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Children of the stars Olbers’ paradox • • • • • Why is the sky dark at night? If the Universe were static, infinite, eternal and uniformly filled with stars, and we looked it any direction, our line of sight would eventually encounter a star Stars get fainter as distance squared, but their number increases as distance squared The sky should be uniformly bright, just like the surface of a star, but it is obviously not! The reason why the sky is dark is that the Universe is not infinitely old, or that the look-back time is greater than the age of the observable Universe Expansion of the Universe • Around 1920, Edwin Hubble discovered that most nebulae are galaxies similar to ours and that they run away from each other • This is, in fact, an expansion of space itself. The larger the distance between two galaxies the faster they recede from each other, as described by the Hubble law • Uniform expansion also means that there is no center to it! Geometry of space • • • • Geometry of the Universe is described with the aid of Einstein’s General Relativity (GR) GR assumes that the presence of mass curves space around it and we sense this effect as gravity GR also assumes that the Universe has a curvature that determines its geometry and expansion Curved space can be finite but have no bounds. This can be explained by the example of a twodimensional creature living on a three-dimensional surface • The Big Bang theory predicts that the hot gas left over from the time of creation of the Universe should be observable as a leftover background radiation coming from all directions Because the Universe expands, the CBR should be observable in infrared and at radio wavelengths and have a temperature of only ~ 3 K The CBR was accidentally discover by Penzias and Wilson in mid-nineteen sixties and then very accurately measured and mapped by the orbiting Cosmic Background Explorer (COBE) The CBR temperature and other characteristics follow theoretical predictions extremely well CBR is commonly interpreted as radiation from the Big Bang The cosmic background radiation (CBR) • • • • The Big Bang • • • • The Universe was dominated by radiation over the first million years. Free electrons from ionization by gamma rays made matter opaque for photons After 1 million years, the Universe became cool enough for neutral atoms to form. The gas became transparent for radiation in this recombination phase At this time, the temperature of the Universe was ~3000 K. We observe photons from this epoch as the CBR After recombination, the Universe became matter dominated and first galaxies were formed out of gas condensations Children of the stars, children of the Universe… • 10-5 sec after the Big Bang, the Universe was filled with gamma-ray photons at well over 1 trillion K and at density comparable to that of an atomic nucleus. Photons produced particles of matter and antimatter which annihilated and • • • became photons again Protons, neutrons and electrons that matter is made of were produced in the first 4 sec of the Universe’s history. H and He were made by the end of the third minute. Only some lithium (Li) and beryllium (Be) was made because stable elements with atomic weights of 5 and 8 do not exist In 30 min., the Universe expanded and cooled enough, so that all nuclear reaction stopped. All elements heavier than Be were made later by nuclear fusion in stars or supernova explosions