The Big Bang: what happened, and when did it happen?

Where are we now?

• Observations of galaxies show the universe is expanding

• The theory of General Relativity contains solutions corresponding to expanding universes

These results lead to 2 questions:

• How long ago did the expansion start?

• What was happening in those early times (Big Bang)

The age of the universe

In a Friedmann universe, the age depends on what sort of a(t) we have

A consistency check for cosmological theories: are our estimates for the age

of the universe consistent with independent measurements of the

age of objects?

• Age of the Earth

• Age of globular cluster stars

Connection with textbook: Skip (for now) discussionof cosmological constant and Dark Energy.

Will return to later, since they are an importantPart of modern cosmology

The “Big Bang”

• Friedmann equation predicts a=0 in remote past

• This happened 14 Gyr ago if Omega=0• Happened (2/3)*14 Gyr ago if Omega =1• At that time, universe infinitely compressed• From that instant on, there was expansion of

universe, density drops, temperature drops, like aftermath of explosion

• Big Bang

The Big Bang

The Big Bang was not like an explosion, in that it didn’t “explode

into nothing”. At the time of the BB, the universe was probably infinite in extent; the scale has

gotten bigger with time. Even if it was finite (K>0), it was unbounded

A Reality Check

• All of this sounds pretty weird (and it’s about to get weirder), but it isn’t “made up”

• We have Hubble’s Law: the universe IS expanding

• We have the equations of General Relativity, exhaustively tested in physics experiments

• More to come

The Big Bang from the inside out; start at t=0 and see what happens

• First few seconds: really weird stuff• First three minutes: whole universe hot and dense as

center of Sun. Nuclear reactions everywhere• 700,000 years after BB: universe cools to point where

hydrogen atoms combine from protons and electrons, making universe transparent

• Few hundred million years after BB: first ghostly protogalaxies

• One billion years after BB: birth of the quasars• 5 billion years after BB: galaxies as they are today

The Big Bang from the inside out; start at t=0 and see what happens

• First few seconds: really weird stuff• First three minutes: whole universe hot and dense as

center of Sun. Nuclear reactions everywhere• 700,000 years after BB: universe cools to point where

hydrogen atoms combine from protons and electrons, making universe transparent

• Few hundred million years after BB: first ghostly protogalaxies

• One billion years after BB: birth of the quasars• 5 billion years after BB: galaxies as they are today

An external view of the Big Bang (?!*)

The Cosmic Background Radiation• For first 400,00 years, everywhere hot gas, conditions like solar

photosphere• Matter radiated and absorbed light• Light was emitted and absorbed by matter

As the universe expanded, the gas cooled

Question: what would have happened to the radiation as the universe expanded? Redder or

bluer?

When the gas cooled to about 3000K, recombination occurred. E+p H, neutral gas. Absorption of light was now negligible, universe became transparent

BIG REALIZATION was that this radiation should still be here, redshifted by z=1000. There should be a glow everywhere in the sky, describable by a blackbody with a temperature of 3K.

The Cosmic Background Radiation

The afterglow of the Big Bang

The “surface of last scattering”

Cosmic Microwave Background was discovered in 1965

This really is a map of the sky, black zero emission, green that of

a blackbody of 2.7K!

To see ripples in the CMB, you really have to go down in sensitivity

Results from spacecraft WMAP, released on February 11, 2003

From these tiny fluctuations grew the structure we see in the universe today

Clusters of galaxies, galaxies, and stars

So what kind of universe do we live in? open, closed…?

Recall that it depends on the mean density

Via the parameter

Determining the mean density of the universe

The answer is….

• For “bright matter” omega = 2.2E-03

• All “baryonic matter” (from abundance of light elements), omega = 0.02 – 0.05

• Taking account of dark matter in clusters of galaxies, omega = 0.33

• Best estimate is that omega < 1

• Abstract of Turner article

The truly weird feature of this is that most of the matter in the universe is not even Baryonic, and thus is of an unknown form.

Taken at face value, this result indicates that omega is < 1, and we live in an open universe. This was the status a few years ago.

Then things got even stranger