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Unformatted text preview: Recap Big Bang Recap Big Bang What are the 3 main experimental observations which support the Big Bang model ? Explain the effect of the hyperweak force in the first fractions of a second of Big Bang ? When did the atoms form ? When did the galaxies form ? Recap the Creation of the Universe Recap the Creation of the Universe What are the two possible scientific answers to the question “How was the Universe created ?” Explain how matter appeared in the Universe. How do you answer the question “How many Universes are there ?” The Fate of the Universe The Fate of the Universe Possible Universe Models Observing the Furthest Galaxies Galactic Masses Invisible Masses Dark energy The history of the Universe Possible Universe Models Possible Universe Models Two forces at work: the Big Bang expansion and the gravitational attraction of the galaxies Three possible models: Open Flat Closed and Pulsing (or Oscillating) Possible Universe Models Possible Universe Models
space constant expansion rate Open Flat Big Bang Closed age of the Universe time Big Crunch Hubble Diagram Revisited (I) Hubble Diagram Revisited (I)
The difficulty in “closed” U. using Hubble “open”U. diagrams to decide the age of the Universe is related “flat” U. to the inaccuracy (critical density) of the “cosmic yardstick”. Today’s estimates favor an “open” Universe. Galaxies distance Galaxies speed Observing the Furthest Galaxies Observing the Furthest Galaxies The Cepheids Type Ia supernovas The Cepheid variable stars are aged stars are known to brighten and dim regularly in a way which reveals their instrinsic brightness and implicitly their distances. Cepheid stars are visible only at relatively small distances (best bellow 80 million lightyears). provide a more powerful phenomenon which has a brightness independent of the galactic size. They occur in binary star systems approaching their end of life. If one star reaches the white dwarf stage and the other the red giant stage, the white dwarf will pull material to grow its size. When it reaches 1.4 solar masses it explodes. This mechanism always happens the same way in every galaxy. 1992 Alan Sandage and his collaborators at the Space Telescope Institute in Baltimore use Cepheid and type Ia supernova observations in the galaxy IC 4182 to calibrate the supernovabased “yard stick”. They gave the Universe an age around 15 billion years, in agreement with the fact that the age of some stars was close to 14 billion years. From Closed to Open Universe From Closed to Open Universe The answer on the fate of the Universe relies on observations on the furthest galaxies at over 10 billion light years from us. In 1980s Sandage and his collaborators favored a “closed Universe” model based on the belief that the gravitational attraction of the galaxies will at some point reverse the expansion. He proposed a “Pulsing Universe” model, with a 40 billion interval between two Big Bangs. Since 1998 the general belief shifted towards an “open Universe”, which has currently has an accelerated expansion. In that year two groups, the HighZ Supernova Search Team and the Supernova Cosmology Project, have analyzed a total of 58 very remote type IA supernovas and have concluded that for the last 6 billion of years the universal expansion was actually speeding up. Galactic Masses Galactic Masses The answer to the question of the closeness of the Universe could also come from measurements of the galactic masses. If these masses are large the average density of the Universe could be higher than the “critical” density, needed to close the Universe. The mass of a typical spiral galaxy is about 109 solar masses. This result, based on brightness observations, is not too accurate for non spiral galaxies. A better method is to calculate galactic masses from the observations on their dynamics. Our Milky Way rotates around its axes in about 200 million years and its mass is about 5 times bigger than the mass of its stars. A lot of work was done and today we have complete galactic masses studies for galaxies of various types, sizes and brightness. Those studies were then combined with studies of the intergalactic distances and lead to 1031 grams per cubic centimeter (31 orders of magnitude smaller than the density of water). That result is about 0.7% of the critical density, that would close the Universe. Intergalactic material Invisible Matter Invisible Matter Neutrinos Using hydrogen’s characteristic 21 cm radiation, one can map not only our galaxy but also some intergalactic clouds approaching the Milky Way. X ray observations confirmed the existence of the intergalactic material which can be charged and moving at high speeds. In addition to the intergalactic clouds it is conceivable that the Universe contains plenty of isolated black dwarfs, neutron stars and black holes. Photonic and neutrinic radiation is dense. While photons have zero mass, neutrinos seem to have a small but significant mass. V.Liubimov in Russia and F.Reines in U.S. lead groups which in 1975 announced a neutrino mass between 2535 eV. That in itself would make the invisible mass 19 times larger than the visible mass ! The existence of other elementary particles, such as the supersymmetric particles, is still a valid speculation. “Dark” Energy The latest estimates support a model where the visible and invisible matter account for 2740% of the critical density of the Universe. Knowing that the Hubble diagram measurements indicate a density close to the critical density, the dominant component in the mass in this Universe is a mysterious dark energy. Explaining dark energy: Whatever it is, the new component of the matter/energy of the Universe must be dark and gravitationally repulsive (it does not influence individual galaxies motion). The existence of this dark energy would make the Universe older than without it. the vacuum energy (with virtual particles production) or on the quintessence theory (A.Linde’s chaotic inflationary model). still far from a precise explanation Short History of the Universe Short History of the Universe The radiative stage The inflationary mechanism A few tens of thousands of years old Universe is dominated by ponderal matter. For about 9 billion years the Universe expanded under the original push of the Big Bang and slowed down by the intergalactic gravitational attraction. In the last 6 billion years as the vacuum between the clusters of galaxies increased the dark energy became dominant and dictated an enforced acceleration. The fate of the Universe is different in the vacuum or in the quintessence scenario. New observations are needed to decide which model provides the correct scenario. In the Superstring Cosmology one hasto also account for the fact that by increasing the size of the 5dimensional bubble, in time, all galaxies will become invisible from our glaxy. Einstein’s Cosmological Constant The main theoretical model in cosmology is Einstein’s general relativity. When he formulated it he introduced a cosmological constant which was adjusted to make the Universe stationary. When observations showed that the Universe is actually expanding, Einstein declared that the cosmological constant was his greatest “blunder”. However, his constant returned as a possible explanation for the “missing mass” and for the distribution of galaxies. Since 1998, we know that the universal expansion is accelerating in agreement with the existence of a non zero cosmological constant (see C.Hogan et al in Scientific American, January 1999). The cosmic “constant” changes its value in time. ...
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