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Expansion of the Universe, Ch. 26, 52 Probably the most profound discovery of all! This is the beginning of modern cosmology- -study of structure and evolution.

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Presentation on theme: "Expansion of the Universe, Ch. 26, 52 Probably the most profound discovery of all! This is the beginning of modern cosmology- -study of structure and evolution."— Presentation transcript:

1 Expansion of the Universe, Ch. 26, 52 Probably the most profound discovery of all! This is the beginning of modern cosmology- -study of structure and evolution of Universe as a whole. Its also what I study.

2 AF-1374, “cosmology” career Before I move on, let me point out confusion among lay public. Cosmology≠Cosmotology, Astrology≠Astronomy

3 Galaxies looks static… …but how can we tell if galaxies are moving? A grain of sand at arm’s length covers 10,000 galaxies

4 The Doppler Effect Moving emitter of waves, Red or Blueshift  0  0  0 blue shift no shift red shift Christian Doppler (see Chap 26) “line of sight” effect

5 Take a Spectrum you know, Hydrogen Energy of photon absorbed = h v = hc / =  E = difference between energy levels in atom

6 ÅÅ HH HH HH Hydrogen NO SHIFT (v=0) Blueshift (v toward us) Redshift (v away from us) Doppler shift of spectral lines  

7 Getting Quantitative: Doppler shift of spectral lines Example: if v = 90 km/s then v/c = 3 x 10  4, for H    = 6563  Å  = z*   2 Å  = 6565  Å, Notice that stretching not just shifting, red moves farther than blue, z=v/c for v<

8 Question: Can you measure a doppler shift (redshift) from a single line? a) yes b) no answer yes and no! You can do it, but you need to know which line it is or you can make a big error

9 Vesto Slipher and the Redshifts , Lowell Observatory, Flagstaff, AZ, Measured doppler shift of Andromeda, first Spiral nebula. Rushing towards us at 300 km/sec, i.e., blueshifted!! Expect ~20 km/sec like most stars t=D/V=2 x10 19 km / 300 km/sec = sec=3x10 9 yrs before impact with Milky Way! --By 1917 Slipher measured 25, in all directions, 21 of 25 redshifted (<1% chance of so many redshifts If random). Some as high as 1,100 km/sec (across solar system/year!) 20 out of 20 measured next year all redshifted It was assumed the shifts would be smaller, mixed blue and red. How to make sense of this pattern of large redshifts?

10 Smaller Galaxies Tended to have larger redshifts

11 Hubble identified the brightest stars and clusters in each galaxy, assumed they were of the same luminosity, and calibrated that luminosity with Cepheid variables. By 1929 Hubble and Humason Had measured the distance to 24 of Slipher’s galaxies Einstein visits Mt. Wilson, inch, Mt. Wilson Meanwhile…Edwin Hubble Decided to look for a pattern in the redshifts of galaxies by Measuring their distances

12 The farther away a galaxy is … Cepheid star Star Cluster Galaxy far, far away… Twice as far =half the angle =1/4 as bright 4 times as far =1/4 the angle =1/16 as bright 8 times as far =1/8 the angle =1/64 as bright Star cluster ~100x luminosity of Cepheid, so can be seen 10 times farther away

13 Hubble’s (Amazing) Diagram! /sec Plotted galaxy distance against apparent velocity… He saw relationship, the farther away a galaxy is, the faster it is receding! This relationship is just what would happen if the space between galaxies (i.e., the Universe) was expanding (I.e. stretching)! He measured an expansion rate of H=+500 km/sec for every Megaparsec of distance /sec Recession speed Distance

14 Arrows with explosion center Do we smell? Last time I said it was *unlikely* (anti- Copernican), but there is actually *evidence*. You can see in this plot that galaxies thin out with distance, if this model were correct. NOT what we see in HDF, etc. This is a schematic of what Hubble saw.

15 Announcements Hw #5 Lab #2

16 distance velocity Hubble’s Law: In an expanding Universe, velocity =H 0 x distance Measured slope, gives expansion rate of the Universe today. Twice as far, twice as fast

17 Redshift Caused by Expansion Not Individual Motions Candles emit light with some wavelength. Expansion stretches wavelength, light redder. Redshift, z. We often refer to an apparent velocity, v=cz, analogous to the Doppler shift with velocity, v, but be aware that the interpretation is space stretching NOT motion (e.g., people on an escalator vs. people walking up the stairs—there is a pattern of motions)

18 Question: How do we know the wavelength a galaxy’s light should have before its redshifted by expansion? a)We don’t know b) we move toward the galaxy c) we measure it in a lab d) we derive it from physics answer, c)

19 More Toy Universes Infinite rubber band with knots in it: a good 1-D analogy The redshifts that we see from distant galaxies are caused by this expansion, not just by galaxies moving through space (which would require a conspiracy!)

