Astronomy 162: Introduction to Stars, Galaxies, & the Universe Prof. Richard Pogge, MTWThF 9:30

Lecture 38: "The First Three Minutes"

Key Ideas

Physics of the Early Universe

Informed by experimental & theoretical physics

Later stages confirmed by observations

The Cosmic Timeline

Unification of the forces just after the Big Bang

Separation of the forces as the Universe expands & Cools

Inflationary Epoch explains smoothness & flatness

Emergence of Matter beginning at t=10^-6 seconds

Recombination & the emergence of the visible Universe

The Big Bang's Hot Past

• Today the Universe is low density, dark, and rather cool.
• We see that it is continuing to expand

The Universe 14 Gyr Ago:

• The Universe was smaller, denser, & hotter
• The Universe was Opaque and filled with radiation (photons)

How far back into the Universe's past can we go?

Binding & Loosing

• Energy needed to unbind (break up) matter.

Can define a Binding Temperature:

• Temperature equivalent to the binding energy
• Matter at this temperature "melts" (unbinds)

Example:

• In massive star cores, nuclei melt at temperature of ~10 Billion K.

Typical Sizes & Binding Energies

The Fundamental Forces of Nature

• Long-Range Force, weakest in Nature

• Long-Range, 10³⁹ times stronger than gravity

Weak Nuclear Force:

• Range <10^-17 meters, 10²⁸ times stronger than gravity

Strong Nuclear Force:

• Range <10^-15 meters, 10⁴¹ times stronger than gravity

Unification of the Forces

• EM & Weak forces unify at high energies (10¹⁵K)
• Verified in particle accelerator experiments.

Grand Unified Theory (GUTs):

• Strong + Electroweak Force.
• Predicted, but no experimental basis (yet?)

"Dreams of a Final Theory"

• Gravity should unify with the GUTs force at very high energies.
• Much higher than in any possible accelerator we might build.
• However, these energies could occur in the Early Universe.

The Cosmic Timeline

• Particle accelerators probe matter at states similar to some of these early phases.
• Astronomers can look for the evidence of these early phases in the present Universe (e.g., Cosmic Background, primordial amounts of deuterium & helium)

The Planck Epoch

• All 4 forces unified into a single Superforce

• 1 force rules all of physics

We cannot say much else about the time before this, as we as do not yet have a quantum theory of gravity to guide us.

The Grand Unification Epoch

• Gravity separates from the Superforce
• Strong & Electroweak Forces unified into the GUTs force.

The Universe at this phase is a hot, dense particle soup of quarks, antiquarks, & photons in equilibrium with each other.

The Inflationary Epoch

• Strong Force separates from the GUTs force
• EM and Weak forces are still unified

The rapid separation of the Strong Force from the GUTs Force triggers a rapid inflation of the Universe.

The Inflationary Universe

• The expansion greatly slows down (back to normal) after Inflation.

Inflation helps to explain why the Universe is so smooth and flat:

• Smooth: Cosmic Background Radiation is smooth to 1 part in 10⁵
• Flat: current observations suggest W₀=1

Four Forces at Last!

• Electroweak force separates into the Electromagnetic & Weak forces.
• All forces are now separate.

Conditions now become right for matter to exist separate from photons, instead of as a hot soup of matter and photons in equilibrium.

Quark Freeze-out

• Free quarks combine into hadrons (primarily protons & neutrons)
• Equilibrium between particle-antiparticle pairs and photons

Matter as we would recognize it today begins to emerge.

Nucleon Freeze-out

• Protons & neutrons decouple from photons and exist as free particles.
• electrons & positrons in equilibrium with photons.
• neutrinos & nucleons in equilibrium

Free neutrons are stable during this epoch.

Neutrino Decoupling

• Neutrinos decouple from matter and radiation, and stream out into space
• These form a Cosmic Neutrino Background (predicted but not yet observable with current technology)

Free neutrons are no longer stable:

• Decay into protons, electrons, and neutrinos
• Left with about 1 neutron for every 5 protons.

The Epoch of Nucleosynthesis

Fusion of protons and the remaining free neutrons:

• Formation of ²H (Deuterium) & ⁴He
• End up with ~75% ¹H, 25% ⁴He
• Also end up with traces of ²H, ³He, Li, Be, B

We cannot observe this epoch directly, but we can look for the products of primordial nucleosynthesis in the present-day Universe, as described in the previous lecture.

The Epoch of Recombination

Electrons & nuclei combine into neutral atoms:

• Universe becomes transparent
• Photons stream out into space
• Origin of the Cosmic Background Radiation.

This represents the earliest epoch of the Universe we can observe directly using photons.

Previous to this, the Universe is opaque to photons.

The "Dark Ages" and Emergence of Galaxies

• No visible or infrared light because there were no stars ("dark")
• The hydrogen and helium in the Universe are neutral
• Universe is mostly opaque to UV photons because of absorption by neutral H and He.

Time of rapid evolution:

• Matter density drops by factor of ~10 Million.
• Matter starts organizing itself into large-scale structures via gravitational collapse.

At t= 500 Myr - 1 Gyr, T=30 K:

• First generation of stars form, ending the "Dark Ages"
• Quasars first form
• First heavy metals made by the first supernovae

Present: t=14 Gyr, T=2.725 K

• Galaxies, stars, planets, us...
• Metals from supernovae of massive stars.

What about the Beginning?

The current frontier appears to be before the Electroweak Epoch (t=10^-12 sec), during the period of rapid Inflation.

This will be the astrophysics theory of the 21st Century (or maybe the 22nd ...)

Note:

The title of this lecture is borrowed from the title of an outstanding popular-science book by physicist Steven Weinberg (who shared the 1989 Nobel Prize in Physics for his work on Electroweak Unification). This book and Weinberg's Dreams of a Final Theory will greatly reward reading by students who have taken Astronomy 162.