Post on 10-Apr-2018
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Sean Carroll
The Future of
Theoretical Cosmology
100 years from now,
what will we be thinking
and how will we be thinking it?
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Focus on three kinds of questions
Evolution
How did galaxies and
clusters form?
What is the distribution
of the dark matter?
What is the chemicalevolution of the
universe?
How did supermassive
black holes form?
Can we disentangle
lensing effects from
tensor modes in the
CMB?
Was Friedmann right?
Composition
What kind of particle is
the dark matter?
Can we detect/produce
dark matter astrophysically
or in the lab?What the hell is the
dark energy?
Does dark energy evolve?
What is the origin of
ultra-high-energy cosmic
rays?
What is the origin of the
matter/antimatter
asymmetry?
Origins
Did the universe inflate?
What is the origin of
the cosmological
perturbations?
Is there a gravitational-wave background?
What is the role of
extra dimensions, if
any?
Are there multiple
universes with
different conditions?
What happened before
the Big Bang?
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4% Ordinary Matter22% Dark Matter74% Dark Energy
Composition
Questions
Prediction: We
will completely
understand this.
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Every slice of the pie
chart is problematic.
Ordinary Matter: Where are there more baryons
than antibaryons? Why comparable to the
dark matter density?
Dark Matter: What is it? Can we detect it directly,
or indirectly, or make it in the lab? How many
components are there?
Dark Energy: What is it? Is it evolving? Why
isn't there much more? Why now?
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Baryogenesis
The good news is that many
baryogenesis scenarios are tied tofeasible particle-physics
experiments (CP violation etc).
Electroweak baryogenesis: we need to understandthe Higgs sector better, to understand the
electroweak phase transition.
GUT Baryogenesis: grand unification predicts that theproton should decay. It hasn't yet, but it might.
Leptogenesis: massive neutrinos may violate lepton
number, later processed into a baryon asymmetry.
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Dark Matter: well-motivated candidates
Weakly Interacting Massive Particles (WIMPs)
- in equilibrium early; freeze-out after becomingnonrelativistic (cold)
- must be neutral, color singlets; likely EW scale
- perfectly suited to collider experiments
- both directly and indirect searches
Axions
- light pseudoscalars predicted by Peccei-Quinn
solution to the strong-CP problem- produced out of equilibrium, by vacuum
misalignment or topological-defect radiation
- colliders no good, need dedicated experiments
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Bonus: understanding the WIMP sector, and directly
detecting it, tests general relativity at T ~ 10 GeV.
Best current test
of Friedmann eq.
in the early
universe: Big BangNucleosynthesis,
at 1 MeV - 50 keV.
So we can push theknown history of the
universe back by
a factor of 10,000.
Expansion
rate
Scale factor -->
WIMP freeze-out
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Dark Energy: well-motivated candidates
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Dark Energy: ill-motivated candidates
Vacuum energy, a/k/a cosmological constant-- a strictly constant energy density inherent
in empty spacetime
Dynamical dark energy
-- evolution characterized by equation-of-state
parameter w = p/r
Modified gravity
-- Friedmann eq. is wrong, but only at late times
Nothing
-- We're just going about it wrong
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The dark energy is probably vacuum energy.
Requires dramatic fine-tuning, but every alternative
requires even more. Observational signature:
constant energy density (w = -1, andw'= 0).
If it is vacuum energy,
cosmological observations
won't tell us anything;
we'll have to understand
fundamental physics
(extra dimensions, susy),
probably through
accelerator experiments.But knowing whether
it is vacuum is of
paramount importance!
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An introverted
dark sector?
ordinary
matter
dark energy
dark
matterStandard Model
SU(3)xSU(2)xU(1)
gravity
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An interactive
dark sector?
ordinary
matter
dark energy
dark
matter
evolution?
perturbations?
variable-mass particles?Chaplygin gas?
scattering?
annihilation?
mass-varying neutrinos?
variable constants?
5th forces?
Standard Model
SU(3)xSU(2)xU(1)
SU(2)? (wimps)
anomalies?
(axions)
baryogenesis?
gravity
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Origins Questions
Inflation is the guiding principle
behind much thought about thevery early universe. From a tiny
starting patch at 1016 GeV,
accelerated expansion creates
a smooth, flat universe that
grows into our own.
