Tuesday, April 17, 2012


1)From Einstein’s general theory of relativity, predicted that space-time began at the big bang singularity and
would come to an end either at the big crunch singularity (if the whole universe recollapsed), or at a singularity inside
a black hole (if a local region, such as a star, were to collapse). Any matter that fell into the hole would be destroyed
at the singularity, and only the gravitational effect of its mass would continue to be felt outside.

2) when quantum effects were taken into account, it seemed that the mass or energy of the matter would eventually be
returned to the rest of the universe, and that the black hole, along with any singularity inside it, would evaporate
away and finally disappear.

        Could quantum mechanics have an equally dramatic effect on the big bang and big
crunch singularities? What really happens during the very early or late stages of the universe, when gravitational
fields are so strong that quantum effects cannot be ignored? Does the universe in fact have a beginning or an end?
And if so, what are they like?        

According to “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, any matter or radiation in it gets
cooler. (When the universe doubles in size, its temperature falls by half.) Since temperature is simply a measure of
the average energy – or speed – of the particles, this cooling of the universe would have a major effect on the matter
in it. At very high temperatures, particles would be moving around so fast that they could escape any attraction
toward each other due to nuclear or electromagnetic forces, but as they cooled off one would expect particles that
attract each other to start to clump together. Moreover, even the types of particles that exist in the universe would
depend on the temperature. At high enough temperatures, particles have so much energy that whenever they collide
many different particle/antiparticle pairs would be produced – and although some of these particles would annihilate
on hitting antiparticles, they would be produced more rap-idly than they could annihilate. At lower temperatures,
however, when colliding particles have less energy, particle/antiparticle pairs would be produced less quickly – and
annihilation would become faster than production.

At the big bang itself the universe is thought to have had zero size, and so to have been infinitely hot. But as the
universe expanded, the temperature of the radiation 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 (extremely light particles that are affected only by the weak force and
gravity) and their antiparticles, together with some protons and neutrons. As the universe continued to expand and
the temperature to drop, the rate at which electron/antielectron 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 with each other to produce more photons, leaving only a few electrons left over. The
neutrinos and antineutrinos, however, would not have annihilated with each other, because these particles interact
with themselves and with other particles only very weakly. So they should still be around today. If we could observe
them, it would provide a good test of this picture of a very hot early stage of the universe. Unfortunately, their
energies nowadays would be too low for us to observe them directly.

If neutrinos are not massless, but
have a small mass of their own, as suggested by some recent experiments, we might be able to detect them
indirectly: they could be a form of “dark matter,” like that mentioned earlier, with sufficient gravitational attraction to
stop the expansion of the universe and cause it to collapse again.

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, and electrons and nuclei no
longer had enough energy to overcome the electromagnetic attraction between them, they would have started
combining to form atoms. The universe as a whole would have continued expanding and cooling, but 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 gravitational 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,
and in this way disklike rotating galaxies were born. Other regions, which did not happen to pick up a rotation, would
become oval-shaped objects called elliptical galaxies. In these, the region would stop collapsing because individual
parts of the galaxy would be orbiting stably round its center, but the galaxy would have no overall rotation.

the hydrogen and helium gas in the galaxies would break up into smaller clouds that would collapse
under their own gravity. As these contracted, and the atoms within them collided with one another, the temperature
of the gas would increase, until eventually it became hot enough to start nuclear fusion 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 stable in this state for a long time as stars like our sun, burning hydrogen
into helium and radiating the resulting energy as heat and light. More massive stars would need to be hotter to
balance their stronger gravitational attraction, making 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.

The outer regions of the star may sometimes get blown off in a
tremendous explosion called a supernova, which would outshine all the other stars in its 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, and would provide
some of the raw material for the next generation of stars. Our own sun contains about 2 percent of these heavier
elements, because it is a second- or third-generation star, 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, but a small amount of the heavier elements collected together to form the bodies that now orbit the
sun as planets like the earth.

                                                                           (from :- A Brief History of Time - Stephen Hawking )

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