Dark Matter and the Universe

Mark Pipe(Based on slides from Ed Daw)

What is dark matter ?And why is it interesting ?

Dark matter is as dark as anything can be. It NEITHER emits NOR absorbs light.

This is not dark matter! It absorbs light, so you can see it.

M33 galaxy~920 kpc (3 million light years) from Earth

Determine the speed of rotation of material aboutthe centre of the galaxy and plot this speed against

distance from the galactic centre.

~15 kpc (50,000 light years)

1. Galaxy rotation curvesIf we can’t see it, how do we know it is there?

1.4GHz frequency = 21cm wavelength

Astronomy with the 21cm Hydrogen Line

The hydrogen atom naturally produces

radiation at a frequency of around 1.4GHz.

Relative motion of source causes wavelength to be

Doppler shifted

A

B

C

(21.10611405413 cm)

Arecibo Observatory, Puerto Rico

305m

Astronomy with the 21cm Hydrogen Line

And the result is:BUT: this does not agree with the predictions of Newtonian classical mechanics and Kepler’s laws!

arXiv:astro-ph/9909252v1

UNLESS:The galaxy is much larger than the visible disk - about ten times as massive!

2. Gravitational lensing

LENSING OBJECTUSSOURCE

NOLENS

LENS

EINSTEIN - Gravity is the curvature of space-time caused by massive objects. Objects moving through space-time, INCLUDING LIGHT, will follow the curves that have been created!

NEWTON - Gravity is a force between massive objects. Light is massless therefore is not affected by gravitational fields.

336 h-1 kpc, 1 arc min.

Gravitationallensing by

galaxy clusters

Image from the Hubble Space

Telescope

Reconstructed massdistribution for

galaxy cluster 0024+1654

[From Tyson et al., Astrophys. Journ.498, L107-L110, 1998 May 10]

Peculiar velocities are motions of galaxies ‘above and beyond’ what is expected from Hubble expansion. They occur due to gravitational fields of massive bodies.

3. Peculiar velocity measurements

The Great Attractor is a region of space that causes peculiar velocities in excess of 700km/s in objects over a region 100s of millions of light years across.

Inferred mass > 10,000 Milky WaysVisible mass < 1,000 Milky Ways

4. Structure formationHow did the universe end up with so much structure?

Forces like electromagnetism cannot do this because the positively and negatively charged particles are interspersed.

Gravitation did it! In order to create the structure that we see in the universe today we require a large amount of matter whose non-gravitational interactions are very weak. Dark matter!

A recent structure formation simulation

From the researchgroup of Ben Moore,

http://krone.physik.unizh.ch/~moore/

5. WMAP satellite results

http://map.gsfc.nasa.gov/

Results:Baryonic matter: 4.6%±0.1%Dark matter: 23.3%±1.3%Dark energy: 72.1%±1.5%

Dark energy• Constant energy density spread uniformly

over all space

• 72% of the universe

• An extremely odd contribution to physics...

...gas particles. What happens when the bottle expands ?

To see just how odd, consider a bottle of ordinary matter.

An expanding bottle of matter

The number of particles in the box stays the same, AND the average energy per particle drops as the particles lose energy due to the expansion (in a gas bottle, the particles lose energy to the receding walls).

Each represents a packet of energy. As the box, which represents a portion of the universe, expands, the number of energy packets increases.

It is as if energy were appearing out of nowhere.

An expanding bottle of ‘dark energy’

So the total energy in dark energy grows with the volume of empty space (vacuum) in the universe. As the

universe expands, its total energy increases.

Another weird property. For objects separated by huge distances ‘dark energy’ tends to oppose the pull of gravity.

We don’t understand it !

There are many candidates for dark energy, one being the ‘cosmological constant’ proposed by Einstein as a

modification to his theory of general relativity

This means that dark energy is causing the rate of expansion of the universe to increase. This is actually

how it was first discovered in experiments.

Ideas for dark matter1. Conventional astronomical objects?

Besides, it’s hard to make that much ordinary matter. People talk about NUCLEOSYNTHESIS BOUNDS - it is hard to synthesise that many nuclei in the early universe.

Objects made of ordinary matter, ATOMS.E.g. brown dwarfs, low luminosity stars, black holes, gas, dust...

Even if they could be produced, it is hard to form as much structure as we see in the universe with only ordinary matter as your building block - its self interactions are too strong, washing out any initial inhomogeneities that might form structure in the early universe.

