Lecture 11: The Big Bang

Galaxies: islands of stars making up the universe

Edwin Hubble

First to realise that galaxies lie outside the Milky Way …

… by measuring their distances using Cepheid variables as ‘standard candles’

Edwin P. Hubble (1889-1953) was trained as a lawyer, before boredom made him turn to astronomy instead

Cepheid variables oscillate in brightness with a regular periodthat depends on their luminosity. By measuring this period,they can be used as standard candles.

Cosmic Speedometer

When a galaxy is receding, light waves travelling to us are red-shifted

Hubble measured the spectrum of these galaxies and found the spectral lines to be red-shifted

The faster the recession, the greater the red-shift

Hubble’s Law

Hubble then noticed a correlation between the distance of the galaxies and the speed at which they are moving away from us

Hubble constant graph

Expansion of the Universe

… ‘winding’ backwards, the universe must have had a beginning

Georges Lemaître (1894-1966) was a Belgian Catholic priest who was fond of saying there is no conflict between science and religion

Georges Lemaître

Proposed that the universe began with the explosion of a ‘primeval atom’

His model was improved by George Gamow and others, who proposed that elements were forged during this hot and dense stage

Known as the Big Bang theory today; coined by Fred Hoyle who proposed a rival theory ...

Steady State Model

Universe is expanding, but maintainsa constant average density

Matter is continually being created in the voids to form new stars, galaxies

Universe has no beginning and no end

But disproved with the discovery of …

Cosmic Microwave Background (CMB)

Background radiation from the sky that is isotropic (same strength in all directions)

Corresponds to a temperatureof just 2.7 Kelvins

Identified as the radiation left over from the Big Bang explosion

Arno Penzias (right) and Robert Wilson of Bell Laboratories, next to the horn antenna with which they discovered the CMB in 1965

Cosmic Background Explorer (COBE)

The COBE was the world’s most sensitive thermometer, built to probe the heat radiation left over from the Big Bang. In 1992, it detected fluctuations of just a few millionths of a degree in the CMB.

CMB Maps by COBE of the Entire Sky

This dipole signal is due to the motion of the Earth through space. The CMB is blue-shifted in the direction of Earth’s motion, and red-shifted in the opposite direction

After the dipole signal has been subtractedout, we are left with a hot central bandrunning across the sky, which is due to ourown Milky Way

The final CMB map is obtained after both the dipoleand galaxy signals have been removed. The resultis a map of regions that are a few millionths of adegree hotter (pink) or colder (blue) than average

These tiny fluctuations have evolved into clusters of galaxies today

Tests of the Big Bang Theory

Expansion of the universe

Cosmic microwave background

Relative abundances of hydrogen, deuterium, helium and lithium

Obtaining the Age of the Universe

Extrapolate the current expansion rate (Hubble constant) back to the Big Bang

– 10 to 20 billion years old

Look for the oldest stars (in globular clusters)

– 11 to 18 billion years old

Best current estimate is 13.4 ± 1.6 billion years

M10 Globular Cluster

Problems with the Big Bang Theory

Flatness problem

– why is the geometry of universe so close to being flat?

Horizon problem

– why is the universe so isotropic?

Smoothness problem

– why is the universe so homogeneous?

Why does the universe appear so uniformeven on opposite sides of the sky, whichcould never have been in causal contactwith each other?

Alan Guth: Inflation

Lasted between 10-35 and 10-32 seconds after the Big Bang

Universe expanded by a factor of 1050, from smaller than an atom to biggerthan a galaxy

It was driven by vast amounts of energy released when a ‘symmetry breaking’ phase transition occurred

Alan Guth of MIT was only 32 when he developed the theory of inflation in 1979

Inflation to the Rescue

Flatness problem

Horizon and smoothness problems

In each successive frame, the sphere is inflated by a factor of three. By the fourth frame, it looks like a flat plane. Thus, inflation drives the geometry of the universe toward flatness

Without inflation, the universe today would consist of apatchwork of different regions. Instead, it is very uniform

Inflation had the effect of expanding a small region to a very large one. Since we are inside such a region, our neighbour-hood appears uniform

Summary: Timeline of the Universe

Timeline of the Universe 2

Possible Fates of the Universe

Will the Universe Recollapse?

Gravitational pull of the galaxies on each other is slowing down the rate of expansion

Required density for the universe to recollapse is 4.5 x 10-30 g/cm3

Observed density of luminous material (stars, galaxies) is about 3 x 10-31 g/cm3

Dark Matter

But there may be about 10 times this amount in the form of ‘dark matter’

So the universe is very close to the threshold for recollapse! (This is related to the flatness problem)

Measurements of the velocities of stars ina galaxy show that there must be morematter in the galaxy than is apparent.This ‘dark matter’ is known to form aninvisible halo around the galaxy

Possible forms of dark matter

If the universe recollapses ...

If the universe expands forever ...

“Some say the world will end in fire, others say in ice.” – Robert Frost