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The Big Bang was the initiation of the continuing expansion of the universe from a state of high density and temperature. It was first proposed as a physical theory in 1931 by Roman Catholic priest and physicist Georges Lemaître when he suggested the universe emerged from a "primeval atom". Various cosmological models of the Big Bang explain the evolution of the observable universe from the earliest known periods through its subsequent large-scale form. These models offer a comprehensive explanation for a broad range of observed phenomena,including the abundance of light elements,the cosmic microwave background (CMB) radiation,and large-scale structure. The overall uniformity of the universe,known as the flatness problem,is explained through cosmic inflation:a sudden and very rapid expansion of space during the earliest moments. However,physics currently lacks a widely accepted theory of quantum gravity that can successfully model the earliest conditions of the Big Bang.
Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model,or simply cosmology,provides a description of the largest-scale structures and dynamics of the universe and allows study of fundamental questions about its origin,structure,evolution,and ultimate fate. Cosmology as a science originated with the Copernican principle,which implies that celestial bodies obey identical physical laws to those on Earth,and Newtonian mechanics,which first allowed those physical laws to be understood.
In physics,a redshift is an increase in the wavelength,and corresponding decrease in the frequency and photon energy,of electromagnetic radiation. The opposite change,a decrease in wavelength and increase in frequency and energy,is known as a blueshift,or negative redshift. The terms derive from the colours red and blue which form the extremes of the visible light spectrum. The main causes of electromagnetic redshift in astronomy and cosmology are the relative motions of radiation sources,which give rise to the relativistic Doppler effect,and gravitational potentials,which gravitationally redshift escaping radiation. All sufficiently distant light sources show cosmological redshift corresponding to recession speeds proportional to their distances from Earth,a fact known as Hubble's law that implies the universe is expanding.
Observations show that the expansion of the universe is accelerating,such that the velocity at which a distant galaxy recedes from the observer is continuously increasing with time. The accelerated expansion of the universe was discovered in 1998 by two independent projects,the Supernova Cosmology Project and the High-Z Supernova Search Team,which used distant type Ia supernovae to measure the acceleration. The idea was that as type Ia supernovae have almost the same intrinsic brightness,and since objects that are farther away appear dimmer,the observed brightness of these supernovae can be used to measure the distance to them. The distance can then be compared to the supernovae's cosmological redshift,which measures how much the universe has expanded since the supernova occurred;the Hubble law established that the farther away that an object is,the faster it is receding. The unexpected result was that objects in the universe are moving away from one another at an accelerating rate. Cosmologists at the time expected that recession velocity would always be decelerating,due to the gravitational attraction of the matter in the universe. Three members of these two groups have subsequently been awarded Nobel Prizes for their discovery. Confirmatory evidence has been found in baryon acoustic oscillations,and in analyses of the clustering of galaxies.
A gravitational lens is matter,such as a cluster of galaxies or a point particle,that bends light from a distant source as it travels toward an observer. The amount of gravitational lensing is described by Albert Einstein's general theory of relativity. If light is treated as corpuscles travelling at the speed of light,Newtonian physics also predicts the bending of light,but only half of that predicted by general relativity.
A non-standard cosmology is any physical cosmological model of the universe that was,or still is,proposed as an alternative to the then-current standard model of cosmology. The term non-standard is applied to any theory that does not conform to the scientific consensus. Because the term depends on the prevailing consensus,the meaning of the term changes over time. For example,hot dark matter would not have been considered non-standard in 1990,but would have been in 2010. Conversely,a non-zero cosmological constant resulting in an accelerating universe would have been considered non-standard in 1990,but is part of the standard cosmology in 2010.
Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline,James Keeler,said,Astrophysics "seeks to ascertain the nature of the heavenly bodies,rather than their positions or motions in space–what they are,rather than where they are." Among the subjects studied are the Sun,other stars,galaxies,extrasolar planets,the interstellar medium and the cosmic microwave background. Emissions from these objects are examined across all parts of the electromagnetic spectrum,and the properties examined include luminosity,density,temperature,and chemical composition. Because astrophysics is a very broad subject,astrophysicists apply concepts and methods from many disciplines of physics,including classical mechanics,electromagnetism,statistical mechanics,thermodynamics,quantum mechanics,relativity,nuclear and particle physics,and atomic and molecular physics.
The Lambda-CDM,Lambda cold dark matter,or ΛCDM model is a mathematical model of the Big Bang theory with three major components:
- a cosmological constant,denoted by lambda (Λ),associated with dark energy
- the postulated cold dark matter,denoted by CDM
- ordinary matter
The expansion of the universe is the increase in distance between gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion,so it does not mean that the universe expands "into" anything or that space exists "outside" it. To any observer in the universe,it appears that all but the nearest galaxies recede at speeds that are proportional to their distance from the observer,on average. While objects cannot move faster than light,this limitation applies only with respect to local reference frames and does not limit the recession rates of cosmologically distant objects.
