Lecture 25' Dark Energy

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Lecture 25. Dark Energy

• Nov 15, 2003

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Measure the Deceleration

• At very large distances, the expansion of the
  universe will deviate from Hubbles law due to
  deceleration

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Does Gravity alone Influence the Expansion?

• Recent observations of white dwarf supernovae in
  very distant galaxies have yielded unexpected
  results.
• remember, white dwarf supernovae make very good
  standard candlesthe supernovae are apparently
  fainter than predicted for their redshifts

• At a given cosmological redshift
• galaxies should be closer to us
• i.e. shorter lookback time
• for greater Universal mass densities
• these supernova are farther back in time than
  even the models for an ever-expanding (coasting)
  Universe predict
• This implies that the Universal expansion is
  accelerating!
• there must be an as yet unknown force which
  repels the galaxies
• a dark energy

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Accelerating Universe?
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What if Gravity alone Determine the Fate of the
Universe?

• Gravity is always an attractive force
• After the big bang, gravity always tries to slow
  things down!
• So the expansion of the universe is always
  decelerating
• If the initial expansion is fast enough, the
  universes expansion might not be halted, but it
  would still be decelerating
• Is there any way that the universe could have an
  accelerating expansion?

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Einsteins Biggest Blunder

• Einstein was the first person to realize this
• He believed in steady-state cosmology the
  universe is stable, either expanding nor
  contracting
• But his theory of general relativity does not
  allow it unless there is a mysterious repulsive
  force to balance the gravitational attraction
• Einstein then introduced a term in his equation
  of gravity, called a Cosmological Constant, or ?
• Soon after Hubbles discovery of the expansion of
  the universe, Einstein abandoned his idea of
  steady universe, and called the introduction of
  the cosmological constant the biggest blunder of
  his life or is it?

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Fig. 18-20, p.377
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What is Dark Energy?

• Dark Energy (a.k.a. vacuum energy) is a repulsive
  force in the universe that causes the universe to
  accelerate
• It is anti-gravity, in the sense that it is
  repulsive
• Einsteins cosmological constant ? is the
  simplest form of dark energy, but it could be
  more complex than a simple constant, in
  particular, it could be evolving with time
• By measuring the accelerating of the universe, we
  can measure the total amount of dark energy, and
  the contribution of dark energy to the total
  density of the universe.

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The Total Density Parameter

• Ototal OM Odark energy

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The density budget
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How Mass Density and Dark Energy affectthe
Expansion of the Universe
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The Fate of the Universe

• Modern Observation shows
• The total density Ototal 1 the universes
  geometry is flat
• The mass density OM 0.3, the universe is
  light-weighted, and will expand forever
• The dark energy density Odark energy 0.7, the
  universe is dominated by
• dark energy, and
• is accelerating

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Expansion History

• Currently the universe is dominated by dark
  energy, probably in the form of cosmological
  constant
• But at high-redshift, the universe is smaller and
  denser, so matter will
• dominate. The universe
• would be decelerating
• then can we observe
• it?

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So, What is Dark Energy?

• We have no idea!
• One of the most important tasks of cosmology in
  the next decade is to understand the nature of
  dark energy, especially by studying the evolution
  of dark energy with time.

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Other Evidence of Dark Energy

• Dark energy affect the expansion rate of the
  universe, so we can also probe dark energy by
  looking at how things grow in the universe
• Distribution of cosmic microwave background
• Growth of large scale structure of the universe
• These tests have so far provided strong evidence
  that the current universe is dominated by dark
  energy

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Dark Energy Splitting the Universe
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Dark Energy Splitting the Universe
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Cosmological Parameters

• The basic properties of the universe are
  characterized by a set of cosmological parameters
• A large fraction of cosmologists are devoted to
  measuring these cosmological parameters
• Hubble Constant, Ho
• Mass density parameter, OM
• Baryonic density parameter, Ob
• Total density parameter, Ototal
• Dark energy density parameter, Odark energy
• Age of the universe, t0
• Etc.

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Cosmological Parameters

• The basic properties of the universe are
  characterized by a set of cosmological parameters
• A large fraction of cosmologists are devoted to
  measuring these cosmological parameters
• Hubble Constant, Ho 70 km/s/Mpc
• Mass density parameter, OM 0.3
• Baryonic density parameter, Ob 0.04
• Total density parameter, Ototal 1
• Dark energy density parameter, Odark energy 0.7
• Age of the universe, t0 13.7 billion years
• Etc.

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The End of the Universe?

• In a dark energy dominated universe
• The expansion will continue to accelerate
• The density of the universe will be very low -
  empty
• The temperature of the universe will be very low
  - cold
• Most of the galaxies will fade away from our view
  - dark
• We can only see our closest neighbors in the
  Local Groups Andromeda, Magellanic Clouds etc.
• The End of the Universe is Frozen in Time