FROM THE VERY SMALL TO THE VERY LARGE

PARTICLE PHYSICS AND THE COSMOS

Michael DineJuly 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.

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Dr. Tyson, who introduced himself as the Frederick P. Rosedirector of the Hayden Planetarium, had invited five"distinguished" cosmologists into his lair for a roastingdisguised as a debate about the Big Bang. It was part ofseries in honor of the late and prolific author IsaacAsimov (540 books written or edited). What turned out to beat issue was less the Big Bang than cosmologists'pretensions that they now know something about theuniverse, a subject about which "the public feels somesense of ownership," Dr. Tyson said.

"Imagine you're in a living room," he told the audience."You're eavesdropping on scientists as they argue aboutthings for which there is very little data."

Dr. James Peebles, recently retired from Princeton, whom he called"the godfather"; Dr. Alan Guth from the MassachusettsInstitute of Technology, author of the leading theory ofthe Big Bang, known as inflation, which posits a spurt of akind of anti-gravity at the beginning of time; and Dr. PaulSteinhardt, also of Princeton, who has recently beenpushing an alternative genesis involving collidinguniverses.

Rounding out the field were Dr. Lee Smolin, a gravitationaltheorist at the Perimeter Institute for Theoretical Physicsin Waterloo, Ontario, whom Dr. Tyson described as "alwaysgood for an idea completely out of left field - he's hereto stir the pot"; and Dr. David Spergel, a Princetonastrophysicist.

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 ellipticalOrbits, 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 RelativityNow, 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 BackgroundIn 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 Ground-Based 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 15-inch 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

forcesTo 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: • Three kinds of neutrino

• Neutrons, protons made up of quarks

Stanford Linear Accelerator

HOW DO WE KNOW HOW THESUN SHINES?

ASTRONOMERS AND PHYSICISTSLOOKING AT THE SURFACE OF THE SUN CAN MEASURE ITS TEMPERATURE AND FIGURE OUT WHAT ITS MADE OF. THEN THEY FIGURE OUT HOW THINGSWORK 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, asSmolin 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

n

e+

Neutrino reactions stop; neutrons decay.

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

Tea in Room 218 Jadwin Hall, at 4 p.m.