Edmund Bertschinger

1
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

2
Elementary ParticlesThe 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!
3
Matter particles grouped into sets
Electron (stable) up quark Electron
neutrino down quark Muon (unstable) charm
quark Muon neutrino strange quark Tauon
(unstable) bottom quark Tau neutrino top quark
1 2 3
Nature provides 3 copies for no apparent reason.
In addition, every particle has an antiparticle.
4
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- ? gg (e- electron, e antielectron)
• gg ? e e- (Particles are not conserved!)

5
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 protonsneutrons
• Etc.

6
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.

7
Particles are not particles

• Theyre waves! Electron microscope!
• No, theyre particles! Photoelectric effect
• No, theyre waves!
• Compromise theyre wavicles! (wave packet)
• Sometimes particles behave like particles,
  sometimes like waves!

8
Particles are field excitations
Electron field with no electrons Electron field
for a beam of many electrons Electron field
of a localized electron
9
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.

10
Dark matter after the big bang
11
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., gg ? e e-

12
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.

13
Neutralino production requires high particle
energies

• Emc2 is true only for particles at rest!
• energy E, mass m, speed of light c
• E2 (mc2)2 (pc)2 is always true
• momentum pEv/c2, speed v
• n n ? c0 c0 requires E(n) gt m(c0) gtgt m(n)
• ? produce c0 c0 in hot early universe

14
Quantum mechanics Heisenberg uncertainty
principle

• Its impossible to measure both position and
  momentum (proportional to 1/wavelength) exactly
  for a wavicle
• Its 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!

15
The particle loophole

• Particles can materialize out of nothing
  (vacuum), live a short time, then disappear.
• Nothing ? e e- ? Nothing
• Virtual Particles

16
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!

17
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!

18
Powering the big bangCosmic Inflation (Alan
Guth, 1981)
Recall from lecture 1 Separation between pair
of matter particles R(t) If dR/dt gt 0 and CR2
gt k, eventually k becomes tiny and can be
neglected to good approximation. Exponential
growth of prices inflation
19
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!

20
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.
• k0 klt0 kgt0
• Inflation predicts k0 as now observed to 1
  accuracy!

21
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.)

e-
e
BH
22
After a few billion years

• Exponential stretching causes the quantum waves
  to behave classically (roughly, Heisenbergs
  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

23
Cosmic Microwave Background Radiation Maps
Observation, Theory
Simulated map at WMAP resolution made in
1995 (different false color scheme, statistical
comparison only)
WMAPs results were judged the top scientific
breakthrough of 2003!
24
CMBR Angular Power SpectrumCosmic
SonogramTop Temperature fluctuations vs.
angular scale(data points and theory)Bottom
Cross-correlation of temperature and linear
polarizationvs. angular scaleFrom Bennett et
al. 2003, WMAP
25
Conclusions

• Cosmic inflation refines the big bang theory.
• Its 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 1017
  seconds.
• Dark matter should be produced in the lab AND
  detected from space mañana.

26
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)