Dark Matter in our Galaxies

Niveditha RamasubramanianAdvisor: Prof. Kenneth Lanzetta

Date : 19-OCT-2015

Layout of the presentation1) Evidences - Galaxy clusters, Galactic rotation curves, Gravitational

lensing, Cosmic Microwave background radiation2) Modified Newtonian dynamics 3) Dark matter candidate4) Method of detection - Direct and Indirect5) Experimental layout of CRESST6) Results from CRESST

Evidence: Galaxy Clusters - 1933Fritz Zwicky measured the mass of the cluster of galaxy by two methods and compared them.

1) Observed the doppler shift and from that the velocity and used virial theorem to get the mass of the cluster.

2) Observed the total light output and compared that with a model of kapteyn stellar system’s light vs mass and obtained the mass of the cluster

The mass obtained by the two methods were not in agreement with each other. And he proposed existence of “Dark” Matter - those that do not interact with light

Evidence: Galactic rotation curves - 1970Regular Newtonian dynamics suggests that we move away from the centre of mass of the galaxy.

Vera rubin and Kent Ford studied various spiral galaxies and studied that the velocity of stars remained constant after a point. And on further observational points to the right, it continued to be constant and did not show any trend of decreasing velocities.

Evidence: Galactic rotation curves - 1970

Modified Newtonian Dynamics-To account for the evidences provided by galactic rotation curves, M. Milgrom suggested a modification to newtonian dynamics as an alternative to the existence of dark matter in 1983

- He suggested a new substitution for acceleration in the Newtonian dynamics which looks like

Where a0 = 1.2*10-8 cm/s2

This modification explained the galactic rotation curves to great accuracy. But failed to explain other evidences.

Evidence - Gravitational lensing- From GR, we know that massive objects bend space time and light travels

through this curvature. “Light is bent by the gravity”

- By observing the distortion in background galaxies, we can find about the mass of the cluster.

Evidence - Gravitational lensing- This when compared with the mass of the galaxy measured by luminosity

measurement has large discrepancies.

Most likely Dark Matter candidate - Weakly Interacting Massive ParticleS (WIMPS)

- These are cold dark matter which are relic from falling out of thermal equilibrium as universe expanded.

- We think that it interacts via weak forces by knowing the relic density and the required cross-section of annihilation to account for this.

Methods of detectionDirect detection - In this method, a large tank of super cooled crystal is kept in an underground laboratory and event by event analysis of particle (electron, photon, WIMP) interaction with the crystal.

Simultaneous measurement of scintillation, ionization and nuclear recoil signals helps in distinguishing each particle and ideally give the number of “pure” wimp signal. There will also be a directional variation of detection with months as the solar system moves through the halo of dark matter.

Laboratories: LUX, CRESST, particle and astroparticle xenon detector.

Indirect Measurement - Dark matter particles are thought to be majorana particles. That is, they are their own antiparticle. In places, where there are high density of dark matter,like the interior of the sun, earth, centre of galaxies, they might interact with themselves, annihilate and produce gamma rays or other particle-antiparticle pairs from the standard model. And some experiments, look for evidences of these products of annihilation.

Experiments : EGRET, PAMELA

Cryogenic Rare Event Search with Superconducting Thermometers (CRESST)

- present at Grandsaso, Italy.- search for WIMPS- direct detection- 1400m below the surface of the earth - Nice video here

http://www.cresst.de/video_en.html

CRESST

CRESST - Overall view

CRESST - general setup

CRESST - Crystal Module

CRESST - Expected range and output

Results - 2013 and 2015

ReferencesGeneral idea and Dark matter evidences:

http://www.cpt.univ-mrs.fr/~cosmo/EC2012/Situation12_a.html

http://background.uchicago.edu/~whu/beginners/introduction.html

https://medium.com/starts-with-a-bang/five-reasons-we-think-dark-matter-exists-a122bd606ba8

http://www.learner.org/courses/physics/unit/text.html?unit=10&secNum=3

Direct and Indirect measurement

http://arxiv.org/pdf/1203.2566v1.pdf

http://arxiv.org/abs/1003.0904

CRESST

http://arxiv.org/pdf/1205.4882v3.pdf

http://arxiv.org/pdf/1509.01515v1.pdf

http://www.cresst.de/material.html