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 Out Too 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 ~ kg or 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 Satellite Gamma-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 Field Lots 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 Gravity Astronomers 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 explanation Spectacular 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 observations Bullet 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