RULED OUT as dark matter.

We have looked! Space telescopes, gravitational lensing. These objects are out there but not in the required quantities.

There are elementary particles other than the usual protons, neutrons, and electrons, but very few make good candidates for dark matter.

Candidates need to:• Remain stable for at least 1014 years.• Have only weak couplings to other matter, except

for gravitation.• Have been produced in large numbers in the early

universe.

Ideas for dark matter2. Other elementary particles?

Neutrinos?

• Known to exist! Produced in nuclear reactions, for example, in the sun, in supernova explosions, in the big bang.

• Interact by gravity and nothing stronger• Known to be plentiful in the universe. Trillions pass through

the human body every second.

BUT:• They are too light - experiments limit neutrino masses to

less than 1 millionth that of an electron.• They inhibit formation of structure, because they are

produced thermally in nuclear reactions. Therefore they are ‘hot’, or ‘high kinetic energy’ particles, which therefore do not easily form bound states through gravitational interactions.

RULED OUT as dark matter.

Axions - masses of around 1 thousanth-billion-billionth that of an electron, (~10-45 kg).10 thousand billion per litre at our location.

WIMPs - ~10-1000 times the mass of a proton (~10-25 kg). A few tens per litre at our location. WIMPs predicted by a huge extension to the standard model of particle physics (supersymmetry).

...other candidates: Kaluza-Klein particles, scalar particlesfrom little Higgs theories, axinos...

Most experimental interest is focussed on WIMPs

Ideas for dark matter3. Exotic particle candidates

Theoretical particles, outside of the standard model of particle physics

Weakly Interacting Massive ParticlesA general class of heavy particle that interacts through the weak

nuclear force and gravity ONLY.

WIMP

Nucleus

WIMPs occasionally interact with atoms of ordinary matter via the weak nuclear force causing a nuclear recoil.

Dark Matter

Stars

WIMPs exist in the form of a halo that encompasses the visible mass of a galaxy.

TargetE.g. liquid Xe, Ge crystal,

gaseous CS2 etc...

Signal detectorE.g. PMT - light detector,TES - phonon detector,

MWPC - charge detector

WIMP

Scintillation?Phonons?Ionisation?

Detecting nuclear recoils

ZEPLIN IIBoulby - UK

ZEPLIN IIIBoulby - UK

EDELWEISSModane - France

CDMSSoudan - Minnesota

Ionisation

ScintillationPhonons

WIMP

NUCLEUS

CRESST IIGran Sasso - Italy

DRIFT IIBoulby - UK

DAMA/LIBRAGran Sasso - Italy

XMASSKamioka - Japan

XENONGran Sasso - Italy

Backgrounds - what unwantedguests excite the detector ?

• Cosmic rays - showers of particles from space.

• Radioactive isotope decays near or in the detector.

• Gamma rays - electron recoils...high rate, but scatter off electrons. Discrimination techniques.• Neutrons - nuclear recoils...just like WIMPS! Good shielding, radiopure detector.

Shielding from cosmic rays• Boulby mine in North Yorkshire• 1.1 km deep working potash mine

Cosmic ray induced backgrounds reduced by factor of 1 million

Shielding from cosmic rays

ZEPLIN III - A liquid xenon

light/charge detector

distributions for each species. It turns out [12] that the dis-crimination is improved by working at moderate electricfields which increases the separation between the two distri-butions and improves the statistical uncertainties of theionisation signal. Some discrimination against nuclearrecoil signals from neutron elastic scattering is obtainedby having good 3-D position reconstruction which canidentify the multiple scattering expected from the muchhigher cross-sections for neutron scattering than for WIMPscattering. E!cient measurement of the ionisation relies onachieving a long lifetime against trapping for free electronsin the liquid. This requires ultrapure xenon as free fromelectronegative impurities as possible. The target volumes

must be constructed as high vacuum vessels and a dedi-cated gas purification system is needed.

