The universe is 14 billion years (giga years: Gy) old. Eric Chaisson has divided the history of the universe into seven epochs; we are calling them ages. The seven ages are shown below along with a characteristic symbol for each age drawn by Mrs. Chaisson. [My: mega (million) years, Gya: giga (billion) years ago]

Any history can be divided into arbitrary number of ages. History of the Western Christian World is usually divided into three main ages: ancient, medieval and modern. Indian or Islamic history could be divided into other ages or epochs. The number of divisions and the name of each division largely depend on the taste and preferences of the community who are adopting that division. So the seven ages we are dealing with here are not written in stone like the Ten Commandments. You could prefer a different taxonomy. I like the seven ages, personally, because it is poetic. It reminds me of the seven ages of human life described by Shakespeare and the seven heavens mentioned in many religious texts. And I could also divide the course of the Brahmaputra river in exactly seven stages using a reasonable dose of wishful thinking.

It is difficult to characterize the seven ages of the universe and even more difficult to delineate one from the other. The division, first of all, is anthropocentric because the history culminates in the emergence of human culture. But maybe millions of other intelligent cultures emerged and got extinct in the past and maybe there are many other cultures like us right now around other stars and in other galaxies. We do not know. The point of this division is not to say that this is the only history of the universe, all we are saying is that this is our history.

This history is a story of evolution, that is the history of the emergence of more and more complex structures and entities in the last 14 Gy. Initially there were only particles in a gaseous conglomeration during the Particle Age. The gas clouds condensed here and there to give rise to the first galaxies during the Galactic Age. Within each galaxy many more condensations of clouds gave rise to countless stars during the Stellar Age. And the molecular gas and dust leftover from the formation of each star created planetary systems around almost every star during the Planetary Age.

Note that the galactic, stellar and planetary ages make sense only in the context of our history, that is the history of human beings. Our galaxy formed 13 Gy ago, our sun formed around 5 Gy ago, our planet formed around 4.5 Gy ago. One age is coming after another. But of course many more galaxies formed before or after the formation of our galaxy. And countless stars and planets are forming even as we speak.

These three ages are shown in the context of the whole universe in this figure. Trillions of galaxies formed from the particles within the first few billion years of the universe. The galaxies are arranged in a cosmic web. The nodes of the web have many more galaxies than the filaments. Only one such galaxy is our galaxy located in a supercluster of galaxies. The first inset is thus representative of the Galactic age. The second inset shows only our galaxy with the position of the sun specified. Finally the third inset shows a close-up of our planetary system.

Our earth is the third distant planet from the sun. Complex chemical processes took shape on the surface and oceans of this planet between 4–5 Gya during the Chemical Age. Life emerged from complex chemistry around 3.5 Gya during the biological age. Finally , the Cultural Age begins with the emergence of human-like animals around 1 Mya, although anatomically modern humans emerged only around 300 ky ago.

Try to relate these seven ages with the concept of entropy and evolution. One age is more complex than the preceding age. Galaxies are more complex (ordered) than random particles, stars more complex than galaxies, planets more complex than stars, chemical processes more complex than geological processes, biological processes even more complex and, finally, cultures are the most complex of all. The evolution of local and temporary complexities in the universe supports the global and permanent increase of entropy.