Ultimate questions of cosmology:

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MANY WORLDS IN ONE
Alex Vilenkin
Tufts Institute of Cosmology
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THE UNIVERSE BEYOND THE HORIZON
Cosmic horizon 4x1010 light years. What lies
beyond? Remote regions are strikingly different
from what we observe here. Theoretical study of
these unobservable regions may explain some
features of the visible universe.
Did the universe have a beginning? (if time
permits).
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Genesis circa 1980

• Present

• 14 billion yrs
• 10 billion yrs

Galaxies

• 1,000,000 yrs

Atoms

Nuclei
1 minute
Big bang
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Questions about the beginning

• Why was the Universe so hot?
• Why was it expanding?
• Why was it so homogeneous?
• What is the origin of small inhomogeneities?
• What happened before the big bang?

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Questions about the beginning

• Why was the Universe so hot?
• Why was it expanding?
• Why was it so homogeneous?
• What is the origin of small inhomogeneities?
• What happened before the big bang?

Addressed in the theory of inflation. Key
role is played by false vacuum.
Guth (1981)
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Properties of false vacuum

• Large energy (mass) density
• Large tension.
• Repulsive gravity

gt 1022 kg /cm3
Mass of the Moon
Decays into a hot fireball of particles
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Cosmic inflation
Guth (1981) Linde (1982) Albrecht Steinhardt
(1982)
time

Doubling time 10-35 sec
Doubling time
In 100 doubling times expansion by a factor
1,000,000,000,000,000, 000,000,000,000,000.
-37
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Cosmic inflation
Guth (1981) Linde (1982) Albrecht Steinhardt
(1982)
time

EXPLAINS

• High temperature
• Expansion
• Homogeneity and flatness
• Small inhomogeneities

Doubling time
Mukhanov Chibisov (1981)
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Evidence for inflation
WMAP
Flat geometry at 1 accuracy
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BEYOND THE HORIZON
Eternal inflation
A.V. (1983) A. Linde (1986)
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False vacuum decays through bubble nucleation
You are here
Bubble universes
False vacuum
We cannot travel to other bubbles.
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Spacetime of a bubble universe
time
A moment of time
Big bang
Nucleation
space
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• An unlimited number of bubbles will be formed in
  the
• course of eternal inflation.
• Everything that can happen will happen in such a
  universe.

• It contains an infinite number of earths with
  histories
• identical to ours, and all possible
  variations.
• (Many worlds in one.)
• 
  Ellis Brundrit (1979)
• 
  Garriga A.V. (2001)
• If inflation is accepted, these conclusions are
  very hard to
• avoid.

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Different bubbles may be characterized by
different values of the constants of Nature
Newtons gravitational constant Mass of electron,
proton, etc. Charge of electron,
Appear to be fine-tuned for our existence.
Neutron mass decrease by 1 no
atoms, increase by 1 only hydrogen.
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Different bubbles may be characterized by
different values of the constants of Nature
Newtons gravitational constant Mass of electron,
proton, etc. Charge of electron,
Appear to be fine-tuned for our existence.
Fine-tuning can be explained if the constants
can take a wide variety of values.
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String theory suggests different
vacua with different values of the constants.
Bousso Polchinski (2000) Susskind (2003)
Bubbles of all types will be produced in the
course of eternal inflation.
What we observe is determined by chance
anthropic selection.
How can we test this?
The multiverse
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String theory suggests different
vacua with different values of the constants.
Bousso Polchinski (2000) Susskind (2003)
Bubbles of all types will be produced in the
course of eternal inflation.
What we observe is determined by chance
anthropic selection.
How can we test this?
We can make statistical
predictions. A case study the
cosmological constant.
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Cosmological constant mass density of our
vacuum.
OBSERVATIONS
Suppose varies from one bubble to another.
Repulsive vacuum gravity suppresses galaxy
formation. No galaxies unless
.
Weinberg (1987)
ANTHROPIC PRINCIPLE the observed value
should be in this range. (guaranteed to be
true.)
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What is the height of the first man I am going to
see when I walk out into the street?
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What is the height of the first man I am going to
see when I walk out into the street?
Shortest man Gul Mohammed, 0.56 m
Guinness Book Tallest man Robert
Wadlow, 2.72 m of Records
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What is the height of the first man I am going to
see when I walk out into the street?
Shortest man Gul Mohammed, 0.56 m
Guinness Book Tallest man Robert
Waldow, 2.72 m of Records
Height distribution of men in US
Number of men
Prediction at a 95 confidence level
95
height (meters)
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PRINCIPLE OF MEDIOCRITY
We expect to be in the 95 range of the
distribution.
A.V. (1995), Efstathiou (1995), Martel, Shapiro
Weinberg (1998)
Now apply the same technique to .
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PRINCIPLE OF MEDIOCRITY
We expect to be in the 95 range of the
distribution.
A.V. (1995), Efstathiou (1995), Martel, Shapiro
Weinberg (1998)
Good agreement with the observed value.
No plausible alternatives.
Our first evidence for the multiverse?
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DOUBTS
Even if we calculate the probability very
precisely, all we can do is predict a range.

Agreement with the data may be just dumb luck.

Try to make more predictions.
Dark matter density
Peak at observed value
Aguirre, Tegmark, Rees Wilczek (2006)
Neutrino mass
Peak at 1 eV
Pogosian, Tegmark A.V. (2006)
Other particle masses and interactions
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NEW PICTURE OF THE UNIVERSE

• Inflation is a never ending process, with
  bubbles like ours
• constantly being formed.

• Constants of nature may take different values in
  different
• bubbles. We can make statistical predictions.

• Some predictions are already supported by the
  data.

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DID THE UNIVERSE HAVE A BEGINNING?
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If inflation is eternal to the future, could it
also be eternal to the past?

• THEOREM
• A universe which is on average expanding
  faster than some minimum rate cannot be
  past-eternal.

Borde, Guth and A.V. (2001)
t

• Inflation must
• have a beginning.

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If inflation is eternal to the future, could it
also be eternal to the past?

• THEOREM
• A universe which is on average expanding
  faster than some minimum rate cannot be
  past-eternal.

Borde, Guth and A.V. (2001)
t
What happened before inflation? And what
happened before that?
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E 0 Q 0

• A.V. (1982, 84)
• Hartle Hawking (1983)
• Linde (1984)

• Spontaneous nucleation of universes from
  nothing.

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E 0 Q 0

• A.V. (1982, 84)
• Hartle Hawking (1983)
• Linde (1984)

• Spontaneous nucleation of universes from
  nothing.

No time before. No cause required. But the laws
of physics should be there
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Spacetime of a bubble universe
time
A moment of time
Big bang
Nucleation
space
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Unlimited number of pocket universes.
Nucleation
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