ZEPLIN-III achieves good 3-D position reconstructionby using an array of 31 200 diameter photomultipliers asshown in the lower panel in Fig. 1. The pattern of signalsseen in the PMTs can provide sub-cm 2-D spatial resolu-tion in the horizontal r, h plane even for single electronsextracted from the liquid [34]. Resolution in the z co-ordi-nate at the !50 lm level is obtained from the time intervalbetween the S1 and S2 signals. The 3-D position recon-struction is then used to define the fiducial volume withoutreliance on any physical surfaces. As shown in [11] thisallows a fiducial region of diameter 31.2 cm containing

Liquid xenon

PMT(x31)

PMT wire grid (-2kV)

cathode wire grid (down to -35kV)

anode mirror (up to +5kV) xenon gas gap

25cm

38.6cm

3.5cm

0.5cm

5.2 cm

WIMP

S1

S2

PMT anode outputs

boundary offiducial volume

Fig. 1. Cross-sections of the target volume showing the key system design concepts and the event interaction process. The top panel shows a side view withkey design features labeled, including a boundary box for the fiducial volume. The bottom panel provides a top view of the PMT arrangement and theradial fiducial boundary.

48 D.Yu. Akimov et al. / Astroparticle Physics 27 (2007) 46–60

Electric Field

ZEPLIN III installation

More shieldingLead to shield against

gamma rays

Wax & plastic to slowdown (moderate) neutrons

Current limits on spin-independent WIMP-nucleon coupling

We can determine a maximum size and

interaction strength of WIMPs from the fact that

they have not been detected

Current limits on spin-independent WIMP-nucleon coupling

Currently, the race for the highest sensitivity to WIMPs is being lead by the CDMS and XENON groups using competing technologies, bolometric Ge crystals and two phase xenon, with the UK ZEPLIN III close behind.

Limitation of liquid and solid dark matter detectors

To make sensitive dark matter detectors, youneed lots of target nuclei all set up to recoil

Lots of target atoms veryclose together make it

impossible to tell wherethe WIMP came from

diffuse target atoms provides information on the WIMP direction of

incidence

WIMP

nuclear recoil ~ nm

nuclear recoil ~mm

WIMP

Benefits of detecting the direction of incidence.

Dark Matter

Stars

If we can show that the nuclear recoils in the detector are correlated with the motion of our planet through the WIMP

halo this is solid proof we are detecting WIMPs.

DRIFT II - Worlds first directional dark matter detector

!"#$%&'()'*+&',-.!*/..0'1&2&324%'4$25"1&'46'2+&'783$$9'

7&55&:'8;1'5+"&:1";#)'

<'

=' >'

!"#$

%&'()*)#"+,

-#."%/.%0(#,112#,

/%()0"%.)34,

+35"/#,

6#*.,

78!9,

6#*.,*)#(:,

/%;#, 9#4."%(,

/%.<3:#,

!"#$%&'$()'#"*(

+$,%%#&#*(

-./0(

1#$2'"(

3%24(

5&'6%(5'&#$%'27(

1#$2'"(

#"#$%&27(

8+9:(

8+9(

!"#$%&'()'*+,-'"./&%01/"2.'".'/3&'45+!6'7&/&1/2%)(

Gaseous dark matter detector - 1m3 prototype

A 100kg directional dark matter detector

7.7 Alternative DRIFT Array 185

configuration of DRIFT detectors as illustrated in Figure 7.7. It is assumed that,

in operational mode, such an array would be adequately shielded from external

neutrons and the dominant background source would again be produced from

internal components, principally the stainless steel vessels. Thus, for the purpose

of this simulation, background neutrons were fired isotropically from all stainless

steel vessels using the neutron energy emission spectra for 1 ppb U and 1 ppb Th

as previously described in Section 7.4.

Figure 7.7: A 5× 5× 25 self-vetoing DRIFT array in which modules are posi-tioned 50 cm apart from one another. For simulation purposes the entire arrayhas been placed inside a large cavern with sufficient space to simulate variousshielding infrastructures. The modelled NaCl walls have a thickness of 3 m,adequate to produce over 99.9% of neutrons that penetrate the rock-laboratoryboundary in the JIF facility.

Within the Monte Carlo, a total of ∼ 8.6× 105 neutrons were emitted from

the stainless steel. Each time a nuclear recoil energy deposition occurred in any

of the modules’ fiducial volumes, the event data were recorded and outputed to

an ASCII file. In order to determine the array’s potential for vetoing events, the

50m

Conclusions• Dark Energy comprises 72% of the universe, and

this fraction is very poorly understood.

• For current physical models of the universe to make sense, roughly a quarter of its mass must be invisible matter.

• The alternative is a complete rewrite of the laws of physics, including Newton’s laws.

• Experiments to try and detect dark matter are underway! They are very hard, but a lot of fun.

Thanks for listening!