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Flamsteed Astronomy Society |
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Successes and Problems of Big Bang Cosmology — Prof Michael Joyce, September 23, 2005 |
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Successes of Big Bang Cosmology The Big Bang theory has been very successful in making predictions about two factors — the Cosmic Microwave Background radiation (CMB), and also the abundances of the elements observed to exist in the Universe, created in ‘nucleosynthesis’ very early in the Big Bang. The Cosmic Microwave Background (CMB) has been key in both the establishment and development of the Big Bang theory. In 1948 George Gamow and others used Big Bang theory to predict that very high temperatures from the early ‘flash’ of the Big Bang should have left an afterglow, or residue detectable today as a general background of microwave radiation appearing to come from all parts of the sky. Gamow predicted the CMB would have a temperature now of about 11˚K — it has been red-shifting to lower and lower temperatures for over 10 billion years. Further theoretical work refined the predicted temperature to about 2 to 5˚K but the CMB wasn’t actually detected until 1965 when Penzias and Wilson found it at about 3˚K by accident during tests with a Bell Labs experimental antenna. The detection of the CMB was of profound importance. It established the Big Bang theory. The rival Steady State theory couldn’t explain the CMB convincingly and gradually fell by the wayside. The second great success for the theory has been the prediction of the abundance of the elements formed by nucleosynthesis in the Big Bang itself. The theory predicts that elements would form in the proportions — 75% Hydrogen, 25% Helium, and traces of Deuterium, Lithium, and Beryllium, and this has been verified in observations so far. Just one free parameter in the theory is able to account correctly for the abundance of three elements.
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Variations in the CMB (COBE/DMR Science Team) |
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Large-scale structure in galaxy clusters(Smithsonian Astrophysical Observatory) |
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The Problems of Big Bang Cosmology Developments especially over the last 10 years however, are challenging the theory. The first area of difficulty concerns the large-scale structure (LSS) of galaxy clusters. Since the 1980s the extension of red-shift surveys has been revealing the clustering of galaxies extending up to the largest scales we can observe. Most recently the Sloan Digital Sky Survey SDSS is surveying 2 million galaxies. Galaxies cluster in ’walls’ and ’sheets’ and there are huge empty gaps — the Universe is inhomogeneous. It is not the same everywhere. How is this to be explained? Such large-scale structures must have resulted from the effects of gravity acting on small ’seed fluctuations’ in the original distribution of matter from the Big Bang. Seed fluctuations like these should have imprinted themselves on the patterns of radiation also, and those variations should be detectable in the Cosmic Microwave Background. From 1965 there was intensive work looking for fluctuations in the CMB but none was detected until 1992. Then, data collected by the COBE satellite (Cosmic Microwave Background Explorer) found fluctuations in the CMB of the order of 1 part in 100,000 — consistent with the theory. A mechanism for the formation of LSS had been verified. Since then the race has been to improve the measurement of fluctuations in the CMB and join-up the CMB and LSS data. The latest CMB data is from the WMAP satellite (Wilkinson Microwave Anisotropy Probe) which has added much to the understanding of CMB fluctuations, but the second instalment of data from WMAP is overdue. Are there difficulties? In any case, what mechanism produced the large-scale structure of galaxy clusters? Cosmologists have been developing simulations to model the clustering process, but simulations that result in anything like the structures we see today require the existence of ‘dark matter’ to produce the observed structures. Dark matter would have to be matter that we have not yet detected. It may be of three kinds — Unobserved normal or ‘baryonic’ matter (brown dwarves or ‘Jupiters’ etc) not yet observed by us. Astronomers think there cannot possibly be enough undetected normal matter to ‘balance the books’. CDM ‘Cold’ non-baryonic dark matter that exerts a gravitational influence but does not interact with E-M radiation, including light etc ‘Hot’ dark matter either in the form of neutrinos or some other exotic sub-atomic particle that will be very difficult to detect. This notion is beloved of fundamental particle physicists who welcome the possibility of convergence between the physics of the very small and the very large.
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(picture Mike Dryland) |
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