Cosmology is a field of astronomy that deals with the universe as a whole, an academic field that addresses questions such as the following: what is the universe composed of, and what is its structure? How was the universe created and how has it evolved? What is the ultimate fate of the universe These are some of the most fundamental questions that have also been the subject of religion and philosophy for the past millennia. However, it was only the past few centuries that cosmology became accepted as a topic and object of scientific inquiry, and currently an abundance of research in this field has brought us to a stage where we can now expect to find substantive answers. The scope of cosmology? is immense in both time and space since it studies the universe itself. Modern cosmology surprising simultaneously encompasses both the microcosmic world of particles as well as the macrocosmic universe. By integrating both microcosmic and macrocosmic worlds in its range of study, cosmology has shown us the features of the early universe, and presented us with significant answers regarding the origin of substance. Up to recent times, cosmology lacked sufficient comparative observational data and relied mostly on theoretical research based on the theory of relativity. However, energized by contemporary astronomy’s astounding new developments in observatory technology, cosmology has achieved remarkable developments in the past two decades. Observation has contributed to the establishment of the Big Bang theory which widely accepted today.
The first contribution to cosmology based on observed evidence was the recession movement of galaxies discovered by Slipher in 1928 based on the analysis of the spectrums of around 40 galaxies. Next, in 1929, Hubble verified the redshift in the galaxy’s spectrum, and discovered that this redshift-measured-velocity of the galaxy receding is proportional to the galaxy’s distance, and this became known as Hubble’s law. Hubble’s law indicates that the universe is expanding. Hubble’s law can be expressed as the formula v=Hd, where v is the receding velocity of the galaxy, d is the distance, and the H is the Hubble constant. In order to determine the distance based on the redshift, it is essential to know the value of the Hubble constant H. From the Hubble constant, we can obtain information on the age of the universe and the critical density that will allow us to determine the geometric structure of the universe, and hence the task of obtaining a reliable H value has emerged as the most important challenge for cosmology based on astronomical observation. Because it is exceedingly difficult to measure the distance to a galaxy, for a long time we have only been able to make only a very uncertain estimate of the Hubble constant to be around ?50-100km/sec/Mpc, but the results of the observation of supernovas using the Hubble telescope has finally determined that the constant is 67km/sec/Mpc. Tracking back into the past of Hubble’s expanding universe, we can assume that there must have been a time when the universe was a single point, and if the expansion has taken place at a constant rate, the age of the universe can be estimated to be around 15.6 billion years old.
After Hubble’s law, the next astronomical observation important to cosmology was the discovery of cosmic background radiation in 1965. Based on the theoretical research regarding the Big Bang theory, it anticipated that the current universe would be full of black body radiation with a temperature of around 3K, and such background radiation was indeed verified through actual observation. Such cosmic background radiation is nearly consistently distributed throughout the universe, not only supporting the theory that the universe was created by a Big Bang but also evidencing the isotropy and consistency of the universe. This coincides with the Cosmological Principle, the basic working assumption behind attempts to establish a model of the evolving universe.
Before the discovery of Hubble’s law, Einstein proposed a static model of the universe based on his own theory of relativity, but once Hubble’s law was discovered, it was necessary to revise this into a model of an expanding universe. Thereafter, Friedman also presented a model based on the theory of relativity and corresponding to Hubble’s law. According to Friedman’s model, it is possible to envision an open universe, a closed universe, and a flat universe depending on the curvature of the universe. Another major challenge for modern cosmology is to use observational astronomy to determine which of these three models would be an accurate reflection of the current universe. Although this is an extremely difficult project, several methods have been proposed. ?If the density of the universe is larger than the critical density, it is a closed universe, and if the density is less, it is an open universe. Accordingly, we can reach a conclusion if we can determine the density of the current universe, but to do so, we must first know the material content of the whole universe . The results of up to the present indicate that the observed density of the universe does not exceed a few percentages of the critical mass and therefore it seems to be an open universe. However, since this density is only the lower limit value and since it has been discovered that there is an immense amount of dark mass that is not actually visible, it is difficult to reach a conclusion.
It is believed that the universe was created by a great, hot explosion called the Big Bang. In the first moments, the density and temperature must have been so high that we do not have a theory capable of explaining the physical phenomena under such conditions. However, regarding the process of evolution from the Big Bang to the present, we do know that between the first 10-45to 10-30seconds, the universe underwent an epoch of rapid inflation in which it expanded more than 1050 times, then passed through a hadron epoch, a lepton epoch,a radiation dominated epoch, and a matter dominated epoch before reaching the present time.