Huge-LQG

http://en.wikipedia.org/wiki/Huge-LQG

The Huge Large Quasar Group, (Huge-LQG, also called U1.27), is alarge quasar group that consists of 73 quasars and measures 4 billion light-years across. Until the discovery of the Hercules–Corona Borealis Great Wallin November 2013,^[1][2][3] it was identified as the largest and the most massive known structure in the observable universe.^[4][5][6]

The Huge-LQG was discovered in November 2012 by a team led by Dr.Roger G. Clowes at the University of Central Lancashire. The astronomersused data from the Sloan Digital Sky Survey.

Contents

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• 1 Size
• 2 Cosmological principle
• 3 Criticism
• 4 See also
• 5 References
• 6 Further reading
• 7 External links

Size[edit]

The Huge-LQG is estimated to be approximately 1.24 gigaparsecs (4 billion light-years) in its longest dimension, by 640 Mpc and 370 Mpc in the others^[7] and is one of the largest known structures in the universe. It has a mass of 6.1×10¹⁸ M_☉. The Huge-LQG was initially named U1.27 due to its average redshift of 1.27, and is located in the sky in the constellation of Leo.^[8]

The Huge-LQG is 615Mpc from the Clowes–Campusano LQG (U1.28), a group of 34 quasars discovered in 1991.

Cosmological principle[edit]

Main article: Cosmological principle

The cosmological principle implies that at sufficiently large scales, the universe is approximately homogeneous, meaning that the statistical fluctuations in quantities such as the matter density between different regions of the universe are small. However, different definitions exist for the homogeneity scale above which these fluctuations may be considered sufficiently small, and the appropriate definition depends on the context in which it is used. Yadav et al have suggested a definition of the homogeneity scale based on the fractal dimension of the universe; they conclude that, according to this definition, an upper limit for the homogeneity scale in the universe is 260/h Mpc.^[9]Some studies that have attempted to measure the homogeneity scale according to this definition have found values in the range 70–130/h Mpc.^[10][11][12]

The Sloan Great Wall, discovered in 2003, has a length of 423Mpc,^[13] which is marginally larger than the homogeneity scale as defined above.

The Huge-LQG is three times longer than, and twice as wide as the Yadav et al. upper limit to the homogeneity scale, and has therefore been claimed to challenge our understanding of the universe on large scales.^[6]

However, due to the existence of long-range correlations, it is known that structures can be found in the distribution of galaxies in the universe that extend over scales larger than the homogeneity scale.^[14]

Criticism[edit]

The claim by Clowes et al. that the Huge-LQG poses a challenge to the cosmological principle has been explicitly criticised in a more recent paper,^[12] which shows that, contrary to this claim, the quasar distribution becomes homogeneous on large scales as expected from the study by Yadav et al.^[9] and that there is therefore no challenge to the cosmological principle. In addition, the algorithm used to identify the Huge-LQG also regularly produces false positive identifications of structures even in simulated homogeneous random distributions of points. It is therefore argued that the Huge-LQG does not constitute a real structure at all.

Nevertheless, Clowes et al. found independent support for the reality of the structure from its coincidence with Mg II absorbers (once-ionised magnesium gas, commonly used to probe distant galaxies). The Mg II gas suggests that the Huge-LQG is associated with an enhancement of the mass, rather than being a false positive. This point is not discussed by the critical paper.^[12]