The Runaway Universe: The Influence of Dark Energy

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The Runaway UniverseThe Influence of Dark Energy

• Geraint F. Lewis
• University of Sydney

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A little history
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Redshift
Edwin Hubble
Each galaxys redshift can be assigned a
velocity. This is the Doppler Shift.
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Redshift vs Distance
Freedman et al 2001
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Just what is expansion?
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The Scale Factor
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The Friedmann Equations
In words The rate of change of R (related to the
velocity galaxies move apart) depends upon the
density of stuff in the Universe. The rate of
change of the rate of change (related to the
acceleration of galaxies in the expansion)
depends upon the density and pressure of stuff
in the Universe.
is the density
is the pressure
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Equation of State
The equation of state tells us how much energy is
in the form of pressure (ie random motions)
compared to the stored energy.
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Energy in the Universe

• For normal matter, the energy in pressure is
  very small compared to the rest mass energy (
  ) and so

The pressure of photons (i.e. light, radio,
X-rays etc) corresponds to a large amount of
energy.
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Evolution of the Universe
A Universe with only matter
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What is the mix of our Universe?

• To determine the mix of energies in the Universe,
  we need to measure the scale factor, R(t), over
  the history of the Universe.
• The shape of R(t) can then be compared with
  theoretical models to measure the components.
• To do this, we must make use of some fundamental
  properties of light and of the redshift.

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Standard Candles

• The brightness of an object diminishes as the
  square of the distance.
• If we know how luminous an object truly is, we
  can calculate its distance based on its apparent
  brightness.
• As the speed of light is finite, we can calculate
  how long ago the light was emitted i.e.
  calculate the t in R(t).

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The Redshift (again)

• In Cosmology, the redshift of light is not due to
  the Doppler effect.
• It is more correct to see it as the wavelength of
  light stretching as the Universe expands.

Scientific American
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Exploding Stars
Supernova 1994D
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Cosmological Supernovae
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Cosmological Supernovae
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Cosmological Supernovae
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What is in the Universe?
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What is this Dark Energy?

• Dark energy cannot be normal matter, or even
  photons
• To explain the supernova results most of the
  Universe must be something else, something
  causing an accelerated expansion.
• What causes accelerated expansion?

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Dark Energy
Accelerated expansion occurs with some negative
values of the equation of state. But what is
negative pressure?
Casimir Effect
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Accelerated expansion

• The supernova results are consistent with an
  equation of state of -1.
• This is the same as Einsteins Cosmological
  Constant.
• What does this mean for the future of the
  Universe?

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The future of the Universe
http//physics.uoregon.edu/jimbrau/astr123/Notes/
ch27/expanding.jpg
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The Future is not so bright
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The Big Rip?
If the equation of state is less than -1,
sometimes called phantom energy, then the
Universe is in trouble. The accelerated expansion
will tear apart the Galaxy, the Solar System, the
Earth and eventually its atoms.
New Scientist
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So, what is the Equation of State?

• Currently, the data from supernova suggest that
  the equation of state is about -1.
• But what if the equation of state is not a
  constant, what if it changes over time? In the
  future, it may change into a phantom energy-like
  form, sending the Universe towards a Big Rip.
• How do we find out what are the properties of
  dark energy?

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SNAP and OWL
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Is any of this right?

• Is our cosmology correct?
• Is General Relativity correct?
• Is dark energy just a mirage?
• Dont be surprised if there is a radical change
  in our ideas in the next few years!

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The End