Our Origins:
The History Of the Universe.


The most accepted theory for the description of the Universe is called The Big Bang Theory. Although it has its shortcomings, as we will discuss it later, it has become one of the main pillars on which modern physics rests. And it is the central part of study in the subject we call these days Cosmology.

It may seem odd that two apparently very unrelated subjects, particle physics and the study of the Universe as a whole, can be united in a single subject of study: the Cosmology.

Over the last three decades, physicist dedicated to the study of elementary particles and cosmologists have discovered that they have affine interests. But let us try to understand how this could happen. Cosmology, actually has been in modern physics since the 1920's when astronomers first estimated the age of the real size of the Universe. The american astronomer Edwin Hubble managed to measure the distance between galaxies and by doing so he made an unexpected discovery: the galaxies were pulled away from each other at a speed proportional to the distance separating them. In other words, Hubble found that the Universe was expanding!

The most important implication of this expansion is that the matter density in the past was greater than it is today. Therefore, at one given moment, all matter was concentrated in a matter density practically infinite. From today's actual expansion speed, we may calculate that the moment of maximum density, and which we identify it as the actual birth of the Universe, the so called Big Bang, ocurred some fifteen billion years ago.

In 1967, several radioastronomers discovered a radio wave emission coming from all directions of the sky. This indicated that the actual temperature of the Universe is of about 2.7 Kelvin. This temperature is extremelly low because the Universe has cooled off over the years until now. However, if we extrapolate backwards in time, the resulting temperature of the Universe during the Big Bang, must have been almost infinite.

In the first moments of the Universe existance, the physical conditions were totally different to the ones we have these days. Matter was formed by all classes of exotic particles, some of them would never exist again. So, it is in this way, that cosmology needs of the most fundamental and advanced theories of particle physics to predict the actual structure of today's Universe.

In modern Cosmology, Einstein's General Relativity plays an imprtant role. In Einstein's theory, space and time are not two different unrelated entities. On the contrary space and time are united in a new and fundamental concept: the space-time. The space-time has four dimensions. According to general relativity, this space-time is curved, and gravity is the manifestation of this curvature. A massive object deforms the space-time and a particle attracted by it does not follow a straight line, but it follows a trajectory determined by the curvature of the space-time.

An important issue to consider, is the fact that general relativity does not describe quantum phenomena. This is the main reason, the Big Bang theory can only be valid for times larger than the so called Planck time. The Planck time is equivalent to 0.00000000000000000000000000000000000000000001 seconds (10E-44 sec.) This is an extremely short lapse of time. Consider for example an atom, the light will cross an atom in 10E26 Planck units!. Before 10E-44 sec., unknown quantum effects must have driven the creation of the Universe.

As we said before, the 2.7 Kelvin radiation is a relic of the Big Bang. And it is the current temperature of the Universe. If we extrapolate backwards into the past, the cosmic temperature rises. We can distinguish several critical moments, some of them corresponds to important radical changes that sealed the fate of the Universe. So let us follow the history of these events as we believe they ocurred:




Before 10E-44 sec., the Universe was in a physical state totally unknown to us. It was a moment dominated by a very strong curvature of space-time and quantum phenomena. It is very possible that at that moment all four fundamental forces were united in a single more fundamental one.

From 10E-44 sec. to 10E-35 sec., the temperature descended from 10E32 to 10E27 Kelvin. We can speculate that at those temperatures the strong, weak, and electromagnetic interactions were unified in a single one. About 10E-34 sec., inflation should have taken place. Inflation is a small but very important modification to the standard Big Bang model, it is introduced in order to solve some of the shortcomings of the theory as we will discuss in another ocassion later on the series.

After inflation, the Universe was a cocktail of quarks, antiquarks, and all kind of particles annhilating with each other. At 10E-12 sec., the temperature has droped to 10E15 Kelvin; at that moment another phase transition takes place: the weak interactions separate from the electromagnetic ones.

When the age of the Universe was 10E-6 sec. and its temperature in the order of 10E13 Kelvin, the quarks combined with each other to form the first hadrons, mainly the nuclei particles: protons and neutrons and their antiparticles. But when the temperature descended a little more, these particles were annihilated by the antiparticles, in this way huge amounts of energy were realesed.

After the first second has gone, the temperature is of about 10E10 Kelvin. In that moment the last antiparticles were annihilated. After that moment, the Universe consisted mainly of protons, neutrons, photons, neutrinos and antineutrinos.

The first atomic nuclei were formed 3 minutes after the Big Bang took place. After that, and for the next half a million years, the Universe continued in expansion, until it reached a temperature of 5000 Kelvin. Then the first atoms were formed and the Universe became transparent; light was free and the shining left from that event is what we see today as the background radiation. The galaxies and the first stars were formed probably a little after this event.

This is, roughly speaking the history of our Universe. Of course there are some problems with the theory as with any other theory. But the main ideas have been outlined. This model is the standard point of departure to other better theories. After all we cannot run without learning how to walk, can we?

































                 






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