Dark Matter, Dark Energy, and the Fate of the Universe

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Dark Matter, Dark Energy, and the Fate of the
Universe
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Mass within Suns orbit 1011 MSun
Observable stars and gas clouds few 109
MSun
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Dark matter and dark energy

• Dark Matter An undetected form of mass that
  emits little or no photons, but we know it must
  exist because we observe the effects of its
  gravity
• Dark Energy An unknown form of energy that is
  causing the universe to expand faster over time

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What is the Universe made of?

• Normal Matter 4.4
• Normal Matter inside stars 0.6
• Normal Matter outside stars 3.8
• Dark Matter 25
• Dark Energy 71

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Spiral galaxies all tend to have flat rotation
curves indicating large amounts of dark matter
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The visible portion of a galaxy lies deep in the
heart of a large halo of dark matter
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measure the velocities of galaxies in a cluster
from their Doppler shifts
Mass is 50 x larger than the mass in stars!
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Clusters contain large amounts hot gas emits x
rays Temperature of hot gas tells us cluster
mass 85 dark matter 13 hot gas 2
stars
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Gravitational lensing of background galaxies also
tells us the mass
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What is dark matter made of?

• Ordinary Dark Matter (MACHOS)
• Massive Compact Halo Objects
• dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
• Weakly Interacting Massive Particles
• mysterious neutrino-like particles

The Best Bet
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MACHOs do not cause enough lensing events to
explain all the dark matter
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Why Believe in WIMPs?

• Theres not enough ordinary matter
• WIMPs could be left over from Big Bang
• Models involving WIMPs explain how galaxy
  formation works

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?
Gravity of dark matter is what caused
protogalactic clouds to contract early in time
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WIMPs dont contract to center because they dont
emit photons, so they can not radiate away their
orbital energy
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Maps of galaxy positions reveal extremely large
structures superclusters and voids
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WIMP models agree better with observations
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Fate of universe depends on the amount of dark
matter
Critical density of matter
Lots of dark matter
Not enough dark matter
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Amount of dark matter is 25 of the critical
density suggesting fate is eternal expansion
Not enough dark matter
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But expansion appears to be speeding up!
Dark Energy?
Not enough dark matter
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Brightness of distant white-dwarf supernovae
tells us how much universe has expanded since
they exploded
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Accelerating universe is best fit to supernova
data