Time Scale 1

13.7 billion to 1 billion years ago

This is an image of some of the farthest galaxies ever seen from a time when the universe was about 400 million years old. Using NASA’s Hubble Space Telescope, the image shows some of the first galaxies that were formed shortly after the big bang occured.   

Credit: Photo by NASA/ESA/S. Beckwith(STScI) and The HUDF Team, Courtesy of NASA>

How did the universe come to be? When did this happen? For most of us, the processes that created the universe are so foreign and bizarre to our everyday experiences that they stretch our imaginations to their limits. Physics, astronomy, and mathematics have provided most of the evidence for the story of our cosmic genesis.

The story begins some 13.7 billion years ago with the “Big Bang”, where the entire contents of the universe emerged from nothingness. Exactly how and why this came to be is not known. Our universe is believed to have been originally compressed into a single point smaller than the size of an atom, a point of unimaginable heat and density. Matter, energy, time, and space were created in that first millisecond of the “Big Bang”. This was followed by a rapid expansion of the universe, a primordial, cosmic plasma “soup” in which a battle ensued between matter and antimatter particles annihilating each other into oblivion. A small amount of that matter won out, and makes up the ‘stuff’ of the world we know.

Over the course of billions of years, as the very early universe continued to expand and cool, new components of the universe appeared in sequence: subatomic particles, simple atoms (e.g. helium and hydrogen), clouds of gas (nebulae), and then stars. Gravity continued to cluster stars into galaxies, and hypernovae explosions produced heavier elements, scattering them through the universe.

Among the trillions of stars that formed, an enormous grouping of over 200 billion stars formed, our Milky Way galaxy. One star on an outer arm of the Milky Way is our sun. As our sun coalesced from its solar nebula about 4.6 billion years ago, the planets of our solar system -including the earth – began to form from the accumulation of gas and dust into small particles. Around 4.5 billion years ago, the early earth was catastrophically struck by a Mars-sized planet – tilting the earth on its axis, and, from the debris, creating the moon.

During the next several hundred million years, the earth developed its liquid oceans, rocky continents, and a primitive, oxygen-poor atmosphere. Out of these conditions the earliest life emerged, almost certainly in the oceans. The earliest life forms were simple, single-celled organisms with DNA that could reproduce. By 3.5 billion years ago, cyanobacteria, simple life forms without a nucleus, were producing oxygen through photosynthesis, converting our atmosphere to one that ultimately supported more complex life, including cells with a nucleus and, ultimately, sexual reproduction.


Event 1

13.798 billion years ago

13.7 billion year old temperature fluctuationsSome 13.798 billion years ago, our universe (matter, energy, space, and time) came into existence. Exactly how and why this happened is not really understood. It is believed that all of the matter that now exists in the universe was originally compressed into a point smaller than the size of an atom called a singularity, which rapidly expanded. This expansion, first faster than the speed of light, has been dubbed “The Big Bang.” The matter of the universe first consisted of a superhot plasma from which the first subatomic particles formed. As the universe expanded, in its first few minutes, it began to cool, forming the first primeval atoms: hydrogen, helium, and lithium.

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Event 2

13.5 billion years ago

First StarsSoon after the Big Bang (in cosmic terms), perhaps 200 million years later, the matter in the universe had expanded and cooled, and began to coalesce by gravity into denser clouds of gas. As these clouds of gas collapsed even further, they reached sizes, densities, and temperatures large enough to begin nuclear reactions and ignite to become true stars.


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Event 3

13.4 billion years ago

Crab nebulaAncient alchemists dreamed of converting simpler materials into gold (discovering the so-called “philospher’s stone” they thought could accomplish this). Although alchemists were never able to do this, such alchemy happens in the universe all the time. The earliest stars were composed only of the lightest, simplest elements of hydrogen and helium and smaller amounts of deuterium and lithium. It is thought that many of these stars were of enormous size, some hundreds of times larger than our sun. All stars burn through nuclear fusion. Stars smaller than our sun only produce helium from hydrogen in this fusion process. Somewhat larger stars go on to produce heavier elements through fusion, including elements such as carbon, nitrogen, oxygen, sodium, aluminum, silicon, potassium, calcium and iron. The stellar giants (stars over ten times the mass of our sun) tend to have a relatively short life before they explode (such explosions are called “supernovae”).  During this cataclysmic end, these stars producing all of the heaviest elements found in nature (such as nickel, copper, zinc, silver, gold, mercury, lead, and uranium), seeding the universe with clouds (nebulae) of the dust of these heavier elements.

