Lecture 11: The Big Bang. Galaxies: islands of stars making up the universe.
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Transcript of Lecture 11: The Big Bang. Galaxies: islands of stars making up the universe.
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Lecture 11: The Big Bang
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Galaxies: islands of stars making up the universe
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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.
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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
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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
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Hubble constant graph
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Expansion of the Universe
… ‘winding’ backwards, the universe must have had a beginning
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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 ...
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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 …
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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
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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.
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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
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These tiny fluctuations have evolved into clusters of galaxies today
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Tests of the Big Bang Theory
Expansion of the universe
Cosmic microwave background
Relative abundances of hydrogen, deuterium, helium and lithium
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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
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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?
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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
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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
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Summary: Timeline of the Universe
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Timeline of the Universe 2
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Possible Fates of the Universe
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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
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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
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Possible forms of dark matter
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If the universe recollapses ...
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If the universe expands forever ...
“Some say the world will end in fire, others say in ice.” – Robert Frost