Today: “Nucleosynthesis… another phase change in early universe…

and why is the Universe so Flat?”

HW for next time:

Onion, “the nucleus and forces of nature”

chapters 3 & 4

reminder: use the Glossary

Session 7: 2/6/07 ? min

geometry of universe:

determined by total density of matter & energy

Density = Critical

Density > Critical

Density < Critical

why is the density ~ critical…i.e. why is our universe so“flat?”

Inflation flattens overall geometry like blowing up a balloon,

overall density of matter plus energy becomes almost critical

…during the first 3 minutes, Big Bang nucleosynthesis created

deuterium (d), 3He, 4He, traces of Li…but nothing else

at t ~1 minute, universe cooled to temperature T ~ 1MeV, nuclear energies

E = kT, 1 MeV ~ 1010 °K

1/40 eV ~10,000 °K

too cool for n↔p equilibrium since mn > mp by 1.3 MeV,

mn ~ mp = 1 GeV = 1000 MeV

so neutrons weakly decay n → p e beta decay

between 1 and 3 minutes,

temperature high enough to fuse n + p → d, then d + t → He t=tritium=3H

not too high to dissociate weakly-bound d 2.2 MeV binding energy

nor to destroy very stable 4He (=α) 24 MeV binding energy

at t = 1 min, Big Bang predicts protons:neutrons = 7:1

after 3 minutes, universe too cool to fuse a neutron and proton to deuterium,

but all neutrons already bound up in helium! 4He2 = α= 2 protons + 2 neutrons

(read Steve Weinberg’s “The First Three Minutes”)

In hot primordial plasma, protons and neutrons combine making ultra-stable, long-lasting helium

but universe only hot enough up to ~ 3 minutes of age

one of the reaction chains… JET, next ITER

Big Bang theory prediction: 75% H, 25% He (12/16 vs 4/16 by mass) Matches observations of primordial gases in star forming regions

Big Bang theory: correctly predicts trace abundances of elements 2H = d, 3He, Li?

(will discuss horizontal axis when we discuss at dark matter)

But, can you observing the Big Bang for yourself? the snow on the TV…darkness of the night sky…evidence for Big Bang

first Kepler, thenOlbers’ Paradox

If universe were

1) infinite

2) unchanging

3) everywhere the same

stars would cover the night sky

Night sky is dark because the universe changes with time

As we look out in space, we can look back to a time when there were no stars

Night sky is dark because the universe changes with time

As we look out in space, we can look back to a time when there were no stars

…now, in summary

a brief history of the universe

as motivation for our study of particle physics

Four known forces in universe:

Strong Force Electromagnetism

Weak Force

Gravity

Do forces unify at high temperatures?

Yes!Fermilab & Cern

Proton Decay (GUT?)

Who knows?(String Theory)

Planck Era

Before Planck time (~10-43 sec)

No theory of quantum gravity

GUT Era

Lasts from Planck time (~10-43 sec) to end of GUT force (~10-38 sec)

Electroweak Era

Lasts from end of GUT force (~10-38 sec) to end of electroweak force (~10-10 sec)

Quark Era

Amounts of matter and antimatter nearly equal

(Roughly 1 extra proton for every 109 proton-antiproton pairs!)

Era of Nucleo-synthesis

Begins when matter annihilates remaining antimatter at ~ 0.001 sec

Nuclei begin to fuse

Era of Nuclei

Helium nuclei form at age ~ 3 minutes

Universe has become too cool to blast helium apart

Era of Atoms

Atoms form at age ~ 380,000 years

Background radiation released

Era of Galaxies

Galaxies form at age ~ 1 billion years

RESERVE

PHOTONS DECOUPLING

from H & He atoms once formed

interactive

History of Universewith black body spectra red shifted

for each era

interactive

Light from disconnected regionscoupled after inflation

interactive

The early universe must have been extremely hot and dense