Hume A. Feldman is a physicist specializing in cosmology and astrophysics. He is a Fellow of the American Physical Society and a professor and chair (2013-2023) of the Department of Physics and Astronomy [1] at the University of Kansas.
Hume A. Feldman is a physicist specializing in cosmology and astrophysics. He is a Fellow of the American Physical Society and a professor and chair (2013-2023) of the Department of Physics and Astronomy [1] at the University of Kansas.
Feldman graduated from the University of California at Santa Cruz in 1983. He got his PhD at Stony Brook University in NY, 1989 working with Robert Brandenberger. [2] He was then a postdoc at the Canadian Institute for Theoretical Astrophysics, Toronto, 1989–91, a research fellow at the University of Michigan 1991-94 and a Prof. Research in the Physics Department at Princeton University 1994–96.
Hume has been a researcher in the study of the large-scale peculiar velocity field for the past two decades. His explanation of the systematic errors, aliasing and incomplete cancellations of small-scale noise masquerading as large-scale signal lead to the reemergence of peculiar velocities as a premier tool in our arsenal to probe the dynamics and statistics of the large-scale structure of the Universe. He developed a formalism to optimize the determination of cosmic flows from proper distance surveys and enabled for the first time direct comparison of independent surveys and cosmological models as a function of scale, thus establishing the cosmological significance of flow probes.
His work has led to many widely cited results [3] (such as a nearly 3-sigma flows on 100 Mpc/h scales), [4] renewed discussion of imposing flow constraints on cosmological models and the redesign of proper distance surveys. He is the coauthor of two recent papers that brought back this field from a decade with no new data (with over 300 and 200 citations, respectively). Hume was a coauthor of the best-cited article on cosmological perturbations (>4000 citations) developing a gauge invariant formalism that is widely considered to be the gold standard in this sub- discipline. [5] His seminal work on the approximation of the matter power spectrum from redshift surveys (>1000 citations) [6] has opened the door to a whole industry of cosmological probes and N-point functions determination in Fourier space. He was a coauthor on the "Loitering Universe" [7] series of papers that predicted an accelerating universe as a solution to the age problem in 1992 and which included a scalar field that acted like an effective cosmological constant or a quintessence field years before the supernovae type IA results.
He worked extensively on the constraints on galaxy bias, matter density, and primordial non-Gaussianity in redshift surveys and his detection of the bispectrum signal was the first observational confirmation of the Gravitational Instability Model. He helped develop an artificial neural network formalism to interpolate the fully non-linear power spectrum of matter fluctuation and provided the community with a fast and accurate (<1% errors) software to determine the non-linear power spectrum given an input cosmology.
His APS Fellow Citation reads:
For his contributions to cosmology, particularly cosmological perturbations, the statistical and dynamical properties of the large scale structure of the universe, the innovative treatment of cosmic peculiar velocity fields, and the imposition of constraints on cosmological parameters. Nominated by: Division of Astrophysics
He travels widely for presentations to a diverse array of groups, from elementary to high school students to churches, interest groups, legislators and professional societies. He became involved in the Creationism/Intelligent Design in Kansas in 1999 when the nearly successful attempts of various fundamentalist groups to force the teaching of religion in public schools began in earnest. Hume was one of the leaders of a group of academics and educators that addressed, publicized and confronted the issue head on by organizing conferences, [8] [9] public forums and workshops, and through political advocacy including testimonials to the Kansas legislatures, providing high school science teachers with the tools and knowledge to discuss these issues locally to parents, students and the interested public.
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 physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the early universe. The inflationary epoch is believed to have lasted from 10−36 seconds to between 10−33 and 10−32 seconds after the Big Bang. Following the inflationary period, the universe continued to expand, but at a slower rate. The re-acceleration of this slowing expansion due to dark energy began after the universe was already over 7.7 billion years old.
The cosmic microwave background is microwave radiation that fills all space in the observable universe. It is a remnant that provides an important source of data on the primordial universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dark. However, a sufficiently sensitive radio telescope detects a faint background glow that is almost uniform and is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum. The accidental discovery of the CMB in 1965 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s.
