Edmund Bertschinger

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The Cosmic Menu ofDark Matter and Dark Energy

• Edmund Bertschinger
• MIT Department of Physics and
• Kavli Institute for Astrophysics and
• Space Research

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Contents of the Universe Today
Credit NASA/GSFC
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Observations of the Early UniverseFossil Relics

• Galaxies
• Cosmic Microwave Background Radiation
• Atoms, Dark Matter and Dark Energy

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The Most Distant GalaxiesObserved
withHubbleanimations
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Cosmic Microwave Background Radiation
Discovery 1965 Penzias Wilson ATT Bell
Labs 1978 Nobel Prize
2003 Wilkinson Microwave Anisotropy Probe (WMAP)
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A Cosmic Sonogram

• The Big Bang began with a burst of
  near-exponential expansion cosmic inflation
• Quantum mechanical fluctuations in energy made
  ripples in the gas Animation
• Fluctuations in radiation Sound waves!
• Light as sound?! Yes!
• Cosmic redshift Doppler shift in an expanding
  universe

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Dark Matter and Dark Energy

• Dark Matter Invisible stuff that gravity draws
  into galaxies. Crucial element of galaxy
  formation.
• Dark Energy Invisible stuff that gravity does
  not draw into galaxies. Instead, this substance
  accelerates the recent expansion of the Universe.
• To date the sole evidence for these substances
  comes from astrophysics. Despite their similar
  names, they probably are completely unrelated.

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Dark Matter Exists around Galaxies
GM(r)rVc2(r)
Dark matter
Credits Left J. Kormendy (NGC 4216). Right
K. Begeman (NGC 3198) and K. Freeman
Although diffuse, dark matter is crucial for
galaxy formation
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How did Galaxies Form?

• Seed Fluctuations from Early Universe
• Inflationary epoch 10-35 s after Big Bang
• Two outcomes large flat universe, quantum
  fluctuations
• Gravitational Amplification
• Dark Matter
• Inferred for 50 years, not yet detected on earth
• Crucial for galaxy formation!
• Atomic matter
• Only 5 of the mass-energy in the Universe, but
  all that we directly see!

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Matthias Steinmetz
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What is the Dark Matter?

• Elementary particles with low thermal speed,
  otherwise could not seed galaxy formation
• Not atomic matter (primordial nucleosynthesis) or
  neutrinos (too light, too hot)
• Consensus view Cold, very weakly interacting
  particles
• A new heavy spin-1/2 particle (neutralino)?
• Large Hadron Collider, 2007-8
• A new ultralight spin-0 field (axion)?
• Searches are underway!

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Dark Matter and Dark Energy

• Dark Matter Invisible stuff that gravity draws
  into galaxies. Crucial element of galaxy
  formation.
• Dark Energy Invisible stuff that gravity does
  not draw into galaxies. Instead, this substance
  accelerates the recent expansion of the Universe.
• To date the sole evidence for these substances
  comes from astrophysics. Despite their similar
  names, they probably are completely unrelated.

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Dark Energy Drives Galaxies Apart
Kinetic Energy Gravitational Energy
Constant For sphere of radius R(t), v(t)
dR/dt,
or
Dark Energy dilutes little or not at all as it
expands!
? v increases as R and t increase, acceleration!
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Science Breakthrough of 1998Dark Energy
Acceleration
Deceleration
m-M log(Distance) z Redshift Velocity
Acceleration
Credit High-z Supernova Team
Deceleration
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What is the dark energy?

• A new fundamental field in the universe?
• Quintessence particles whose de Broglie
  wavelength is billions of light years!
• A cosmological constant?
• Proposed by Einstein in 1917
• Retracted as his greatest blunder
• Energy density of empty space!
• Maybe General Relativity is wrong?
• String theory might change gravity across the
  observable universe

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We dont know what the dark energy is! But we
want to find out!

• Why is the energy density of the vacuum so small,
  when virtual particles should make it huge? If
  not Huge, why not Zero?
• Why is dark energy comparably abundant to matter
  today, when it was negligible 10 billion years
  ago? Why now?
• What is the ultimate fate of our Universe?

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Additional Credits and Information

• Credits
• Hubble Deep Field photo courtesy Space Telescope
  Science Institute
• WMAP (Wilkinson Microwave Anisotropy Probe)
  images courtesy NASA/Goddard Space Flight Center
• Numerical simulation of galaxy formation (gas and
  dark matter) courtesy Mattias Steinmetz
• Books
• Origins Fourteen Billion Years of Cosmic
  Evolution, Neil deGrasse Tyson and Donald
  Goldsmith
• Just Six Numbers The Deep Forces that Shape the
  Universe, Martin J. Rees
• An Introduction to Modern Cosmology, Andrew
  Liddle (advanced undergraduate textbook)