UniVerse

(astronomy) The totality of astronomical things, events, relations, and energies capable of being described objectively.

The universe comprises everything in existence, including all matter and energy, and the enormous volume which contains them. The observable universe currently spans about 6 × 10²² km (4 × 10²² mi), and contains 2 × 10⁵⁰ to 1 × 10⁵¹ kg (4 × 10⁵⁰ to 2 × 10⁵¹ lb) of matter, yielding an average density of a few atoms per cubic meter. Most of the universe, then, is empty space; the matter is distributed thinly throughout, forming objects and structures at a variety of different sizes.

Constituents

This article will start the cosmic survey with the more familiar objects, following a sequence of increasing size. A number of lesser-known and less tangible entities will complete the survey.

Baryonic matter

Most of the matter encountered in everyday life is in the form of atoms. An atom consists of a positively charged nucleus of protons and neutrons, surrounded by clouds or shells of negatively charged electrons. The protons and neutrons are responsible for most of the mass of the atom. Since both protons and neutrons belong to a class of subatomic particles known as baryons, this ordinary form of atomic matter is called baryonic matter in astronomical contexts. See also Baryon.

A large fraction of the visible matter elsewhere in the universe—planets, stars, nebulae, galaxies—is also baryonic in nature, but the relative proportions of the chemical elements are very different from here on Earth. Hydrogen is by far the most abundant element in the universe, representing 75% of the total baryonic mass. Helium is also plentiful at about 23% of the total mass. All the heavier elements make up the remaining 2%.

Stars and stellar evolution

The most plentiful denizens of the nearby universe, as seen in the night sky, are the stars. These points of light are objects much like the Sun, which appear faint due to their great distance from Earth. Most visible stars are enormous balls of hot gas (primarily hydrogen and helium) held together by their own gravitation. They are powered by nuclear reactions deep in their interiors, where temperatures and pressures are high enough to fuse hydrogen atoms into helium, releasing energy in the process. See also Carbon-nitrogen-oxygen cycles; Nuclear fusion; Proton-proton chain.

Once a star has used up all of its core hydrogen, it must change its internal structure to burn new fuel sources. A low-mass star (less than a few solar masses) will burn helium for a time, but as the helium fuel is exhausted and its internal furnace wanes, the outer layers of the star are ejected into space, and the core will gradually shrink into a tiny, dense ember, a white dwarf, glowing only from its residual heat.

Higher-mass stars will burn helium, then carbon, and then a succession of even heavier elements, each for a progressively shorter time, until fusable material of any sort abruptly runs out and the star collapses catastrophically. Much of the interior mass of the star is compacted into an ultradense core; the outer layers rebound off this core and explode into space, forming an exceptionally bright type II supernova, shining for several weeks at a billion times the luminosity of the Sun. The stellar core is usually left behind as a neutron star, a small, rapidly rotating body consisting almost entirely of neutrons. Powerful magnetic fields on the surface of a neutron star produce radio waves which appear to blink on and off as the star spins, and the neutron star is observed as a pulsar. In the most extreme cases, the stellar core is compressed so far that it collapses to an infinitesimal point, forming a black hole. See also Black hole; Neutron star; Nucleosynthesis; Pulsar; Star; Stellar evolution.

Solar system

The Sun is accompanied by a number of smaller objects of various sizes and compositions. The Sun's gravitational domain extends out to almost half a parsec (1 parsec equals 3.1 × 10¹³ km or 1.9 × 10¹³ mi) but the planets, the best-known elements of the solar system, lie much closer, within about 5 × 10⁹ km (3 × 10⁹ mi) of the center. See also Planet; Solar system.

Extrasolar planets

In recent years, strong evidence has been mounting that the Sun is not the only star with a planetary system. At least 60 nearby stars, similar in type to the Sun, exhibit subtle periodic shifts in their motion which are most likely due to the gravitational influence of small orbiting companions.

Interstellar material

The “empty” space between the stars actually contains significant amounts of matter—some of it distributed continuously, some of it concentrated in enormous dark clouds—collectively known as the interstellar medium. Also scattered throughout space are denser clouds of molecular hydrogen (H₂), which contain traces of carbon monoxide (CO) and more complex molecules, and are sprinkled with the heavier elements. These clouds often are the site of star formation, when external gravitational disturbances or shock waves trigger the collapse of portions of these clouds. See also Interstellar matter; Molecular cloud.

Galaxies

Stars and interstellar matter are not distributed uniformly throughout the universe but cluster together in vast units known as galaxies, each containing from 10 million to 1 trillion stars. Galaxies are categorized into three types—spiral, elliptical, and irregular—with numerous subclasses based on size and structure. The Milky Way Galaxy is a typical spiral galaxy—a flattened disk of some 10¹¹ stars. See also Galaxy, external.

