“Dark Matter in Modern Cosmology”

Sergio Colafrancesco

SummaryIntroduction

Dark Matter probes

Future of Dark Matter

Hystorical background and gained evidence

Motivations Dark Matter candidates

Types of probes Analysis of neutralino annihilations

Problems in DM probes

Multi-approch of DM problem The alternative approch:modified gravity

Introduction

Dark Matter Scientific revolution

DM

Local Global

Close to the plane of the Galaxy

Baryonic

Low amount

Dominating mass component

Large structures

Hystorical background and gained evidence

The problem

Radial velocities of galaxies in Coma cluster

Zwicky (1933)

Unexpected large velocity dispersion (бv)

Mean density ~ 400 times greater

Huge amount of “Dunkle Kalte Materie” (Cold Dark Matter)

Smith (1936) Mass of Virgo cluster

Unexpected high mass Excess of mass

“Great mass of internebular material within the cluster”

Babcock(1939) Spectra of M31

Unexpected high rotational velocity in the outer regions

High mass to light ratio in the periphery

Strong dust absorption

Oort(1940) Rotation and surface brightness of one edge-on SO galaxy (NGC3115)

“Distribution of mass in this system appears to bear almost no relation to that of light”

Kahn & Woltjer(1959) Motion of the galaxy M31 and of the Milky Way

M31 and the Galaxy started to move apart ~ 15Gyr ago

The mass of the Local Group had to be greater than the sum of galaxies masses

Missing mass in the form of hot gas (T~5•105 k)

Rotation curve of M31

Roberts & Whitehurst (1975)

No Kleperian drop-off High mass to light ratio in the outermost regions(› 200)

Missing mass exist in cosmologically significant amounts

Confirmation of the presence of unknown matter by indipendent sources (beginning of the 1980’s)

Dynamics of galaxies and of stars within galaxies

Mass determinations of galaxy clusters based on gravitational lensing

X-ray studies of clusters of galaxies

N-body simulations of large scale structure formation

The CMB contribution

Theory of fluctuations to explain the formation of structures

Expected amplitude of the baryonic density fluctuations at the epoch of recombination

First detection of the CMB (1965): relic emission coming from the epoch of recombination

COBE(1992): the amplitude of the fluctuations appears to be lower than expected

Solution: Non-baryonic dominating DM component

Dark Matter candidates

Neutrinos High velocities HOT DARK MATTER

No galaxy can be formed

Hypothetical non baryonic particles

Low velocities

COLD DARK MATTER

Astro-particle connection

Search of the nature of Cold Dark Matter

Properties of CDM candidates

Fluid on galactic scales and above

Must behave sufficiently classically to be confined on galactic scales

Dissipationless

Collisionless

Cold

Upper and lower bounds on the mass of the particle

Most important candidates

NeutralinosSterile neutrinos

Light DM

Lightest particle of the minimal supersymmetric extension of the Standard Model (MSSM)

Lightest right-handed neutrino

Motivations

Galaxy rotation curves

Dwarf galaxy mass estimators

Lensing reconstruction of the gravitational potential of galaxy clusters and large scale structures

Combination of global geometrical probes of the Universe(CMB) and distance measurements (Sne)

Galaxy cluster mass estimators

Large scale structure simulations

Dark matter probes

Types of probes

Inference probesPresence, the total amount and the spatial distribution of DM in the large scale structures

Dynamics of galaxies

Hydrodynamics of hot intra-cluster gas

Gravitational lensing distortion of background galaxies

Physical probes Nature and physical properties of DM particles

Astrophysical signals of annihilation or decay

Wide range of frequencies

Analysis of neutralino annihilations

Focus

Particle: neutralino (Mχ range: few GeV to a several hundreds of GeV )

Astrophysical laboratories:Galaxy cluster

Dwarf spheroidal galaxies

Neutralino annihilation

у-ray emission

Synchrotron radiation

Bremsstrahlung radiation

Inverse Compton Scattering (ICS)

Neutrinos

SED

mass

cross section

composition

A general view

General informations

Annihilation rate: R = n (r) <>

n (r) = n g(r)

Annihilation cross section: <>

Wide range of values (theoretical upper limit <> < 10-22 (Mχ/TeV)-2

cm3/s)

Particles produced

Annihilation χ-χ

Quarks, leptons vector bosons and Higgs bosons

Depending on physical composition

Decay

Secondary electrons and positrons

Energy losses

Spatial diffusion(relevant on galactic and sub-galactic scales)

SED

Gamma rays emission:

Decay: Continuum spectrum

Bremsstrahlung and ICS of secondary e±

Coma cluster:

Draco dwarf galaxy:

Radio emission:Synchrotron emission of secondary e±

Diffuse radio emissionComa cluster:

ICS of CMB: from microwaves to gamma-ray

Secondary e± up-scatter CMB photons that will redistribuite over a wide frequency range up to gamma-ray frequencies

ICS of CMB: SZ effect from DM annihilation

Secondary e± up-scatter CMB photons to higher frequecies producing a peculiar SZ effect

Heating: Secondary e± produced heat the intra-cluster gas by Coulomb collisions

The radius of the region in which DM produce an excess heating increases with neutralino mass

Cosmic rays: Neutralino annihilation in nearby DM clumps produce cosmic rays that diffuse away

Future of Dark Matter

Problems in DM probes

Direct and indirect probes for DM have not yet given a definite answer

Some of the anomalies are not easy to explain within canonical DM models

DM that has no standard model gauge interactions

The DM induced signals are expected to be confused or overcome by other astrophysical signals

Ideal systems Multi approach

Multi approach of DM problem

Multi - frequency

Multi - messenger

Multi - experiment

The alternative approach:modified gravity

Mismatch between the predicted gravitational field and the observed one

When effective gravitational acceleration is around or below: a~10-7 cms-2 (weak gravitational field)

Newtonian theory of gravity break down?