The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. It was first proposed in 1927 by Roman Catholic priest and physicist Georges Lemaître. 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.
The ultimate fate of the universe is a topic in physical cosmology,whose theoretical restrictions allow possible scenarios for the evolution and ultimate fate of the universe to be described and evaluated. Based on available observational evidence,deciding the fate and evolution of the universe has become a valid cosmological question,being beyond the mostly untestable constraints of mythological or theological beliefs. Several possible futures have been predicted by different scientific hypotheses,including that the universe might have existed for a finite and infinite duration,or towards explaining the manner and circumstances of its beginning.
Russell Alan Hulse is an American physicist and winner of the Nobel Prize in Physics,shared with his thesis advisor Joseph Hooton Taylor Jr.,"for the discovery of a new type of pulsar,a discovery that has opened up new possibilities for the study of gravitation".
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
General relativity is a theory of gravitation developed by Albert Einstein between 1907 and 1915. The theory of general relativity says that the observed gravitational effect between masses results from their warping of spacetime.
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:6×10−10 J/m3,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.
Modified Newtonian dynamics (MOND) is a hypothesis that proposes a modification of Newton's second law to account for observed properties of galaxies. It is supported by a minority of astrophysicists as an alternative to the more widely accepted hypothesis of dark matter in terms of explaining why galaxies do not appear to obey the currently understood laws of physics.
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.
Euclid is a wide-angle space telescope with a 600-megapixel camera to record visible light,a near-infrared spectrometer,and photometer,to determine the redshift of detected galaxies. It was developed by the European Space Agency (ESA) and the Euclid Consortium and was launched on 1 July 2023.
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 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.
Jamie S. Farnes is a British cosmologist,astrophysicist,and radio astronomer based at the University of Oxford. He studies dark energy,dark matter,cosmic magnetic fields,and the large-scale structure of the universe. In 2018,it was announced by Oxford that Farnes may have simultaneously solved both the dark energy and dark matter problems,using a new negative mass dark fluid toy model that "brings balance to the universe".
