Cosmology the Formation and Evolution of Galaxies

Cosmology

The Formation and Evolution of Galaxies

One of the greatest and possibly most important questions about our universe is how it originated. Astronomer Sandy Faber of California's Lick Observatory states that, "Galaxies are the building blocks of the universe, and therefore one of the greatest questions of modern astronomy is to understand how they form." (Chaikin)

Galaxy is usually understood as a cluster or aggregate of stars - combined gas and dust - held together by gravity. Galaxies come in three main types: ellipticals, spirals, and irregulars. (The Galaxies) There are a number of theories about the formation of galaxies. Two predominant theories are the Bottom-Up and Top-Down theories. The Bottom-Up theory states that the Universe started from small particles of mass that came together to form galaxies, which evolved into clusters and then super-clusters. (ibid) The second theory - the Top-Down theory - states that "vast pancaked-shaped clouds of matter provided the seeds for the formation of galaxies." (ibid) This theory is supported by the fact that,

In December 1991, radio astronomers in New Mexico reported the detection of a huge mass of primordial hydrogen gas near the edge of the observable universe that has the structure of a flat disk or pancake, with a mass of 500-trillion solar masses and measuring 5-million light years across." (ibid)

However, Astronomers are uncertain about the sequence of events that preceded the formation and evolution of galaxies. There is controversy about which came first - black holes or galaxies and clusters. While some scientists are of the opinion that black holes appeared at the centre of existing star groups, others think that the black holes formed first and drew stars around them.

Black holes are "objects with a gravitational field so strong that its escape velocity exceeds the speed of light. In other words, a black hole is an object whose mass and size are such that nothing, not even light, can escape its gravity." (Wikipedia: Black Holes) In support of the black hole theory, there is evidence that the oldest clusters of stars were formed by the action of black holes.

Globular clusters contain the oldest known stars in the Universe - M15 is 13-billion years old; G1 is at least 10-billion years. The presence of black holes at their centre supports the idea that black holes came first, says astrophysicist Martin Ward of the University of Leicester, UK. (New class of black holes)