Quasars

Supermassive black holes, similar to the stellar-sized black holes which form during the supernova explosion of a massive star but containing 10⁶ to 10⁷ solar masses instead of only a few, are thought to exist at the centers of many galaxies—including, perhaps, the Milky Way. As stellar or interstellar matter is drawn into the black hole, it forms an accretion disk around the central point, where it is heated by friction to temperatures of millions of kelvins and glows brightly with a luminosity equivalent to 10¹² Suns. At the far reaches of the universe, these galaxy cores appear to observers as starlike point sources, and were thus labeled quasistellar objects or quasars when first discovered. See also Quasar.

Groups, clusters, and large-scale structure

Galaxies themselves are usually bound together in groups (up to about 50 galaxies) and clusters (50 to thousands of galaxies) spanning regions 2–10 Mpc in diameter. Clusters accumulate into yet larger agglomerations called superclusters. Groups and clusters seem to be concentrated in thin sheets, surrounding enormous voids with very few galaxies. Superclusters sit at the edges and vertices where several surfaces intersect.

Antimatter

All subatomic particles have oppositely charged antiparticle counterparts. When a particle and its corresponding antiparticle collide, they annihilate one another, converting all their mass to energy. Both matter and antimatter are expected to have been formed in the early universe, but clearly not in precisely equal amounts, since the universe (at least the part that observers can see) is predominantly composed of matter. See also Antimatter.

Nonbaryonic particles

Although most of the matter observed is baryonic in nature, several kinds of nonbaryonic matter also exist in the universe or have been predicted by particle physicists, and may also account for a substantial fraction of the mass of the universe.

Neutrinos are electrically neutral, very low mass particles that are generated in nuclear reactions. They were once suspected to be completely massless entities, but theoretical and experimental evidence is now mounting that they have a tiny but nonzero mass, so that they could exert a small gravitational influence on the universe at large. See also Neutrino.

Other particle species, also appear in cosmic contexts. WIMPs, a sort of slow-moving superneutrino, have been proposed as being responsible for the missing dark mass component of the universe. See also Weakly interacting massive particle (WIMP).

Dark matter

A large fraction of the mass of the universe is in some form which cannot be seen but which is evident from its gravitational effect on the motions of bright objects, such as stars and galaxies. This dark matter, or “missing mass,” is present on many different scales, from galaxies to clusters to the universe as a whole.

Energy

Energy is a physical entity as real as matter, but somewhat less tangible, which makes it more difficult to categorize easily. Like matter, energy comes in many different forms, which can be readily transformed from one to another. Energy can also be converted to and from matter.

The four basic forces of nature—gravity, electromagnetism, and the strong and weak nuclear forces—may in fact be different facets of a single fundamental force. Particle physics experiments show that under conditions of extremely high energy, the weak nuclear force and electromagnetism merge into a single “electroweak” force. Theoretical efforts are being made to devise a grand unified theory under which the strong force and gravity are also incorporated. Such a unified force may have existed during the moments following the big bang and then fragmented into separate forces as the universe expanded and cooled. See also Electroweak interaction; Fundamental interactions; Grand unification theories; Standard model.

Origin and fate

The universe is a dynamically evolving system. By closely studying the current distribution and motion of matter and energy, and collecting the “fossil light” from distant objects, scientists have constructed a fairly consistent picture of the creation of the universe, the big bang theory, which explains the observations fairly well.

Big bang

The observable universe originated some 9–18 billion years ago in a fiery cataclysm termed the big bang. This was not an explosion of compressed matter and energy into a previously empty space. Instead, space itself, as well as everything contained therein, sprang from a single point of infinite density and temperature, and grew to the volume observed today. As it expanded, it cooled, allowing familiar forms of matter to condense from the high-energy “soup” of subatomic particles that constituted the very early universe.

Cosmological redshifts

Distinct spectral features, characteristic of the different chemical elements that are present, appear at well-established locations in the spectrum of an astronomical object, but the entire spectrum is shifted toward shorter wavelengths (bluewards) for an object approaching the observer, or toward longer wavelengths (redwards) for a receding object. The amount of this Doppler shift is closely related to the actual relative speed of the object. See also Doppler effect.

With the exception of a few nearby galaxies, all of the galaxies exhibit redshifts of varying amounts, implying that they are all moving away from the Earth. Edwin Hubble extended this work by determining the distances to these galaxies, and found that the redshift was directly proportional to the distance; that is, translating redshift into recession velocity, the more distant the galaxy, the faster it is moving away from the Earth. The constant of proportionality relating speed and distance now bears Hubble's name, and an accurate determination of this Hubble constant (H₀) is a central pursuit of modern astronomy. Recent studies seem to be converging towards a value of H₀ = 72 km/(s)(Mpc).

