Download Matter particles

What the Largest Structures in
the Universe can tell us about the
Smallest
Edmund Bertschinger
MIT Department of Physics and
Kavli Institute for Astrophysics and
Space Research
Elementary Particles:
The “periodic table” of physics

Matter particles


Quarks


Feel strong force
Leptons

Do not feel strong
force
Force carriers

Spin
Two known types:




Four known types:
Photons


Carry weak nuclear force
Gravitons


Carry strong nuclear force
W,Z0


Carry electromagnetic forces
Gluons


Spin
Carry gravity (in principle)
Higgs (not yet discovered)

Gives matter particles mass
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Many more types are expected to be found this decade!
Matter particles grouped into sets
1
Electron (stable)
Electron neutrino
up quark
down quark
2
Muon (unstable)
Muon neutrino
charm quark
strange quark
3
Tauon (unstable)
Tau neutrino
bottom quark
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 momentum
e + g  e’ + g’ (requires electric charge)

Change the particles (alchemy!)
e + W+  ne (requires weak charge)

Produce matter/antimatter pairs, or be
produced when matter and antimatter
annihilate
e+ + e-  g+g (e- = electron, e+ = antielectron)
g+g  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 charges
Pi meson = (up + anti-up) and (down + anti-down)
 Quantum superposition!

Baryons: three quarks whose strong charges
add to zero
Proton = (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!
packet)
(wave
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., g+g  e+ + e11
Dark matter: neutralino c0 (chi-zero)
Weak forces change one kind of matter
particle into another
e- + W+  ne (requires weak charge)
Supersymmetric forces (hypothetical new
forces) change matter particles into
force carriers and vice-versa.
Lightest supersymmetric particle, c0 , 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
n + n  c0 + c0 requires E(n) > m(c0) >> m(n)
 produce c0 = c0 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 forcecarriers
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!
e-
e+
BH
Inflation makes virtual particles
become real, then stretches their waves!
(The key feature of both is an “event horizon”.)
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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: Crosscorrelation of
temperature and
linear polarization
vs. 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 10-35 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)
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