The Big Bang
The universe begins with a cataclysm that generates space and time, as well as all the matter and energy the universe will ever hold. For an incomprehensibly small fraction of a second, the universe is an infinitely dense, hot fireball. The prevailing theory describes a peculiar form of energy that can suddenly push out the fabric of space. At 10-35 to 10-33 seconds a runaway process called "Inflation" causes a vast expansion of space filled with this energy. The inflationary period is stopped only when this energy is transformed into matter and energy as we know it.
The Universe Takes Shape
After inflation, one millionth of a second after the Big Bang, the universe continues to expand but not nearly so quickly. As it expands, it becomes less dense and cools. The most basic forces in nature become distinct: first gravity, then the strong force, which holds nuclei of atoms together, followed by the weak and electromagnetic forces. By the first second, the universe is made up of fundamental particles and energy: quarks, electrons, photons, neutrinos and less familiar types. These particles smash together to form protons and neutrons.
Formation of Basic Elements
Protons and neutrons come together to form the nuclei of simple elements: hydrogen, helium and lithium. It will take another 300,000 years for electrons to be captured into orbits around these nuclei to form stable atoms.
The Radiation Era
The first major era in the history of the universe is one in which most of the energy is in the form of radiation -- different wavelengths of light, X rays, radio waves and ultraviolet rays. This energy is the remnant of the primordial fireball, and as the universe expands, the waves of radiation are stretched and diluted until today, they make up the faint glow of microwaves which bathe the entire universe.
Beginning the Era of Matter Domination
At this moment, the energy in matter and the energy in radiation are equal. But as the relentless expansion continues, the waves of light are stretched to lower and lower energy, while the matter travels onward largely unaffected. At about this time, neutral atoms are formed as electrons link up with hydrogen and helium nuclei. The microwave background radiation hails from this moment, and thus gives us a direct picture of how matter was distributed at this early time.
Birth of Stars and Galaxies
300 million years
Gravity amplifies slight irregularities in the density of the primordial gas. Even as the universe continues to expand rapidly, pockets of gas become more and more dense. Stars ignite within these pockets, and groups of stars become the earliest galaxies. This point is still perhaps 12 to 15 billion years before the present.
Birth of the Sun
5 Billion Years Before the Present (BP)
The sun forms within a cloud of gas in a spiral arm of the Milky Way Galaxy. A vast disk of gas and debris that swirls around this new star gives birth to planets, moons, and asteroids . Earth is the third planet out.
3.8 Billion Years BP
The Earth has cooled and an atmosphere develops. Microscopic living cells, neither plants nor animals, begin to evolve and flourish in earth's many volcanic environments.
Primitive Animals Appear
700 Million Years BP
These are mostly flatworms, jelly fish and algae. By 570 million years before the present, large numbers of creatures with hard shells suddenly appear.
The First Mammals Appear
200 Million Years BP
The first mammals evolved from a class of reptiles that evolved mammalian traits, such as a segmented jaw and a series of bones that make up the inner ear.
Dinosaurs Become Extinct
65 Million Years BP
An asteroid or comet slams into the northern part of the Yucatan Peninsula in Mexico. This world-wide cataclysm brings to an end the long age of the dinosaurs, and allows mammals to diversify and expand their ranges.
Homo Sapiens Evolve
600,000 Years BP
Our earliest ancestors evolve in Africa from a line of creatures that descended from apes.
Supernova 1987A Explodes
170,000 Years BP
A star explodes in a dwarf galaxy known as the Large Magellanic Cloud that lies just beyond the Milky Way. The star, known in modern times as Sanduleak 69-202, is a blue supergiant 25 times more massive than the Sun. Such explosions distribute all the common elements such as Oxygen, Carbon, Nitrogen, Calcium and Iron into interstellar space where they enrich clouds of Hydrogen and Helium that are about to form new stars. They also create the heavier elements (such as gold, silver, lead, and uranium) and distribute these as well. Their remnants generate the cosmic rays which lead to mutation and evolution in living cells. These supernovae, then, are key to the evolution of the Universe and to life itself.
Crab Supernova Appears
A new star in the constellation Taurus outshines Venus. Chinese, Japanese, and Native American observers record the appearance of a supernova. It is not, however, recorded in Europe, most likely as a consequence of lack of study of nature during the Dark Ages. The remnants of this explosion are visible today as the Crab Nebula. Within the nebula, astronomers have found a pulsar, the ultra-dense remains of a star that blew up.
Galileo Builds the First Telescope
Five years after the appearance of the great supernova of 1604, Galileo builds his first telescope. He sees the moons of Jupiter, Saturn's rings, the phases of Venus, and the stars in the Milky Way. He publishes the news the following year in "The Starry Messinger."
