Edmund Bertschinger

MIT Department of Physics andKavli Institute for Astrophysics and

Space Research

What the Largest Structures in the Universe can tell us about

the Smallest

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Elementary Particles:The “periodic table” of physics

Matter particles Spin

Two known types: Quarks

Feel strong force Leptons

Do not feel strong force

Force carriers Spin

Four known types: Photons

Carry electromagnetic forces Gluons

Carry strong nuclear force W,Z0

Carry weak nuclear force Gravitons

Carry gravity (in principle) Higgs (not yet discovered)

Gives matter particles mass

Many more types are expected to be found this decade!

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Matter particles grouped into sets

Electron (stable) up quarkElectron neutrino down quark

Muon (unstable) charm quarkMuon neutrino strange quark

Tauon (unstable) bottom quarkTau neutrino top quark

Nature provides 3 copies for no apparent reason. In addition, every particle has an antiparticle.

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What force carriers can do to matter particles: chemistry

Change the momentume + e’ + ’ (requires electric charge)

Change the particles (alchemy!)e + W+ e (requires weak charge)

Produce matter/antimatter pairs, or be produced when matter and antimatter annihilate

e+ + e +(e = electron, e+ = antielectron)

+ e+ + e(Particles are not conserved!)

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Composite particles

Mesons: quark-antiquark pairs which do not annihilate because the quarks have different strong chargesPi meson = (up + anti-up) and (down + anti-down)

Quantum superposition!

Baryons: three quarks whose strong charges add to zeroProton = (up + up + down)

Atomic nuclei: protons+neutrons Etc.

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Outstanding problems of particle physics

Why is the periodic table so complicated?“The search for unified field theories”

Supersymmetry

Why are the elementary particle masses so light but not zero? “The mass problem”

Higgs particle

Astrophysics and cosmology are unlikely to help answer these questions.

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Particles are not particles

They’re waves! Electron microscope!

No, they’re particles! Photoelectric effect

No, they’re waves!

Compromise: they’re wavicles! (wave packet)

Sometimes “particles” behave like particles, sometimes like waves!

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Particles are field “excitations”

Electron field with no electrons:

Electron field for a beam of many electrons:

Electron field of a localized electron:

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Why is astrophysics relevant?

The early universe was the most powerful particle accelerator ever.

Cosmic expansion has stretched wavicles whose wavelength was microscopic, to be larger than the observable universe today.

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Dark matter after the big bang

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The universe was denser, hence hotter, in the past

Thermodynamics: compressing a gas makes it hotter, if the heat is trapped in the gas

Hot gas energetic particles many particles can be produced by collisions

e.g.+ e+ + e

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Dark matter: neutralino 0 (chi-zero)

Weak forces change one kind of matter particle into another

e + W+ e (requires weak charge)

Supersymmetric forces (hypothetical new forces) change matter particles into force carriers and vice-versa.

Lightest supersymmetric particle, 0 , is predicted to be stable.

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Neutralino production requires high particle energies

E=mc2 is true only for particles at rest!energy E, mass m, speed of light c

E2 = (mc2)2 + (pc)2 is always true momentum p=Ev/c2, speed v

0 + 0 requires E() > m(0) >> m()

produce 0 = 0 in hot early universe

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Quantum mechanics: Heisenberg uncertainty principle

It’s impossible to measure both position and momentum (proportional to 1/wavelength) exactly for a wavicle

It’s also impossible to measure the energy (proportional to 1/frequency) in an arbitrarily short time.

These hold for any kind of wave, not just quantum wavicles!

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The particle loophole

Particles can materialize out of nothing (vacuum), live a short time, then disappear.

Nothing e+ + e Nothing

Virtual Particles

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Effects of virtual particles

All “static” forces (gravity, electrostatic, magnetostatic, etc.) carried by virtual force-carriers

Virtual particles interact with real particles to modify their interactions (“plasma screening” or “confinement”)

Virtual particles contribute nonzero energy to the vacuum (empty space).

The problem: they contribute Infinite energy!

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Virtual particles in cosmology

The universe has no preferred axis of orientation spin-0 force-carriers (e.g. Higgs field) can contribute a residual nonzero energy

Vacuum or “false” (temporary) vacuum energy

Could explain dark energy

Could also power the big bang itself!

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Powering the big bang:Cosmic Inflation (Alan Guth, 1981)

Recall from lecture 1:

Separation between pair of matter particles R(t)

If dR/dt > 0 and CR2 > k, eventually k becomes tiny and can be neglected to good approximation.

Exponential growth of prices = inflation

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Consequences of cosmic inflation

A region smaller than a peso gets stretched to become larger than our observable universe

Any initial small-scale roughness is smoothed to an imperceptibly small amount Explains why the universe is so homogeneous and isotropic!

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Consequences of cosmic inflation

Any initial k constant becomes negligibly small compared with (dR/dt)2. In general relativity, k determines the geometry of space. k = 0 is Euclidean space.

k=0 k<0 k>0 Inflation predicts k=0 as now observed to 1%

accuracy!

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Consequences of cosmic inflation

Quantum fluctuations of the spin-0 force-carrier that drives inflation lead to very weak fluctuations of density after inflation. Similar to Hawking radiation from black holes!

Black holes make virtual particles

become real!

Inflation makes virtual particles

become real, then stretches their waves!

(The key feature of both is an “event horizon”.)

BHe+

e

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After a few billion years…

Exponential stretching causes the quantum waves to behave classically (roughly, Heisenberg’s uncertainty is relatively unimportant for very big things)

The waves push around matter and radiation, creating small ripples which then amplify into all structure we see in the universe

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Cosmic Microwave Background Radiation Maps: Observation, Theory

Simulated map at WMAP resolution made in 1995(different false color scheme, statistical comparison only)

WMAP’s results were judged the top scientific breakthrough of 2003!

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CMBR Angular Power Spectrum:Cosmic Sonogram

Top: Temperature fluctuations vs. angular scale(data points and theory)

Bottom: Cross-correlation of temperature and linear polarizationvs. angular scale

From Bennett et al. 2003, WMAP

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Conclusions

Cosmic inflation refines the big bang theory. It’s predictions have so far been well confirmed; no

other theory has explained all that inflation does. Results suggest a new very high mass spin-0 field

existed in the early universe. Success increase confidence that we can

understand the universe from age 1035 to 10+17 seconds.

Dark matter should be produced in the lab AND detected from space “mañana.”

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For additional information

The Fabric of the Cosmos: Space, Time, and the Texture of Reality, Brian Greene

The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory, Brian Greene (more advanced than The Fabric of the Cosmos)

The First Three Minutes: A Modern View of the Origin of the Universe, Steven Weinberg (a slightly outdated classic)

The Inflationary Universe: The Quest for a New Theory of Cosmic Origins, Alan H. Guth (advanced but without math)