Uncertainties in the DM properties in our galaxy

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COUPP
(Chicagoland Observatory for Underground Particle
Physics)

• A bubble chamber sensitive to Weakly Interacting
  Massive Particles (WIMPs)

2 Kg of CF3I, that can be superheated to respond
to very low energy nuclear recoils like those
expected from WIMPs while being totally
insensitive to minimum ionizing particles
http//collargroup.uchicago.edu/news/coupp.html
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COUPP

• Detection of single bubbles in a superheated
  liquid, induced by high dE/dx nuclear recoils in
  heavy liquid bubble chambers

Stereo view of a typical event in 2 kg chamber

• Choice of three triggers pressure, acoustic,
  motion

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COUPP

• Detection of single bubbles in a superheated
  liquid, induced by high dE/dx nuclear recoils in
  heavy liquid bubble chambers

neutron-induced nucleation in 20 c.c. CF3Br (0.1
s real-time span) Movie available from
http//cfcp.uchicago.edu/collar/bubble.mov

• Choice of three triggers pressure, acoustic,
  motion

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Dark matter indirect detection

• Dark Matter particles can annihilate in regions
  with a large density. We can try to detect the
  annihilation products.

Neutrinos
Details dependent on the DM model
Relevant final states
Photons
Electron-Positron
At the centre of the Milky Way
Or in other galaxies
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Dark matter indirect detection

• Dark Matter particles can annihilate in regions
  with a large density. We can try to detect the
  annihilation products.

Neutrinos
Details dependent on the DM model
Relevant final states
Photons
Electron-Positron
Inside the Sun
Even inside the Earth!
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Dark matter in particle accelerators

• Future particle accelerators can also help
  decrypting the nature of Dark Matter

Dark matter particles could be produced in the
very energetic events at the Large Hadron
Collider at CERN
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Dark matter related experiments around the world
(2007)
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Things to take home
MOST OF THE MATTER IN THE UNIVERSE IS DARK
WE HAVE NOT IDENTIFIED IT YET
STAY TUNED FOR ANVANCES IN THE NEAR FUTURE
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DAMA/LIBRA

• Searching for annual modulation in the DM
  detection signal

The Earths velocity through the DM halo has a
seasonal dependence
This induces a 7 summer-winter modulation in
the detection rate
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DAMA/LIBRA
(Gran Sasso, Italy)

• Searching for annual modulation in the DM
  detection signal

The Earths velocity through the DM halo has a
seasonal dependence
DAMA claimed evidence for the detection of Dark
Matter particles (WIMPs) but this has not been
confirmed by any other experiment.
This induces a 7 summer-winter modulation in
the detection rate
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CDMS
(Soudan Underground Lab., USA)

• A discrimination of DM signal using simultaneous
  measurement of ionization and phonons in
  Germanium crystals.

Deep underground for protection against cosmic
rays. Extra shielding (lead and polyethylene) to
protect from background due to natural
radioactivity.
It is so far the most sensitive Dark Matter
experiment.
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