The universe can be described as the sum of all matter and energy that exists and the space-time in which they are located and in which all events occur or could occur. Models of the theorists' universe extrapolate, as far as possible, physics based on empirical observations of the basic components of the universe and their interactions. In contrast, observational cosmologists study a finite part of the same space-time, limiting themselves (nearly) to that part of space-time which is directly observable. Both theorists and observers base their models by assuming fundamental theoretical principles and laws that govern matter, energy, space, and time. These principles and laws are constantly tested empirically. It is presently unknown whether or not the observers' universe corresponds to the whole universe in the theorists' sense.A majority of cosmologists believe that the observers' universe is an extremely tiny part of the theorists' "whole" Universe. To theoretical cosmologists, a universe is a speculative model of all of space-time which is connected together, including all matter and energy in it and space-time events. This model is either required to be consistent with that part of space-time that can be seen or otherwise observed to have occurred by empirical observation, or it can be a more general, hypothetical model. This observable part of space-time can be referred to as the Universe, the known universe, observable universe, or visible universe and is studied by observational cosmologists. The term the Universe sometimes refers to the theoretical model of which the observable universe is only a small part.The age of the universe, according to the Big Bang theory, is the time elapsed between the Big Bang and the present day. The current scientific consensus holds this to be about 13.7 billion years.The most agreed upon model of the universe has time beginning at the Big Bang, and does not speculate about what may have existed "before". However there are alternative possibilities. In some cosmological models (such as steady state theory or static universe) there is no Big Bang, and the universe has infinite age: however, the current scientific consensus is that the observational evidence overwhelmingly supports the occurrence of a Big Bang. There are also cosmological models (such as the cyclic model) in which the universe has existed forever but has undergone a repeated series of Big Bangs and Big Crunches. If these models are correct, then the age of the universe described in this article can be taken as the time since the last Big Bang. In cosmology, the cosmic microwave background radiation (most often abbreviated CMB but occasionally CMBR, CBR or MBR, also referred as relic radiation) is a form of electromagnetic radiation discovered in 1965 that fills the entire universe. (It has a thermal 2.725 kelvin black body spectrum which peaks in the microwave range at a frequency of 160.4 GHz, corresponding to a wavelength of 1.9 mm) Most cosmologists consider this radiation to be the best evidence for the hot big bang model of the universe. In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and has strong negative pressure According to the Theory of Relativity, the effect of such a negative pressure is qualitatively similar to a force acting in opposition to gravity at large scales. Invoking such an effect is currently the most popular method for explaining recent observations that the universe appears to be expanding at an accelerating rate, as well as accounting for a significant portion of the missing mass in the universe. Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and quintessence, a dynamic field whose energy density can vary in time and space. Distinguishing between the alternatives requires high-precision measurements of the expansion of the universe to understand how the speed of the expansion changes over time. The rate of expansion is parameterized by the cosmological equation of state. Measuring the equation of state of dark energy is one of the biggest efforts in observational cosmology today. In astrophysics, dark matter is a theoretical form of matter that does not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence may be inferred from its gravitational effects on visible matter. Among the observed phenomena consistent with the existence of dark matter are the rotational speeds of galaxies and orbital velocities of galaxies in clusters, gravitational lensing of background objects by galaxy clusters such as the Bullet cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter also plays a central role in structure formation and Big Bang nucleosynthesis, and has measurable effects on the anisotropy of the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than is directly observable, indicating that the remainder is dark. The composition of dark matter is unknown, but may include new elementary particles such as WIMPs and axions, ordinary and heavy neutrinos, dwarf stars and planets collectively called MACHOs, and clouds of nonluminous gas. Current evidence favors models in which the primary component of dark matter is new elementary particles, collectively called non-baryonic dark matter. The dark matter component has vastly more mass than the "visible" component of the universe. At present, the density of ordinary baryons and radiation in the universe is estimated to be equivalent to about one hydrogen atom per cubic metre of space. Only about 4% of the total energy density in the universe (as inferred from gravitational effects) can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. Some hard-to-detect baryonic matter makes a contribution to dark matter, but constitutes only a small portion. Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics. It has been noted that the names "dark matter" and "dark energy" serve mainly as expressions of our ignorance. After the Big Bang, the universe had a period when it was remarkably homogeneous, as can be observed in the Cosmic Microwave Background, the fluctuations of which are less than one part in one hundred thousand. The most accepted view today is that all the structure we observe today was formed as a consequence of the growth of primordial fluctuations. The primordial fluctuations caused gas to be attracted to areas of denser material, and star clusters and stars. One consequence of this model is that the location of galaxies indicates areas of higher density of the early universe. Hence the distribution of galaxies is closely related to the physics of the early universe.