Isaac Newton Describes Gravity
At the age of 23, young Isaac Newton realizes that gravitational force accounts for falling bodies on earth as well as the motion of the moon and the planets in orbit. This is a revolutionary step in the history of thought, as it extends the influence of earthly behavior to the realm of the heavens. One set of laws, discovered and tested on our planet, will be seen to govern the entire universe.
Albert Einstein Publishes Theory of Relativity
The first of his many seminal contributions to twentieth century science, relativity recognizes the speed of light as the absolute speed limit in the universe and, as such, unites the previously separate concepts of space and time into a unified spacetime. Eleven years later, his General Theory of Relativity replaces Newton's model of gravity with one in which the gravitational force is interpreted as the response of bodies to distortions in spacetime which matter itself creates. Predictions of black holes and an expanding Universe are immediate consequences of this revolutionary theory which remains unchallenged today as our description of the cosmos.
Edwin Hubble Discovers that the Universe is Expanding
Edwin Hubble discovers that the universe is expanding. The astronomer Edwin Hubble uses the new 100-inch telescope on Mt. Wilson in Southern California to discover that the farther away a galaxy is, the more its light is shifted to the red. And the redder a galaxy's light, the faster it is moving away from us. By describing this "Doppler shift," Hubble proves that the universe is not static, but is expanding in all directions. He also discovers that galaxies are much further away than anyone had thought.
Discovery of Quasars
Allan Sandage and Thomas Matthews find sources of intense radio energy, calling them Quasi Stellar Radio Sources. Four years later, Maarten Schmidt would discover that these sources lie at the edge of the visible universe. In recent years, astronomers have realized that they are gigantic black holes at the centers of young galaxies into which matter is heated to high temperatures and glows brightly as it rushes in.
Discovery of Microwave Background Radiation
Scientists at the Bell Telephone Laboratories discovered microwave radiation that bathes the earth from all directions in space. This radiation is the afterglow of the Big Bang.
Discovery of Pulsars
A graduate student, Jocelyn Bell, and her professor, Anthony Hewish, discover intense pulsating sources of radio energy, known as pulsars. Pulsars were the first known examples of neutron stars, extremely dense objects that form in the wake of some supernovae. The crab pulsar is the remnant of the bright supernova recorded by Native Americans and cultures around the world in the year 1054 A.D.
Light from Supernova 1987A Reaches Earth
The light from this supernova reaches earth, 170,000 years after is parent star exploded. Underground sensors in the United States and Japan first detect a wave of subatomic particles known as neutrinos from the explosion. Astronomers rush to telescopes in the southern hemisphere to study the progress of the explosion and perfect models describing the violent deaths of large stars.
Hubble Space Telescope Launched
The twelve-ton telescope, equipped with a 94-inch mirror, is sent into orbit by astronauts aboard the space shuttle Discovery. Within two months, a flaw in its mirror is discovered, placing in jeopardy the largest investment ever in astronomy.
Big Bang Confirmed
Astronomers use the new Cosmic Background Explorer satellite (COBE) to take a detailed spectrum of the microwave background radiation. These studies showed that the radiation is in nearly perfect agreement with the Big Bang theory. Two years later, scientists used the same instrument to discover minute variations in the background radiation: the earliest known evidence of structure in the universe.
Hubble Space Telescope Repaired
Astronauts aboard the space shuttle Endeavor succeed in correcting Hubble's flawed optics, ushering in a spectacular new age of astronomy from space. Hubble's greatest legacy so far: detailed images of galaxies near the limits of the visible universe.
The Stellar Era Ends
100 Trillion Years in the Future
Astronomers assume that the universe will gradually wither away, provided it keeps on expanding and does not recollapse under the pull of its own gravity. During the Stelliferous Era, from 10,000 years to 100 trillion years after the Big Bang, most of the energy generated by the universe is in the form of stars burning hydrogen and other elements in their cores.
The Degenerate Era
100 Trillion to 1037 Years in the Future
This era extends to Ten Trillion Trillion Trillion years after the Big Bang. Most of the mass that we can currently see in the universe is locked up in degenerate stars, those that have blown up and collapsed into black holes and neutron stars, or have withered into white dwarfs. Energy in this era is generated through proton decay and particle annihilation.
The Black Hole Era
1038 to 10100 Years in the Future
This era extends to Ten Thousand Trillion Trillion Trillion Trillion Trillion Trillion Trillion Trillion years after the Big Bang. After the epoch of proton decay, the only stellar-like objects remaining are black holes of widely disparate masses, which are actively evaporating during this era.
The Dark Era
Times Later than 10100 Years in the Future
At this late time, protons have decayed and black holes have evaporated.Only the waste products from these processes remain: mostly photons of colossal wavelength, neutrinos, electrons, and positrons. For all intents and purposes, the universe as we know it has dissipated.