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 ( ) was trained as a lawyer, before boredom made him turn to astronomy instead Cepheid variables oscillate in brightness with a regular period that 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 ( ) 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 maintains a 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 temperature of 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 subtracted out, we are left with a hot central band running across the sky, which is due to our own Milky Way The final CMB map is obtained after both the dipole and galaxy signals have been removed. The result is a map of regions that are a few millionths of a degree 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 uniform even on opposite sides of the sky, which could never have been in causal contact with each other?

Alan Guth: Inflation  Lasted between and seconds after the Big Bang  Universe expanded by a factor of 10 50, from smaller than an atom to bigger than 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 a patchwork 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 g/cm 3  Observed density of luminous material (stars, galaxies) is about 3 x g/cm 3

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 in a galaxy show that there must be more matter in the galaxy than is apparent. This ‘dark matter’ is known to form an invisible 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