A Zeldovich pancake is a theoretical condensation of gas out of a primordial density fluctuation following the

Big Bang The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including th ...

. In 1970, Yakov B. Zeldovich showed that for an

ellipsoid An ellipsoid is a surface that can be obtained from a sphere by deforming it by means of directional Scaling (geometry), scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface; that is, a Surface (mathemat ...

of gas on a supergalactic scale, an approximation can be used that will model the collapse as occurring most rapidly along the shortest axis, resulting in a pancake form. This approximation assumes that the

of gas is sufficiently large that the effect of pressure is negligible and only gravitational attraction needs to be considered. That is, the gas will collapse without being significantly perturbed by outward pressure. This assumption is especially valid if the collapse occurs before the recombination era that resulted in the formation of

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

atoms. In 1989, Zeldovich and S. F. Shandarin showed that initial overlapping density fluctuations of random Gaussian fields would result in "dense pancakes, filaments, and compact clumps of matter". This model became known as a top-down model of galactic formation, with the supergalactic condensation fragmenting into protogalaxies. The formation of plane concentrations would compress the gas through

shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...

s generated during the collapse, increasing the temperature. At a higher level, the collapse of larger structures according to the Zeldovich Approximation is known as second-generation pancakes or superpancakes. At still higher levels there is a transition to a hierarchical clustering model in which a hierarchy of collapsing structures exist. The Truncated Zeldovich Approximation allowed the method to be applied to these hierarchical, or bottom-up models of cosmological structure. This approach truncates the power law fluctuation spectrum at large values of ''k'' before applying the Zeldovich Approximation. Generalizations of the Zel'dovich Approximation have also been shown to apply in the case of a non-zero

cosmological constant In cosmology, the cosmological constant (usually denoted by the Greek capital letter lambda: ), alternatively called Einstein's cosmological constant, is a coefficient that Albert Einstein initially added to his field equations of general rel ...

. The first example of a Zeldovich pancake may have been identified in 1991 using the

Very Large Array The Karl G. Jansky Very Large Array (VLA) is a centimeter-wavelength radio astronomy observatory in the southwestern United States built in the 1970s. It lies in central New Mexico on the Plains of San Agustin, between the towns of Magdalena, Ne ...

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

References

External links

* {{Cite web, last=Gough, first=Alex, date=2021-09-22, title=A universe of pancakes, url=https://astrobites.org/2021/09/22/zeldovich-pancakes/, website=Astrobites, language=en-US Large-scale structure of the cosmos 1970 in science

See also

References

External links