AST4320 - Cosmology and extragalactic astronomy Lecture 14 The Too Big to Fail Problem The Nature of Dark Matter 1
Previously on AST4320: Missing Satellite Problem (see review by Weinberg et al. 2013, arXiv:1306.0913) Left: simulated dark matter distribution in dark matter halo with M=10 12 M sun . Circles denote 9 most massive substructures or ` satellites ’. Right : Spatial distribution of observed Milky Way ` satellites’ . 2
Observational Constraints on Dark Matter Halo Profiles (see review by W. De Blok, arXiv:0910.3538) (Oh et al. 2011; THINGS* survey. Colored points are the dwarfs.) (* The HI Nearby Galaxy Survey)
The Nature of the Dark Matter 4
The Nature of the Dark Matter What is the dark matter, and why is it `cold’? Cosmic microwave background and observed large-scale structure in the Universe (i.e. clustering of galaxies) provide constraints on content of Universe: Ordinary matter (baryons, leptons, photons) make up ~ 4% of Universal energy density. `Dark energy’ accounts for ~73%. Dark matter accounts for the remaining ~23%. ` Just as the chocolate frosting glues the sprinkles together on the cupcake, dark matter binds baryons together to form galaxies, galaxy groups, and galaxy clusters. ’ A. Peter, 2013, arXiv:1201.3942 5
The Nature of the Dark Matter What is the dark matter, and why is it `cold’? Dark matter is not: • baryonic: evidence from cosmic microwave background, large scale structure, and also from Big-Bang Nucleosynthesis (maybe more on this in later lecture) 6
The Nature of the Dark Matter What is the dark matter, and why is it `cold’? Dark matter is not: • baryonic: evidence from cosmic microwave background, large scale structure, and also from Big-Bang Nucleosynthesis (maybe more on this in later lecture) • composed of `light’ (m X < keV) particles. BB. 7
The Nature of the Dark Matter What is the dark matter, and why is it `cold’? Dark matter is not: • baryonic: evidence from cosmic microwave background, large scale structure, and also from Big-Bang Nucleosynthesis (maybe more on this in later lecture) • composed of `light’, m X < keV, particles. These particles would be `relativistic’ when T of the Universe was ~ 1 keV. This would suppress growth of structure on `small’ scales at levels that are at odds with Lyman alpha forest (next lecture) constraints. This is illustrated on the next slide. 8
Constraints on (Warm) Dark Matter Observational constraints mass power spectrum `primordial’ P(k) `Meszaros’ suppression r H matter-radiation equality T~ eV `r H ‘ relativisitic WDM m ~ keV Lyman alpha forest indicates that m DM > keV 9
Constraints on other Properties of Dark Matter Constraints on electro-magnetic charge Constraints on self-interaction. ` Self-interaction’ refers to interactions among (different species of) dark matter particles, mediated by e.g. `dark gauge bosons’. 10
Constraints on other Properties of Dark Matter “Bullet Cluster”: two merging clusters. Pink : hot X-ray emitting gas. Blue : dark matter in the cluster, determined from measuring the lensing signal (lecture~20) from the visible-light images of the galaxies. 11
Constraints on other Properties of Dark Matter Constraints on electro-magnetic charge. Constrained by small-scale fluctuations in Cosmic-Microwave Background (see Sigurdson et al. 2004) Constraints on self-interaction. `Self-interaction’ refers to interactions among (different species of) dark matter particles, mediated by e.g. `dark gauge bosons’. Could alter predicted structure of dark matter halos. 12
Constraints on other Properties of Dark Matter Constraints on self-interaction. `Self-interaction’ refers to interactions among (different species of) dark matter particles, mediated by e.g. `dark gauge bosons’. Could alter predicted structure of dark matter halos. Example : Recent example of self-interacting dark matter as a solution to the `cusp-core’ problem (with velocity dependent collision cross-section). 13
Constraints on other Properties of Dark Matter Density profiles in cosmological simulations that have self-interacting Dark Matter (SIDM). Slope of density profile flattens from Cusp to Core. Vogelsberger et al. 2012 Example of self-interacting dark matter as a solution to the `cusp-core’ problem 14 (with velocity dependent collision cross-section).
