The foundations of cosmology are inherently consigned to some degree of speculation.
Apparently, it doesn't require special proof that the science of cosmology has very little effect on the daily life of an average individual. Although it seems overwhelmingly distant from our immediate concerns and mundane worries, cosmological ideas have, for centuries, played an essential role in shaping philosophical views and therefore they have heavily influenced numerous aspects of religious and political life.
Cosmology, as a science, limits itself to the study of the universe as a whole; its contents, structure, and evolution. Cosmological beliefs are based on the conclusions drawn from astronomical observations and mathematical models, but they still substantially influence the media and raise public interest.
The study of cosmology has changed from a speculative enterprise into a data-driven science that is part of a modern standard physical theory and supported by a wealth of observations. Nevertheless, some theoretical proposals are being made for the very early stages of the universe that have no observational support; and sometimes it may be impossible ever to obtain this. Thus, in some respects it remains a principle-driven enterprise, with observation subordinate to theory. Which means that the foundations of this science are inherently consigned to some degree of speculation.
In this book we are going to undertake a breathtaking journey into the very roots of the philosophy of cosmology in order to appraise rigorously this degree of possible speculation. This will allow us to make an attempt to define the ultimate limits of human knowledge in order to form a sober view of what, exactly, we can and cannot know.
We hope that our effort will escape the possible accusations in agnosticism and we will distance ourselves from the famous saying of Socrates "I know that I know nothing" by trying to estimate the true limits of our knowledge while appreciating tremendous progress of science that took place since the days of this great Greek thinker.
The first difficulty that the philosophy of cosmology encounters is the uniqueness of the universe. The most fundamental issue is that there is only one universe. This essential uniqueness of its object of study sets cosmology apart from all other sciences. In particular, the unique initial conditions that led to the current state of the universe that we have the honor to observe today were somehow "set" by the time that physical laws as we know them started governing the evolution of both the universe and its contents. We are not able to alter these unique initial conditions in any way. They are given to us as absolute and unchangeable.
One of the major implications of this is that the universe itself cannot be subjected to physical experimentation. Obviously we cannot re-run the universe with the same or altered conditions to see what would happen if they were different, so we cannot carry out scientific experiments on the primary object of our study, the universe itself. Furthermore, due to its uniqueness the universe cannot be observationally compared with any other universes.
For example, the laws of inheritance, which laid the foundation for modern genetics, derived by Gregor Mendel, needed tests on over 28,000 pea plants. His experiments wouldn't be possible if he had only one pea plant to examine.
Unfortunately, like having only one pea, we have only one universe to study, and, moreover, one that we can only partially observe. Because we cannot compare our universe with any other universes, we are considerably limited in our ability to derive certain laws that would apply to the whole group of objects that we aren't even sure exists.
This example may illustrate the intriguing thought that the concept of Laws of Physics' once it applies to only one object is questionable. We cannot scientifically establish `laws of the universe' that might apply to the class of all such objects, for we cannot test any proposed law except in terms of being consistent with one object. Indeed the concept of a law' becomes doubtful when there is only one given object that is possible to study. The basic idea of a physical law is that it applies to a group of objects, all of which have similar characteristics despite some possible variations. These variations result from different initial conditions for the systems on which the law acts. Scientific experiments allow us to vary the initial conditions of the systems we wish to test. This is not possible in the case of cosmology because we cannot re-run the universe in the lab.
We can observe the laws of physics locally and we can confirm that they look the same on the small scale anywhere in the universe, but we have a certain difficulty extrapolating them on the higher level of hierarchy of the organization of the universe. For example, Newton's Laws of Gravity work perfectly on the level of our solar system, but they can not be applied with the same degree of certainty once we examine the orbital speeds of stars around the galactic center, which turned out to be higher than expected, or behavior of the galaxies in the clusters that stay together despite the fact that their visible mass wouldn't be able to hold them together, and some other issues. Even though modern cosmology explains this by the presence of the missing mass that was dubbed "dark matter" in the halos of the galaxies, there are still some alternative theories like modified Newton's gravity (MOND) that challenge mainstream cosmology from time to time, quite upsetting the advocates of Lambda Cold Dark Matter model that currently is in general agreement with observed phenomena.
On a higher level, the laws of gravity cannot explain why the universe is expanding and even accelerating in its expansion. There is a need for new laws that would describe the missing energy responsible for such expansion, dubbed "dark energy" either in a form of cosmological constant or quintessence. Although, such new laws that may provide reasonable explanations cannot be checked because we cannot observe them on any other object but our universe itself.
Because of the restriction of a global solution to a local neighborhood, we can employ as a solution the hypothesis that we have zillions of "mini-universes" on which may test the laws that control the local nature of the universe. But a mini-universe is not the universe itself; it is a small part of the whole. By examining these "mini-universes" and seeing if they are essentially the same everywhere, we can, to some degree, check that the laws of physics are the same everywhere in the universe (a key feature of all cosmological analysis), and secondly that the universe is practically the same in all areas and directions. But the latter feature is what has to be explained by a "law of the universe". Verifying homogeneity does not explain why it is the case.
