The origin of the universe

Early theories (origin of the earth):

• The Nebular Hypothesis: first argued by the German philosopher Immanuel Kant, revised by the mathematician Laplace in 1796. The planets formed from a cloud of material around a young, slowly rotating sun.
• In 1950, Otto Schmidt (Russia) and Carl Weizsäcker (Germany) revised it: the sun was surrounded by a solar nebula of mostly hydrogen and helium plus dust; friction and collisions made a disc-shaped cloud, and planets formed by accretion.
• Later scientists turned to the bigger question: the origin of the universe.

The Big Bang Theory (also called the expanding universe hypothesis):

• In the beginning, all matter was in one place as a 'tiny ball' (a singular atom) of unimaginably small volume, infinite temperature and infinite density.
• At the Big Bang it exploded, leading to huge expansion that continues today. The book dates it 13.7 billion years ago (the latest estimate is about 13.8 billion). Expansion was very rapid in the first fractions of a second, then slowed; some energy turned into matter.
• Within the first three minutes, the first atoms began to form.
• Within 3,00,000 years, the temperature fell to 4,500 K, atomic matter formed, and the universe became transparent.

Evidence: in the 1920s Edwin Hubble showed that galaxies are moving apart: the universe is expanding. (Hubble gave the evidence for expansion; he did not propose the Big Bang theory itself.) Expansion means the space between galaxies grows.

• The balloon model: dots drawn on a balloon move apart as it is blown up, like galaxies. It is only partly right, because the dots also grow, while real galaxies do not.
• An alternative was Fred Hoyle's steady state idea: the universe is roughly the same at any time. With growing evidence of expansion, scientists now favour the expanding universe.

From the video: the teacher warns students not to mix up the two names: Hubble supplied the evidence of an expanding universe; the Big Bang theory is the explanation built on such evidence.

Example (new): think of raisins in a cake that is rising in the oven. Every raisin moves away from every other one as the dough expands, yet the raisins themselves stay the same size, which is closer to the real universe than the balloon.