richard e. hughes dark matter & glast; p.1 the lure of dark matter richard e. hughes department...

Dark Matter & GLAST; p.1Richard E. Hughes

The Lure of Dark Matter

Richard E. HughesDepartment of Physics

“The most incomprehensible thing about the universe is that it is comprehensible”

- Albert Einstein


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 don’t 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 can’t SEE. This matter is imaginatively referred to as “Dark Matter”.


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 Kepler’s 3rd Law:The squares of the times to complete one orbit are proportional to the cubes of the The squares of the times to complete one orbit are proportional to the cubes of the

semi-major axis of the ellipse.semi-major axis of the ellipse.


Newton’s 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 toRotation of stars around

galactic centersRotations of galaxies in

clusters of galaxies

( )r

GMrv r=2


The Milky Way GalaxyOur 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


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:


The Milky Way GalaxyThe 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 centerNear the Orion armBetween the arms

Perseus and Sagittarius


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).


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 thespeeds of stars as one moves from the center, but we don’t

see it.


How about other galaxies?NGC 6503: Galaxy in Constellation Draco


Yet another galaxy…


Whatever it is, its DARK!


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(<r) 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!

Why are the rotation curves flat?


Example of Gravitational Lensing

Foreground cluster of galaxies CL0024+1654 (constellation Pisces)

Blue galaxy behind the cluster

“lensed” copy of blue galaxy


Example of Gravitational Lensing


Gravitational Lensing


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


What the Universe is Made Of


What and where is the dark matter?

The dark matter can’t be in the central disk of galaxies. Why?Interstellar clouds would be much thinner (due to gravitational forcesof 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 holes2) Dim, old white dwarfs which are no longer bright3) Proto-stars in which fusion did not start

What most of the dark matter SEEMS to be:Some new form of elementary matter


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.


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


“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!


Gamma-ray Large Area Space Telescope


Viewing the universe in many different wavelengths

See http://www.ipac.caltech.edu/Outreach/Multiwave/gallery3.html for image citations.


GLAST in Action!


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!


GLAST MissionGLAST 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 - >300 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)

LAT FoV

GBM FoV


GLAST LAT Overview: Design

e+ e–

Si Trackerpitch = 228 µm8.8 105 channels12 layers × 3% X0

+ 4 layers × 18% X0

+ 2 layers

Data acquisition

Grid (& Thermal Radiators)

Flight Hardware & Spares16 Tracker Flight Modules + 2 spares16 Calorimeter Modules + 2 spares1 Flight Anticoincidence DetectorData Acquisition Electronics + Flight Software

3000 kg, 650 W (allocation)

1.8 m 1.8 m 1.0 m

20 MeV – 300 GeV

CsI CalorimeterHodoscopic array8.4 X0 8 × 12 bars

2.0 × 2.7 × 33.6 cm cosmic-ray rejection shower leakage correction

ACDSegmented scintillator tiles0.9997 efficiency

minimize self-veto


Gamma Ray Bursts 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 990123

Distance: 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

BATSE map of its 2704 detected GRBs

Artists conception of a GRB


Active Galactic Nuclei (AGN) 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?

Hubble Heritage image of M87

Schematic diagram of an AGN


GLAST is an International Mission

Germany

FranceSweden Italy

USA Japan

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


Why study -rays ?

Gamma-rays carry a wealth of information -rays offer a direct view into Nature’s largest accelerators the Universe is mainly transparent to -rays: can probe

cosmological volumes. -rays readily interact in detectors, with a clear signature. -rays are neutral: no complications due to magnetic

fields; point directly back to sources, etc.


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 can’t decay into something else UNLESS: it meets its antiparticle.

richard e. hughes dark matter & glast; p.1 the lure of dark matter richard e. hughes department...

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