1.
Physical cosmology
–
Physical cosmology is the study of the largest-scale structures and dynamics of the Universe and is concerned with fundamental questions about its origin, structure, evolution, and ultimate fate. These advances made it possible to speculate about the origin of the universe, a few researchers still advocate a handful of alternative cosmologies, however, most cosmologists agree that the Big Bang theory explains the observations better. Cosmology draws heavily on the work of many areas of research in theoretical. Areas relevant to include particle physics experiments and theory, theoretical and observational astrophysics, general relativity, quantum mechanics. Modern cosmology developed along tandem tracks of theory and observation, in 1916, Albert Einstein published his theory of general relativity, which provided a unified description of gravity as a geometric property of space and time. At the time, Einstein believed in a universe. This is because masses distributed throughout the universe gravitationally attract, however, he realized that his equations permitted the introduction of a constant term which could counteract the attractive force of gravity on the cosmic scale. Einstein published his first paper on relativistic cosmology in 1917, in which he added this cosmological constant to his equations in order to force them to model a static universe. However, this so-called Einstein model is unstable to small perturbations—it will eventually start to expand or contract, the Einstein model describes a static universe, space is finite and unbounded. It was later realized that Einsteins model was just one of a set of possibilities, all of which were consistent with general relativity. The cosmological solutions of general relativity were found by Alexander Friedmann in the early 1920s and his equations describe the Friedmann–Lemaître–Robertson–Walker universe, which may expand or contract, and whose geometry may be open, flat, or closed. In the 1910s, Vesto Slipher interpreted the red shift of spiral nebulae as a Doppler shift that indicated they were receding from Earth, however, it is difficult to determine the distance to astronomical objects. One way is to compare the size of an object to its angular size. Another method is to measure the brightness of an object and assume an intrinsic luminosity, due to the difficulty of using these methods, they did not realize that the nebulae were actually galaxies outside our own Milky Way, nor did they speculate about the cosmological implications. In 1929, Edwin Hubble provided a basis for Lemaîtres theory. Hubble showed that the nebulae were galaxies by determining their distances using measurements of the brightness of Cepheid variable stars. He discovered a relationship between the redshift of a galaxy and its distance and he interpreted this as evidence that the galaxies are receding from Earth in every direction at speeds proportional to their distance. Given the cosmological principle, Hubbles law suggested that the universe was expanding, two primary explanations were proposed for the expansion
2.
Big Bang
–
The Big Bang theory is the prevailing cosmological model for the universe from the earliest known periods through its subsequent large-scale evolution. If the known laws of physics are extrapolated to the highest density regime, detailed measurements of the expansion rate of the universe place this moment at approximately 13.8 billion years ago, which is thus considered the age of the universe. After the initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, giant clouds of these primordial elements later coalesced through gravity in halos of dark matter, eventually forming the stars and galaxies visible today. Since Georges Lemaître first noted in 1927 that a universe could be traced back in time to an originating single point. More recently, measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, the known physical laws of nature can be used to calculate the characteristics of the universe in detail back in time to an initial state of extreme density and temperature. American astronomer Edwin Hubble observed that the distances to faraway galaxies were strongly correlated with their redshifts, assuming the Copernican principle, the only remaining interpretation is that all observable regions of the universe are receding from all others. Since we know that the distance between galaxies increases today, it must mean that in the past galaxies were closer together, the continuous expansion of the universe implies that the universe was denser and hotter in the past. Large particle accelerators can replicate the conditions that prevailed after the early moments of the universe, resulting in confirmation, however, these accelerators can only probe so far into high energy regimes. Consequently, the state of the universe in the earliest instants of the Big Bang expansion is still poorly understood, the first subatomic particles to be formed included protons, neutrons, and electrons. Though simple atomic nuclei formed within the first three minutes after the Big Bang, thousands of years passed before the first electrically neutral atoms formed, the majority of atoms produced by the Big Bang were hydrogen, along with helium and traces of lithium. Giant clouds of primordial elements later coalesced through gravity to form stars and galaxies. The framework for the Big Bang model relies on Albert Einsteins theory of relativity and on simplifying assumptions such as homogeneity. The governing equations were formulated by Alexander Friedmann, and similar solutions were worked on by Willem de Sitter, extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This singularity indicates that general relativity is not a description of the laws of physics in this regime. How closely models based on general relativity alone can be used to extrapolate toward the singularity is debated—certainly no closer than the end of the Planck epoch. This primordial singularity is itself called the Big Bang, but the term can also refer to a more generic early hot. The agreement of independent measurements of this age supports the model that describes in detail the characteristics of the universe. The earliest phases of the Big Bang are subject to much speculation, in the most common models the universe was filled homogeneously and isotropically with a very high energy density and huge temperatures and pressures and was very rapidly expanding and cooling
3.
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, 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, over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the center of the Solar System. In developing the law of gravitation, Sir Isaac Newton built upon Copernicuss work as well as observations by Tycho Brahe. Further observational improvements led to the realization that our Solar System is located in the Milky Way galaxy and 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, 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, giant clouds later merged through gravity to form galaxies, stars, and everything else seen today. Some physicists have suggested various multiverse hypotheses, in which the Universe might be one among many universes that likewise exist, the Universe can be defined as everything that exists, everything that has existed, and everything that will exist. According to our current understanding, the Universe consists of spacetime, forms of energy, the Universe encompasses all of life, all of history, and some philosophers and scientists suggest that it even encompasses ideas such as mathematics and logic. The word universe derives from the Old French word univers, which in turn derives from the Latin word universum, the Latin word was used by Cicero and later Latin authors in many of the same senses as the modern English word is used. Another synonym was ὁ κόσμος ho kósmos, synonyms are also found in Latin authors and survive in modern languages, e. g. the German words Das All, Weltall, and Natur for Universe. The same synonyms are found in English, such as everything, the cosmos, the world, 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, the model is based on general relativity and on simplifying assumptions such as homogeneity and isotropy of space. The Big Bang model accounts for such as the correlation of distance and redshift of galaxies, the ratio of the number of hydrogen to helium atoms. The initial hot, dense state is called the Planck epoch, 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 density of electromagnetic radiation decreases more quickly than does that of matter because the energy of a photon decreases with its wavelength
4.
