The Universe Through Time Guest Contributors: Christine and Birgitte Reisæter Amundsen


Te universe begins 13.7 billion years ago with an event known as the Big Bang in a very high energy and immensely hot environment. Both time and space are created in this event. Te universe passes through many different epochs, where most activity and change takes place in the first second. In a tiny fraction of a second, 10-43


seconds after the Big


Bang, the cosmos goes through a superfast expansion called ‘inflation’, expanding from the size of an atom to that of a grapefruit. At 10-32


seconds (s), the universe is a seething,


hot soup of electrons, quarks, and other particles created spontaneously out of pure energy; the temperature is 1027 10-6


s and a temperature of 1013 and neutrons. By three minutes the universe is a 108 °C. At °C quarks clump into protons °C superhot


fog, and protons and neutrons combine to form the first atomic nuclei – mostly hydrogen nuclei (single protons) and nuclei of helium-4 (comprising 2 protons and 2 neutrons), along with tiny fractions of other light isotopes. Nothing much then happens as 300,000 years go by and


the universe cools to around 6,000°C (about the surface temperature of our sun). Now electrons combine with protons and neutrons to form the first atoms, 75% hydrogen and 25% helium. From 300,000 to 150 million years, the universe is literally in the Dark Age; although photons exist, no stars have yet formed to give off light. After some hundred million years, the temperature is -200°C and gravity makes hydrogen and helium gas coalesce to form giant clouds. Matter clumps together forming the first proto-galaxies and, within them, smaller dense clumps of gas start to collapse under their own gravity, becoming hot enough to trigger nuclear fusion between hydrogen atoms, thus giving birth to the very first stars. Te lights are on! Nuclear fusion in stars forms heavier elements such


as carbon, oxygen, silicon and iron. Te first stars are supermassive ones, a hundred times more massive than our sun. Tey are short-lived, however, and explode in massive supernova events creating even heavier elements, sending material into space ready to be used in future generations of stars and planets. After a few billion years more, the rate of expansion of the universe begins to accelerate, caused by a mysterious force known as ‘dark energy’, the nature of which is unknown. At nine billion years, the solar system forms. Our sun, its


eight planets, and all the asteroids, comets and Kuiper Belt objects (e.g., Pluto) and Oort Cloud objects form from the debris left behind by earlier generations of stars. Ten billion years after the Big Bang, the first life appears on Earth in the form of simple cells, perhaps after impacting comets and asteroids contributed organic molecules to Earth. Life spreads across the globe. Today, the expansion of the universe and the recycling of


star material into new stars continue. Te visible universe contains trillions of galaxies, each comprising billions of stars. Te characteristic temperature of the universe is approximately


292


-270°C, or more exactly 2.725 K (Kelvin) above absolute zero according to the latest measurements obtained by looking at the cosmic microwave background (CMB). Te ‘standard model’ of the make-up of the universe suggests that it is made of 70% dark energy that permeates all of space and tends to increase the rate of expansion of the universe; 26% dark matter, an extremely hard-to-detect unknown substance which emits no light, heat, radio waves, nor any other kind of radiation; and 4% normal matter, which is the matter we can see, account for and explain with all our experiments and instruments. Within our own Milky Way, thousands of exoplanets have been discovered orbiting other stars. From Earth, we are trying to unravel the mysteries of the cosmos, and we are asking: is anybody out there? In one to two billion years from now, the sun is significantly


brighter. On Earth a runaway greenhouse effect is triggered, and our planet is heating up. Life on Earth is impossible but luckily in the Kuiper Belt formerly icy worlds are melting, so that liquid water is present beyond the orbit of Pluto. Perhaps Eris, a trans-Neptunian object, is our new home? After five billion years, the sun enters into the red giant phase of its evolution, and it consumes Mercury, Venus and Earth, before it shrivels into a white dwarf. In seven billion years from now the Milky Way and Andromeda have merged to form a huge galaxy, with its bright core dominating the night sky. Now, the future is not so clear. Will the universe collapse and end with a Big Crunch, or expand forever, becoming increasingly cold and empty? Te Big Crunch model is a possibility if the average density of the universe is sufficiently large to stop the expansion, so that it begins the process of collapsing onto itself. In this way, the universe resets, and it may trigger the next Big Bang – like the mythical Phoenix, in death it is reborn. Te Big Freeze model is an end-result if the average density of the universe is not enough to stop the expansion and it continues to expand at an ever increasing speed. It will steadily get colder and colder until the temperature throughout the universe reaches absolute zero. Galaxies, stars and matter are pulled so far apart that the stars would eventually lose access to the raw material needed for star formation. Tis leaves the universe to its ultimate fate as cold, dead, empty space, containing only radiation. When the last star is extinguished, the lights go out for good.


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