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Event 4

13 billion years ago

The Milky Way, with our Sun's locationStars, attracted to other stars by gravity, formed clusters of stars that formed gigantic spinning discs called galaxies that are usually spiral-shaped (like our own Milky Way) or more elliptical (football-shaped), possibly formed by the merging of two galaxies or more. It is believed that the first galaxies formed around 13 billion years, and the Milky Way may be this old. It now appears that most if not all galaxies contain a supermassive black hole at their center, and there is a growing suspicion that these galaxies and black holes may be necessarily linked.

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Event 5

4.6 billion years ago

Moon formation (possibly how our moon was formed)Around 4.6 billion years ago, our solar dust cloud (nebula) began to be compressed, probably from a supernova explosion from another star system. As this nebula collapsed, this dust contracted to form a large spinning flattened disk of very hot gas, with a bulge at the center that would ultimately become our sun. The outer layer of gas (the “accretion disk”) cooled off sufficiently that solid matter starts to condense into very small particles of metal and then rock. These dust particles accumulated by gravity into larger and larger chunks of matter, called planetoids. At a certain size (perhaps tens of miles in diameter), heat from countless impacts and the additional heat of radioactive decay of matter inside these bodies totally melted them. Strong gravitational forces quickly concentrated most of the matter at the core of this spinning disk of gas and particles, about 99 percent, into our local star, the sun. The sun became a giant nuclear fusion reactor and ignited, generating enormous amounts of heat and light. The residual matter of the solar system, formed from the particles in the outer accretion disk, eventually became the planets, the moons, the asteroids, the meteors, and the comets. In the early solar system, these bodies were like giant billiard balls, often colliding, sometimes merging, sometimes breaking apart. Our moon was almost certainly the product of one of these massive collisions.

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Event 6

4.5 billion years ago

Formation of Earth's ContinentsIt is likely that after a hard rocky crust formed on the earth it was periodically re-melted from impacts with other large planetary bodies in the early solar system. Finally, the earth hardened into rocky plates that continued to move through the course of the earth’s history. The core of these plates, called cratons, became the cores of the continents, composed of lighter granites riding over the heavier basalt oceanic crust. Much of the water that made up the oceans is believed to have come from comets impacting the early earth as well as water vapor produced by early volcanoes. It is believed that the earth’s early atmosphere contained almost no free oxygen (compared to about 20 percent today), but was composed primarily of water vapor (about 80 percent), carbon dioxide, sulphur dioxide, carbon monoxide, sulphur, chlorine, nitrogen, hydrogen, ammonia, and methane. (For comparison, our modern atmosphere is composed primarily of nitrogen (78 percent) and oxygen (21 percent).

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Event 7

3.8 billion years ago

DNA moleculeThe most remarkable event in the history of our Solar System was the emergence of Life. How self-replicating organisms emerged from an inorganic world is one of the great mysteries of science. All known modern life forms (about 1.75 million species) reproduce, grow, eat, excrete, breathe, and usually move. We humans are just one of these species of life, all of which are carbon-based.

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Event 8

3 billion years ago

CyanobacteriaAll over the world, starting about 3 billion and peaking about 2.3 billion years ago, something remarkable happened. The earth rusted. Amazingly, the tiny iron minerals in the once-stagnant oceans rusted or oxidized with the addition of oxygen to these seas, forming iron oxide. And that oxygen was apparently produced by lowly single-celled bacteria!


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Event 9

2.2 billion years ago

Imagine a world so cold that all of the surface water of the oceans froze solid, the entire earth covered with ice. The average temperature would have been 74 degrees below zero, with temperatures at the equator as cold as Antarctica is today. This may have happened at least three times in the earth’s history, first at around 2.2 billion years ago, then at 710 million years ago, and again about 640 million years ago. Simple forms of life could have survived in the seas below the ice layer. These super-cold phases may have been caused by a periodic reduction in “greenhouse gases” (carbon dioxide and methane) in the atmosphere (such gases tend to warm the planet, so a reduction of these gases would cool the planet), combined with the fact that the sun was also dimmer in the past. Such extreme super-cold phases may have prevented more advanced forms of life to evolve during the first few billion years of the earth’s history.

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Event 10

1 billion years ago

Sponge or Spongia officinalisWe belong to the kingdom Animalia, better known as the animals. Unlike plants, animals usually move around freely and find and ingest their food for energy (rather than relying on the sun for photosynthesis). Animals are multicellular organisms (having many cells), and are eukaryotes (each cell having a nucleus.) It is thought that the earliest animals, as well as sexual reproduction, first emerged around 1 billion years ago. Tracks and burrows in rocks of this age suggest that some of the earliest multicellular animal life (the Metazoa) were worm-like, and would have been the most complex form of life at the time.

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