Download THE BIG BANG - Santa Cruz Institute for Particle Physics

Modern Physics: A Conclusion
Michael Dine
May, 2003
Standard Model
Quarks
Leptons
Gauge Bosons (force carriers): photons, W§,Zo,gluons
Leptons
e, me= .511 Mev/c2
m mm= 105 MeV/c2
t mt= 1780 MeV/c2
Neutrinos: ne nm,nt
MASS?
tm= 10-8 sec
tt= 10-11sec
PDG Wall Chart
Beta Decay in the Standard Model:
u
u
d
d
d
u
We
n
Neutrinos from the sun?
Starting in 1960’s: Ray Davis (Nobel Prize, 2002):
search for neutrinos by looking at induced radioactivity
in a large tank of cleaning fluid in Homestake Gold Mine (SD).
About half the expected flux.
Subsequent experiments: confirmed this ``solar neutrino deficit”.
Problem? The sun? Or the neutrinos?
Possibility: neutrinos ``oscillate” on their trip from sun to earth.
Like spin ½. Spin up $ ne.
Spin down $ nm
y = ( cos(w t) , sin(w t) )
Only electron neutrinos interact with the chlorine in Davis expt
Sudbury Neutrino Observatory: Can count all neutrino types.
Neutral currents: involve the Z particle; all neutrinos
couple in the same way.
Result: solar model correct, neutrinos do oscillate.
Suggests neutrino masses of order .01 eV/c^2.
Further evidence for neutrino masses: from oscillations
in the atmosphere. Recently also from accelerators.
Beyond the Standard Model?
What questions does the Standard Model
Fail to Answer?
•The Standard Model has about 17 parameters. Where do
they come from?
•Why are there three generations of quarks and leptons?
Neutrino mass can be described within the Standard Model,
but it implies that there is some new, higher scale of physics.
•The strong CP Problem: dn = 10-16e cm £ q, q < 10-9. Why?
•The Hierarchy Problem: Why is MW ¿ Mp?
•How does gravity (General Relativity) fit in? What tames
the bad high energy behavior which seems to plague
quantum theories of gravity? What resolves the tension
between quantum mechanics and black holes pointed out by
Hawking?
Some Ideas for What Lies Beyond:
•Supersymmetry: a symmetry between fermions and bosons
-- could provide the dark matter
•String Theory – unifies general relativity and the Standard
Model
•Axions – could be the dark matter
A HYPOTHETICAL
SPECTRUM
~~~~
u,d,s,c,..
600
l
~ ~~
e, m,t
400
200
~ ~ ~
g, Z, H
Production of supersymmetric particles
g
l
l
g
e+
g,Z
~
m
~
m
e-
Electrons
Selectrons
m=5 10-4mp s=1/2 m=200 mp? s=0
Quarks
m= 10-3-175 mp
Photons
m=0 s=1
Higgs particles
m=115 mp? s=0
Squarks
m=500 mp? S=0
Photinos
m=100mp? s=1/2
Higgsinos
m=80 mp? S=1/2
Open String
Closed String
New York Times: April, 2003
Reports a debate among cosmologists about
the Big Bang.lll1.html
Dr. Tyson, who introduced himself as the Frederick P. Rose
director of the Hayden Planetarium, had invited five
"distinguished" cosmologists into his lair for a roasting
disguised as a debate about the Big Bang. It was part of
series in honor of the late and prolific author Isaac
Asimov (540 books written or edited). What turned out to be
at issue was less the Big Bang than cosmologists'
pretensions that they now know something about the
universe, a subject about which "the public feels some
sense of ownership," Dr. Tyson said.
"Imagine you're in a living room," he told the audience.
"You're eavesdropping on scientists as they argue about
things for which there is very little data."
Dr. James Peebles, recently retired from Princeton, whom he called
"the godfather"; Dr. Alan Guth from the Massachusetts
Institute of Technology, author of the leading theory of
the Big Bang, known as inflation, which posits a spurt of a
kind of anti-gravity at the beginning of time; and Dr. Paul
Steinhardt, also of Princeton, who has recently been
pushing an alternative genesis involving colliding
universes.