Chronology of the universe
–
The chronology of the universe describes the history and future of the universe according to Big Bang cosmology. The metric expansion of space is estimated to have begun 13.8 billion years ago, the time since the Big Bang is also known as cosmic time. The solar system formed at about 4.6 billion years ago, the far future, after cessation of stellar formation, with various scenarios for the ultimate fate of the universe. Little is understood about physics at this temperature, different hypotheses propose different scenarios, in inflationary cosmology, times before the end of inflation does not follow the traditional big bang timeline. Models attempting to formulate processes of the Planck epoch are speculative proposals for New Physics, examples include the Hartle–Hawking initial state, string landscape, string gas cosmology, and the ekpyrotic universe. Between 10−43 second and 10−36 second after the Big Bang As the universe expanded and cooled and these can be regarded as phase transitions much like condensation and freezing phase transitions of ordinary matter. The grand unification epoch began when gravitation separated from the gauge forces, the non-gravitational physics of this epoch would be described by a so-called grand unified theory. The grand unification epoch ended when the GUT forces further separate into the strong, while decelerating expansion would magnify deviations from homogeneity, making the universe more chaotic, accelerating expansion would make the universe more homogeneous. Inflation ended when the field decayed into ordinary particles in a process called reheating. The time of reheating is usually quoted as a time after the Big Bang, according to the simplest inflationary models, inflation ended at a temperature corresponding to roughly 10−32 second after the Big Bang. As explained above, this does not imply that the inflationary era lasted less than 10−32 second, in fact, in order to explain the observed homogeneity of the universe, the duration must be longer than 10−32 second. In inflationary cosmology, the earliest meaningful time after the Big Bang is the time of the end of inflation, in inflationary cosmology, the electroweak epoch began when the inflationary epoch ended, at roughly 10−32 seconds. There is currently insufficient observational evidence to explain why the universe contains far more baryons than antibaryons, a candidate explanation for this phenomenon must allow the Sakharov conditions to be satisfied at some time after the end of cosmological inflation. While particle physics suggests asymmetries under which conditions are met. After cosmic inflation ends, the universe is filled with a quark–gluon plasma, from this point onwards the physics of the early universe is better understood, and the energies involved in the Quark epoch are directly amenable to experiment. If supersymmetry is a property of our universe, then it must be broken at an energy that is no lower than 1 TeV, the electroweak symmetry scale. The masses of particles and their superpartners would then no longer be equal, between 10−6 second and 1 second after the Big Bang The quark–gluon plasma that composes the universe cools until hadrons, including baryons such as protons and neutrons, can form. At approximately 1 second after the Big Bang neutrinos decouple and begin traveling freely through space and this cosmic neutrino background, while unlikely to ever be observed in detail since the neutrino energies are very low, is analogous to the cosmic microwave background that was emitted much later
5.
Planck epoch
–
Planck units have significance for theoretical physics since they simplify several recurring algebraic expressions of physical law by nondimensionalization. They are relevant in research on unified theories such as quantum gravity and this occurs at energy 7009195465531414000♠1. 22×1019 GeV, at times 6956538999999999999♠5. 39×10−44 s and length 6965162000000000000♠1. 62×10−35 m. All systems of measurement feature base units, in the International System of Units, for example, the base unit of length is the metre. In the system of Planck units, the Planck base unit of length is simply as the Planck length, the base unit of time is the Planck time. For example, Newtons law of gravitation, F = G m 1 m 2 r 2 = m 1 m 2 r 2 can be expressed as F F P =2. In order for this last equation to be valid, F, m1, m2, and r are understood to be the dimensionless numerical values of these quantities measured in terms of Planck units. This is why Planck units or any use of natural units should be employed with care, referring to G = c =1, Paul S. Wesson wrote that. Physically it represents a loss of information and can lead to confusion, key, L = length, M = mass, T = time, Q = electric charge, Θ = temperature. As can be seen above, the attractive force of two bodies of 1 Planck mass each, set apart by 1 Planck length is 1 Planck force. Likewise, the distance traveled by light during 1 Planck time is 1 Planck length, yet relative to other units of measurement such as SI, the values of the Planck units are only known approximately. This is mostly due to uncertainty in the value of the gravitational constant G, hence the value of c is now exact by definition, and contributes no uncertainty to the SI equivalents of the Planck units. The same is true of the value of the vacuum permittivity ε0, due to the definition of ampere which sets the vacuum permeability μ0 to 4π × 10−7 H/m and the fact that μ0ε0 = 1/c2. The numerical value of the reduced Planck constant ħ has been determined experimentally to 44 parts per billion, G appears in the definition of almost every Planck unit in Tables 2 and 3. Some physicists argue that communication with extraterrestrial intelligence would have to such a system of units in order to be understood. Unlike the metre and second, which exist as units in the SI system for historical reasons. Natural units help physicists to reframe questions, Frank Wilczek puts it succinctly, We see that the question is not, Why is gravity so feeble. But rather, Why is the protons mass so small, for in natural units, the strength of gravity simply is what it is, a primary quantity, while the protons mass is the tiny number. From the point of view of Planck units, this is comparing apples to oranges, because mass and electric charge are incommensurable quantities
