Upload
karan
View
28
Download
1
Tags:
Embed Size (px)
DESCRIPTION
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. Matter particles Spin Two known types: Quarks Feel strong force Leptons Do not feel strong force. - PowerPoint PPT Presentation
Citation preview
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
2
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!
3
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.
1
2
3
4
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!)
5
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.
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
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!
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.+ e+ + e
12
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.
13
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
14
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!
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 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
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.
k=0 k<0 k>0 Inflation predicts k=0 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”.)
BHe+
e
22
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
23
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!
24
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
25
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.”
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)