It has been suggested that this article or section be merged into Large-scale structure of the cosmos. (Discuss)

Structure formation refers to a fundamental problem in physical cosmology. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 11 billion years ago. However, looking in the sky today, we see structures on all scales, from stars and planets to galaxies and, on much larger scales still, galaxy clusters, and enormous voids between galaxies. How did all of this come about from the nearly uniform early universe? Wikipedia does not have an article with this exact name. ... Astronomy and cosmology examine the universe to understand the large-scale structure of the cosmos. ... Cosmology, as a branch of astrophysics, is the study of the large-scale structure of the universe and is concerned with fundamental questions about its formation and evolution. ... The deepest visible-light image of the cosmos, the Hubble Ultra Deep Field. ... In cosmology, the cosmic microwave background radiation (most often abbreviated CMB but occasionally CMBR, CBR or MBR) is a form of electromagnetic radiation discovered in 1965 that fills the entire universe. ... The Pleiades star cluster A star is a massive, compact body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. ... A planet is generally considered to be a relatively large mass of accreted matter in orbit around a star. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light years in diameter and approximately 60 million light years distant. ... Galaxy groups and clusters are super-structures in the spread of galaxies of the cosmos. ...

Cosmological Fluctuations

Structure formation includes two key ingredients. The first is the realization that while observations indicate that the cosmic microwave background radiation is nearly uniform, there are very small temperature fluctuations at one part in 100,000. To put this in perspective, the same level of fluctuations on a topographic map of the United States would show no feature higher than a few meters high. However, these fluctuations are critical, because they provide the seeds from which the largest structures within the universe can grow and eventually collapse to form galaxies and stars. COBE(Cosmic Background Explorer) provided the first detection of the intrinsic fluctuations in the cosmic microwave background radiation in the 1990s, and confirmed that these fluctuations were consistent with the predictions of the inflationary model of the universe. Temperature is also the name of a song by Sean Paul. ... Example of a topographic map with contour lines Topographic maps, also called contour maps, topo maps or topo quads (for quadrangles), are maps that show topography, or land contours, by means of contour lines. ... The Cosmic Background Explorer (COBE), also referred to as Explorer 66, was the first satellite built dedicated to cosmology. ... Inflation is the idea—first proposed by Alan Guth (1981)—that the nascent universe passed through a phase of exponential expansion (the inflationary epoch) that was driven by a negative pressure vacuum energy density. ...

Dark Matter

The second key ingredient is dark matter. One of the key realizations made by cosmologists in the 1970s and 1980s was that the majority of the matter content of the universe was composed not of atoms, but rather a strange mysterious form of matter known as dark matter. Dark matter interacts through the force of gravity, but does not emit or absorb radiation, and hence interacts only very feebly via the weak interaction (at best) with itself or with ordinary matter. In cosmology, dark matter refers to matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. ... Matter is commonly defined as the substance of which physical objects are composed. ... Properties For alternative meanings see atom (disambiguation). ... Gravity is a force of attraction that acts between bodies that have mass. ... Radiation in physics is a process of emission of energy or particles. ... The weak nuclear force or weak interaction is one of the four fundamental forces of nature. ...

Dark matter turns out to play a key role in accelerating the process of structure formation precisely because it feels only the force of gravity. As a result, dark matter begins to collapse into a complex network of dark matter halos well before ordinary matter, which is impeded by pressure forces. Without dark matter, the epoch of galaxy formation would occur substantially later in the universe than is observed. Beyond the visible, inner portion of the galactic halo lies a much larger region, known as the dark matter halo which contains large amounts of dark matter. ... In astrophysics, the questions of galaxy formation and evolution are: How, from a homogeneous universe, did we obtain the very inhomogeneous one we live in? How did galaxies form? How do galaxies change over time? The formation of galaxies is still one of the most active research areas in astrophysics...

The Millennium Simulation

The famous millennium simulation [1] does to some extent explain how large scale structure can be formed from initial fluctuation.

Initial Conditions

The initial conditions for the universe are thought to arise from the scale invariant quantum mechanical fluctuations of cosmic inflation. The perturbation of the background energy density at a given point in space is then given by an isotropic, homogeneous Gaussian random field of mean zero. This means that the spatial Fourier transform of ρ – has the following correlation functions Cosmic inflation is the idea, first proposed by Alan Guth in 1981, that the nascent universe passed through a phase of exponential expansion (the inflationary epoch) that was driven by a negative pressure vacuum energy density. ... Isotropy (the opposite of anisotropy) is the property of being independent of direction. ... Look up Homogeneous in Wiktionary, the free dictionary. ... A Gaussian random field is random field involving Gaussian probability density functions of the variables. ... In statistics, mean has two related meanings: the average in ordinary English, which is more correctly called the arithmetic mean, to distinguish it from geometric mean or harmonic mean. ...

,

where δ⁽³⁾ is the three dimensional Dirac delta function and is the length of . Moreover, the spectrum predicted by inflation is nearly scale invariant, which means The Dirac delta function, often referred to as the unit impulse function and introduced by the British theoretical physicist Paul Dirac, can usually be informally thought of as a function δ(x) that has the value of infinity for x = 0, the value zero elsewhere. ... In physics, scale invariance is the feature of physical objects of laws that do not change if the space is magnified, i. ...

,

where n_s − 1 is a small number. Finally, the initial conditions are adiabatic or isentropic, which means that the fractional perturbation in the entropy of each species of particle is equal.