Explains: homogeneity, isotropy, flatness, absence of
monopoles, nearly scale-free primordial fluctuations
Predictions: - fluctuations should not be precisely scale-free
- tensor gravity-wave fluctuations should exist
along with scalar fluctuations; potentially
observable in CMB B-mode polarization
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A deep conceptual issue about
inflation: Does it really provide
more natural initial conditions?
Basic issue: Entropy of our current
universe is about Stoday ~ 10100, and the
entropy of the early radiation-dominated
universe was Srad ~ 1088
. But the entropyof a tiny inflationary patch is only Sinfl ~ 10
10.
So: if we are going to randomly fluctuate into some state,
shouldn't it be a high-entropy state, not a low-entropy one?
Moral: we really do need to understand the pre-inflationary
universe, i.e. have a theory of initial conditions.
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A possible solution: us as a baby universe
If there is a pre-existing
empty, static spacetime
(or whatever), quantum
fluctuations can nucleatebubbles of false vacuum
that then grow into
universes of their own.
False-vacuum bubbles are
naturally low entropy.
us
(primeval atom)
background
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A natural consequence: the multiverse
If one bubble pinches
off, it will just keep
happening, creating
an infinite fractal
landscape of universes.
The babies may or
may not be essentially
the same; low-energy
physics could be
different from one
child to the next.
Note time-symmetry.
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The multiverse and environmental selection
String theory might plausibly predict that there can beregions of space with utterly different physical properties.Perhaps 10500 different vacuum states.
Imagine that:
Then we could never observeregions where the vacuumenergy is large enough to ripus to shreds the ultimate
selection effect.
There are many distinct
domains throughout space.
They each have a different
vacuum energy.
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But is the multiverse testable?
Scientific theories must make testable predictions.
But every theory also makes untestable predictions.
The multiverse is not a theory; it's a prediction.
To make all this respectable, we
don't need to observe the multiverse;
we need to understand the laws ofphysics sufficiently well to know
whether they really predict a fractal
universe on ultra-large scales.
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Cosmology depends on fundamental physics.
We really need a theory of everything (or everythingrelevant, up to MPlanck); will we get one?
Particle accelerators
increase in energy by
103 every 40 years.
We'll reach the Planck
scale around 2200 --
not within the scope of
this talk.
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Evolution Questions
[Tegmark]
We're pretty
good atpower-spectrum
issues,
especially
in the linearregime.
Less good at
the nonlinear
universe:
galaxies and
clusters
(and stars!).
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The real issue is dynamic range: important processes
stretch from atomic physics to cluster dynamics.
Clusters of galaxies:
mass ~ 1046 g
timescale ~ 1016 secsize ~ 1024 cm
Atoms:
mass ~ 10-24 g
timescale ~ 10-10 sec
size ~ 10-8 cm
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Numerical simulations
are the way forward,and modern work is
increasingly including
more and more physical
processes. (Not justsimple dark-matter
gravitational dynamics.)
But there is a lot of room
for improvement --
and it will come!
[Virgo consortium]
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Quantum computation: intrinsically massively parallel.
Three classical bits can be
in any of eight states:
(000), (001), (010), etc.
Three quantum bits (qubits) are naturally in
superpositions of all eight possibilities:
|y > = a|000> + b|001> +g|010> + d|011> +
e|100> + z|101> + h|110> + q|111>
Operating a quantum computer with 300 qubits
is like simultaneously running as many classical
processors as there are particles in the universe.
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Utterly new techniques: Genetic Algorithms.
Eventually, computers will be deciding how to do
the simulations, as well as doing them. They will
be functioning as theoretical cosmologists!
Define a fitness landscape to determine thesuccess of a program. (E.g., fitting the data.)
Run multiple algorithms.
Allow fittest algorithms to reproduce with mutations.
Repeat as you fit the data better and better.
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The last hundred years have given us a remarkable picture
of the universe; the last ten years have brought it intosharp focus.
We are blessed with puzzles about the evolution,
composition, and origin of the universe but they
don't seem completely intractable.
Theoretical work is driven by data, so we never really know
what's coming.
Scientific cosmology was born and matured in the 20th
century. The 21st is unlikely to be as groundbreaking but
there will be plenty of surprises. Right now we don't
even know what questions we'll be asking.
Conclusions