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Transcript
Lecture 29 Cosmology December 14, 2015 1 2 Structure of the Universe • Does clustering of galaxies go on forever? – Looked at very narrow regions of space to far distances. – On large scales the universe appears homogeneous (smooth, same in all directions) Figure 26.1, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall 3 Cosmological Principle • The universe looks homogeneous on the largest scales • The universe is isotropic • Both are assumptions about the universe 4 Implications • The universe can have no edge • The universe can have no center 5 Obler’s Paradox • Assume – cosmological principle – universe is infinite – universe is unchanging in time. • Results – The entire sky should be bright (BUT this is not observed!!) Figure 26.3, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall 6 Motions of Stars • Radial Motion (Radial Velocity) – Motion along the line of sight – Measured using Doppler shift of spectral lines. – Can measure for very distant stars • Transverse (Proper) Motion – Motion of star across the sky. – Measured by observing change in position of star over a long period of time. – Only detectable for nearby stars. Figure 17.4, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall 7 The Expanding Universe • The universe is not unchanging over time, but is expanding – Primary evidence comes from Hubble’s Law • Hubble’s Law – The further away an object is, the faster it is moving away. – predicts universe had a beginning ~13 billion years ago 8 • How does the expanding universe solve the paradox? – We can see only a finite distance (~13 billion light years). – For more distant objects the light has not had time to reach us. – Edge of universe (from our perspective) is the cosmic light horizon 9 Why does the observable universe have an “edge”? A. We can only see as far as light has traveled over the 100% age of the universe. B. The density of neutrinos at the “edge” becomes so large that photons cannot penetrate this barrier. C. Dust prevents us from seeing beyond a certain distance. D. There are so many galaxies in the universe that they block our view. 0% A. B. 0% C. 0% D. 10 The Big Bang • Universe starts out almost infinitely small • Goes through period of inflation, increases size by factor of 1050 11 Inflation Explains Why the Universe Appears Flat • The universe does not appear to be curved. • Perhaps it is so large that we cannot observe the curvature at this time. Figure 27-3, Freedman, Geller, Kaufmann 9th ed. Universe, © 2011 W. H. Freeman and Company 12 From what evidence do astronomers deduce that the Universe is expanding? A. They can see the disks of galaxies getting 93% smaller over time. B. They see a redshift in the spectral lines of distant galaxies. C. They detect cosmic background X-ray radiation. D. They can see distant galaxies dissolve, pulled apart by the expansion of space. 5% 2% 0% 0% E. All of the above. A. B. C. D. E. 13 What is meant by inflation in the early Universe? A. The force of gravity suddenly grew stronger in the 95% distant past. B. Protons expanded to the size of stars, which was how our Sun formed. C. The Universe increased dramatically in size in an extremely brief period of time. D. The number of galaxies that we see at large distances is much greater than the number we can see near to us. 5% 0% 0% E. The diameter of distant galaxies is much greater0% than the diameter of galaxies near to us. A. B. C. D. E. 14 Evidence for the Big Bang • Universe would be very hot (> 1 trillion K) right after the Big Bang (photon dominated). • Early universe would emit black body radiation. • As universe expanded, wavelengths were stretched • Radiation would appear much cooler = Cosmic Microwave Background Radiation. Figure 26.6, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall 15 Cosmic Microwave Background • CMB has now cooled to about 3 K Figure 17.7, Arny and Schneider, 5th ed. Explorations, © 2008 The McGraw-Hill Companies 16 Wilkinson Microwave Anisotropy Probe (WMAP) • Launched June 2001 • Measured fluctuations in Cosmic Microwave Background Image by NASA/WMAP Science Team 17 Temperature and Density Fluctuations of Early Universe Image by NASA/WMAP Science Team 18 What is meant by the cosmic microwave background? A. It is radiation from distant quasars. 91% B. It is radiation from hot gas in intergalactic space. C. It is radiation from the first stars formed when the Universe was young. D. It is radiation created during the early days of the Universe. E. It is the explanation of Olbers’ paradox. 4% 4% 2% 0% A. B. C. D. E. 19 History of Universe 20 Creating Matter • Universe is initially dominated by photons • Matter and anti-matter created via pair production. – Most pairs annihilate each other. – Inflation is so fast that some pairs are separated – But more matter than anti matter survives – Why? • By 300,000 yrs from Big Bang, universe becomes matter dominated. 21 Galaxies Form • Galaxies form from density fluctuations in the early universe. – Early universe would have been very uniform. – Minor fluctuations in density form stars and galaxies. – First stars form ~ 100 to 400 million years after Big Bang 22 Formation of Large-Scale Structure Figure 18.14 Bennett et al, The Essential Cosmic Perspective, 7th ed. (Pearson, 2015) 23 Small irregularities in the cosmic microwave background are important because A. they show us that the Big Bang explosion was very nonuniform. 63% B. they were the seeds of the quasars we can still see today. C. they are thought to have led to the creation of galaxy 38% superclusters. D. they are thought to contain most of the elusive “missing matter.” 0% A. B. 0% C. D. 24 Results from WMAP • Universe is “flat” (parallel lines never meet) • Universe is composed of: – Baryonic matter (matter made of atoms) – Cold dark matter (massive particles as yet undiscovered) – “Dark Energy”???? • Universe is 13.7 billion years old Figure 18.17 Bennett et al, The Essential Cosmic Perspective, 7th ed. (Pearson, 2015) 25 Is the Universe Accelerating? • Recent observations of supernova indicate that the universe may be accelerating instead of slowing down. • Universe may expand forever. Figure 17.10, Arny and Schneider, 5th ed. Explorations, © 2008 The McGraw-Hill Companies 26 How Will the Universe Evolve? • Depends on density (gravitational pull) and velocity of galaxies. • Critical density = density of universe needed to stop expansion 27 Future of Universe Figure 18.15 Bennett et al, The Essential Cosmic Perspective, 7th ed. (Pearson, 2015) 28 • Open Universe (unbound, or low density) – Density Critical Density – Not enough gravity to stop expansion. – Universe expands forever. Figure 26.14, Chaisson and McMillan, 6th ed. Astronomy Today, © 2008 Pearson Prentice Hall 29 • Closed Universe (bound, or high density) – Density > Critical Density – Gravity is stronger than expansion. – Universe will initially expand, stop, and eventually fall back into itself. 30 Predicting the Fate of the Universe • If we know the density of the universe, we can predict the future of the universe – Measure mass in large volume of space and divide mass by volume • Current Estimate: The Universe is open; it will expand forever. It may also be accelerating. 31 Evidence for acceleration Figure 18.16 Bennett et al, The Essential Cosmic Perspective, 7th ed. (Pearson, 2015) 32 What will happen if the universe is closed ? A. The universe will expand past its maximum size, then fragment into mini-universes. 98% B. The universe will eventually fall back in on itself, heading toward a “Big Crunch.” C. The universe will expand forever. D. The universe will reach a maximum size and remain there, like a balloon being blown up. 2% 0% A. B. C. 0% D.