An inhomogeneous cosmology is a physical cosmological theory which,unlike the currently widely accepted cosmological concordance model,assumes that inhomogeneities in the distribution of matter across the universe affect local gravitational forces enough to skew our view of the Universe. When the universe began,matter was distributed homogeneously,but over billions of years,galaxies,clusters of galaxies,and superclusters have coalesced,and must,according to Einstein's theory of general relativity,warp the space-time around them. While the concordance model acknowledges this fact,it assumes that such inhomogeneities are not sufficient to affect large-scale averages of gravity in our observations. When two separate studies claimed in 1998-1999 that high redshift supernovae were further away than our calculations showed they should be,it was suggested that the expansion of the universe is accelerating,and dark energy,a repulsive energy inherent in space,was proposed to explain the acceleration. Dark energy has since become widely accepted,but it remains unexplained. Accordingly,some scientists continue to work on models that might not require dark energy. Inhomogeneous cosmology falls into this class.
While the presence of any mass bends the path of light passing near it,this effect rarely produces the giant arcs and multiple images associated with strong gravitational lensing. Most lines of sight in the universe are thoroughly in the weak lensing regime,in which the deflection is impossible to detect in a single background source. However,even in these cases,the presence of the foreground mass can be detected,by way of a systematic alignment of background sources around the lensing mass. Weak gravitational lensing is thus an intrinsically statistical measurement,but it provides a way to measure the masses of astronomical objects without requiring assumptions about their composition or dynamical state.
In physical cosmology and astronomy,dark energy is an unknown form of energy that affects the universe on the largest scales. Its primary effect is to drive the accelerating expansion of the universe. Assuming that the lambda-CDM model of cosmology is correct,dark energy is the dominant component of the universe,contributing 68% of the total energy in the present-day observable universe while dark matter and ordinary (baryonic) matter contribute 26% and 5%,respectively,and other components such as neutrinos and photons are nearly negligible. Dark energy's density is very low:7×10−30 g/cm3,much less than the density of ordinary matter or dark matter within galaxies. However,it dominates the universe's mass–energy content because it is uniform across space.
Geraint Francis Lewis,FLSW is a Welsh astrophysicist,who is best known for his work on dark energy,gravitational lensing and galactic cannibalism. Lewis is a Professor of Astrophysics at the Sydney Institute for Astronomy,part of the University of Sydney's School of Physics. He is head of the Gravitational Astrophysics Group. He was previously the Associate Head for Research at the School of Physics,and held an Australian Research Council Future Fellowship between 2011 and 2015. Lewis won the 2016 Walter Boas Medal in recognition of excellence in research in Physics. In 2021,he was awarded the David Allen Prize of the Astronomical Society of Australia for exceptional achievement in astronomy communication.
In cosmology,baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter of the universe,caused by acoustic density waves in the primordial plasma of the early universe. In the same way that supernovae provide a "standard candle" for astronomical observations,BAO matter clustering provides a "standard ruler" for length scale in cosmology. The length of this standard ruler is given by the maximum distance the acoustic waves could travel in the primordial plasma before the plasma cooled to the point where it became neutral atoms,which stopped the expansion of the plasma density waves,"freezing" them into place. The length of this standard ruler can be measured by looking at the large scale structure of matter using astronomical surveys. BAO measurements help cosmologists understand more about the nature of dark energy by constraining cosmological parameters.
UrošSeljak is a Slovenian cosmologist and a professor of astronomy and physics at University of California,Berkeley. He is particularly well-known for his research in cosmology and approximate Bayesian statistical methods.
A black hole cosmology is a cosmological model in which the observable universe is the interior of a black hole. Such models were originally proposed by theoretical physicist Raj Kumar Pathria,and concurrently by mathematician I. J. Good.
Marc Kamionkowski is an American theoretical physicist and currently the William R. Kenan,Jr. Professor of Physics and Astronomy at Johns Hopkins University. His research interests include particle physics,dark matter,inflation,the cosmic microwave background and gravitational waves.
Christopher Michael Hirata is a Japanese-American cosmologist and astrophysicist.
Ruth Durrer is a professor of Cosmology at the University of Geneva. She works on the cosmic microwave background,brane cosmology and massive gravity.
References
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- ↑ Pedersen, Eske M.; Yao, Ji; Ishak, Mustapha; Zhang, Pengjie (2020). "First Detection of the GI-type of Intrinsic Alignments of Galaxies Using the Self-calibration Method in a Photometric Galaxy Survey". The Astrophysical Journal Letters. 899 (1): L5. arXiv: 1911.01614 . Bibcode:2020ApJ...899L...5P. doi: 10.3847/2041-8213/aba51b .
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- ↑ "Provost's Award for Faculty Excellence in Graduate Research Mentoring".
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- ↑ "Robert S. Hyer Research Award – Unit – TSAPS". engage.aps.org.