A great deal of the research in this area is focused on components of our own Milky Way, since it is the easiest galaxy to observe. The observations which must be explained in, or at least not at odds with, a theory of galactic evolution, include:

the stellar disk is quite thin, dense, and rotates
the stellar halo is very large, sparse, and does not rotate (or has perhaps even a slight retrograde rotation), with no apparent substructure
halo stars are typically much older and have much lower metallicities than disk stars (there is a correlation, but there is no absolute connection between these data)
some astronomers have identified an intermediate population of stars, variously called the "metal weak thick disk", the "intermediate population II", et al. If these are indeed a distinct population, they would be described as metal-poor (but not as poor as the halo stars), old (but not as old as the halo stars), and orbiting very near the disk, in a sort of "puffed-up", thicker disk shape.
globular clusters are typically old and metal-poor as well, but there are a few which are not nearly as metal-poor as most, and/or have some younger stars. Some stars in globular clusters appear to be as old as the universe itself (by entirely different measurement and analysis methods)!
in each globular cluster, all the stars were born at virtually the same time (except for a few globulars that show multiple epochs of star formation)
globular clusters with smaller orbits (closer to the galactic center) have orbits which are somewhat flatter (less inclined to the disk), and less eccentric (more circular), while those further out have orbits in all inclinations, and tend to be more eccentric.
High Velocity Clouds, clouds of neutral hydrogen are "raining" down on the galaxy, and presumably have been from the beginning (these would be the necessary source of a gas disk from which the disk stars formed).

The earliest modern theory of the formation of our galaxy (known by astronomers as ELS, after the initials of the authors of that paper, Olin Eggen, Donald Lynden-Bell, and Allan Sandage describes a single (relatively) rapid monolithic collapse, with the halo forming first, followed by the disk. Another view published in 1978 (known as SZ for its authors, Leonard Searle and Robert Zinn) describes a more gradual process, with smaller units collapsing first, then later merging to form the larger components. An even more recent idea is that significant portions of the stellar halo could be stellar debris from destroyed dwarf galaxies and globular clusters that once orbited the Milky Way. The halo would then be a "new"er component made of "recycled" old parts!

In recent years, a great deal of focus has been put on understanding merger events in the evolution of galaxies. Rapid technological progress in computers have allowed much better simulations of galaxies, and improved observational technologies have provided much more data about distant galaxies undergoing merger events. After the discovery in 1994 that our own Milky Way has a satellite galaxy (the Sagittarius Dwarf Elliptical Galaxy, or SagDEG) which is currently gradually being ripped up and "eaten" by the Milky Way, it is thought these kinds of events may be quite common in the evolution of large galaxies. The Magellanic Clouds are satellite galaxies of the Milky Way that will almost certainly share the same fate as the SagDEG. A merger with a fairly large satellite galaxy could explain why M31 (the Andromeda Galaxy) appears to have a double core.

Giant elliptical galaxies are probably formed by mergers on a grander scale. In the Local Group, the Milky Way and M31 (the Andromeda Galaxy) are gravitationally bound, and currently approaching each other at high speed. Since we cannot determine the speed of M31 perpendicular to the line from us to it, we do not know if it will collide with the Milky Way. If the two galaxies do meet they will pass through each other, with gravity distorting both galaxies severely and ejecting some gas, dust and stars into intergalactic space. They will travel apart, slow down, and then again be drawn towards each other, and again collide. Eventually both galaxies will have merged completely, streams of gas and dust will be flying through the space near the newly formed giant elliptical galaxy. Out of the gas ejected from the merger, new globular clusters and maybe even new dwarf galaxies may form and become the halo of the elliptical. The globulars from both M31 and the Milky Way will also form part of the halo; globulars are so tightly held together that they are largely immune to large scale galactic interactions. On the stellar scale, little will happen. If anybody is around to watch the merger, it will be a slow, but magnificent event, with the sight of a distorted M31 spectacularly spanning the entire sky. M31 is actually already distorted: the edges are warped. This is probably because of interactions with its own galactic companions, as well as possible mergers with dwarf spheroidal galaxies in the recent past - the remnants of which are still visible in the disk populations.

While we have learned a great deal about ours and other galaxies, the most fundamental questions about formation and evolution remain only tentatively answered.

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