Recent developments in the understanding of Dark Matter

Liverpool Physics Teachers Conference 20th June 2013 Recent developments in the understanding of Dark Matter Phil James Liverpool John Moores University Astrophysics Research Institute

OUTLINE OF TALK Astrophysical evidence for dark matter The possible nature of dark matter Direct detection of dark matter The current status of dark matter successes, problems and alternatives

Astrophysical evidence for dark matter involves galaxies Spiral galaxies

Astrophysical evidence for dark matter involves galaxies rapidly rotating disc systems

A near-ir view of our galaxy 2MASS all-sky image: J Carpenter

M87 Elliptical galaxies are very different from spirals old stars only, little rotation

A dwarf spheroidal galaxy

The first evidence for Dark Matter Fritz Zwicky

The Coma Cluster of galaxies

Zwicky 1937

Gravitational lensing in galaxy cluster Cl0024 (HST/STScI)

Dark matter in clusters Clusters of galaxies seem to have more mass than their observable contents (galaxies and X-ray emitting gas) Mass can be measured from galaxy velocities, gravitational lensing or the temperature of the X-ray gas. All show the same discrepancy

Dark matter in spiral galaxies Evidence comes from rotation velocities of stars and gas orbiting the outskirts of their host galaxies

Dark matter in spiral galaxies Evidence comes from rotation velocities of stars and gas orbiting the outskirts of their host galaxies Velocities come from a feature in the spectrum, plus the Doppler effect we can use star formation regions or atomic gas

Do spiral galaxy rotation curves look like the predictions from Newtonian gravitation?

No!

Dark matter in spiral galaxies Most spiral galaxy rotation curves show no decline at large radii Implies that spiral galaxies sit at the centre of huge haloes of dark matter If correct, the dark matter outweighs all the normal matter (gas and stars) in these galaxies

Dark matter in elliptical galaxies

Dark matter in elliptical galaxies Harder to measure than in spirals: No HI gas Stars concentrated towards galaxy centres No emission lines in spectrum Recent ingenious observations using planetary nebulae, with surprising results (PN.S on the William Herschel Telescope, Isaac Newton Group, La Palma)

Romanowsky et al. 2004: Velocities of planetary nebulae in elliptical galaxies

Romanowsky et al. 2004

If Romanowsky et al. are correct, elliptical galaxies may have much less dark matter than do spiral galaxies This would be a problem for some theories of formation of elliptical galaxies

Merging of two disk galaxies. Simulation by J. Dubinski

Later paper on a bright elliptical galaxy, NGC 4374. Same technique applied, using Planetary Nebula Spectrograph, but with very different results. This galaxy does seem to have a dark matter halo.

Napolitano et al. 2010

There seem to be two types of elliptical galaxy in terms of DM propertieswhy?

Dark matter in dwarf galaxies

The faintest dwarf galaxies also need dark matter, and may be the most DM-dominated of all

The possible nature of dark matter MACHOs vs WIMPs

One method used the stars of the LMC

Microlensing technique (suggested by Paczynski, used by Alcock and collaborators) found some dark objects between us and the LMC. However, too few to make up the Galactic dark matter. Generally accepted now that MACHOs cannot make up the bulk of the dark matter

So the WIMPs win but what particles? No shortage of candidates for weaklyinteracting particles that could comprise the Dark Matter neutrinos known to exist, plus axions, supersymmetric counterparts of known particles ( neutralinos ) and many others First astrophysical constraints from Marc Aaronson s three stars

Subsequent observations find high velocity stars (~10 km/s) in all dwarf spheroidal galaxies These are the most DM-dominated galaxies known, and those for which the constraints against hot dark matter are the strongest These dwarf galaxies are almost pure DM, so uncertainties in stellar properties, and effects of normal matter on DM are unimportant

Dark Matter the astrophysical evidence Lots of DM in galaxy clusters (velocities, lensing) Lots of DM in spiral galaxies (rotation rates) Evidence for DM in elliptical galaxies less certain Dwarf galaxies possibly the most DMdominated of all, and may give the best tests of DM properties Generally accepted that we live in a Universe with 5% normal matter, 27% Dark Matter and 68% Dark Energy

Direct detection of DM Need high-mass, high-sensitivity detectors, well-shielded from background radiation Current instruments are 10s of kg of pure crystalline solids (e.g. sodium iodide), germanium, silicon,or liquefied noble gases (e.g. xenon) Placed in deep mines, e.g. Boulby mine in north Yorkshire, or the Gran Sasso laboratory in Italy Some methods can determine the direction of motion of a particle, others are nondirectional

Direct detection of DM Recent results from Cryogenic Dark Matter Search experiment in the US. On 15 th April 2013, they reported the detection of 3 candidate DM detection events from 140 kg-days of observation with a silicon-based detector. Estimated 5% chance of it being a statistical fluctuation. If correct, the best estimate of the particle mass is 8.6 GeV.

DM and the LHC

DM and the LHC Chance of detecting lightest, stable, supersymmetric particle through energy/ momentum imbalance in events detected by, e.g., Compact Muon Solenoid No evidence for such events from LHC yet; have narrowed possible parameter space

Dark Matter decays in space? Alpha Magnetic Spectrograph on ISS Detects an unexplained high-energy excess of positrons over electrons May result from decaying dark matter, but there are other explanations

Current status of dark matter- successes, problems and alternatives

CDM predictions large scale structure Qualitatively excellent agreement, Quantitatively requires some free parameters Millennium Survey, Springel et al.

CDM predictions numbers of satellite galaxies Possible substructure problem for cosmology Cold Dark Matter simulations predict that our Galaxy should have ~500 dwarf companions, not the ~20 observed Fixed by invoking strong intergalactic UV radiation, causing photo-ionisation and preventing gas from collecting in low-mass haloes

CDM predictions numbers of satellite galaxies

Alternatives to dark matter Several suggested alternatives to dark matter Most resilient of these is Milgrom s Modified Newtonian Dynamics, MOND Essentially Newtonian, not GR, at least in Milgrom s formulation; involves putting a floor on the gravitational acceleration at very low values Accelerations involved well below any laboratory, or even solar system, tests Works well for spiral and dwarf galaxies; does not explain the DM problem in galaxy clusters

However, most are not convinced, and 90% of astrophysicists working on cosmology and galaxy properties explicitly or implicitly assume the existence of dark matter in all of their research

The Bullet Cluster conclusive evidence for dark matter? Interacting pair of galaxy clusters found by Clowe et al. (2006) Appear to have passed through one another, with the dark matter and galaxies emerging as two separate clumps, but the gas left behind to show where the collision occurred As expected for weakly-interacting dark matter and strongly-interacting gas:

The Bullet Cluster conclusive evidence for dark matter? Looks extremely interesting, but there are some concerns: Only one example of such an object could be a chance alignment or random occurrence which happens to give this appearance Modelling of the system by the Clowe et al. deduces that the collision occurred at 4700 km/second