Lecture 11: The Big Bang

1. Lecture 11: The Big Bang

2. Galaxies: islands of stars making up the universe

3. 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.

4. 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

5. 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

6. Hubble constant graph

7. Expansion of the Universe … ‘winding’ backwards, the universe must have had a beginning

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

9. 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 …

10. 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

11. 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.

12. 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

13. These tiny fluctuations have evolved into clusters of galaxies today

14. Tests of the Big Bang Theory • Expansion of the universe • Cosmic microwave background • Relative abundances of hydrogen, deuterium, helium and lithium

15. Obtaining the Age of the Universe • Extrapolate the current expansion rate (Hubble constant) back to the Big Bang • Look for the oldest stars (in globular clusters) • Best current estimate is 13.4 ± 0.2 billion years M10 Globular Cluster

16. 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?

17. Alan Guth: Inflation Alan Guth of MIT was only 32 when he developed the theory of inflation in 1979 • 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 New evidenceinflation

18. 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

19. Summary: Timeline of the Universe

20. Timeline of the Universe 2

21. Possible Fates of the Universe

22. 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

23. 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