In my books, I tried to review the development of science through history. However, I think that an addition to cover the more recent developments would be in order so we are aware of the cutting edge…
Einstein
Einstein, amongst many other significant achievements, worked on his theories of relativity through 1905 to 1916. This story deserves to be told: After deriving the many equations describing his cosmology, he reduced them to just one equation by assuming the density of universe to be the same (the cosmological principle) by considering a huge chunk of the cosmos.
Then to his dismay, he realized that the equation indicated an expanding universe. He has always visualized it as an everlasting, static one. Thus, he went on to modify his equation to account for a non expanding, static universe. He did this by providing a fudge factor called the cosmological constant. Thus, his Universe became a steady, none expanding, and none contracting one. However, a while later, two scientists in turn, the Russian Alexander Friedman and the Belgian Georges Lemaitre tried to persuade him that mathematics indicated an expanding space. He utterly refused them. Lemaitre was not taken aback. He pushes again, this time his effort coincided with that of the findings of the Hubble telescope and Hubble`s derivation of his constant. These findings strongly point to an expanding Universe. This, coupled with data obtained from the Cosmic Microwave Background Radiation (CMB, the remnant vibrations from the Big Bang) persuaded him. He accepted the idea of an expanding universe and declared his cosmological constant the biggest blunder of his life.
The special relativity points to a mass-energy equivalence (E=mc2) while the general relativity points to a space-time. Newton`s law of gravity had left a question open: How could gravity effect over vast distances of space? Einstein explains this in a brilliant manner by the concept and the warping of space-time. His theory has been proven by Michelson-Morley experiment and the solar eclipse of 29 May 1919. The foundation for such an achievement was already in place: One should mention the eminent French scholar, Henry Poincare for his contribution in proposing the backbone of the special theory of relativity as well as the contribution of Hendrik Lorentz in various transformations. At this stage I would like to point to a link that underscores the foundation stones leading to Einstein crowning the efforts. (http://mathpages.com/rr/s8-08/8-08.htm)
Minkowski Time Cone
Einstein`s equations for General relativity are not easy to follow. His work becomes lucid when seen by the geometric model forwarded by his former teacher Herman Minkowski. The illustration is in three dimensions where one of them is time. It elucidates the steps followed by the Lorentz
Transformations. (http://www.nobelprize.org/educational/physics/relativity/transformations-1.html)
The resultant figure in this scheme is a cone.
(http://www.phy.syr.edu/courses/modules/LIGHTCONE/minkowski.html)
Everything that happens in the universe can be traced in this cone. An event follows a lifeline in this cone. The sides of the cone are limits delineated by the speed of light. According to the theory of relativity, time is nonexistent. What we perceive as the passage of time is due to the second law of thermodynamics which states that the entropy always increases. To put simply; disorder in the Universe goes on. Everything around us gets old. If a glassware brakes, there is disorder, parts will not get back, the cells of biological life degrades due to such disorder. When we witness these, we conceive the development of disorder (increase in entropy) as a passage of time. Thus the term “arrow of time” indicates the time progressing in the direction of increasing disorder. This process is accompanied by a loss of energy, mostly in the form of heat. Therefore the Universe in future will end in a “heat death”. (ie the Universe has diminished to a state of no thermodynamic free energy and therefore can no longer sustain processes that consume energy.)
Is there no way out?
It seems there may be… Henry Poincare, whom I mentioned above, has forwarded a theory called recurrence. This states that after a sufficiently long but finite time certain systems will return to a state very close to the initial state. (http://en.wikipedia.org/wiki/Poincar%C3%A9_recurrence_theorem)
A fairly recent theory suggests that the arrow of time may be reversed for very short time intervals when the speed of light is reached(Like in receding velocity at the edge of the Universe.
Big Bang
Coming to the creation of the Universe, there seems to be a large consensus on the Big Bang theory, which states that all started with the explosion of a tiny, point like region of the universe. The original theory had some shortcomings, but these seem to be rectified by the subsequent recount of the event through the concept of inflation… These state that the universe is expanding in an increasing rate of acceleration.
I would like to point out at this stage that there have been steady state theories. Einstein himself had forwarded one in a handwritten manuscript which was overlooked by other scientists. The research paper was titled “About the Cosmological Problem”.Therein Einstein proposed a revision of steady state model I mentioned above. This was still with a cosmological constant, but now the constant was responsible for the creation of new matter as the Universe expanded. Thus, the average density of the system never changed. He stated: “If one considers a physically bounded volume, particles of matter will be continually leaving it. For the density to remain constant, new particles of matter must be continually formed in the volume from space.”
This is consistent with the Steady State model of cosmology, proposed later in 1949 and with today’s modern understanding of dark energy. One such theory has been suggested by Dr. Nuri Saryal. I am including the reference to it simply because think the math involved are sound and there is interesting reasoning. (http://www.metu.edu.tr/~saryal/astrophysics.html)
Another very interesting proposal is that the Universe will expand to its radius.
https://en.wikipedia.org/wiki/Schwarzschild_radius
This is to satisfy a zero total energy of the Universe. Then on it may become a steady state Universe.
Forces of Nature
There are four forces in nature. The weak nuclear force, the strong nuclear force, the electromagnetism and the gravity. It is known that they were one force at the creation but the first three separated from the fourth one later on. This was baffling. Einstein himself spent the last thirty years of his life to get all together and reach a Unified Theory with no success.
The quantum physics of the world of small, when applied to the universe, produced promising results in the concept of quantum field comprising the first three forces in quanta. The quantum field is like any other field in existence, each group of elements having their quantum field. Then scientists strived to combine the fourth force. This resulted in what we may call in mathematics a singularity. Then, it was asked at what size of particles such a combination could occur. The result was of a confounding size: approximately 10-35 meters which is called the Planck length. (Dealing with a 5.15500 × 1096 kg/m3 of Planck density). At such scales, mathematics may be reasonable tools, but physics requires verification. No device would be able to cope with such infinitesimal sizes. Thus, many physicists gave up…
String Theory
I shall make use of the illuminative book by Brian Greene to relate the following two ideas:
(http://en.wikipedia.org/wiki/The_Hidden_Reality)
Those who carried on came up with a theory called the String Theory. This assumes more dimensions to the universe than three of space and one of time that are usually used. These strings cannot be seen by even the most powerful devices available today. They are filaments of energy rather than the dots of quantum field quanta. Their mass and charge are determined by their frequency of vibration and their pattern of vibration. However, their shape and energy content determines the behavior of the medium. This theory is very promising in the sense that it alleviates many mathematical singularities because of the shapes of the strings assumed to form the medium. It also envelopes the existing theories, starting with Newton and then Einstein and then the quantum field. The particles of quantum field are treated as dots where there are different shapes, masses and spins of strings. One must choose the particular elements of a certain field on which the work is to be done. An example is the Standard Model. It is a quantum field theory containing 57 distinct quantum fields. For example the fields for neutrino, the photon, quarks of different type and the like. Nevertheless, the quantum field theory may be utilized in bridging some of the concepts brought to light by the string theory even if there is no physical experimental verification.
Multiverse
Another striking aspect is the notion of multiverse. Thinking of the fabric of the strings prior to big bang, it conceivable to think of a multitude of big bangs to occur. Thus multiple universes would form. The environmental conditions of all these universes would not be the same. However, as I mentioned in my books, the existence of biological life on earth is a very special event whose probability of presence is zero. But, when one thinks of many universes and many galaxies in such universes, that very low probability goes towards one that is reasonable.