Cosmological Structure Formation A Short Course. I. Why Dark Matter, and why Cold? Chris Power. Zwicky (1933) measured the radial velocities for eight galaxies in the Coma cluster and found an unexpectedly large velocity dispersion of ~1000 km/s.
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
I. Why Dark Matter, and why Cold?
He used the Virial Theorem to deduce that the mass density of Coma would have to be ~400 times that of the luminous matter -- although he assumed a Hubble parameter of ~500 km/s/Mpc. For present day value of Hubble parameter mass discrepancy of ~50.
What caused this mass discrepancy? What could resolve it?Evidence for Missing Matter
The time derivative of the virial leads to the virial theorem
For a gravitationally bound system, RHS tends to zero, so twice the kinetic energy should equal the potential energy.The Virial Theorem
“If this overdensity is confirmed we would arrive at the astonishing conclusion that dark matter is present with a much greater density than luminous matter”.
“From these considerations it follows that the large velocity dispersions in Coma (and in other clusters of galaxies) represents an unsolved problem”
… but waited for “New Physics” to resolve the discrepancy…Evidence for Missing Matter
The Russian school led by Yakob Zel’dovich, who argued for “top-down” or the “adiabatic” scenario
The American school led by Jim Peebles, who argued for “bottom-up” or the “isothermal” scenario
Both assumed baryons + radiation
Neither could work…
Adiabatic scenario predicted fragmentation of structure from pancakes
But also predicted enormous anisotropies in the CMB - inconsistent with observations.
Isothermal scenario predicted formation of GC-mass structures that merged
But no plausible mechanism for producing perturbations of this kindCosmology’s Early Years
Credit: M. S. Roberts