Chapter 18CosmologySlide 3Observations of the UniverseSlide 5Motion of GalaxiesAge of the UniverseAre We at the Center of the Universe?Olbers’s ParadoxSlide 10The Cosmic HorizonSlide 12The Size of the UniverseThe Cosmic Microwave Background“Stretching” RadiationSlide 16Composition of the Oldest StarsEvolution of the UniverseSlide 19Slide 20The Density of the UniverseSlide 22Slide 23A Cosmological Repulsion?Slide 25The Curvature of the UniversePositive CurvatureNegative CurvatureFlat CurvatureMeasuring the Curvature of SpaceSlide 31Slide 32Makeup of the UniverseThe Origin of the UniverseRadiation, Matter, and AntimatterHistory of the UniverseSlide 37Slide 38Slide 39Slide 40Formation of GalaxiesSlide 42The Inflationary UniverseSlide 44Chapter 18CosmologyCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Cosmology•Cosmology is the study of the structure and evolution of the Universe as a whole–How big is the Universe?–What shape is it?–How old is it?–How did it form?–What will happen in the future?Cosmology•What we seem to know now:–The Universe is expanding and is filled with a very low-energy background radiation–The radiation and expansion imply the Universe began some 13.7 billion years ago–The Universe began as a hot, dense, violent burst of matter and energy called the Big BangObservations of the Universe•In the early years of the 20th century, astronomers envisioned the Universe as a static place with only the Milky Way and a few companions•It was not until the 1920s that astronomers realized the Universe was filled with other galaxies millions of light-years apart and that the Universe was expandingObservations of the Universe•No matter which way you look (ignoring the zone of avoidance), you see about the same number of galaxies•The galaxies are not spread smoothly, but clump into groups•This “smooth clumping” implies a similar distribution for the whole Universe (contrast this with the sky’s Milky Way implying a disc-shaped galaxy)Motion of Galaxies•In general, a galaxy obeys the Hubble law: speed of recession is proportional to the galaxy’s distance, the proportionality given by the Hubble constant•The motion away is due to the expansion of space itself – not like bomb fragments going through the air, but like buttons attached to an expanding balloonAge of the Universe•Running the Universe’s expansion backward implies all mass becomes confined into a very small volume, what was once called the “Primeval Atom”•Assuming galaxies have always moved with the velocities they now have, the Hubble Law gives age for Universe of 14 billion years with H = 70 km/s/MpcAre We at the Center of the Universe?•The recession of distant galaxies often leads to the misconception that the Milky Way is the Universe’s center•However, because space is expanding, no matter where you are located, galaxies will move away from you – there is no preferred center•This lack of a preferred location is called the cosmological principleOlbers’s Paradox•In 1823, Heinrich Olbers offered Olbers’s Paradox: If the Universe extends forever and has existed forever, the night sky should be bright – but of course it isn’t•Olbers reasoned that no matter which direction you looked in the sky a star’s light should be seenOlbers’s Paradox•Resolution: Finite age and speed of light means only a finite volume of starlight is available – the night sky is darkThe Cosmic Horizon•The age of the Universe limits the distance we can see since the speed of light is finite•In a static Universe, this distance is directly determined from its age and the speed of light•The maximum distance one can see (in principal, but not necessarily in practice) is called the cosmic horizonThe Cosmic Horizon•The space within the horizon is called the visible (or observable) Universe – there may very well be more to the Universe beyondThe Size of the Universe•The distance to the cosmic horizon gives a rough measure of the radius of the (visible) Universe•For a 14 billion-year-old Universe, this radius is 14 billion light-yearsThe Cosmic Microwave Background•The proposed very-dense early Universe implied that it must have been very hot, perhaps 10 trillion K•It was proposed that as the Universe expanded and cooled, the radiation that existed at that early time would survive to the present as microwave radiation•This radiation was accidentally discovered by Arno Penzias and Robert Wilson in 1965 and has since then been referred to as the cosmic microwave background (CMB)“Stretching” RadiationThe Cosmic Microwave Background•The CMB follows a perfect blackbody spectrum with a temperature of 2.725 K (about 3 K, a bit above absolute zero)Composition of the Oldest Stars•Current theory suggests that the early Universe consisted of protons, neutrons, and electrons•The initial hot and dense state allowed nuclear reactions to create helium•Based on estimates of the early Universe’s expansion rate, about 24% of the matter should be transformed to helium – in good agreement with what is observed in old stars in the Milky Way and other galaxies•Similar measurement of deuterium (2H) and lithium also support the hot, dense early Universe ideaEvolution of the Universe•The Universe is currently expanding, but what of its future:–Will it expand forever–Will it stop expanding and collapseEvolution of the Universe•Expanding forever means that as all the stars consume their hydrogen, the Universe will become black and empty – this scenario is the open universe•A Universe that collapses as a “Big Crunch” might lead to another “Primeval Atom”, leading perhaps to the birth of another universe – this scenario is the closed universe•The expansion speed of the Universe becomes zero when the Universe has reached infinite size – this scenario is the flat universeEvolution of the Universe•The energy content of the Universe depends on what type of universe we are in–An open universe has positive total energy–A flat universe has zero total energy–A closed universe has negative total energy–In principal, if we measure the energy content of the Universe, we can tell what type it is–The energy content of the Universe is the sum of its positive kinetic energy of expansion and its negative energy of gravitational binding (basically its mass contentThe Density of the