FROM THE VERY SMALL TO THE VERY LARGE

FROM THE VERY SMALL TO THE VERY LARGE Theoretical High Energy Physics in the 21 st Century Michael Dine June, 2007

New York Times Reports a debate among cosmologists about the Big Bang. lll 1. 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 20 th-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=3 minutes • 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.

Physical Law – What are we after? Newton: F=ma FG= M 1 M 1/R 2 Probably the most famous physical laws. UNIVERSAL

Newton could use his laws to explain the motion of the planets, the moon. Haley – comets.

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.

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 19 th 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 19 th 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= m. M/R 2 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 is 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!

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: Aside: we understand this as a quantum phenomenon; the “measurement” was done a long time ago.

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.

LOOKING STILL DEEPER By the 1940’s, much progress, but much not well understood: • Photons -- the quantum mechanics of electrodynamics (QED) • 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. Over the next 50 years, numerous elementary particles, understanding of the basic constituents of matter and the forces between them – the Standard Model. Critical interplay between theory and experiment.

Success of the Standard Model is extremely successful Experimental discovery of all of its matter constituents and force carriers Simple common approach to describe all (relevant) forces: gauge principle Self-consistent at the level of quantum corrections

Detailed Comparison with Experiment

One missing piece: the Higgs particle • In SM, responsible for the masses of the quarks, leptons, W§, Z 0 • Mass not predicted by the model, but know something from experiment. Peter Higgs • Should be discovered at the LHC. • Much work on this by H. Haber

The Standard Model Higgs Boson time [year] Last missing particle in SM (EW symmetry breaking – mass) Light SM Higgs preferred MH = 126 +73 -48 Ge. V < 280 Ge. V (95% CL) Higgs Search at LEP: mass limits: obs. mh > 114. 4 Ge. V exp. m > 115. 3 Ge. V h

Fundamental open questions Is There a Higgs particle? The hierarchy problem: why is Higgs mass so small? (dimensional analysis: m. H » 1019 mp ) from LEP 114. 4 Ge. V/c 2 < m. H< 1 Te. V/c 2 Lots of parameters – masses, couplings of quarks, leptons. Where do they come from? Can’t just add general relativity to the SM; not clear consistent with quantum mechanics. SM describes only a tiny fraction of our universe – what is the dark matter ? What about the dark energy from theory

One possible new phenomenon: Supersymmetry • A new symmetry among the elementary particles. ``Fermions ! bosons; bosons ! fermions. Not only a new symmetry of nature, but if this idea is right, then it explains what the dark matter is! [Banks, Dine, Haber]

An attractive Extension: Supersymmetry Symmetry between Fermions ↔ Bosons (matter) (force carrier) . . . doubled particle spectrum. . . ☹

Consequences of Supersymmetry • Automatically provides an explanation of the dark matter • Predicts one of the fundamental constants of nature. • Provides an understanding of why there is more matter than antimatter in the universe • Predicts lots of phenomena observable at the LHC.

Interaction Strength in Supersymmetry without SUSY. . . BUT some Standard Model Problems solved. . . extension in string theory is candidate for Grand Unified Theory. . . lightest SUSY particle stable ⇨candidate for dark matter. . . unification of forces. . . with SUSY 1 Te. V Interaction energy in Ge. V

l c l c q q g q l l c l q Production and decay of superparticles at the LHC. Here, jets, Leptons, missing energy.

In a broad class of supersymmetric models, the lightest new particle is stable (R-parity); typically the partner of the Higgs or Z boson or photon. Produced in early universe. N W+ N W-

Range of supersymmetry parameters consistent with dark matter density; here partner of photon is essentially the dark matter.

I am a fan of the supersymmetry hypothesis; I'm not alone. About 12, 000 papers in the SPIRES data base. If true, quite exciting: a new symmetry of physics, closely tied to the very nature of space and time. Dramatic experimental signatures. A whole new phenomenology, new questions. But neither the limited evidence nor these sorts of arguments make it true; there is good experimental as well as theoretical reason for skepticism. This is not the only explanation offered for the hierarchy, and all predict dramatic phenomena in this energy range. • Large extra dimensions • Warped extra dimensions • Technicolor • It’s just that way (anthropic? )

Hypothetical answers to a set of fundamental questions: • Too many parameters • Charge quantization • Quantum general relativity • Dark Matter • Dark energy • Baryogenesis

STRING THEORY String theory has pretensions to attack the remaining problems on this list: • A consistent theory of quantum gravity • Incorporates gauge interactions, quarks and leptons, and other features of the Standard Model. • Parameters of the model can be calculated, in principle. • Low energy supersymmetry emerges naturally – all of this proliferation, which seemed artificial, almost automatic.

Has string theory delivered? • String theory is hard. We don’t have a wellunderstood set of principles. Some problems of quantum gravity are resolved, but many of the challenges remain. • String theory seems able to describe a vast number of possible universes, only a small fraction of which are like ours. • Until recently, no progress on one of the most difficult challenges to particle physics: the dark energy, but this has changed.

We are at the dawn of a very exciting era. We may resolve some of our fundamental questions.