Zeldovich pancake

Short description: Theoretical condensation of gas out of a primordial density fluctuation following the Big Bang

A Zel'dovich pancake is a theoretical condensation of gas out of a primordial density fluctuation following the Big Bang. In 1970, Yakov B. Zel'dovich showed that for an ellipsoid 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.[1] This approximation assumes that the ellipsoid 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 atoms.[2]

In 1989, Zel'dovich 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".[3] This model became known as a top-down model of galactic formation, with the supergalactic condensation fragmenting into protogalaxies.[4] The formation of plane concentrations would compress the gas through shock waves generated during the collapse, increasing the temperature.[5]

At a higher level, the collapse of larger structures according to the Zel'dovich 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 Zel'dovich 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 Zel'dovich Approximation.[6] Generalizations of the Zel'dovich Approximation have also been shown to apply in the case of a non-zero cosmological constant.[7]

The first example of a Zel'dovich pancake may have been identified in 1991 using the Very Large Array in New Mexico.[8][9]

## See also

* Astronomy:Baryon acoustic oscillations - Fluctuations in the density of the normal matter of the universe
* Galactic orientation - Spatial orientation of a galactic plane
* Astronomy:Lyman-alpha blob - Astronomical object type

## References

1. ↑ Longair, M. S. (2007). Galaxy Formation. Springer. pp. 475–476. ISBN 978-3-540-73477-2.
2. ↑ Zeldovich, Y. B. (1970). "Gravitational instability: an approximate theory for large density perturbations". Astronomy and Astrophysics 5: 84–89. Bibcode: 1970A&A.....5...84Z.
3. ↑ Shandarin, S. F.; Zeldovich, Ya. B. (1989). "The large-scale structure of the universe: Turbulence, intermittency, structures in a self-gravitating medium". Reviews of Modern Physics 61 (2): 185–220. doi:10.1103/RevModPhys.61.185. Bibcode: 1989RvMP...61..185S.
4. ↑ Subramanian, Kandaswamy; Swarup, Govind (1992). "A cluster of protogalaxies at redshift 3.4?". Nature 359 (6395): 512–514. doi:10.1038/359512a0. Bibcode: 1992Natur.359..512S.
5. ↑ Sunyaev, R. A.; Zeldovich, Ya. B. (August 1972). "Formation of Clusters of Galaxies; Protocluster Fragmentation and Intergalactic Gas Heating". Astronomy and Astrophysics 20: 189–200. Bibcode: 1972A&A....20..189S.
6. ↑ Pauls, Jennifer L.; Melott, Adrian L. (May 1995). "Hierarchical pancaking: why the Zel'dovich approximation describes coherent large-scale structure in N-body simulations of gravitational clustering". Monthly Notices of the Royal Astronomical Society 274 (1): 99–109. doi:10.1093/mnras/274.1.99. Bibcode: 1995MNRAS.274...99P.
7. ↑ Barrow, John D.; Goetz, Guenter (September 1989). "Newtonian no-hair theorems". Classical and Quantum Gravity 6 (9): 1253–1265. doi:10.1088/0264-9381/6/9/010. Bibcode: 1989CQGra...6.1253B.
8. ↑ Wilford, John Noble (1991-12-10). "Giant 'Pancake' Is Clue To Origin of Universe". The New York Times. https://www.nytimes.com/1991/12/10/science/giant-pancake-is-clue-to-origin-of-universe.html?sec=health&spon=&pagewanted=all.
9. ↑ Uson, Juan M.; Bagri, Durgadas S.; Cornwell, Timothy J. (1991-12-09). "Radio detections of neutral hydrogen at redshift Z=3.4". Physical Review Letters 67 (24): 3328–3331. doi:10.1103/PhysRevLett.67.3328. PMID 10044706. Bibcode: 1991PhRvL..67.3328U.

## External links

* Gough, Alex (2021-09-22). "A universe of pancakes" (in en-US). https://astrobites.org/2021/09/22/zeldovich-pancakes/.

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