Universe 20

[Ermengardi Atkisson]

Theuniverse is all of space and time[a] and their contents.[10] It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of energy and matter, and the structures they form, from sub-atomic particles to entire galactic filaments. Space and time, according to the prevailing cosmological theory of the Big Bang, emerged together 13.7870.020 billion years ago,[11] and the universe has been expanding ever since. Today the universe has expanded into an age and size that is physically only in parts observable as the observable universe, which is approximately 93 billion light-years in diameter at the present day, while the spatial size, if any, of the entire universe is unknown.[3]

Some of the earliest cosmological models of the universe were developed by ancient Greek and Indian philosophers and were geocentric, placing Earth at the center.[12][13] 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 universal gravitation, Isaac Newton built upon Copernicus's work as well as Johannes Kepler's laws of planetary motion and observations by Tycho Brahe.

Further observational improvements led to the realization that the Sun is one of a few hundred billion stars in the Milky Way, which is one of a few hundred billion galaxies in the observable universe. Many of the stars in a galaxy have planets. At the largest scale, galaxies are distributed uniformly and the same in all directions, meaning that the universe has neither an edge nor a center. At smaller scales, galaxies are distributed in clusters and superclusters which form immense filaments and voids in space, creating a vast foam-like structure.[14] Discoveries in the early 20th century have suggested that the universe had a beginning and has been expanding since then.[15]

From studying the effects of gravity on both matter and light, it has been discovered that the universe contains much more matter than is accounted for by visible objects; stars, galaxies, nebulas and interstellar gas. This unseen matter is known as dark matter,[16] (dark means that there is a wide range of strong indirect evidence that it exists, but we have not yet detected it directly) having come into existence alongside the rest of the physical universe before gradually gathering into a foam-like structure of filaments and voids and allowing other forms of matter to form together into visible structures. The ΛCDM model is the most widely accepted model of the universe. It suggests that about 69.2%1.2% of the mass and energy in the universe is dark energy which is responsible for the acceleration of the expansion of the universe, and about 25.8%1.1% is dark matter.[17] Ordinary ('baryonic') matter is therefore only 4.84%0.1% of the physical universe.[17] Stars, planets, and visible gas clouds only form about 6% of the ordinary matter.[18]

There are many competing hypotheses about the ultimate fate of the universe and about what, if anything, preceded the Big Bang, while other physicists and philosophers refuse to speculate, doubting that information about prior states will ever be accessible. Some physicists have suggested various multiverse hypotheses, in which the universe might be one among many.[3][19][20]

The physical universe is defined as all of space and time[a] (collectively referred to as spacetime) and their contents.[10] Such contents comprise all of energy in its various forms, including electromagnetic radiation and matter, and therefore planets, moons, stars, galaxies, and the contents of intergalactic space.[21][22][23] The universe also includes the physical laws that influence energy and matter, such as conservation laws, classical mechanics, and relativity.[24]

The word universe derives from the Old French word univers, which in turn derives from the Latin word universus, meaning 'combined into one'.[31] The Latin word 'universum' was used by Cicero and later Latin authors in many of the same senses as the modern English word is used.[32]

The prevailing model for the evolution of the universe is the Big Bang theory.[38][39] The Big Bang model states that the earliest state of the universe was an extremely hot and dense one, and that the universe subsequently expanded and cooled. The model is based on general relativity and on simplifying assumptions such as the homogeneity and isotropy of space. A version of the model with a cosmological constant (Lambda) and cold dark matter, known as the Lambda-CDM model, is the simplest model that provides a reasonably good account of various observations about the universe.

