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transcript
2/25/2016
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Dark matter: summary
Gravity and detecting Dark Matter
Massive objects, even if they emit no light,
exert gravitational forces on other massive objects.
We study the motions (dynamics) of visible objects like stars in
galaxies, and look for effects that are not explicable by the mass of the
other light emitting or absorbing objects around them.
m1
m2
r12
http://www.hep.shef.ac.uk/cartwright/phy111/ppt/dark_matter_intro.ppt
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Doppler effect: measure velocities from shift of spectral lines
Fritz ZwickyCalifornia Institute of Technology
Dark matter
He measured the speed with which the galaxies in Coma move. To his
surprise, he found enormous speeds—thousands of kilometers per
second — fast enough to rip the cluster apart.
Why was the cluster not tearing itself up? Zwicky concluded that the
cluster must be filled with additional unseen matter that holds the
galaxies together with its gravitational force.
Coma Cluster: 1000 galaxies
321 million light years away
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Fritz Zwicky
measurements
Were redone with
much better
instruments.
Measure the
velocities of
galaxies in a
cluster from their
Doppler shifts.
The mass we find
from galaxy
motions in a
cluster is about 50
times larger than
the mass in stars!
Measure velocities of stars and gas clouds orbiting the galaxy center.
One would expect that outer stars should behave much like the planets of
our solar system. Inner planets rotate faster and outer planets rotate
slower (Keplerian motion). In galaxies, however, both inner and outer stars
rotate at about the same speed.
http://www.haystack.mit.edu/edu/pcr/Astrochemistry/3 - MATTER/Dark Matter.ppt
Further evidence for dark matterRotation curves
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1973: Further evidence for dark matter
Problem with galactic simulations
https://www.learner.org/courses/physics/unit/text.html?unit=10&secNum=2
James Peebles Jeremiah Ostriker
Princeton University
Jeremiah Ostriker and James Peebles
used numerical simulation to study how
galaxies evolve: they programmed 300
mass points into their computer to
represent groups of stars in a galaxy
rotating about a central point.
Ostriker and Peebles found that in a time less than an orbital period, most of
the mass points would collapse to a bar-shaped, dense concentration close
to the center of the galaxy with only a few mass points at larger radii.
However, if they added a static, uniform distribution of mass three to 10
times the size of the total mass of the mass points, they found a more
recognizable structure would emerge.
Gravitational Lensing of Light
Bending of light in gravitational fields can make lenses out of massive objects
LENSING OBJECT
USSOURCE
NO
LENS
LENS
Strong or close lens, expect a ring of light, or a ring of images in the
presence of the lens.
When not resolved, expect increased intensity.
http://www.hep.shef.ac.uk/cartwright/phy111/ppt/dark_matter_intro.ppt
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Gravitational lensing by dark matter
Sometimes galaxies are lensed by other galaxies.
Other times they were lensed by invisible objects – dark matter.
By measuring the distortion of the galaxies, scientists were able to “weigh” the dark matter.
They found that it accounts for 90% of the mass of the universe.
https://www.nsf.gov/od/lpa/news/press/00/pr0029.htm
Cosmic shear: the light from distant galaxies is distorted by dark matter.
2006: Bullet cluster
http://chandra.harvard.edu/press/06_releases/press_082106.html
Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical:
NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map:
NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.
Therefore, during the collision the
dark matter clumps from the two
clusters moved ahead of the hot
gas, producing the separation of
the dark and normal matter seen
in the image. If hot gas was the
most massive component in the
clusters, as proposed by
alternative theories of gravity,
such an effect would not be
seen. Instead, this result shows
that dark matter is required.
The hot gas in each cluster was slowed by a drag force, similar to air resistance,
during the collision. In contrast, the dark matter was not slowed by the impact
because it does not interact directly with itself or the gas except through gravity.
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The visible
portion of a
galaxy lies
deep in the
heart of a
large halo of
dark matter.
http://www.sjsu.edu/people/monika.kress/courses/sci255/
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85% of matter in the universe is of unknown nature
Normal matter: ~15% of total matter
Dark matter~85% of total matter
?
We know it is out there but we do not know what it is.
Dark Matter: An undetected form of mass that emits little or no photons,
but we know it must exist because we observe the effects of its gravity.
What dark matter is not: MACHOS hypothesis have been ruled out
MACHOS: MAssive Compact Halo Objects:
dim stars (white dwarfs, drown dwarfs, neutron
stars), black holes, and Jupiter-sized planets,
http://www.jcschroder.com/phy111/machos.htm
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What do we know about dark matter?
Mostly have “negative” information from astrophysics and searches for
new particles:
• No electric charge
• No colour charge (property of quarks and gluons that is related to the
particles' strong interactions)
• No strong self-interaction
• Does not seem to decay:
stable, or very long-lived
• Not a particle in the Standard Model
of particle physics
×
Dark matter candidate particle zoo
WIMP: Weakly Interacting Massive Particle
SuperWIMPS: superweakly-interacting massive particles produced in the
late decays of other particles
Axion, Peccei–Quinn symmetry
Kaluza-Klein (KK) photon and graviton are from universal extra dimension models
Neutralino and gravitino are particles of supersymmetric modelsWIMPZILLA (nonthermal dark matter)
proton mass
https://indico.in2p3.fr/event/10162/session/5/contribution/10/material/slides/0.pdf
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http://www.ippp.dur.ac.uk/~ross/invisibles13/talks/78-marrod_xe1n_undagoita/slides/78-0-Marrodan_Invis_Durham2013.pdf
How to search for dark matter particles
Make dark matter particlesBuild a trap for dark matterSearch for things dark matter can decay to
Supersymmetry and WIMPs:The lightest (stable) supersymmetries particle is your dark matter
WIMP: neutralino (combination of –inos)
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The Large Hadron Collider (LHC) CERN (Conseil Européen pour la Recherche Nucléaire)
LHC: 27-kilometre ring of superconducting magnets with a number of accelerating
structures to boost the energy of the particles along the way.
The world's largest and most powerful particle collider: 13 TeV (1012 eV).
The largest, most complex experimental facility ever built.
The largest single machine in the world. Cost: 3 billion euro.
Credit: CERN
Slide from: T. Kono (ATLAS), BW2011 workshop
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http://www.ippp.dur.ac.uk/~ross/invisibles13/talks/78-marrod_xe1n_undagoita/slides/78-0-Marrodan_Invis_Durham2013.pdf
How to search for dark matter particles
Make dark matter particlesBuild a trap for dark matterSearch for things dark matter can decay to
Direct detection: How to detects WIMPs?
http://cdms.berkeley.edu/Education/DMpages/essays/essays/essays/science/images/NucRecoilAtoms.jpg
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http://www.hep.ucl.ac.uk/darkMatter/
Earth is moving through dark matter halo ( dark matter not moving)
Dark matter signal
should vary during
the year
MAX signal ~ June 2
Smallest
signal
Earth’s orbital speed
around the Sun is 30 km/s
Scientific American 288, 50-59 (March 2003)
Detection principle:
Measure the recoil energy imparted to detector nuclei through
WIMP-nucleon collisions.
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https://indico.in2p3.fr/event/10162/session/5/contribution/10/material/slides/0.pdf