Astronomy 1143 – Spring 2014

Lecture 30:Dark Matter Revisted…..

Key Ideas:

Dark matter observations• 23-27% of the Universe (M~0.3)

• Cold dark matter – particle in nature• Cannot be made from protons/neutrons present

during BBN

Dark matter candidates – • Possibles: WIMPS, axions, gravitinos• Not possibles: brown dwarfs/planets/white

dwarfs/neutron stars/black holes, neutrinos

Key Ideas

Determining the nature of dark matter• Attempts to see annihilations producing -rays• Attempts to detect particle interactions here on

Earth, such as nuclear rebound

Dark Matter vs new form of gravity• DM proposed to explain motions of stars and

galaxies and gravitational lensing – alternate explanation?

• Bullet Cluster argues for DM rather than MOND

Possibilities Ruled OutToo few gravitational microlensing events for the

dark matter to be black holes, neutron stars, white dwarfs, brown dwarfs or other “lumps” with the mass of stars

Too much D in the Universe for there to be so much “normal” matter

Galaxy formation shows that DM is cold (or at most warm) not hot

Therefore a weakly interacting massive (non-baryonic) particle is preferred…

D and He as densitometers

Prediction if dark matter were baryons

The Particle Zoo

Protons, neutrons, electrons – the components of ordinary matter

Neutrinos – important for nuclear reactions, very small cross-section, but very small mass

Many other particles out there• E.g. muons, pions, Z bosons, …• Many are unstable, and we don’t ordinarily

encounter them

WIMPs

We need a particle that

• is massive = “cold” (speeds <<<< c) mass of ~10-25 kg or 10-20 protons

• interacts very weakly or not at all

• has a high density in the Universe

• is stable for a long time or forever

Weakly Interacting Massive Particles are predicted by particle physics models

Ways to Detect Dark Matter

WIMPs & Annihilation

Particle models born out of attempts to understand the forces of nature

Examples include axion, neutralino, sneutrino

Some of theories predict dark matter particles will annihilate each other with a very small cross-section.

These annihilations will produce gamma-rays

Need a lot of dark matter to see this – look at Galaxy

Fermi SatelliteGamma-ray Satellite

Launched June 11, 2008

Gamma-rays come from many sources: decay of radioactive nuclei, explosions of massive stars, gas going into black holes, as well as possible dark matter annihilations

Predicted Gamma-Ray Signal from Dark Matter Annihilations

No Signal So Far

No detection of gamma-rays from DM annihilation in 1 yr Fermi data

Rules out some dark matter candidates, but leaves many, many more

But there is no guarantee that the dark matter particle annihilates

Another technique, nuclear recoil, could possibly detect dark matter, depending again on the cross-section

Nuclear Recoil Experiments

Dark matter particles hit and bounce off of nuclei of atoms – not absorbed or emitted, but energy is transferred

Energy is measured by photons or by heat emitted from nucleus

Expected rates are 1 per day per kg• Backgrounds (interactions from non-DM particles

are a huge problem)• Need very large detectors, preferably

underground

Why is Direct Detection so Difficult?

Dark matter doesn’t interact well with normal matter

Event rates are very, very low

Background events are very high – for example• Muons slamming into your nuclei

• Solution, go underground• Radioactive decays producing neutrons in your

material• Solution, attempt to use inert(er) material

• Neutrinos• Solution, attempt to determine direction

Direct Detection Experiments & Theory

Possible Detections?

Latest & Greatest Results from LUX experiment

State of the FieldLots of work, both theoretical and experimental,

is ongoing

No signal accepted by most scientists

It is possible that the dark matter particle cannot be detected by either nuclear recoils or annihilation signals

However, other experimental work in particle physics, such as the Large Hadron Collider, will provide evidence which model of particle physics is correct

Dark Matter or New Form of Gravity?

Can we explain the motions of stars, gas and galaxies by rewriting the equations of gravity from Newton/Einstein?

Leads to so-called “MOND” (Modified Newtonian Dynamics) theories

We know that our equations are wrong on the quantum scale, could they also be wrong on very large scales

Example: New Form of GravityAstronomers were puzzled by the

advance in the perihelion of Mercury

Wrong answer: Some proposed that there was a small solar system body near the Sun that was affecting Mercury’s orbit

Right answer: Einstein’s theory of General Relativity

Example: “Dark” Matter

Astronomers were puzzled by the orbit of Uranus, as it sometimes was moving faster and sometimes slower than expected

Right answer: Some proposed that there was a solar system body that was affecting Uranus’ orbit

1846: Neptune found in the predicted position

Is “Dark Matter” the only possible explanation?It is not easy to believe that we are unaware of

the nature of something that has 5-6 times more mass than “normal” matter

However, many lines of evidence are pointing to the same conclusion!

Possible counter explanation: • Neither Newton nor Einstein got the law of gravity

quite right. • On galaxy-sized scales, gravity stronger than

what Law of Universal Gravity states

Bullet Cluster-- suggests that DM, not Law of Gravity, is the explanationSpectacular example of gravitational lensing showing

evidence for dark matter

Two clusters of galaxies colliding – most of the normal matter is gas that is now between the clusters.

• Gas pancakes where the clusters collide• Galaxies pass right by each other• Dark matter pass right by as well

Map based on the lensing shows that there is a lot of mass centered on the two groups of galaxies

If we had the Law of Gravity wrong, the center of mass should be where the gas is!

Colliding Clusters

Bullet Cluster

Hot X-ray gas. Visible mass pretty much all here

Mass of cluster is here, according to lensing. This is where the DM should be

MOND has trouble explaining observationsBullet Cluster shows that the gravity is not

from the “normal” matter, but MOND requires that normal matter explain all motions

It is very difficult to find a theory of MOND that explains what we see over a large range of masses and distances

Important for stimulating theoretical and experimental work