1446 Introductory Astronomy II Chapter 18A

1. 1446 Introductory Astronomy IIChapter 18A Cosmology I R. S. Rubins Fall 2011

3. Prologue 1 • “Cosmologists are addressing some of the problems that people attempted to resolve over the centuries through philosophical thinking, but we are doing so on systematic observation and quantitative methodology.” • “As citizens of the universe we cannot help but wonder how the first sources of light formed, how life came into existence, and whether we are alone as intelligent beings in this vast space.” Abraham Loeb in Scientific American, Nov. 2006

4. Prologue 2 • “What makes modern cosmology an empirical science is that we are literally able to peer into the past…cosmologists do not need to guess how the universe evolved; we can watch its history through telescopes.” • “We have a snapshot of the universe as it was 400,000 years after the big bang … as well as pictures of individual galaxies a billion years later.” Abraham Loeb in Scientific American, Nov. 2006

5. The Cosmological Principle • At the very largest scales (over distances of about a billion ly), the universe appears to be i. homogeneous (the same at all places); ii. isotropic(the same in all directions at any point). • The cosmological principleimplies that there is i. no edge to the universe; ii.no center to the universe. • The proponents of the discredited steady state universe(Hoyl et al.), believed in a universe (on the largest scales) that was independent of time. • An additional assumption, continually being tested, is that physical theories are unchanged over time and space.

6. A Universe, Finite and Unbounded • We imagine 2-dimensional objects on a 2-dimensional surface. • While all the coins move away from each other as the surface expands, the sizes of the coins do not change because of the strong EM forces holding each coin together.

7. Cosmological Redshift • As the balloon expands, the wavelength of an EM signal increases; i.e. it is redshifted. • The further away the emitting source, the more the received signal will be redshifted.

8. Doppler Redshift vs Cosmological Redshift

9. COBE Data Fits T = 2.73 K From 1989 to 1994, a precise value of 2.73 K for the cosmic microwave background (CMB) was obtained with a far IR spectrometer on the COBE satellite.

10. Motion of the Milky Way 2 More precise measurements made by COBE showed temperature variations of up to ± 3 mK, with the warmer region in the direction of Leo and the cooler region in the direction of Aquarius, indicating Doppler effects.

11. Motion of the Milky Way 1 • The COBE measurements showed that the Earth is moving towards Leo at about 380 km/s. • Taking into account the motion of the Sun around the galactic center, astronomers deduced that the Milky Way Galaxy is moving at 600 km/s relative to the CMB.

12. Ripples in the Background Radiation 1 • To produce the “sudsy” structure of galactic clusters and observed in the universe, cosmological models predicted that there should have been tiny ripples in the temperature of the CMB before the appearance of stars. • The lead investigators of the COBE scientific team, George Smoot and John Mather, made the first measurements of the ripples, and shared the 2006 Nobel Prize in physics. • Improved measurements were made with specialized telescopes, on the ground and in balloons flying high above Antarctica, but the most extensive investigations were those of the Wilkinson Microwave Anisotropy Probe (WMAP), which was an orbiting robotic infra-red telescope. • The ripples in the 3K CMB were found to be roughly10 μK (one hundred thousandths of a degree).

13. The “Boomerang” Experiment, 1998 Following circumpolar winds at almost 40 km above Antarctica for 10 days, microwave detectors cooled to 0.3 K obtained images 40 times sharper than COBE.

14. Ripples in the CMB 1 • In the map of the sky after 5 years of collecting WMAP data, the red regions are warmer than average, and the blue regions cooler, by about 3 x 10–5 K. • The warmer regions are slightly denser, because gravitational collapse causes heating.

15. Ripples in the CMB 2 • The tiny temperature ripples in the WMAP figure are of fundamental importance, because they are the origins of the galaxies formed over a million years later. • The density contrasts observed in the WMAP figure are amplified during the expansion of the universe, because the gravitational force slows the expansion of the denser regions. • The temperature ripples are of the order of ± 0.00003 K . • If applied to a sphere the size of the Earth, it would be equivalent to ripples on the Earth’s surface of less than 100 yards (the length of a football field), which is another way of justifying the cosmological assumption of homogeneity.

16. Development of Structures • The pictures show a computer simulation of how structures emerge in an expanding universe. • For practical reasons, the expansion has been subtracted out, so that the boxes remain the same size. • During expansion, the denser regions expand more and more slowly, compared to less dense regions, so that the density contrasts grow.

17. Galactic Distributions Observed Simulated

18. “Big Bang” Theory • The term Big Bang Theory was introduced by the Englishphysicist Fred Hoyle in 1949 in a radio broadcast, to distinguish it from the Steady State Theory that he believed in at the time. • In 1931, Georges LeMaitre, Belgian physicist and Catholic priest, independently derived equations obtained over ten years earlier by the Russian, Alexander Friedmann, which showed that Einstein’s General Theory of Relativity could give an expanding universe. • LeMaitre proposed that the universe could have begun as a tiny point which expanded and cooled with time. • George Gamow, who had studied with Friedman in Russia, and later escaped to the USA, deduced that radiation created at the Big Bang should still be visible today. • This radiation is known as the Cosmic Microwave Background.

19. Questions about Big-Bang Theory • 1. Was the Big Bangan explosion in previously empty space? • 2.What happened before the “Big Bang”? • 3. Do the restrictions of Einstein’s Special Theory of Relativitymean that galaxies cannot recede faster than c, the speed of light in free space. • 4. Does the expansion of the universe cause everything within it to grow proportionately in size? • 5. Do those measurements, which show the universe to be about 13.7 billion years old, mean that the radius of the observable part of the universe is 13.7 billion light years?

20. Misconceptions Answered • 1. Since time and space are thought to have come into existence with the Big Bang, the latter could not have occurred at a point in space. • 2. For the same reason, the concept of time before the Big Bang has no meaning. • 3.The limiting speed c of special relativity applies to the motion of matter or energy through space, but not to the expansion of space itself. • 4.Because of the gravitational forces between them, the sizes of planets, stars, galaxies, and their clusters remain unchanged by the expansion of space. • 5. By the time a signal reaches us from a distant object, that object will be much further away because of the expansion of space to the estimated radius of 42 billion years..

21. Lookback Time 1 • The lookback timerefers to the time when a signal we see today was emitted by its source.