Constraints on other Properties of Dark Matter Density profiles in cosmological simulations that have self-interacting Dark Matter (SIDM). Vogelsberger et al. 2012 SIDM reduces tension between kinematics in observed and simulated satellites. 15
Summary Empirical Constraints Dark Matter Mass of dark matter particle > keV from Lyman alpha forest observations. Dark matter is at least colder than warm . Collisionless nature of dark matter particle constrained by cluster lensing + X-ray data. Cross-section for `hard-sphere’ elastic scattering though recently some models of self-interacting DM have been put forward that bypass cluster constraints while addressing core-cusp + too big to fail problems in dwarf galaxies Cosmic-Microwave Background limits the charge of the dark matter particle (see Sigurdson et al. 2004) 16
Some Dark Matter Candidates I: WIMPs WIMP : W eakly I nteracting M assive P article s . Popular because: 17
Some Dark Matter Candidates I: WIMPs WIMP : W eakly I nteracting M assive P article s . Popular because: `Electro-weak’ energy scale at ~200 GeV, above which weak and electromagnetic interaction merges into the `electroweak’ interaction. It is thought that new particles* should exist around this mass-scale. This new particle annihilates into quarks + antiquarks in the early Universe, until density and temperature drops sufficiently that annihilation becomes increasingly rare. The comoving number density n X `freezes’ out. The`predicted’ mass density in this relic density of particles - for the standard assumptions for the mass and annihilation coupling strength - comes out at The fact that particle physics considerations alone, can give the correct order of magnitude for WIMP mass density is referred to as WIMP Miracle . * what these particles are depends on the new physics that is introduced at the electroweak scale. Examples of WIMPS are supersymmetric neutralino, Kaluza-Klein photon,... 18
Some Dark Matter Candidates I: WIMPs WIMP : W eakly I nteracting M assive P article s . Popular because: 19
Some Dark Matter Candidates I: WIMPs WIMP : W eakly I nteracting M assive P article s . Popular because: 20
Some Dark Matter Candidates II: Other New Particles Other candidates include: • Axions : hypothetical particle introduced to resolve the strong CP problem in QCD. Caution: I know little about this. There are many reviews on dark matter candidates out there (often with the obscure title `Dark Matter’). I followed Peter’s review that has many references in there. 21
Some Dark Matter Candidates II: Other New Particles Other candidates include: • Axions : hypothetical particle introduced to resolve the strong CP problem in QCD. • Gravitinos : supersymmetric partner of graviton. Not as popular as WIMPs because hard to detect & tuning required to get matter density correct. Caution: I know little about this. There are many reviews on dark matter candidates out there (often with the obscure title `Dark Matter’). I followed Peter’s review that has many references in there. 22
Some Dark Matter Candidates II: Other New Particles Other candidates include: • Axions : hypothetical particle introduced to resolve the strong CP problem in QCD. • Gravitinos : supersymmetric partner of graviton. Not as popular as WIMPs because hard to detect & tuning required to get matter density correct. • Sterile Neutrinos : neutrinos that do not act electroweakly. Introduced to generate mass for `active’ neutrinos, explain neutrino experiment anomalies,... Caution: I know little about this. There are many reviews on dark matter candidates out there (often with the obscure title `Dark Matter’). I followed Peter’s review that has many references in there. 23
Some Dark Matter Candidates II: Other New Particles Other candidates include: • Axions : hypothetical particle introduced to resolve the strong CP problem in QCD. • Gravitinos : supersymmetric partner of graviton. Not as popular as WIMPs because hard to detect & tuning required to get matter density correct. • Sterile Neutrinos : neutrinos that do not act electroweakly. Introduced to generate mass for `active’ neutrinos, explain neutrino experiment anomalies,... • Hidden sector dark-matter : dark sector may be as rich as ordinary standard model, but not `communicate’ much at all. These sectors are referred to as `hidden’ sectors, which may contain `dark photons’. Caution: I know little about this. There are many reviews on dark matter candidates out there (often with the obscure title `Dark Matter’). I followed Peter’s review that has many references in there. 24
HARD Dark Matter Searches. Searches for dark matter can be done in • Colliders : given that dark matter is neutral and weakly interacting, they behave like giant neutrinos in colliders. Missing energy* in collisions may hint at existence of e.g. WIMPs. So far, no evidence for physics beyond standard model. Moreover, even if hints for a WIMP are found, it is unclear whether it would be stable over cosmological times (let go longer than a ns). 25
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