The universe is 15 billion years old. 15 billion years ago, all the stuff in the universe was concentrated in a singularity, a mathematical concept, a speck of infinite density. It exploded. This is what astronomers call the Big Bang, the point at which time began. George Gamow coined the term.
Primordial energy and matter flew in all directions. It cooled. Gas clouds condensed into galaxies. Galaxies are aggregates of stars, the building blocks of the universe. There are 100 billion galaxies. They are distinctive. The Whirlpool and Sombrero look like works of art. Galaxies are categorized according to their structures. The Milky Way is a spiral. M87 is elliptical. The Magellanic Clouds of the Southern Hemisphere are irregular. They are satellites of the Milky Way.
The Milky Way belongs to a Local Group of 31 galaxies. The Andromeda Galaxy M31 is part of the Local Group. M31 is a spiral similar to the Milky Way but larger. It is 2.3 million light-years from us and the fartherest visible object. It is faint.
Numbers were assigned to fuzzy patches in the sky by Charles Messier, an 18th century comet hunter. He cataloged 103 objects so as not to mistake them for comets. Nebulas and galaxies were lumped together. The more thorough New General Catalog (NGC) dates from the 19th century.
Galaxies are found in clusters, and these in turn comprise superclusters. The Virgo and Coma Berenices superclusters are enormous. Our Local Group is part of the Virgo supercluster. In spite of this, the universe is mostly empty.
Proof of the Big Bang came from the work of Edwin Powell Hubble. By applying the Doppler Effect to light, Hubble found that light from galaxies shows a redshift. This suggests that galaxies are receding, travelling away from each other. This is what we mean by the Expanding Universe. If you run it backwards, there is a point at which all galaxies converge. The primeval atom! Furthermore, the farther apart galaxies get, the faster they travel. This is as Hubble's Law. The question becomes whether expansion will continue forever or whether there is enough gravity in the universe to pull it back together. This would be the Big Crunch and suggests an oscillating universe, one which alternately expands and collapses. Black holes may provide the gravity for a Big Crunch. The universe is not expanding in space. Space is being created as the universe expands. The balloon analogy is used, blowing up a balloon with dots on it to represent galaxies.
We ask what there was before the Big Bang. The answer is nothing. There was no space, no time and no events. It was the beginning in the true sense. Penzias and Wilson provided more proof of the Big Bang when they detected its background radiation.
E.P. Hubble was the greatest astronomy of the 20th century. Shapley was great but believed external galaxies were inside our own.
Cosmology was a step I was trying to take since I was a teenager. Carl Sagan's Cosmos series was a breakthrough. To paraphrase Sagan, "The Cosmos is everything that has been, everything that is and everything that will be." Sagan saw man as poised on the shore of the cosmic ocean, intelligent life as a means for the cosmos to know itself. The terms "cosmos" and "universe" are interchangible.