Finally, the concept of probability is problematic in the context of the existence of only one object. Problems arise in applying the idea of probability to cosmology as a whole, but a concept of probability underlies much of modern argumentation in cosmology. For instance, we are talking about very low probability of the observed fine tuning', which means that all physical constants have such precise values that allow them to create conditions not only for life to exist but also for atoms to form. If the constants were different the atoms would never form, the stars would never shine, thermonuclear synthesis of the elements wouldn't be possible and the diversity of the chemical elements that supports life would never emerge.
This assumes both that things could have been different, and that we can assign probabilities to the set of possibilities that have never become a reality in an astronomically provable way. The issue here is to explain in what sense they could have been different with well-defined probabilities assigned to the different theoretical scenarios, if there is indeed only one universe with one set of initial conditions.
We cannot scientifically establish laws of creation of the universe that might determine different initial conditions and resulting probabilities. First of all, it is useful to distinguish between the experimental sciences, physics, chemistry, microbiology for example, on the one hand, and the historical and geographical sciences, astronomy, geology, evolutionary theory for example, on the other. It is the experimental sciences that are usually in mind in discussions of the scientific method. The understanding in these cases is that we observe and experiment on a class of identical, or almost identical, objects and establish their common behavior. The problem then resides in just how identical those objects are. Quarks, protons, electrons, are all exactly identical to each other, and so have exactly the same behavior (indeed this feature underlies well-tested quantum statistics). All DNA molecules, frogs, human beings, and ecosystems are somewhat different from each other, but are similar enough nevertheless that the same broad descriptions and laws apply to them; if this were not so, then we would be wrong in claiming they belonged to the same class of objects in the first place. Water molecules, gases, solids, liquids are in an intermediate category: almost identical, and certainly describable reliably by specific physical and chemical laws.
As regards the geographical and historical sciences, here one explicitly studies objects that are unique (the Rio Grande, the continent of Antarctica, the Solar System, the Andromeda galaxy, etc.) or events that have occurred only once (the origin of the Solar System, the evolution of life on Earth, the explosion of a certain supernova star.). Because of this uniqueness, we can only observe rather than experiment; the initial conditions that led to these unique objects or events cannot be altered or experimented with. However at least in principle there is a class of similar objects out there (other rivers, continents, planetary systems, galaxies, etc.) or similar events (the origin of other galaxies, the evolution of other planetary systems, the explosion of other supernovae, etc.) which we can observe and compare with our specific exemplar, also carrying out statistical analyses on many such cases to determine underlying patterns of regularity. In this respect these topics differ from cosmology.
If we truly cannot carry out such an analysis then that subject poses a legitimate question about the nature of cosmology. One may claim that the dividing line here is that if we convince ourselves that some large-scale physical phenomenon essentially occurs only once in the entire universe, then it should be regarded as part of cosmology; whereas if we are convinced phenomena occurs in many places or times, even if we cannot observationally access them (e.g. we believe that planets evolved around many stars in other galaxies), then the study of that class of objects or events can be distinguished from cosmology precisely because there is a class of them to study.
Some scientists have tried to get around this set of problems by essentially denying the uniqueness of the universe. This is done by proposing the physical existence of `many universes' to which concepts of probability can be properly applied envisaged either as widely separated regions of a larger universe with very different properties in each region (as in chaotic inflation, for example), or as an ensemble of completely disconnected universes with no physical connection whatsoever between them in which all possibilities are realized.
So far there is no hard proof that "other universes" may exist and we have to stick to the statement that the universe we live in is unique at least from our point of view and we need to deal with philosophical implications of such an approach.
 Gravitation is one of the four fundamental interactions in nature, the other three being the electromagnetic force, the weak nuclear force, and the strong nuclear force. Gravitation is the weakest of these interactions, but acts over great distances and is always attractive. Newton's law of gravitation states that every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
 In physics, Modified Newtonian Dynamics (MOND) is a theory that explains the galaxy rotation problem without assuming the existence of dark matter. MOND was proposed by Mordehai Milgrom in 1981 to model the observed uniform velocity data without the dark matter assumption. The most successful relativistic version of MOND, which was proposed in 2004, is known as "TeVeS" for Tensor-Vector-Scalar. Bekenstein, Jacob D.: Modified Gravity vs Dark Matter: Relativistc theory for MOND, JHEP Conference Proceedings, 2005
 Lambda CDM represents the current model of Big Bang cosmology that is aimed to explain cosmic microwave background observations, as well as large scale structure observations and supernovae observations of the accelerating expansion of the universe.
 Thermonuclear fusion is the process that takes place inside the stars throughout their lives and when the stars explode as Super-Novae. Gravity causes the clouds of gas to collapse and form the star. In the core of the star very high pressure and temperature ignite the thermonuclear reaction. By this process multiple nuclei join together to form a heavier nucleus. It is responsible for the diversity of elements in the universe and works to fill in the Periodic Table of Elements. The tremendous energy released in this process makes the stars shine and allow us to enjoy the sunlight.