The theory that describes the modern concepts for the evolution of the Universe is called the theory of the hot Universe. It was first proposed by the American astrophysicist from Russian descent George Gamow and was based on Einstein’s theory of relativity.

According to Gamow’s theory, the expansion of the Universe started with a huge explosion (the Big Bang) of some very compact and very dense state of matter, characterised by extremely high temperature – hence the term “hot Universe”. This initial configuration of the matter has two important characteristics, which are very difficult our logic, based on our everyday experience, to grasp.

The first characteristic is connected to “the beginning”. If we follow the evolution backwards in time, using the current observed parameters of the expansion and the laws of physics, we will discover, that the density of the matter in the very beginning must have been infinitely large. This looks impossible from physics’ point of view and for such a configuration the laws of physics as we know them will not be valid. This is why we still do not have the proper tools to describe this initial configuration. Further more – time itself is not well defined at these incredibly high densities in the early moments of the creation of the Universe. Together with the Universe, with space and all the laws of physics that govern it, in the process of the Big Bang was born time as well. This is why the question “What happened before the Big Bang?” is similar to the question “Which direction is West, looking from the North pole?” It is clear that at the Earth’s pole, the directions East and West have no meaning- you can only go South. East and West will be “born” with the first step you take away from the pole.

The second characteristic is connected with “the place”. It is not at all easy to grasp the fact that the Big Bang has no centre. It happened everywhere at the same time. This is why, even though the galaxies are moving away from each other, they are not moving away from the same central starting point. This characteristic follows directly from the Cosmological principle. If from the Earth we see that the galaxies are “running away” from each other, exactly the same phenomenon will be seen by observers in any other galaxy. This is a fact because of the requirement that Hubble’s law is the same everywhere in the Universe. At first it looks lake there is a contradiction in the above statement. However, let us compare the expansion of the Universe to a balloon that is being blown up and on the surface of which are drawn spirals. In this analogy, each spiral is a galaxy from our Universe. As the balloon is being blown up, each spiral moves away from all other spirals. If we place an “observer” in on of the spirals, they will watch the other spirals moving away from them. If we move the observer in another spiral, they will see exactly the same thing. Thus, we can not speak about a “centre” from which the galaxies started moving away – the spherical surface of the balloon has neither a centre, nor beginning and end. Whatever direction we choose along this surface, we will never reach its end. In this sense the Universe limitless, but not infinite – the volume under this spherical surface is not infinite at all.