Transcript THE BIG BANG - Santa Cruz Institute for Particle Physics

FROM THE VERY SMALL
TO THE VERY LARGE
PARTICLE PHYSICS AND THE
COSMOS
Michael Dine
July 2004
Questions from Professor
Brown’s Lecture
1. How big is the universe?
2. What is the large-scale geometry of
the universe?
3. Why is the universe accelerating?
4. What is dark matter?
5. Why are protons indestructable?
6. Why are quarks confined?
7. Why is the charge of the proton the
same as the charge of the electron?
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.
But Dr. Smolin said the 20th-century revolution was not
complete. His work involves trying to reconcile Einstein's
general relativity, which explains gravity as the
"curvature" of space-time, with quantum mechanics, the
strange laws that describe the behavior of atoms.
"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.
•What is the dark matter?
•Why does the universe contain matter at all?
•What is the dark energy?
•What is responsible for ``inflation”?
•What happened at t=0?
We can’t answer any of these questions
without resolving mysteries of particle
physics. We need to know that laws of nature
which operate at the smallest distances we can
presently imagine. This will be the subject of
this talk.
What is particle physics?
Particle physics is just that – the study of particles. What
are the proton and neutron? What are they made of? What
about the electron? How do all of these particles interact with
each other? The tools: mainly big machines, called particle
accelerators.
Why bother?
• It’s fun.
• Learning about the elementary particles, we learn what are
the laws of nature which operate at very small distance
scales.
More questions:
• Why does studying the behavior of small particles tell
us something about the fundamental laws of nature.?
And don’t we already know all of the laws? And why
do we care, anyway?
• Why do we need big machines in order to study small
particles?
Physical Law
Newton: F=ma
FG= M1M1/R2
Probably the most famous physical
laws. UNIVERSAL
Newton could use his laws to
explain the motion of the
planets, the moon. Haley –
comets.
Explained ``Kepler’s Laws: Planets move in elliptical
Orbits, with the sun at one focus.
Electricity and Magnetism:
Faraday
Conducted experiments which showed that
a changing electric field produces a magnetic
field and vice versa, and that a changing
magnetic field induces current (generators)
Electricity and magnetism aspects of one
related set of phenomena:
ELECTROMAGNETISM.
Electricity and Magnetism:
Maxwell
Wrote down the laws of electricity
and magnetism; ``Maxwell’s equations.”
Light, radio waves (Maxwell predicted),
and other radiation all part of the same
set of phenomena.
HERTZ: RADIO WAVES
So two sets of laws. These describe most of the phenomena of our
day to day experience: gravity, light, electricity, magnetism…
With these, scientists of the late 19th century understood the motion
of the planets in great detail, and made great technical progress.
They started, as well (somewhat inadvertently) to explore the world
of atoms.
The end of the 19th century saw the discovery of the first
elementary particle, by Thompson – the electron.
EINSTEIN
Excited by Maxwell’s equations and also puzzled. There
seemed to be a maximal speed at which light could travel.
Puzzled, also by the problem of the photoelectric effect – the
emission of electrons by light.
Also wondered about the existence of atoms. Were they real, or
just a trick to understand the periodic table?
Einstein’s Extraordinary year:
1905
•Photoelectric effect – the idea that lights come in
packets of energy – the beginnings of the photon
concept
•Explanation of the Brownian motion – basic to
physics, chemistry, biology – clinched the idea that
atoms were real.
•Special relativity – time and space are relative
concepts; depend on the observer. But the speed of
light is absolute: all observers agree about it.
General Relativity
Now, a deeper understanding of the laws of
electricity and magnetism. But Einstein didn’t
know how to reconcile Newton’s laws with the rules
of relativity. E.g. in Newton’s laws, action at a
distance. Didn’t make sense; electricity and
magnetism don’t work this way.
Einstein’s clue: the equality of gravitational and
inertial mass. Inertia – something to do with space
and time. So gravity?
F=ma
FG= mM/R2
Inertia
Gravitation
The equality of gravitational and inertial mass
was first tested in experiments by the Hungarian
scientist Eotvos in the late 1800’s:
Einstein and the General Theory
of Relativity
After almost eleven years of struggle, Einstein announced
his general theory of relativity in 1916. A theory in which
gravity arises as the distortion of space and time by energy.
Proposed three experimental tests:
•Bending of light by the sun
•Perihelion of Mercury
•Red Shift
General Relativity and the
Universe
Gravity was unique among the forces in that it
is always attractive. So it acts on things at the
surface of the earth, on the planets, on stars,
and on the universe as a whole. So Einstein
and others tried to apply his theory to the
universe.
But the universe is complicated, varied. How
to proceed?
Einstein + Copernicus
Assume the universe is homogeneous and isotropic –
no special place or direction.
Einstein’s equations have no Static solutions.
The universe expands!
Einstein was very troubled – remember that at that time (c. 1920)
Astronomers didn’t know about galaxies!
Edwin Hubble, who started out as a lazy, rich kid, became one of
the most important of all astronomers.
HUBBLE (1921)
Galaxies move away from us at a speed proportional to
their distance
The Cosmic Microwave Background
In the past, the universe must have been much
hotter: Big Bang. Gamow, Peebles: if true, there
should be a ``glow” left over from this huge explosion
(but of microwave radiation, not light).