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.
Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit toward the red end of the visible spectrum. The discovery of Hubble's law is attributed to Edwin Hubble's work published in 1929.
In modern physical cosmology, the cosmological principle is the notion that the spatial distribution of matter in the universe is uniformly isotropic and homogeneous when viewed on a large enough scale, since the forces are expected to act equally throughout the universe on a large scale, and should, therefore, produce no observable inequalities in the large-scale structuring over the course of evolution of the matter field that was initially laid down by the Big Bang.
Peculiar motion or peculiar velocity refers to the velocity of an object relative to a rest frame — usually a frame in which the average velocity of some objects is zero.
The Sunyaev–Zeldovich effect is the spectral distortion of the cosmic microwave background (CMB) through inverse Compton scattering by high-energy electrons in galaxy clusters, in which the low-energy CMB photons receive an average energy boost during collision with the high-energy cluster electrons. Observed distortions of the cosmic microwave background spectrum are used to detect the disturbance of density in the universe. Using the Sunyaev–Zeldovich effect, dense clusters of galaxies have been observed.
Observational cosmology is the study of the structure, the evolution and the origin of the universe through observation, using instruments such as telescopes and cosmic ray detectors.
The Lambda-CDM, Lambda cold dark matter, or ΛCDM model is a mathematical model of the Big Bang theory with three major components:
Redshift-space distortions are an effect in observational cosmology where the spatial distribution of galaxies appears squashed and distorted when their positions are plotted as a function of their redshift rather than as a function of their distance. The effect is due to the peculiar velocities of the galaxies causing a Doppler shift in addition to the redshift caused by the cosmological expansion.
Primordial fluctuations are density variations in the early universe which are considered the seeds of all structure in the universe. Currently, the most widely accepted explanation for their origin is in the context of cosmic inflation. According to the inflationary paradigm, the exponential growth of the scale factor during inflation caused quantum fluctuations of the inflaton field to be stretched to macroscopic scales, and, upon leaving the horizon, to "freeze in". At the later stages of radiation- and matter-domination, these fluctuations re-entered the horizon, and thus set the initial conditions for structure formation.
In physical cosmology, structure formation is the formation of galaxies, galaxy clusters and larger structures from small early density fluctuations. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.8 billion years ago. However, looking at the night sky today, structures on all scales can be seen, from stars and planets to galaxies. On even larger scales, galaxy clusters and sheet-like structures of galaxies are separated by enormous voids containing few galaxies. Structure formation attempts to model how these structures were formed by gravitational instability of small early ripples in spacetime density or another emergence.
Andrew R. Liddle is a Principal Investigator at the University of Lisbon. From 2018 to 2020 he was a Visiting Fellow at the University of Waterloo. From 2013 to 2017 he was Professor of astrophysics at the Royal Observatory Edinburgh. Publications include books and over 260 papers. He is a theoretical cosmologist and is interested in understanding the properties of the Universe and how these relate to fundamental physical laws.
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.
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.
In cosmology, the steady-state model or steady state theory is an alternative to the Big Bang theory. In the steady-state model, the density of matter in the expanding universe remains unchanged due to a continuous creation of matter, thus adhering to the perfect cosmological principle, a principle that says that the observable universe is always the same at any time and any place.
Bharat Vishnu Ratra is an Indian-American physicist, theoretical cosmologist and astroparticle physicist who is currently a university distinguished professor of physics at Kansas State University.
CMB spectral distortions are tiny departures of the average cosmic microwave background (CMB) frequency spectrum from the predictions given by a perfect black body. They can be produced by a number of standard and non-standard processes occurring at the early stages of cosmic history, and therefore allow us to probe the standard picture of cosmology. Importantly, the CMB frequency spectrum and its distortions should not be confused with the CMB anisotropy power spectrum, which relates to spatial fluctuations of the CMB temperature in different directions of the sky.
