Large Quasar Group (LQG)
An international team of astronomers, Using data from the Sloan Digital Sky Survey, has found the largest known structure in the universe - large quasar group (LQG). This LQG is so large that it would take a vehicle travelling at the speed of light some 4 billion years to cross it. For comparison, our own galaxy, the Milky Way, is just a hundred thousand light-years across, while the local supercluster of galaxies in which it's located, the Virgo Cluster, is only a hundred million light-years wide.
Cosmological Principle
The usual models of the Universe in cosmology are built on the assumption of the cosmological principle – assumption of homogeneity after imagined smoothing on some suitably large scale. Models depend on the Robertson–Walker metric assumes the homogeneity of the mass–energy density. Further, assumes that any property of the Universe ultimately depends on the mass–energy content then homogeneity naturally asserts that any global property of sufficiently large volumes should be the same within the expected statistical variations.
The Milky Way, our galaxy, is separated from its nearest neighbour, the Andromeda Galaxy, by about 0.75 Megaparsecs (Mpc) or 2.5 million light-years. Whole clusters of galaxies can be 2-3 Mpc across but LQGs can be 200 Mpc or more across. Based on the Cosmological Principle and the modern theory of cosmology, calculations suggest that astrophysicists should not be able to find a structure larger than 370 Mpc.
The new discovered LQG has a typical dimension of 500 Mpc, membership of 73 quasars and mean redshift z¯=1.27. In terms of both size and membership, it is the most extreme LQG found in the DR7QSO catalogue for the redshift range 1.0 ≤ z ≤ 1.8. But because it is elongated, its longest dimension is 1240 Mpc (or 4 billion light years) - some 1600 times larger than the distance from the Milky Way to Andromeda.
Implications
An international team of astronomers, Using data from the Sloan Digital Sky Survey, has found the largest known structure in the universe - large quasar group (LQG). This LQG is so large that it would take a vehicle travelling at the speed of light some 4 billion years to cross it. For comparison, our own galaxy, the Milky Way, is just a hundred thousand light-years across, while the local supercluster of galaxies in which it's located, the Virgo Cluster, is only a hundred million light-years wide.
Cosmological Principle
The usual models of the Universe in cosmology are built on the assumption of the cosmological principle – assumption of homogeneity after imagined smoothing on some suitably large scale. Models depend on the Robertson–Walker metric assumes the homogeneity of the mass–energy density. Further, assumes that any property of the Universe ultimately depends on the mass–energy content then homogeneity naturally asserts that any global property of sufficiently large volumes should be the same within the expected statistical variations.
The Milky Way, our galaxy, is separated from its nearest neighbour, the Andromeda Galaxy, by about 0.75 Megaparsecs (Mpc) or 2.5 million light-years. Whole clusters of galaxies can be 2-3 Mpc across but LQGs can be 200 Mpc or more across. Based on the Cosmological Principle and the modern theory of cosmology, calculations suggest that astrophysicists should not be able to find a structure larger than 370 Mpc.
The new discovered LQG has a typical dimension of 500 Mpc, membership of 73 quasars and mean redshift z¯=1.27. In terms of both size and membership, it is the most extreme LQG found in the DR7QSO catalogue for the redshift range 1.0 ≤ z ≤ 1.8. But because it is elongated, its longest dimension is 1240 Mpc (or 4 billion light years) - some 1600 times larger than the distance from the Milky Way to Andromeda.
Implications
- The occurrence of structure on Gpc-scales from the Huge-LQG and from galaxies implies that the Universe is not homogeneous on these scales.
- The Huge-LQG would also indicate that there is more dark matter in some directions than in others. Such mass concentrations could conceivably be associated with the cosmic (dark) flows on the scales of ∼100–1000 Mpc. Of particular interest is the possibility raised by Tsagas (2012) that those living within a large-scale cosmic flow could see local acceleration of the expansion within a Universe that is decelerating overall.
