Re: Why is the big bang THEORY presented as the only possible sollution?

The Big Bang model isn't the only possible solution, but it is the best one known.

Any decent astronomy textbook should also mention the Steady State theory. This theory predicts that the Universe looks the same everywhere in space and in time. In contrast, the Big Bang theory predicts that the Universe looks the same everywhere in space, but changes over time. (Specifically, the BB theory predicts that the Universe was denser and hotter in the past. Also, when I say the same, this is to be understood as the same if one averages over a suitably large region of space.)

In the 1950s the SS and BB theories divided the astronomers about 50-50; about half of all astronomers thought that the SS was more likely to be correct while about half thought that the BB was more likely to be correct. By the 1960s, new observations spelled the doom of the SS theory. Among other observations, it was discovered that there were more radio galaxies in the past and that there was a uniform microwave glow pervading the Universe (the cosmic microwave background). Both are quite easily explained within the BB theory (indeed the BB theory predicts the existence of the cosmic microwave background); they are essentially impossible to explain within the SS theory. Within a decade or so, most astronomers decided that the BB theory explained existing data better than any other model---a situation that continues to this day.

I think you have also fallen victim to the notion that words mean the same in all contexts. They don't. For instance, if I say that a basketball team has gone cold, I don't mean that the members of the team are in danger of freezing to death. I mean that they cannot make baskets. Similarly, if I say that I believe the time to be about 4:30 p.m., I'm am not expressing any particular faith in my answer. In fact, I'm saying the exact opposite, I'm saying that my answer is the best one I can give, but it could be wrong. (I should point out that I don't wear a watch.)

The same notion applies to the word theory. In common usage it often means a hunch, wild guess. In scientific usage, it is high praise. Few models are elevated to the level of being called a theory, and only the most well-tested are called such. Indeed consider some of the other theories in science: The Atomic Theory (the theory that all matter is composed of atoms), the Cell Theory (the theory that all living things are composed of cells), the Special Theory of Relativity (the theory that describes how different observers see events), the Plate Tectonic Theory (the theory that the Earth's surface is composed of large assemblages of rock that move on the underlying, molten rock), and the Theory of Evolution (the theory that the diversity of life results from natural selection acting organisms).

Now does this mean that these theories (including the Big Bang theory) are 100% correct? No. However, to ask for 100% correctness is something science cannot do. Science does not work with Truth. (Let me give a simple example. Is the Earth flat or round? The answer is that it is neither. It has an extremely complicated shape. However, which is more right or, perhaps more importantly, which is less wrong? Clearly, saying that the Earth is round is nearly correct. Similarly, the BB model explains the evolution of the Universe, though almost certainly not 100% correctly.)

The best we can do is develop models of how the Universe works. We then compare the predictions of those models with observations of the Universe. If the predictions agree with the observations, we gain some confidence in the model. The more observations with which a model agrees, the more confidence we gain. That doesn't mean that model or theory is 100% correct, only that it has explained everything to date. In the future, there may be a new set of observations that the BB theory cannot explain. Indeed, theories must make falsifiable predictions, that is, predictions of phenomena that can never be observed. (The BB makes a number of such falsifiable predictions.) If any such phenomena are observed, it indicates that the model is wrong. To date, nobody has falsified the BB theory (i.e., found something the BB predicts cannot exist).

The other possibility is that there are two theories that explain the same data equally well. That was the situation in the 1950s in astronomy, when the SS and BB models fought it out. To date, nobody has developed an alternative model to the BB that explains all of the astronomical data as well as the BB.

In the absence of any evidence to the contrary and any theory that does a better job of explaining the data, what else can one do? For more information on the BB theory, I encourage you to explore the MadScientist Network and the Cosmology FAQ.