Modern Physics: A Conclusion

Michael DineMay, 2003

Standard Model

QuarksLeptonsGauge Bosons (force carriers): photons, W§,Zo,gluons

Leptons

e, me= .511 Mev/c2

m= 105 MeV/c2

m= 1780 MeV/c2

= 10-8 sec= 10-11sec

Neutrinos: e ,

MASS?

PDG Wall Chart

Beta Decay in the Standard Model:

u

d

d

u

d

u

W-

e

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 $ e. Spin down $

= ( cos( t) , sin( 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 ModelFail 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 £ , < 10-9. Why?•The Hierarchy Problem: Why is MW ¿ Mp?•How does gravity (General Relativity) fit in? What tamesthe 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

, Z, H

~ ~ ~

e, , ~ ~ ~

u,d,s,c,..~ ~ ~ ~

200

400

600

e+

e-

,Z

~

~

Production of supersymmetric particles

g

g

Electrons

m=5 10-4mp s=1/2

Selectrons

m=200 mp? s=0

Quarks

m= 10-3-175 mp

Squarks

m=500 mp? S=0

Photons

m=0 s=1

Photinos

m=100mp? s=1/2

Higgs particles

m=115 mp? s=0

Higgsinos

m=80 mp? S=1/2

Open StringClosed String

Reports a debate among cosmologists about the Big Bang.lll1.html

New York Times: April, 2003

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 forthese 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

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!

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.”

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.

The Temperature Viewed Across the Sky – color variations represent variations in temperature

WMAP RESULTS

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 Ground-Based 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

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.

No where in this picture:•Any explanation of the dark matter•Any explanation why there are baryons•No understanding of inflation, the big bang

Electrons

m=5 10-4mp s=1/2

Selectrons

m=200 mp? s=0

Quarks

m= 10-3-175 mp

Squarks

m=500 mp? S=0

Photons

m=0 s=1

Photinos

m=100mp? s=1/2

Higgs particles

m=115 mp? s=0

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, 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

•General Relativity Plus Quantum Mechanics•The Standard Model*• Supersymmetry (dark matter, baryogenesis)• Candidate mechanisms for inflation• A possible explanation of the dark energy.