The science that studies the structure and development of the Universe as a whole is called cosmology. Its main goal is to understand how the different natural phenomena – from the smallest particle to the largest-scale structures in the Universe - fit into one whole under the operation of the most fundamental forces that we know. This is why the questions that cosmology considers are among the basic question in science. Is the Universe still in its infancy, or has it reached its later years? How will it develop in the future? Will it cease to exist or not? The clarification of the origin of the Universe and how it develops in time is connected to the most important question for humanity as well – what is the origin of the human civilization and what will be its development in the future.

Cosmology is an observational science, which at the moment experiences a true “golden age”. Until recently it was based on only two fundamental observations. One was Hubble’s law, which shows a very important for the Universe fact – the galaxies with higher red shift in their spectral lines are moving away from us with higher velocities. A direct consequence from this fact is that the Universe is expanding. The other fact was the Cosmic microwave background emission, which we will talk about a bit later.

* Towards the end of 2018, the international Astronomical Union, after a vote among its members, changed the name of the Hubble’s law to “Hubble–Lemaître law”, in order to acknowledge the contribution of the Belgian astronomer Georges Lemaître to the discovery of the expansion of the Universe.

The structure of the Universe is different at different scales. In the particle’s scale the structures look one way, and in the largest scales it is completely different. To understand the composition of the Universe as a whole, cosmologists rely on the so called cosmological principle. It states that the laws of physics are the same everywhere in the Universe. It is based on two important facts, which are based on our current knowledge about the Universe. The first is that the Universe is homogeneous at large scales. This means, that a cube of large proportions (200 – 300 million light years, for example), placed randomly in the Universe, will contain the same number of galaxies as any other cube of the same size, placed anywhere else in the Universe. In other words, at these scales, the Universe looks “smooth”. 
The second fact is that the Universe is isotropic, i.e. it looks uniform in all directions and from whichever point of space you look at it. An observer in any other point in the Universe will see the same large- scale structure we see.