Transcript talk29102009

Dark Energy &
Cosmic Horizons
Pim van Oirschot
Table of Contents
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Part I: The standard model of
Cosmology and its problems
Part II: Cosmic Horizons
Part III: Melia’s Horizon
Part I
The standard model of
Cosmology and its problems
The Cosmological Constant
Problem
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The cosmological constant can be interpreted as
the energy density of the vacuum
Quantum Field Theory can predict a value for this
energy density
We can compare this value with the value inferred
from astronomical observations
The latter is 120 orders of magnitude smaller than
predicted by Quantum Field Theory
The Expert’s Opinion
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Frank Wilczek
The biggest and most profound gap in our
current understanding of the physical world
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Sean Carroll
One of the most perplexing
issues in theoretical physics
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Michael Turner
The biggest embarrassment
in theoretical physics
Dark Energy
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A component that causes the
expansion of the Universe to accelerate
is referred to as Dark Energy
The cosmological constant is just one
possibility
Dark Energy is characterised by its
equation of state, w
w = -1 for the cosmological constant
Part II
Cosmic Horizons
Particle Horizon
Harrison, Cosmology - 1981
Event Horizon
Horizons in an expanding Universe
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The Robertson-Walker line element is
conformally equivalent to the Minkowski
metric
The conformal time
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can converge, depending on a(t)
For the concordance model:
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Conformal Convergence
The Hubble sphere
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Hubble’s law V = H D
The Hubble sphere marks the distance
beyond which V exceeds the speed of
light, c
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We can see beyond the Hubble sphere
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A Conformal Diagram
Part III
Melia’s Horizon
A new type of “horizon”
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Observations should indicate that the
equation of state of Dark Energy, w = -1/3
The Robertson-Walker line element should
be written in static coordinates:
Notes on the “cosmic horizon”
1.
The “cosmic horizon” has been
defined before by Hawking as the
apparent horizon of the Universe
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We can see beyond the “cosmic
horizon”, like we can see beyond the
Hubble sphere
Another coincidence?
Only in a Universe with
Only in such a Universe, is c times the age t
always equal to the “cosmic horizon”
Conclusions
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A number of outstanding, fundamental
questions concerning the standard model of
cosmology remain unsolved.
Exploring the nature of Dark Energy might
help us finding an answer to these
questions
The theory of Cosmic Horizons is not in
disagreement with the existence of a nonzero cosmological constant