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STEPHEN W. HAWKING
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THE HOT BIG BANG MODEL
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In order to explain what my paper was about, I shall first describe the generally accepted
history of the universe, according to what is known as the "hot big bang model." This
assumes that the universe is described by a Friedmann model, right back to the big bang.
In such models one finds that as the universe expands, the temperature of the matter
and radiation in it will go down. Since temperature is simply the measure of the average
energy of the particles, this cooling of the universe will have a major effect on the
matter in it. At very high temperatures, particles will be moving around so fast that they
can escape any attraction toward each other caused by the nuclear or electromagnetic forces.
But as they cooled off, one would expect particles that attract each other to start to
clump together.
At the big bang itself, the universe had zero size and so must have been infinitely hot.
But as the universe expanded, the temperature of the radiation would have decreased. One
second after the big bang it would have fallen to about ten thousand million degrees.
This is about a thousand times the temperature at the center of the sun, but temperatures
as high as this are reached in H-bomb explosions. At this time the universe would have
contained mostly photons, electrons, and neutrinos and their antiparticles, together
with some protons and neutrons.
As the universe continued to expand and the temperature to drop, the rate at which the
electrons and the electron pairs were being produced in collisions would have fallen below
the rate at which they were being destroyed by annihilation. So most of the electrons
and antielectrons would have annihilated each other to produce more photons, leaving behind
only a few electrons.
About one hundred seconds after the big bang, the temperature would have fallen to one
thousand million degrees, the temperature inside the hottest stars. At this temperature,
protons and neutrons would no longer have sufficient energy to escape the attraction of the
strong nuclear force.They would start to combine together to produce the nuclei of atoms of
deuterium, or heavy hydrogen, which contain one proton and one neutron. The deuterium nuclei
would have then combined with more protons and neutrons to make helium nuclei, which
contained two protons and two neutrons. There would also be small amounts of a couple of
heavier elements, lithium and beryllium.
One can calculate that in the hot big bang model about a quarter of the protons and neutrons
would have been converted into helium nuclei, along with a small amount of heavy hydrogen
and other elements. The remaining neutrons would have decayed into protons, which are the
nuclei of ordinary hydrogen atoms. These predictions agree very well with what is observed.
The hot big bang model also predicts that we should be able to observe the radiation left
over from the hot early stages. However, the temperature would have been reduced to a few
degrees above absolute zero by the expansion of the universe. This is the explanation of
the microwave background of radiation that was discovered by Penzias and Wilson in 1965.
We are therefore thoroughly confident that we have the right picture, at least back to
about one second after the big bang. Within only a few hours of the big bang, the production
of helium and other elements would have stopped. And after that, for the next million years
or so, the universe would have just continued expanding, without anything much happening.
Eventually, once the temperature had dropped to a few thousand degrees, the electrons and
nuclei would no longer have had enough energy to overcome the electromagnetic attraction
between them. They would then have started combining to form atoms.
The universe as a whole would have continued expanding and cooling. However, in regions that
were slightly denser than average, the expansion would have been slowed down by the extra
gravitational attraction. This would eventually stop expansion in some regions and cause
them to start to recollapse. As they were collapsing, the gravitaional pull of matter
outside these regions might start them rotating slightly. As the collapsing region got
smaller, it would spin faster - just as skaters spinning on ice spin faster as they draw
in their arms. Eventually, when the region got small enough, it would be spinning fast
enough to balance the attraction of gravity. In this way, dislike rotating galaxies were
born.
As time went on, the gas in the galaxies would break up into smaller clouds that would
collapse under their own gravity. As these contracted, the temperature of the gas would
increse until it became hot enough to start nuclear reactions. These would convert the
hydrogen into more helium, and the heat given off would raise the pressure, and so stop
the clouds from contracting any further. They would remain in this state for a long time
as stars like our sun, burning hydrogen into helium and radiating the energy as heat and
light.
More massive stars would need to be hotter to balance their stronger gravitational attraction.
This would make the nuclear fusion reactions proceed so much more rapidly that they would
use up their hydrogen in as little as a hundred million years. They would then contract
slightly and, as they heated up further, would start to convert helium into heavier elements
like carbon or oxygen. This, however, would not release much more energy, so a crisis
would occur, as I described in my lecture on black holes.
What happens next is not completely clear, but it seems likely that the central regions of
the star would collapse to a very dense state, such as a neutron star or black hole. The
outer regions of the star may get blown off in a tremendous explosion called a supernova,
which would outshine all the other stars in the galaxy. Some of the heavier elements
produced near the end of the star's life would be flung back into the gas in the galaxy.
They would provide some of the raw material for the next generation of stars.
Our own sun contains about two percent of these heavier elements because it is a second - or
third - generation star. It was formed some five thousand million years ago out of a cloud
of rotating gas containing the debris of earlier supernovas. Most of the gas in that cloud
went to form the sun or got blown away. However, a small amount of the heavier elements
collected together to form the bodies that now orbit the sun as planets like the Earth.
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Courtesy:
THE THEORY OF EVERYTHING
THE ORIGIN AND FATE OF THE UNIVERSE
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