Lecture 22: The expanding Universe. Astronomy 111 Wednesday November 15, 2017

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1 Lecture 22: The expanding Universe Astronomy 111 Wednesday November 15, 2017

2 Reminders Online homework #10 due Monday at 3pm Then one week off from homeworks Homework #11 is the last one

3 The nature of the Universe The Universe contains hundreds of billions of galaxies More galaxies in the Universe than there are stars in the Milky Way This understanding came only in the last century

4 Charles Messier ( ) Constructed a catalog of 103 nebulae To prevent confusing with comets Most thought these nebulae were part of the Milky Way, which contained the entire Universe

5

6 Further discoveries Herschels found 2500 more galaxies More observations of nebulae followed, but still the nature of the Universe was unknown Some thought the Milky Way was so big, it must be all there is How do we know the size of the Universe?

7 Cosmology Cosmology is the study of the entire Universe: Physics of the Universe Distribution of objects on all scales Motions of objects in the Universe Evolution of the Universe Age, origin, and fate of the Universe

8 The Universe in : Einstein explored the cosmological implications of General Relativity Observational state in 1917: Kapteyn model of the Milky Way was favored by some (but not all) astronomers No agreement on the spiral nebulae First good calibrations of the P-L relation for Cepheids and RR Lyrae variables

9 The Cosmological Principle The Universe is homogeneous and isotropic on the largest scales. Critical assumption underlying cosmology Homogeneous: No special places in the Universe. Isotropic: No special directions

10 Homogeneity When viewed on the largest scales: The average density of matter is about the same in all places in the Universe The Universe is fairly smooth on large scales Does not apply locally: We see planets, stars, galaxies in regions nearby in space The Universe is locally rather lumpy

11 Isotropy When viewed on the largest scales: The Universe looks the same to all observers The Universe looks the same in all directions as viewed by a particular observer Does not apply locally: We see different numbers of local objects in different directions

12 The dynamic Universe Einstein applied the Cosmological Principle to General Relativity and got a surprise: The spacetime of the Universe could not be static and unchanging The Universe must either expand or contract! Astronomers assured him that no such general motion was observed

13 The Cosmological Constant To make the Universe static, he added a new term in his field equations: The Cosmological Constant, L: Repulsive gravitation-like force term Arises from empty space Balances the effects of gravity

14 Discovery of expansion : Vesto Slipher, Lowell Observatory Measured radial velocities from spectra of 25 galaxies Found: 21 of the 25 galaxies show a redshift speeds of some >1000 km/sec Most galaxies are rapidly receding from us ASTR111 Lecture 22

15 Hubble s discovery 1929: Edwin Hubble Measured distances to 25 galaxies: Used Cepheids and brightest stars Compared distances with recession velocities Discovered: Recession velocity gets larger with distance Systematic expansion of the Universe

16 Recession Velocity (km/sec) ASTR111 Lecture 22 Hubble s data (1929) Distance (Mpc)

17 Refinement The 1929 work was very suggestive of an effect, but the uncertainties were large Set about collecting more data: By 1931 Added 8 more galaxies with distance estimates Most distant galaxy had a recession velocity of nearly 20,000 km/sec! Showed a stronger, tighter trend with distance

18 Recession Velocity (km/sec) ASTR111 Lecture 22 Hubble & Humason (1931) 20,000 15,000 10, Data Distance (Mpc)

19 Hubble s Law v H d v = recession velocity in km/sec d = distance in Mpc H = Hubble s Constant = 75 km/sec/mpc In words: The more distant a galaxy, the faster its recession velocity

20 Interpretation Hubble s Law demonstrates that the Universe is expanding in a systematic way: The more distant a galaxy is, the faster it appears to be moving away from us Rate of expansion = Hubble Constant Comments: Hubble s Law is empirical - based only on data Not an exact law

21 Nature of the expansion General expansion of spacetime: All observers in different galaxies see the same expansion around them No center - all observers appear to be at the center What is the recession velocity? NOT motion through space... Expansion of spacetime: galaxies carried along

22 Systematic Expansion ASTR111 Lecture 22 Universe 2x larger Galaxies are 2x further apart

23 Cosmological redshift Most galaxies are receding from us Convenient to express recession velocity as a redshift for the galaxy Redshift (z): Directly observable quantity. Measured accurately from spectral lines shifts Often express cosmic distances as redshifts

24 Redshift distances d v H Distances estimated using Hubble s Law even for very distant objects

25 Mapping the Universe Can map the distribution of galaxies using redshifts Largest map includes ~1,000,000 galaxies Reveals sheets and filaments of galaxies surrounding great voids. Depth is ~500 Mpc (or so)

26 Sloan Digital Sky Survey Dedicated 2.5-meter telescope in New Mexico Image 1/4 of the sky in 5 colors: 100 million galaxies 100 million stars Redshift survey: 1 million galaxies 100,000 quasars

27

28 Distance ladder We need to precisely measure the Hubble constant to determine the expansion rate of the Universe Measure distances to distant objects very accurately, derive H 0 Construct a distance ladder

29 Distance ladder ASTR111 Lecture 22

30 State of the art Einstein s guess about the homogeneity and isotropy of the Universe was brilliant and far ahead of the scanty empirical data of his time Modern observations bear out large-scale homogeneity and isotropy: Large-scale galaxy surveys Maps of the cosmic background radiation

31 What about the Cosmological Constant? 1920s: DeSitter corrects an error in Einstein s math, showed that the L Universe was unstable Friedmann & Lemaître showed that without L, GR predicts that the Universe expands Hubble firmly established cosmic expansion observationally

32 Vesto Slipher Willem de Sitter Georges Lemaître Edwin Hubble Albert Einstein ASTR111 Lecture 22 Alexsandr Friedmann

33 Modern Cosmological Constant In current cosmological theory, L reappears in a somewhat modified form: Introduced as the vacuum energy of space Quantum ground-state of empty space Assumed to be either very very small or zero Distinction: Its use today is arguably better physically motivated than when Einstein used it in 1917

34 Modern Cosmological Constant Recent supernova observations suggest a non-zero L May mean that there is a form of dark energy pushing against gravity Form is almost completely unknown and a subject for 21st century astrophysics Many projects are planned over the next 10 years to characterize dark energy

35 The most distant galaxies Now we know that the Universe is very large and contains hundreds of billions of galaxies Hubble Space Telescope Ultra Deep Field was the first to image these distant galaxies 10,000 galaxies Formed only ~500 million years after the Big Bang!

36

37 Summary Cosmological Principle: The Universe is Homogeneous and Isotropic on Large Scales No special places or directions General Relativity predicts an expanding universe Cosmological Constant

38 Summary Hubble s Law: Galaxies are receding from us Recession velocity gets larger with distance Hubble Constant: Rate of expansion of the Universe Cosmological Redshift: Redshift distances Redshift maps of the Universe

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