Dark Matter in Our Universe: Dark Matter in Our Universe Candi M. Yurkovic Advisor: Professor R. Koopmann May 10, 2002


Slide2: Where Are We?


Slide of where we are in the Universe….: Slide of where we are in the Universe….


Dark Matter in Galaxies and Galaxy Clusters: Dark Matter in Galaxies and Galaxy Clusters Spiral Galaxies Ellipticals Galaxy Clusters


SPIRAL GALAXIESMass Determination Methods: SPIRAL GALAXIES Mass Determination Methods Mass-Luminosity Relation – Given the size and luminosity of a galaxy we can determine the amount of matter it must contain. Note H-R Diagram which plots temperature (x-axis) vs. luminosity (y-axis). Mass associated with luminosity is 10% of a galaxy’s mass….  This suggests DARK MATTER


SPIRAL GALAXIESMass Determination Methods (cont.): SPIRAL GALAXIES Mass Determination Methods (cont.) Rotations – The mass of spiral galaxies at large radii must be obtained using the rotation speed of stars at various distances.  Rotation Curve Method The magnitude of the Doppler Effect determines the velocity at which the matter is rotating around a particular galaxy. The velocities derived from the Doppler Shift are plotted as a function of radius on the rotation curve. Orbital velocity and distance (from the center of a galaxy) are needed to compute the mass.


SPIRAL GALAXIESMass Determination Methods (cont.)Rotation Curve : SPIRAL GALAXIES Mass Determination Methods (cont.) Rotation Curve 'Expected' What we really see'


ELLIPTICAL GALAXIESMass Determination Methods: ELLIPTICAL GALAXIES Mass Determination Methods Mass-Luminosity Relation – Luminous masses are estimated via the mass-luminosity relation.


ELLIPTICAL GALAXIESMass Determination Methods (cont.): ELLIPTICAL GALAXIES Mass Determination Methods (cont.) Velocity Dispersion Method -- Stars in ellipticals have more random orbits than stars in spirals  We measure the velocity of stars with a component of their motion in our line-of-sight .  Yields a lower mass limit  VELOCITY DISPERSION  Assume dispersion is typical for every L-O-S and estimate mass needed to support observed velocity dispersion.


GALAXY CLUSTERSMass Determination Methods : GALAXY CLUSTERS Mass Determination Methods Similar to Velocity Dispersion in Ellipticals – Look at motions of galaxies within a cluster  Lower limit for mass determined by L-O-S velocities. Given range of velocities and cluster size…  How massive must this cluster be to hold itself together with this range of velocities? ** Mass of galaxies in a cluster + DM andlt; what we measure gravitationally  DM between each galaxy!


Possible Dark Matter Candidates: Possible Dark Matter Candidates MACHOs Massive Compact Halo Objects WIMPs Weakly Interacting Massive Particles


A closer look at MACHOs…: A closer look at MACHOs… Possible Candidates - Brown Dwarfs - Low Mass, Faint Red Stars - White Dwarfs - Neutron Stars - Black Holes


A closer look at MACHOs (cont)…: A closer look at MACHOs (cont)… Detection Methods - Hubble Space Telescope - Gravitational Lensing


A closer look at MACHOs (cont)…: A closer look at MACHOs (cont)… Detection Projects - The MACHO Project - The EROS Collaboration - The Optical Lensing Gravitational Experiment (OLGE)


A closer look at WIMPs…: A closer look at WIMPs… What are they? - neutral particles formed during the Big Bang - pass through massive particles without interacting - exert and experience only gravitational (and possibly weak) forces - ie, photinos, neutrinos, gravitinos, axions - only neutrinos have been detected


A closer look at WIMPs (cont)…: A closer look at WIMPs (cont)… Detection Methods and Projects - The AMANDA Project (Antarctica Muon and Neutrino Detector Array) - The Cryogenic Dark Matter Search - The DAMA Experiment (Particle Dark Matter Searches with Highly Radiopure Scintillators at Gran Sasso)


Dark Matter and the Universe: Dark Matter and the Universe OPEN? Expands forever; density of Universe andlt; critical density for an open Universe CLOSED? Gravity slows expansion, halts it, reverses it into contraction, collapses into single point; actual density andlt; critical density FLAT? Density = critical density; enough mass to gravitationally stop Universe from expanding but not enough to collapse it back to a single point


Current Model of the Universe: Current Model of the Universe Recent observations of supernovae blasts in distant galaxies used to test model of the the Universe. Observations suggest that expansion of the Universe is actually accelerating. Universe is most likely FLAT!


HDM and CDM: HDM and CDM Hot Dark Matter - particles with masses of zero or near-zero - travel at/near speed of light - masses between 1 million and 1 thousandth the mass of an electron - primary candidate  Neutrino - HDM may comprise 20% of matter in the Universe Cold Dark Matter - sufficiently massive particles that travel at subrelativistic velocities - ie, WIMPs - masses 10-100 times a proton’s mass - CDM may comprise 70% of matter in the Universe


What if Dark Matter Does Not Exist???: What if Dark Matter Does Not Exist??? Basis of Newton’s Universal Law of Gravitation  the gravitational force between 2 objects is directly proportional to the product of their mass, and is inversely proportional to the sequence of the distance between them Current DM evidence assumes this theory applies to the large scale of galaxies Some astronomers find this assumption hard to make  MOND (Modified Newtonian Dynamics) F=ma does not hold  fits to observed rotation curves for more than 100 galaxies consistent