Within the first fraction of a second of the universe's existence, the four fundamental forces had separated. As the universe continued to cool from its inconceivably hot state, various types of subatomic particles were able to form in short periods of time known as the quark epoch, the hadron epoch, and the lepton epoch. Together, these epochs encompassed less than 10 seconds of time following the Big Bang. These elementary particles associated stably into ever larger combinations, including stable protons and neutrons, which then formed more complex atomic nuclei through nuclear fusion.[41][42]

According to the general theory of relativity, far regions of space may never interact with ours even in the lifetime of the universe due to the finite speed of light and the ongoing expansion of space. For example, radio messages sent from Earth may never reach some regions of space, even if the universe were to exist forever: space may expand faster than light can traverse it.[50]

Because humans cannot observe space beyond the edge of the observable universe, it is unknown whether the size of the universe in its totality is finite or infinite.[3][57][58] Estimates suggest that the whole universe, if finite, must be more than 250 times larger than a Hubble sphere.[59] Some disputed[60] estimates for the total size of the universe, if finite, reach as high as 10 10 10 122 \displaystyle 10^10^10^122 megaparsecs, as implied by a suggested resolution of the No-Boundary Proposal.[61][b]

Assuming that the Lambda-CDM model is correct, the measurements of the parameters using a variety of techniques by numerous experiments yield a best value of the age of the universe at 13.799 0.021 billion years, as of 2015.[2]

Over time, the universe and its contents have evolved. For example, the relative population of quasars and galaxies has changed[62] and the universe has expanded. This expansion is inferred from the observation that the light from distant galaxies has been redshifted, which implies that the galaxies are receding from us. Analyses of Type Ia supernovae indicate that the expansion is accelerating.[63][64]

General relativity describes how spacetime is curved and bent by mass and energy (gravity). The topology or geometry of the universe includes both local geometry in the observable universe and global geometry. Cosmologists often work with a given space-like slice of spacetime called the comoving coordinates. The section of spacetime which can be observed is the backward light cone, which delimits the cosmological horizon. The cosmological horizon, also called the particle horizon or the light horizon, is the maximum distance from which particles can have traveled to the observer in the age of the universe. This horizon represents the boundary between the observable and the unobservable regions of the universe.[81][82]

An important parameter determining the future evolution of the universe theory is the density parameter, Omega (Ω), defined as the average matter density of the universe divided by a critical value of that density. This selects one of three possible geometries depending on whether Ω is equal to, less than, or greater than 1. These are called, respectively, the flat, open and closed universes.[83]

The fine-tuned universe hypothesis is the proposition that the conditions that allow the existence of observable life in the universe can only occur when certain universal fundamental physical constants lie within a very narrow range of values. According to this hypothesis, if any of several fundamental constants were only slightly different, the universe would have been unlikely to be conducive to the establishment and development of matter, astronomical structures, elemental diversity, or life as it is understood. Whether this is true, and whether that question is even logically meaningful to ask, are subjects of much debate.[89] The proposition is discussed among philosophers, scientists, theologians, and proponents of creationism.[90]

The observable universe is isotropic on scales significantly larger than superclusters, meaning that the statistical properties of the universe are the same in all directions as observed from Earth. The universe is bathed in highly isotropic microwave radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.72548 kelvins.[7] The hypothesis that the large-scale universe is homogeneous and isotropic is known as the cosmological principle.[116] A universe that is both homogeneous and isotropic looks the same from all vantage points and has no center.[117][118]

Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously,[122] and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space while still permeating then enough to cause the observed rate of expansion. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to vacuum energy.

A lepton is an elementary, half-integer spin particle that does not undergo strong interactions but is subject to the Pauli exclusion principle; no two leptons of the same species can be in exactly the same state at the same time.[143] Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Electrons are stable and the most common charged lepton in the universe, whereas muons and taus are unstable particles that quickly decay after being produced in high energy collisions, such as those involving cosmic rays or carried out in particle accelerators.[144][145] Charged leptons can combine with other particles to form various composite particles such as atoms and positronium. The electron governs nearly all of chemistry, as it is found in atoms and is directly tied to all chemical properties. Neutrinos rarely interact with anything, and are consequently rarely observed. Neutrinos stream throughout the universe but rarely interact with normal matter.[146]

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