Dark Matter, Baryons vs. Non-Baryons, and the Fate of the Universe

Discovery of Dark Matter

• dark matter neither emits nor absorbs light, our primary means of observation

• BUT dark matter can be observed by its gravitational effect on 1) orbits of stars and galaxies and 2) light, whose path it can bend

• Fritz Zwicky notes in 1933 that outlying galaxies in Coma cluster moving much faster than mass calculated for the visible galaxies would indicate

  Hubble Space Telescope Image of the Center of the Coma Cluster (movie)

  Fritz Zwicky

  • finds large orbital speeds (about 1000 km/s)

  • cluster should have flown apart long ago because visible stars in galaxies cannot account for all gravity necessary to keep cluster together

  • calculates cluster must be 9/10's dark matter or ``missing mass''

    

• Zwicky predicts in 1937 that dark matter could be investigated by observing galaxies that act as gravitational lenses, something that came to pass only in the 1990's

  Schematic of Foreground Galaxy Acting as Gravitational Lens on Background Quasar (movie of light bent by mass)

  Cluster of Galaxies Acting as Gravitational Lens on Background Galaxies

  

  To see a movie of a model of a cluster (yellow galaxies) moving in front of background galaxies (faint blue galaxies) and gravitationally lensing them, click here.

  To check out a gravitational lensing demo, click here.

  To read more about gravitational lensing, click here or here.

• Vera Rubin discovers dark matter in galaxies

  

  • velocities of stars far from the center of a galaxy are larger than expected

  • galaxies in clusters move too fast, AND stars in galaxies move too fast

  • additional mass must be present to hold galaxy together

    

But is dark matter the only explanation for these results?

• two examples from our Solar System
  • discovery of Neptune by its gravitational effect on Uranus -- unseen matter

    

    • wobbling of Uranus's orbit led to search for unseen body

    • Neptune discovered as a result, the only planet detected first by its gravitational effect on another planet

    • early unseen mass (or ``dark matter'') problem that lead to actual detection

  • precession of perihelion of Mercury -- incomplete physics

    

    • change in orbit originally thought to be due to unseen body called Vulcan

    • precession later found to result from bending of space by Sun, a consequence of Einstein's General Theory of Relativity (movie)

    • so, unseen mass not required in this case, but a change in known physics

• so, two possible solutions to dark matter problem

  • unknown matter: exotic, non-baryonic particles like WIMPS and neutrinos or hard-to-detect, baryonic matter (black holes, brown dwarfs, cold gas, hot gas, faint galaxies)
    • baryonic example: hot (X-ray emitting!) gas in clusters of galaxies

      

    • baryonic example: cold gas between galaxies not in clusters (Lyman-alpha forest)

      

      Spectra in Direction of Nearby and Distant Quasars Showing Lyman-alpha Forest (absorption features are cold gas clouds of hydrogen in front of each quasar)

    • baryonic example: low surface brightness galaxies

      High surface brightness galaxy (upper left) compared with three low surface brightness galaxies. Low surface brightness galaxies, which have low contrast compared with brightness of sky, are hard to find!

    • baryoninc example: brown dwarfs, white dwarfs, and other stellar remnants -- MAssive Compact Halo ObjectS (MACHOS)
      • MACHOS detected with gravitational microlensing

      • alignment of foreground dark object with background star causes amplification of star's light via lensing

      • but even in optimistic models for halo MACHO population of galaxy, the probability of such an event is ~ 10^-7 ---> have to monitor at least 10⁷ stars over course of year

        Star Brightens and Fades Once (signature of microlensing!)

        Light Curve of Star Brightness vs. Time for Microlensing Event (observed in both red and blue light ---> can rule out variable star)

      • halo white dwarfs may constitute up to 2-4 times as much matter as in all visible stars in Milky Way (although still not enough to account for all dark matter)

    • non-baryonic example: neutrinos

      • three types, at least one has mass (but is it enough?)

      • incredibly numerous, density is about 100/cc

  • OR unknown physics: need adjustments in law of gravity

With dark energy, dark matter helps determine fate of universe (movie)...

• whether universe expands forever (open or flat, dies by cooling or in Big Rip) or collapses in Big Crunch (closed) depends on

  • Hubble constant, which is related to the current rate of expansion, (dr/dt) / r

  • Omega_0,matter (mass density of universe/critical density needed to close universe in absence of dark energy)

  • Omega_0,energy (energy density of universe/critical density and Einstein's ``greatest blunder'')

    Recent observations of distant supernova have suggested that the expansion of the universe is actually accelerating or speeding up, like the red curve, which implies the existence of a form of matter with a strong negative pressure, such as the cosmological constant. This strange form of matter is also sometimes referred to as the "dark energy". If dark energy in fact plays a significant role in the evolution of the universe, then in all likelihood the universe will continue to expand forever.

    

    Component	Mass Fraction (relative to critical density)	Comment
    luminous matter	~0.005	light emitting baryons only
    all baryons	~0.04-0.06	from nucleosynthesis predictions and CMB measurements, factor of ~10x luminous baryons
    all light and dark matter	~0.3	from adding observed light from mass derived from bulk flows, factor of ~5x all baryons
    dark energy	~0.7	from supernovae and CMB measurements