Rounding out the field were Dr. Lee Smolin, a gravitational
theorist at the Perimeter Institute for Theoretical Physics
in Waterloo, Ontario, whom Dr. Tyson described as "always
good for an idea completely out of left field - he's here
to stir the pot"; and Dr. David Spergel, a Princeton
astrophysicist.
"Quantum mechanics and gravity don't talk to each other,"
he said, and until they do in a theory of so-called quantum
gravity, science lacks a fundamental theory of the world.
The modern analog of Newton's Principia, which codified the
previous view of physics in 1687, "is still ahead of us,
not behind us," he said.
Although he is not a cosmologist, it was fitting for him to
be there, he said, because "all the problems those guys
don't solve wind up with us."
Today, you are listening to someone seemingly more out in left field
-- a particle physicist.
Particle physics: seeks to determine the laws of nature at a
``microscopic” – really submicroscopic, level.
What does this have to do with the Big Bang?
EVERYTHING!
With due respect to the New York Times, articles like
this give a very misleading impression.
We know:
•There was a Big Bang
•This even occurred about 13 Billion Years Ago
•We can describe the history of the universe,
starting at t=3minutes
•There is now a huge amount of data and a picture
with great detail.
There are lots of things we don’t know. With due
respect to Lee Smolin, the correct address for
these questions is Particle Physics.
We can’t answer any of these questions without
resolving mysteries of particle physics. These will
be the subject of this talk.
•Why does the universe contain matter at all?
•What is the dark energy?
•What is responsible for ``inflation”?
•What happened at t=0?
•What is the dark matter?
Physical Law and the Universe
•Newton: F=ma
FG= M1M1/R2
--Motion of Planets
• Laws of electricity and magnetism, nuclear physics:
-Understanding of Stars
•Einstein: General Relativity
– Expansion of the Universe
EINSTEIN
•1905: special relativity, photoelectric effect, Brownian Motion
•1916: General Relativity
•
•
Culmination of a 9 year struggle to understand Newton’s
Gravity, starting with ``equivalence principle”
Tests: Precession of Mercury’s Orbit, Bending of Light
Implications for the universe: COSMOLOGY
Einstein + Copernicus
Assume the universe is homogeneous and isotropic –
no special place or direction.
Einstein’s equations have no Static solutions.
The universe expands!
HUBBLE (1929)
Galaxies move away from us at a speed proportional to
their distance
Important times:
t=3 minutes: Synthesis of the light elements
T=MeV
t= 100,000 years: T=1 eV (3 1012/100)-1/2
Now:
t= 1.5 1010 years
T = 3 £ 10-4 eV = 1 eV (105/1.5 1010)2/3
What happens much earlier?
The Cosmic Microwave
Background
Initially, little evidence. Gamow, Peebles: if true, there
should be a ``glow” from the Big Bang.
Objects give off a characteristic spectrum of electromagnetic
radiation depending on their temperature; ``blackbody.”
Discovered by Penzias and Wilson (1969).
Today: thanks to COBE satellite,
best measured black body spectrum in nature.
3 minutes: Synthesis of the Light
Elements
•CMBR: A fossil from t=100,000 years.
•He,Li,De: Produced at t=3 minutes
p
e+
Neutrino reactions stop;
neutrons decay.
n
n
Results of Detailed
Nucleosynthesis Calculations:
• The fraction of the universe made of
``baryons”=protons + neutrons:
• During last two years, an independent
measurement from studies of CMBR:
Very impressive agreement!
The CMBR and the Copernican
Principle
Just how homogeneous and isotropic is the universe?
Reasonably so for galaxies on scales of 100’s of
millions of light years.
What about the CMBR?
-- the temperature is the same to a part in 10,000
in every direction in the sky!
Small variations only observed in 1993. Now
studied with great precision.
