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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