Future Cosmology

8/8/2019 Future Cosmology

1/25

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.


4/25

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


10/25

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.


15/25

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

Future Cosmology

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