- 2102 - The age of the Universe. How did
we determine the age of the Earth? Of the
Solar System, even the age of the total Universe? It is an interesting study. This Review covers some of the highlights.
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----------------- 2102 -
Age of the Universe
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- The Universe
is 13,500,000,000 to 13,900,000,000 years old.
Said another way the Universe is 13.7 billion years old, + or - 0.2
billion years. How did astronomers ever
figure this out?
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- The age
question started with "how old is the Earth ?". Science first tried to figure that out by
calculating how long it would take the oceans to get salty. They knew how fast rock salts dissolved and
they estimated how much water was in the oceans, but their calculations were
not very accurate. Another attempt was
to assume the Sun was hard coal and calculate how long it would burn. At the time there was no knowledge of atoms
or nuclear fusion, or even what the Sun was made of.
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- The real
breakthrough came with radioactive dating.
Once science learned in the laboratories how fast a particular
radioactive material would undergo radioactive decay and turn into another
element they could calculate how long the material had been decaying, that is
its age.
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- They could
do this with rocks in the Earth or meteorites that landed from outer
space. Measure the percentage of the
element that was radioactive, the percentage that had decayed into a new
element through fission versus how much material was there to start with. ( See Reviews # 871 and #872 on Carbon-14 and
Potassium-40 dating).
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- These
calculations performed many times on different rocks determined the Earth to be
4,500,000,000 years old and the Solar System to be 5,000,000,000 years old.
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- To get how
old the Universe was we had to look to the stars that were older than our
Sun. How long a star lives depends on
its size. The bigger the star the
shorter its lifetime.
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- When stars
die they explode into supernovae or into planetary nebulae and leave behind a
carbon star called a White Dwarf. White
Dwarfs have no fuel left to burn so they simply cool down at the natural rate
from thermal radiation. By studying the
White Dwarfs in the Universe astronomers came up with an age of 10 to 20
billion years old. Not too precise.
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- Another
method was to use Globular Clusters of stars.
These clusters of stars were assumed to be all formed at about the same
time and would all be about the same age.
But, as we pointed out big stars die faster that smaller stars. Our Sun is mid-sized and will live about 10
billion years.
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- So,
astronomers could count the sizes of the stars in the cluster. What were the sizes of most of the stars that
are gone, are now White Dwarfs, Neutron Stars, or Black Holes, but no longer
stars on the Main Sequence of burning hydrogen.
Knowing the point on the Main Sequence diagonal on an HR Diagram told
them how old the remaining stars were in the cluster. Still, calculations were not too precise at
10 to 15 billion years old.
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- Another
method arrived at the turn of the century when we discovered that nebulae in
the night sky were actually galaxies.
After studying many galaxies it was determined that most were moving away
from us. In other words the Universe was
expanding. By using Doppler
technology to learn velocity, and
Cepheid variables to learn how far these galaxies were away from us astronomers
could calculate the rate of expansion.
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- Again, the
accuracy was not perfect putting the expansion rate at 50 to 100
kilometers/second per mega parsec. (put in astronomer's metric notations) The most recent calculations put it at 73.5
km/sec/mps. This equates to 47,000 miles
per hour per million lightyears distance.
In other words a galaxy that is 1 million lightyears further away will
be receding away from us 47,000 miles per hour faster.
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- It turns out
that the reciprocal of the expansion rate is the age of the Universe, IF you
assume the expansion rate is a constant.
The reciprocal is distance / velocity which is time. These calculations got us to 10 to 17 billion
years old since the Universe started its expansion with the Big Bang.
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- This all
depended on the expansion rate being constant.
But, what affect did gravity have in slowing up the expansion rate? What effect did Dark Energy have in speeding
up the expansion rate? And, what shape
was the Universe? Can we assume it was a
straight line back to the Big Bang or was it on a spherical curve, or a convex
saddle shape?
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- None of these
answers were available to us until we discovered the Cosmic Microwave
Background radiation. Microwave
satellites could take a high resolution snapshot of the microwave background
that was released when light first left the plasma created with the Big
Bang. This light was released when the
Universe was 380,000 years old.
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- The radiation was at a temperature of 3,000 Kelvin
and was in the frequency range of Gamma Rays.
Today it is Microwaves frequencies and only 2.73 Kelvin temperature. The temperature has cooled be a factor of
1000 and the Universe has expanded be a factor of 1000 since CMB was
released.
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- Quantum
fluctuations in the early Universe created granularity in the radiation. It was not perfectly smooth. The variations were small, only one part in
100,000 in temperature. But they were
the same across the entire sky which told astronomers that the Universe was
nearly flat geometrically.
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- The granules
were also about 1 arc second in size which corresponds to the granularity in
the beginning expanding to what it is today.
The granules correspond to the density of galaxies and voids across the
cosmos. Gravity caused the denser regions to become stars, blackholes, and
galaxies. The rarified regions became
voids in the bubbles of galaxies and galaxy strings that we see today.
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- From the CMB
calculations the Universe is 13.7 billion years old. Astronomers determined that about half the
time of expansion gravity dominated and was slowing the rate of expansion.
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- However, in
the last half of the expansion Dark Energy is dominate and the acceleration
rate is speeding up. Will it speed up
forever? Will the Universe die a cold
void totally dispersed throughout space?
Or, will Dark Energy reverse itself and the Universe collapse into
another Big Bang to repeat things all over again? We don’t know, but we are still learning.
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--------------------- Thursday, September 13,
2018 -------------------------
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