Upload
julius-woods
View
216
Download
0
Tags:
Embed Size (px)
Citation preview
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