Objects give off a characteristic spectrum of
electromagnetic radiation depending on their
temperature; ``blackbody.” The temperature then
was 10,000 degrees; today it would be about 3
degrees
Discovered by Penzias and Wilson
(1969).
Today: thanks to COBE satellite, the best
measured black body spectrum in nature.
Artist’s Rendering of COBE
COBE measured the temperature of the universe:
More detailed study of the
CMBR:
From satellites and earth based (balloon)
experiments. Most recently the WMAP
satellite.
Detailed information about the
universe:
COMPOSITION OF THE
UNIVERSE
From studies of CMBR, of distant Supernova
explosions, and from Hubble and GroundBased observations we know:
• 5% Baryons (protons, neutrons)
• 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
violent expansion (inflation) at about 10-30 seconds
after the big bang. What caused this?
But: we’ve gotten ahead of our
story.
We started out talking about laws of nature.
We had Newton, and with him an
understanding of the planets; then Maxwell,
and an understanding of the electromagnetic
spectrum, and now Einstein, and we have
started to think about the universe as a
whole. But a lot happened between 1905
and these discoveries.
New particles, new laws
•
•
•
•
1895 – discovery of the electron
1911 - discovery of the atomic nucleus
1920’s – quantum mechanics
1930’s – the neutron, and understanding of
the atomic nucleus.
• 1930’s – discovery of antimatter.
Rutherford’s Discovers the
Nucleus
"It was quite the most incredible event that ever happened to me
in my life. It was almost as incredible as if you had fired a 15inch shell at a piece of tissue paper and it came back and hit you."
Applications
• Understanding of atoms, quantum mechanics – all
of our modern technological revolution. Also
important tools for looking at the universe – the
spectrum of light tells us the composition of stars,
planets.
• Nuclear physics: reactors (bombs); also
understanding of the workings of stars.
Together, a revolution in astronomy.
LOOKING STILL DEEPER
By the 1940’s, much progress, but much not
well understood:
• Photons
• The precise laws underlying the nuclear
forces
To go further: theoretical developments
Experiments probing distances smaller
than the size of nuclei
Quantum Electrodynamics
Feynman, Schwinger, Tomanaga:
detailed understanding of how quantum
mechanics and electricity and magnetism
work together. Predictions with awesome
precision. E.g. the magnetism of the
electron explained in terms of the
electron’s charge and mass to one part in
1012.
Looking Deeper
The late 1940’s launched the era of large
particle accelerators. Some of the important
discoveries (also cosmic rays):
• Particles like the electron, but heavier: m t
• Three kinds of neutrino
• Neutrons, protons made up of quarks
Stanford Linear Accelerator
HOW DO WE KNOW HOW THE
SUN SHINES?
ASTRONOMERS AND PHYSICISTS
LOOKING AT THE SURFACE OF THE SUN CAN
MEASURE ITS TEMPERATURE AND FIGURE
OUT WHAT ITS MADE OF. THEN THEY FIGURE
OUT HOW THINGS
WORK INSIDE.
BUT CAN WE SEE INSIDE?
Ray Davis and John Bahcall said yes; look for
neutrinos produced in the nuclear reactions in
the sun.
Homestake Gold Mine, South
Dakota
A huge tank of cleaning fluid!
Chlorine atoms hit by neutrinos
turn into radioactive argonne.
About once a month, Davis and his crew
flushed out the
tank, looked for the radioactivity.
They did this every month for about
30 years! They found only ½ as many
neutrinos as Bahcall said there
should be. Did this mean
We didn’t understand the
sun?
No! We didn’t understand the
neutrinos!
SNO—Sudbury Neutrino Observatory
Where Do We Stand?
• Particle physicists know the laws of nature
on scales down to one-thousandth the size
of an atomic nucleus. The ``Standard
Model”. 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
Back to Our Cosmic Questions
• 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.
• With what we know today, we can figure out the
history of the universe back to about 10-7 seconds
after the big bang!
BUT THAT’S NOT GOOD ENOUGH
• We may soon know the identity of the dark
matter. This probably requires increasing
the energies of our accelerators by a factor
of 10
• Related to this, we may have some idea
where the matter in the universe came from
• Other questions may require theoretical as
well as experimental breakthroughs.
One possible new phenomenon:
Supersymmetry
• A new symmetry among the elementary
particles. ``Fermions ! bosons; bosons !
fermions.
There’s not time to explain why here, but if
this idea is right, then it explains what the
dark matter is!
Electrons
Selectrons
Quarks
Squarks
Photons
Photinos
Higgs particles
Higgsinos
This symmetry proposed to solve puzzles of particle physics,
but it turns out that the photino is – automatically – a natural
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
• Supersymmetry (dark matter, baryogenesis)
• Candidate mechanisms for inflation
• A possible explanation of the dark energy.
WMAP ORBIT
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!
April 10, 2003
Thursday
Princeton Physics Department Colloquium, 4:30 p.m. - Jadwin A-10
Speaker: Michael Dine UCSC
Title: "Bringing String Theory into Contact With Experiment"
Abstract:
String theory bears a striking resemblance to the real world. But
making precise predictions for future experiments is surprisingly
difficult. In this talk, I will explain the difficulties, and outline
the approaches which are being pursued to developing a string
phenomenology. I will also describe some of the insights which string
theory has already provided into long-standing puzzles of particle
physics.
Host: Chiara Nappi