Lecture 37 Cosmology [not on exam] January 16b, 2014

Transcription

1 1 Lecture 37 Cosmology [not on exam] January 16b, 2014

2 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, 6 th ed. Astronomy Today, 2008 Pearson Prentice Hall

3 3 Cosmological Principle The universe looks homogeneous on the largest scales The universe is isotropic Both are assumptions about the universe

4 4 Implications The universe can have no edge The universe can have no center

5 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, 6 th ed. Astronomy Today, 2008 Pearson Prentice Hall

6 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, 6 th ed. Astronomy Today, 2008 Pearson Prentice Hall

7 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 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 is the cosmic light horizon

9 9 The Big Bang Universe starts out almost infinitely small Goes through period of inflation, increases size by factor of 10 50

10 10 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 9 th ed. Universe, 2011 W. H. Freeman and Company

11 11 From what evidence do astronomers deduce that the Universe is expanding? A. They can see the disks of galaxies getting 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. E. All of the above.

12 12 From what evidence do astronomers deduce that the Universe is expanding? A. They can see the disks of galaxies getting 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. E. All of the above.

13 13 What is meant by inflation in the early Universe? A. The force of gravity suddenly grew stronger in the 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. E. The diameter of distant galaxies is much greater than the diameter of galaxies near to us.

14 14 What is meant by inflation in the early Universe? A. The force of gravity suddenly grew stronger in the 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. E. The diameter of distant galaxies is much greater than the diameter of galaxies near to us.

15 15 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, 6 th ed. Astronomy Today, 2008 Pearson Prentice Hall

16 16 Cosmic Microwave Background CMB has now cooled to about 3 K Figure 17.7, Arny and Schneider, 5 th ed. Explorations, 2008 The McGraw-Hill Companies

17 17 Wilkinson Microwave Anisotropy Launched June 2001 Measured fluctuations in Cosmic Microwave Background Probe (WMAP) Image by NASA/WMAP Science Team

18 18 Cosmic Microwave Background The background is nearly uniform at K. If we subtract out that signal, we can see the dipole pattern caused by the Doppler shift due to Earth s motion through the CMB. Image from the COBE data set

19 19 Temperature and Density Fluctuations of the Early Universe If we subtract out the Doppler shift dipole pattern, we re left with only tiny fluctuations on the order of K. Image by NASA/WMAP Science Team See also:

20 20 What is meant by the cosmic microwave background? A. It is radiation from distant quasars. 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.

21 21 What is meant by the cosmic microwave background? A. It is radiation from distant quasars. 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.

22 22 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.

23 23 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 ~ million years after Big Bang

24 24 History of Universe

25 25 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 Image by NASA/WMAP Science Team

26 26 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, 5 th ed. Explorations, 2008 The McGraw-Hill Companies

27 27 How Will the Universe Evolve? Depends on density (gravitational pull) and velocity of galaxies. Critical density = density of universe needed to stop expansion

28 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, 6 th ed. Astronomy Today, 2008 Pearson Prentice Hall

29 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 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.

31 31 What would happen if the density of the Universe were greater than the critical density? A. Matter would never have formed. B. Galaxies would not form. C. Too many galaxies would form. D. All the matter formed would be neutrons. E. The Universe would eventually recollapse.

32 32 What would happen if the density of the Universe were greater than the critical density? A. Matter would never have formed. B. Galaxies would not form. C. Too many galaxies would form. D. All the matter formed would be neutrons. E. The Universe would eventually recollapse.