The Mystery of Dark Matter Maxim Perelstein, LEPP/Cornell U. CIPT Fall Workshop, Ithaca NY, September 28 2013
Introduction Last Fall workshop focused on physics of the very small - elementary particles This time, we ll talk about very large - astronomical - distance scales... and some interesting connections between the two!
Distances in the Universe Solar System 10^12 m Galaxy 10^21 m Cluster of Galaxies 10^23 m Visible Universe 10^26 m Earth 10^7 m Evidence for Dark Matter comes from these scales
Measuring Distances
Doppler Effect in Astronomy Analyze spectra of light from distant stars/galaxies Identify absorption (or emission) line patterns of familiar elements observed on Earth Overall shift of lines - Doppler effect - allows to determine relative velocity of the star/galaxy with respect to Earth
Expanding Universe Light from all distant galaxies is redshifted they are running away from us! (Hubble, 1930 s) Picture: Universe is like an expanding gas of galaxies Extrapolate back in time: there was a time when density was infinite - BIG BANG! Note: Uniformly expanding gas has no center - any observer (far away from the walls) would see the same picture!
Universe: Infinite or Finite? horizon : signals sent from here 14 bln years ago are just reaching us now (R ~ 10^26 m) Light: c=3*10^8 m/sec You Are Here visible universe : inside the horizon Big Bang Theory: Age = 14 billion years Inaccessible universe : outside the horizon. Finite? Not? We have no way to tell.
Evidence for DM on the Galactic Scale Most galaxies (including ours) are orbited by satellites, much like Sun is orbited by planets. Satellites are just baby galaxies.
Rotation Curves Predicted Newton s law of gravitation
Rotation Curves Observed? NO! Kuzio de Naray et al, The Astrophysical Journal Letters, Volume 710, L161-L166 (2010) Two Possible Conclusions: Newton s Law of Gravity is wrong at large distances OR... Visible galaxy is surrounded by gravitating dark matter
Dark Matter Halo v independent of R implies
Gravitational Lensing Einstein s General Relativity: any gravitating mass should bend light Massive objects between source and observer can act as gravitational lenses Observation of lensing provides information about the mass of the lens - a way to weigh distant galaxies
Weak Lensing Distortion of shapes of distant sources Allows to map spatial profile, or distribution, of mass in the distant galaxy (or cluster of galaxies) that acted as the lens
Evidence for DM: Bullet Cluster Observation: profile of gravitating mass is NOT the same as the profile of visible (luminous) mass direct evidence for the existence of dark matter inconsistent with modifications of Newton s law
Summary So Far Motion of galactic satellites breakdown of Newton s law OR extra matter, gravitating but invisible ( dark ) Gravitational lensing must be dark matter Some possibilities: Huge hydrogen clouds? Non-luminous star remnants? Black holes? Sub-stellar mass objects (e.g. MACHOs )? The answer turns out to be much more exciting - stay tuned!
More Evidence for DM: Echoes from the Early Universe Nucleosynthesis : protons+neutrons combined to form He, Li, B,... Recombination : protons+electrons combined to form atoms
Cosmic Microwave Background Light: c=3*10^8 m/sec You Are Here In the early universe, photons are part of charged plasma - a hot gas, at a temperature of about 4000 K at recombination After recombination, photons no longer interact - decouple - and they come straight to us and can be observed! Provide direct information about conditions in the Universe just 400,000 yrs after Big Bang.
CMB:Observations Observing CMB: see Michael Niemack s talk and lab tour later today Photon energy can be measured, and is simply related to the temperature of the gas at the time when it decoupled, in the place where it comes from First conclusion: Universe at 400,000 years of age was remarkably homogeneous - no big lumps! But subtle variations in energy (at the level of 1 part in 100,000) exist, indicating tiny inhomogeneities in the primordial plasma
CMB:Observations Details of CMB inhomogeneities contain a wealth of information about properties of the Universe (average on very large distance scales) Since their discovery in 1992 by the COBE satellite, measuring and interpreting the CMB inhomogeneities has been a major research program 1992 2003 2013 Now we know (among other things): the total (average) density of the Universe, and the total (average) density of ordinary matter (protons, neutrons, electrons).
