Chronology and The Big Bang

Big Bang and Chronology of the universe

The prevailing model for the evolution of the Universe is the Big Bang theory. The Big Bang model states that the earliest state of the Universe was extremely hot and dense and that it subsequently expanded. The model is based on general relativity and on simplifying assumptions such as homogeneity and iso tropy of space. A version of the model with a cosmological constant (Lambda) and cold dark matter, known as the Lambda-CDM model, is the simplest model that provides a reasonably good account of various observations about the Universe. The Big Bang model accounts for observations such as the correlation of distance and redshift of galaxies, the ratio of the number of hydrogen to helium atoms, and the microwave radiation background.



In this diagram, time passes from left to right, so at any given time, the Universe is represented by a disk-shaped "slice" of the diagram. 

The initial hot, dense state is called the Planck epoch, a brief period extending from time zero to one Planck time unit of approximately 10−43 seconds. During the Planck epoch, all types of matter and all types of energy were concentrated into a dense state, where gravitation is believed to have been as strong as the other fundamental forces, and all the forces may have been unified. Since the Planck epoch, the Universe has been expanding to its present form, possibly with a very brief period of cosmic inflation which caused the Universe to reach a much larger size in less than 10−32 seconds.

After the Planck epoch and inflation came the quark, hadron, and lepton epochs. Together, these epochs encompassed less than 10 seconds of time following the Big Bang. The observed abundance of the elements can be explained by combining the overall expansion of space with nuclear and atomic physics. As the Universe expands, the energy density of electromagnetic radiation decreases more quickly than does that of matter because the energy of a photon decreases with its wavelength. As the Universe expanded and cooled, elementary particles associated stably into ever larger combinations. Thus, in the early part of the matter-dominated era, stable protons and neutrons formed, which then formed atomic nuclei through nuclear reactions. This process, known as Big Bang nucleosynthesis, led to the present abundances of lighter nuclei, particularly hydrogen, deuterium, and helium. Big Bang nucleosynthesis ended about 20 minutes after the Big Bang, when the Universe had cooled enough so that nuclear fusion could no longer occur. At this stage, matter in the Universe was mainly a hot, dense plasma of negatively charged electrons, neutral neutrinos and positive nuclei. This era, called the photon epoch, lasted about 380 thousand years.

Eventually, at a time known as recombination, electrons and nuclei formed stable atoms, which are transparent to most wavelengths of radiation. With photons decoupled from matter, the Universe entered the matter-dominated era. Light from this era could travel freely, and it can still be seen in the Universe as the cosmic microwave background (CMB). After around 100 million years, the first stars formed; these were likely very massive, luminous, and responsible for the reionization of the Universe. Having no elements heavier than lithium, these stars also produced the first heavy elements through stellar nucleosynthesis.[35] The Universe also contains a mysterious energy called dark energy, the density of which does not change over time. After about 9.8 billion years, the Universe had expanded sufficiently so that the density of matter was less than the density of dark energy, marking the beginning of the present dark-energy-dominated era.[36] In this era, the expansion of the Universe is accelerating due to dark energy. you can visit our other site.

About Universe

The Universe is all of time and space and its contents. It includes planets, moons, minor planets, stars, galaxies, the contents of intergalactic space, and all matter and energy. The size of the entire Universe is unknown.

The earliest scientific models of the Universe were developed by ancient Greek and Indian philosophers and were geocentric, placing Earth at the center of the Universe. Over the centuries, more precise astronomical observations led Nicolaus Copernicus (1473–1543) to develop the heliocentric model with the Sun at the center of the Solar System. In developing the law of universal gravitation, Sir Isaac Newton (NS: 1643–1727) built upon Copernicus's work as well as observations by Tycho Brahe (1546–1601) and Johannes Kepler's (1571–1630) laws of planetary motion.

Further observational improvements led to the realization that our Solar System is located in the Milky Way galaxy, which is one of many galaxies in the Universe. It is assumed that galaxies are distributed uniformly and the same in all directions, meaning that the Universe has neither an edge nor a center. Discoveries in the early 20th century have suggested that the Universe had a beginning and that it is expanding
at an increasing rate.The majority of mass in the Universe appears to exist in an unknown form called dark matter.

The Big Bang theory is the prevailing cosmological description of the development of the Universe. Under this theory, space and time emerged together 13.799±0.021 billion years ago, with a fixed amount of energy and matter that has become less dense as the Universe has expanded. After the initial expansion, the Universe cooled, allowing the first subatomic particles to form and then simple atoms. Giant clouds later merged through gravity to form galaxies, stars, and everything else seen today.

There are many competing hypotheses about the ultimate fate of the Universe and about what, if anything, preceded the Big Bang, while other physicists and philosophers refuse to speculate, doubting that information about prior states will ever be accessible. Some physicists have suggested various multiverse hypotheses, in which the Universe might be one among many universes that likewise exist.