Is this reasonable?
Remember, the CMBR is light from the time that the
universe was 100,000 years old. At that time,
1,000 times smaller than it is today. When we
look at photons separated by 3^o in the sky, those
photons came from points that were separated by
far greater distance than light any signal could
have traveled between at that time in the history of
the universe – they are ``causally disconnected.”
g
g
So it’s not reasonable!
The solution: ``Inflation”– the universe
underwent a period of very rapid expansion,
probably at about t=10-24 seconds.
Is there evidence for this?
Yes!
The very rapid expansion of the universe
produces small variations in the energy
density (energy in different places), which
lead, eventually, to the formation of galaxies
and stars.
The same small variations appear in the
temperature of the CMBR!
Observational Confirmation
From satellites and earth based (balloon)
experiments. Most recently the WMAP
satellite.
Position of the Lagrange points in the Sun-Earth
System. WMAP is at L2.
WMAP RESULTS
The Temperature Viewed Across the Sky –
color variations represent variations in temperature
COMPOSITION OF THE
UNIVERSE
If 5% of the Universe is Baryons, What is the
Rest?
From studies of CMBR, of distant Supernova
explosions, and from Hubble and GroundBased observations we know:
• 5% Baryons
• 35% Dark Matter (zero pressure)
• 65% Dark Energy (negative pressure)
A Confusing Picture: Where Do
We Stand?
We have a good understanding of the history of the
universe, both from observations and well
understood physical theory, from t=180 seconds.
BUT:
• We don’t know why there are baryons at all!
• We don’t know what constitutes 95% of the
energy of the universe.
• We know that the universe underwent a period of
inflation. But we have little idea what inflation is.
What’s the Problem?
• To answer these questions, we need to know
how the universe behaved when the
temperature was extremely high.
Temperature=energy. So we need to know
about high energies.
• In quantum mechanics, high energies=short
distances. We need to know about the laws of
physics which operate at very short distances.
What do we know?
• Particle physicists know the laws of nature on scales
down to one-thousandth the size of an atomic nucleus.
The ``Standard Model”. This corresponds to
temperatures about 1,000,000 times those of
nucleosynthesis.
• Experiments at higher energy accelerators at CERN
(Geneva) and Fermilab (Chicago) are testing our
understanding at even shorter distances. Expect to
discover new phenomena.
PDG Wall Chart
No where in this picture:
•Any explanation of the dark matter
•Any explanation why there are baryons
•No understanding of inflation, the big bang
So the Standard Model Cannot Be Complete.
What lies beyond?
One possible new phenomenon:
Supersymmetry
A new symmetry among the elementary particles.
Fermions ! bosons;
bosons ! fermions.
Electrons
Selectrons
m=5 10-4mp s=1/2 m=200 mp? s=0
Quarks
m= 10-3-175 mp
Photons
m=0 s=1
Higgs particles
m=115 mp? s=0
Squarks
m=500 mp? S=0
Photinos
m=100mp? s=1/2
Higgsinos
m=80 mp? S=1/2
The ATLAS detector under construction
for the LHC. (Many components at
UCSC).
This symmetry proposed to solve puzzles of particle physics,
but it turns out that the photino is viewed as a leading
candidate for the dark matter. If it exists with the conjectured
properties, it is produced in just the right quantity to be the dark
matter.
Supersymmetry also provides a natural way to understand why
there are baryons in the universe at all (a puzzle first posed by
Andrei Sakharov).
So a better understanding of the laws of nature – in the not too
distant future – might answer two of the puzzles in our list.
What about the others?
Harder: But over time, we may have answers. All require, as
Smolin says, an understanding of quantum mechanics and
gravity (general relativity). Particle physicists do have a theory
which reconciles both: String Theory. This is the subject of
another talk. But String Theory does have
•General Relativity Plus Quantum Mechanics
•The Standard Model*
• Supersymmetry (dark matter, baryogenesis)
• Candidate mechanisms for inflation
• A possible explanation of the dark energy.