Weighing the Universe Space can be curved : e.g. a 2D world on a sphere In curved space, parallel lines can meet, and triangle angles do not add up to 180 What about our 3D space? On everyday distance scales, experience suggests it is flat At cosmological scales (~10^26 m), this is an experimental question, which was only answered recently, by studying CMB Einstein s General Relativity connects the average density of energy (or mass) in the Universe to the curvature: FLAT CLOSED OPEN
CMB:Curvature Determination Need an object of a known size, at a known distance from us Measure its apparent size infer bending of light rays infer curvature The size of hot/cold spots in the primordial gas at recombination time provides just such an object!
CMB:Curvature Determination Need an object of a known size, at a known distance from us Measure its apparent size infer bending of light rays infer curvature The size of hot/cold spots in the primordial gas at recombination time provides just such an object! The Universe Is Flat:
Normal Matter Density Nucleosynthesis : protons+neutrons combined to form He, Li, B,... All matter on Earth consists of: protons, neutrons, electrons, photons, neutrinos Almost all mass is contained in protons and neutrons: CMB hot/cold spot pattern provides information about the density of these particles at recombination time Independent verification is provided by measurements of abundances of He, Li, B, etc., which are related to proton and neutron densities by the theory of nucleosynthesis
Normal Matter Density Nucleosynthesis : protons+neutrons combined to form He, Li, B,... All matter on Earth consists of: protons, neutrons, electrons, photons, neutrinos Almost all mass is contained in protons and neutrons: CMB hot/cold spot pattern provides information about the density of these particles at recombination time Independent verification is provided by measurements of abundances of He, Li, B, etc., which are related to proton and neutron densities by the theory of nucleosynthesis Normal Matter is 4% of the Total
What Is the Universe Made Of? 96% of the Universe is Dark : Dark Matter and Dark Energy (DE is topic for another day!) Dark matter CANNOT be gas, small stars, MACHOs, etc. - all those things are made of protons, neutrons, and electrons, while DM is NOT!
DM and Particle Physics Particle physicists love their Standard Model, considered a great triumph of physics [see my Fall 12 CIPT talk] Most particles in the SM are highly unstable - cannot account for DM Only stable particle that s not constrained by 4% is neutrino, but they are too light to be DM Conclusion: DM must be made out of NEW, as yet undiscovered particles!!! DM? DM particle must be electrically neutral, and interact only weakly with ordinary matter
Example: Supersymmetry DM Candidates Supersymmetry is the idea that each SM particle has a new, much heavier (but still tiny) particle associated with it - superpartner In simple models, the lightest supersymmetric particle - LSP - is stable If electrically neutral, it can be dark matter!
Dark Matter and Colliders collider detector High-energy colliders, such as the Large Hadron Collider (LHC), can produce new dark matter particles! Signature in the detector: missing momentum event : DM production (computer simulation) Not observed so far - another chance in the next LHC run, in 2015-18
Dark Matter Underground There s dark matter all around us! Density depends on the mass of the DM particle: e.g. for SUSY dark matter, expect about 1 per coffee cup DM particles can scatter on ordinary nuclei, deposit (small) energy Detecting requires going deep underground (to shield from cosmic rays), low temperatures (to suppress thermal noise), and large detectors (to increase signal rates) Many experiments around the world - the hunt is on!
Direct DM Searches 10-39 Evolution of the s SI for a 50 GeVêc 2 WIMP WIMP-Nucleon ssi @cm 2 D 10-41 10-43 10-45 10-47 Ê Ê Ê Ú Ê Ê Ú Ê ÚÚ Ê ÚÁ Ì Ì Ûı Ì Û Ì Û Á Ê Cryogenic Detectors Crystals Ï Liquid Argon Ú Liquid Xenon Ù Threshold Detectors 10-49 1990 2000 2010 2020 Year
γ γ Dark Matter in the Sky Indirect Detection Indirect Detection γ γ Fermi Fermi VERITAS VERITAS Super-K Super-K ICECUBE ICECUBE Super K Super K PAMELA PAMELA +, e+, p, p e e, e, p, p AMS AMS
Cosmic Positrons data data consistent with DM origin of the excess, although more prosaic explanations (e.g. pulsars) are also possible standard astrophysics prediction
Conclusions Evidence for dark matter comes from dynamics of galaxies, gravitational lensing, and cosmic microwave background The scientific case for existence of dark matter is rock-solid. It makes ~25% of mass in the Universe It cannot be made of known elementary particles Theories of new elementary particles often have dark matter candidates, e.g. supersymmetry The hunt is on for the DM particle: at colliders, underground, and in the sky Stay Tuned!