Future Cosmology

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  • 8/8/2019 Future Cosmology


    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


    How did galaxies and

    clusters form?

    What is the distribution

    of the dark matter?

    What is the chemicalevolution of the


    How did supermassive

    black holes form?

    Can we disentangle

    lensing effects from

    tensor modes in the


    Was Friedmann right?


    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


    What is the origin of the




    Did the universe inflate?

    What is the origin of

    the cosmological


    Is there a gravitational-wave background?

    What is the role of

    extra dimensions, if


    Are there multiple

    universes with

    different conditions?

    What happened before

    the Big Bang?

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    4% Ordinary Matter22% Dark Matter74% Dark Energy



    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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    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


    - 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.



    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


    -- 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?



    dark energy


    matterStandard Model



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    An interactive

    dark sector?



    dark energy





    variable-mass particles?Chaplygin gas?



    mass-varying neutrinos?

    variable constants?

    5th forces?

    Standard Model


    SU(2)? (wimps)





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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


    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.


    (primeval atom)


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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


    We're pretty

    good atpower-spectrum



    in the linearregime.

    Less good at

    the nonlinear


    galaxies and


    (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


    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.