the big bang theory. galaxies: islands of stars making up the universe
TRANSCRIPT
The Big Bang Theory
Galaxies: islands of stars making up the universe
Edwin Hubble First to realize 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 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 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
1) 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
2) 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
3) The final CMB map is obtained after both the dipole and galaxy signals have been removed. The resultis 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 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
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 neighborhood 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
Your Research Presentation
Groups must do more extensive research into the following topics:
1) Red Shift/Doppler Effect
2) Cosmic Microwave Background Radiation
3) Inflation
4) Steady State Model
5) Cepheid Variables and Hubble Constant
6) Fates of the Universe
Project RequirementsThursday Meet in Computer Lab 322
1) 8 Minute (minimum) Power Point Presentation• Each Group Member must speak• Each slide must be labeled by the creator of the
slide
2) Each group must create one paper poster summarizing their topic
3) Draft slides (one per student) due Thursday, September 15th (50 classwork points)
4) 2nd Draft Slides due Tuesday, September 20th
(50 classwork points)
5) Presentations next Thursday, SEPTEMBER 22nd (100 project points)