The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. The concept of an expanding universe was scientifically originated by physicist Alexander Friedmann in 1922 with the mathematical derivation of the Friedmann equations. The earliest empirical observation of an expanding universe is known as Hubble's law, published in work by physicist Edwin Hubble in 1929, which discerned that galaxies are moving away from Earth at a rate that accelerates proportionally with distance. Independent of Friedmann's work, and independent of Hubble's observations, physicist Georges Lemaître proposed that the universe emerged from a "primeval atom" in 1931, introducing the modern notion 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 astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be observed. Such effects occur in the context of formation and evolution of galaxies, gravitational lensing, the observable universe's current structure, mass position in galactic collisions, the motion of galaxies within galaxy clusters, and cosmic microwave background anisotropies.
The multiverse is the hypothetical set of all universes. Together, these universes are presumed to comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and constants that describe them. The different universes within the multiverse are called "parallel universes", "flat universes", "other universes", "alternate universes", "multiple universes", "plane universes", "parent and child universes", "many universes", or "many worlds". One common assumption is that the multiverse is a "patchwork quilt of separate universes all bound by the same laws of physics."
The universe is all of space and time and their contents. It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of matter and energy, and the structures they form, from sub-atomic particles to entire galactic filaments. Since the early 20th century, the field of cosmology establishes that space and time emerged together at the Big Bang 13.787±0.020 billion years ago and that the universe has been expanding since then. The portion of the universe that can be seen by humans is approximately 93 billion light-years in diameter at present, but the total size of the universe is not known.
In cosmology, the cosmological constant, alternatively called Einstein's cosmological constant, is a coefficient that Albert Einstein initially added to his field equations of general relativity. He later removed it; however, much later it was revived to express the energy density of space, or vacuum energy, that arises in quantum mechanics. It is closely associated with the concept of dark energy.
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 further 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 further away 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.
The fine-tuned universe is the hypothesis that, because "life as we know it" could not exist if the constants of nature – such as the electron charge, the gravitational constant and others – had been even slightly different, the universe must be tuned specifically for life. In practice, this hypothesis is formulated in terms of dimensionless physical constants.
Lawrence Maxwell Krauss is a Canadian-American theoretical physicist and cosmologist who taught at Arizona State University (ASU), Yale University, and Case Western Reserve University. He founded ASU's Origins Project in 2008 to investigate fundamental questions about the universe and served as the project's director.
Saul Perlmutter is a U.S. astrophysicist, a professor of physics at the University of California, Berkeley, where he holds the Franklin W. and Karen Weber Dabby Chair, and head of the International Supernova Cosmology Project at the Lawrence Berkeley National Laboratory. He is a member of both the American Academy of Arts & Sciences and the American Philosophical Society, and was elected a Fellow of the American Association for the Advancement of Science in 2003. He is also a member of the National Academy of Sciences. Perlmutter shared the 2006 Shaw Prize in Astronomy, the 2011 Nobel Prize in Physics, and the 2015 Breakthrough Prize in Fundamental Physics with Brian P. Schmidt and Adam Riess for providing evidence that the expansion of the universe is accelerating. Since 2021, he has been a member of the President’s Council of Advisors on Science and Technology (PCAST).
Adam Guy Riess is an American astrophysicist and Bloomberg Distinguished Professor at Johns Hopkins University and the Space Telescope Science Institute. He is known for his research in using supernovae as cosmological probes. Riess shared both the 2006 Shaw Prize in Astronomy and the 2011 Nobel Prize in Physics with Saul Perlmutter and Brian P. Schmidt for providing evidence that the expansion of the universe is accelerating.
Sean Michael Carroll is an American theoretical physicist who specializes in quantum mechanics, cosmology, and the philosophy of science. He is the Homewood Professor of Natural Philosophy at Johns Hopkins University. He was formerly a research professor at the Walter Burke Institute for Theoretical Physics at the California Institute of Technology (Caltech) department of physics. He also is currently an external professor at the Santa Fe Institute, and he has been a contributor to the physics blog Cosmic Variance, where he has published in scientific journals such as Nature as well as other publications, including The New York Times, Sky & Telescope, and New Scientist. He is known for his atheism, his vocal critique of theism and defence of naturalism. He is considered a prolific public speaker and science popularizer. In 2007, Carroll was named NSF Distinguished Lecturer by the National Science Foundation.
Paul S. Wesson, B.Sc., Ph.D., D.Sc., F.R.A.S was a professor of astrophysics and theoretical physics.
In physical cosmology and astronomy, dark energy is a proposed 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 dominates 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.
Sergei D. Odintsov is a Russian astrophysicist active in the fields of cosmology, quantum field theory and quantum gravity. He received his master's degree and PhD in Physics at the Tomsk State University. Odintsov is an ICREA Research Professor at the Institut de Ciències de l'Espai (Barcelona) since 2003. He also collaborates as group leader at research projects of the Tomsk State Pedagogical University. He is editor-in-chief of Symmetry, and is a member of the editorial boards of Gravitation and Cosmology, International Journal of Geometric Methods in Modern Physics, International Journal of Modern Physics D, Journal of Gravity, Universe, Russian Physics Journal and the Tomsk State Pedagogical University Bulletin. Odintsov also is an advisory panel member of Classical and Quantum Gravity.
John G. Hartnett, is an Australian young Earth creationist and cosmologist. He has been active with Creation Ministries International and is known for his opposition to the Big Bang theory and criticism of the dark matter and dark energy hypotheses.
Ethan R. Siegel is an American theoretical astrophysicist and science writer, who studies the Big Bang theory. In the past he has been a professor at Lewis & Clark College and a blogger at Starts With a Bang, on ScienceBlogs and also on Forbes.com since 2016.
Daniel Wayne Hooper is an American cosmologist and particle physicist specializing in the areas of dark matter, cosmic rays, and neutrino astrophysics. He is a professor of physics at the University of Wisconsin–Madison and the director of the Wisconsin IceCube Particle Astrophysics Center (WIPAC).
Richard Panek is an American popular science writer, columnist, and journalist who specializes in the topics of space, the universe, and gravity. He has published several books and has written articles for a number of news outlets and scientific organizations, including Scientific American, WIRED, New Scientist, and Discover.
The Trouble with Gravity: Solving the Mystery Beneath Our Feet is a nonfiction popular science book by Richard Panek and published by Houghton Mifflin Harcourt on July 9, 2019.