If space itself is expanding uniformly in all directions, and the galaxies are being carried along in this general expansion, then any point in the universe would see all other points moving away. The greater the distance between two points, the more space exists between them, and the faster this distance increases. Hubble's law is therefore a consequence of the uniform expansion of space, which causes more distant galaxies to exhibit larger redshifts because they are receding from the Earth faster.

If space is expanding uniformly in all directions, then it follows that in the past the universe was smaller. At some point in the past, all matter and energy may have existed in a single point of infinite temperature and density. See also Hubble constant; Redshift.

Cosmic background radiation

If this picture is correct, and the current universe was spawned from a primordial fireball, then observers should see a residual afterglow from the era when the matter in the universe was hot and emitting strongly, in much the same way that the surfaces of stars shine today. This microwave background was first observed in 1961. The Cosmic Background Explorer (COBE) satellite mission in 1990 that the spectrum was measured its spectrum over a wide range of wavelengths, establishing the cosmic background radiation temperature at 2.726 K.

Since the cosmic background radiation is an intrinsic characteristic of the universe, observers expect to see it reaching the Earth uniformly from all directions in space. However, reanalysis of the COBE data, as well as more recent observations of the cosmic background radiation, shows minute ripples or anisotropies in the temperature of the background radiation. The amplitude and spatial scale of these ripples are of keen interest, since they probably result from the very first clumps of matter that accreted in the early universe. See also Cosmic background radiation.

Evolution of the universe

The best models for the beginning of the universe start at 10⁻⁴³ second after the big bang itself. At 10⁻⁴³ s, the universe was 10³⁰ times smaller than it is today (barely larger than an atom), and had a mean temperature of 10³⁰ K. At such temperatures, matter as presently known cannot exist, because the energies are so enormous that even the protons and neutrons themselves are torn apart into separate sub-subatomic particles called quarks. The universe was a featureless mixture of quarks, leptons (another class of particles including electrons and neutrinos), and high-energy photons. See also Lepton; Quantum gravitation; Quarks; Supergravity; Supersymmetry.

At 10⁻³⁴ s, when the temperature had dropped to 3 × 10²⁶ K, heavier exotic particles such as magnetic monopoles could have emerged. Around this time, the rapidly enlarging structure of space may have undergone even faster expansion, driven by the energy of space itself. This era of hyperfast inflation helps explain features of the present-day observable universe, such as the uniformity of the cosmic background radiation over the entire sky, and the way in which the average mass density of the universe is high enough for structures like stars and galaxies to form, but not so large that it would recollapse quickly. See also Inflationary universe cosmology; Magnetic monopoles.

When the temperature had dropped further, to about 3 × 10¹² K, protons and neutrons condensed out of the quark mixture. It was still much too hot for electrons to join them and form atoms, but more complex atomic nuclei were created in a fashion similar to stellar core fusion. This era of big bang nucleosynthesis established the initial composition of the universe, about three-quarters hydrogen, one-quarter helium, and small amounts of lithium, from which all subsequent stellar and supernova nucleosynthesis has proceeded.

Until a time about 100,000 years after the big bang, temperatures were still too high for electrons to join these nuclei to make complete atoms. Up to this point, the universe was relatively opaque, since free charged particles like electrons are very good at absorbing light and other electromagnetic radiation. Once the temperature fell below 3000 K, however, atoms formed, and the universe suddenly became transparent to most wavelengths. The cosmic background radiation formed at this point and has had little interaction with matter since. The formation of structure—the development of clumpiness in the universe, which is seen today as stars, galaxies, and clusters—started slightly before matter-radiation decoupling.

Ultimate fate of the universe

The motions of galaxies indicate that the universe has continued to expand after the initial impulse of the big bang, but it is not known whether this expansion will continue. The situation is complicated by the possible presence of an additional cosmological force, predicted by general relativity, which has the opposite effect from gravitational mass. This cosmological constant, represented by Λ, pushes outward instead of pulling inward like gravity, adding an acceleration term to the expansion of the universe. Interest in Λ has revived in recent years in an attempt to explain deviations in redshift velocities of some very distant galaxies. If Λ > 0, as some researchers now think, then this would make an infinite expansion more likely, even with significant amounts of dark matter. Currently favored models yield a topologically flat universe. See also Big bang theory; Cosmology.

A relational DBMS from IBM that runs on the major Unix and Windows servers. By 1997, more than a million seats had been sold. UniVerse includes its own BASIC programming environment and a variety of tools and enhancements for programming UniVerse applications. UniVerse was originally developed by Ardent Software, which was acquired by Informix and then IBM. It is part of IBM's U2 product family. See UniData.

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