20 More Toy Universes Infinite rubber band with knots in it: a good 1-D analogy The redshifts that we see from distant galaxies are caused by this expansion, not just by galaxies moving through space (which would require a conspiracy!)

21 A 2D Toy Universe Surface of a spherical balloon: 2-D analogy of a finite universe. The 3D center of expansion is at the center of the balloon, which is not part of this hypothetical universe: it is in a physically inaccessible dimension! Imagine ants on the surface of an expanding balloon shooting photons of blue light at each other…

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23 Extended Hubble diagram Hubble and Humason extended diagram Reminder: 1 parsec = 3.26 light years, so to get approximate light years, multiply parsecs by 3.

24 Hubble’s original diagram From work in my thesis, ~1996

25 Implications of Expanding Universe: No unique center! Example: infinite loaf of “raisin bread” expanding uniformly, doubles in size. v (cm/s) = H 0 d (cm), H 0 = 1 cm/s/cm Final d (1 sec) 2 cm 5 cm 4 cm 5 cm Speed 1 cm/s 2.5 cm/s 2 cm/s 2.5 cm/s Original d (0 sec) AB = 1 cm AC = 2.5 cm CB = 2 cm CA = 2.5 cm 0 sec 1 sec So whether you sit on A,B, or C, same expansion rate! H 0 =Speed/ Original d 1 cm/s/cm

26 Prior Efforts? Standard Galaxies? Swedish astronomer, Knut Lundmark ( ) plotted the radial velocity of 44 galaxies against their estimated distances. He assumed that Andromeda is 200,000 pc (650,000 ly) away, then made rough determinations of the distances to other galaxies by comparing their sizes and brightnesses to that of Andromeda. Lundmark concluded that there may be a relationship between galactic redshift and distance, but "not a very definite one.” Galaxy are not homogeneous enough! Lundmark-1924Hubble-1929

27 Hubble's law can be also used to determine distances to far-away galaxies 1) Obtain spectrum 2) Measure the redshift, z=v/c 3) Use Hubble's law, v = H 0 d, to compute d. Note, this becomes inaccurate for z>0.1 because H 0 may change as we will discuss

28 TIME: Universe Begin?

29 Hubble constant, H 0, gives the present expansion rate. Its inverse gives approximate age of Universe (all at a point) if no forces are acting on Universe to change the rate (farther galaxies return faster when run clock backwards). From Hubble’s initial value of H 0 =500 km/sec/Mpc we get: 1/H 0 =2 billion years old. Even in 1929, knew this was wrong… Milky Way Time forward Milky Way Time backward Milky Way Time backward Hubble’s Law Says Galaxies were together in the past, gives Universe age

30 Distances from SNe Two populations of stars discovered CCD’s in use, “Hubble wars” Hubble launched NY Times, August, RIESS ET AL. (2011) HUBBLE CONSTANT (KM S -1 MPC -1 ) ± 2.4  13.4 Billion years Age Universe Billions Years WE LIVE IN AN OLD (13.4 ± 0.4 BILLION YEARS) UNIVERSE… In 1929 radioactive dating of rocks showed that the Earth was a few Gyr old and the main sequence turn-off of stars was already more than that, so it was a relief when Baade (chap 53) corrected the scale in 1952

31 My Research on Measuring the expansion rate, H 0.

32 My New Measurement of the Hubble Constant: A New Distance Ladder

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34 Hubble's Law cz=v = H 0 d (H 0 = "H-naught" = Hubble's constant now; it changes with time.) i.e., a galaxy with a redshift of z=0.01 or an apparent velocity v=3000 km/sec is at a distance, d=40 Mpc (130 Million lyr)…which is about 40 times farther than Edwin Hubble could see Cepheids. It also means it takes the light t=d/c=130 Million years to reach us! (Does that mean our measurements are out of date? Yes…) 73±2.4

35 What if there are forces in the Universe (like the mutual attractive gravity of its own galaxies) which are altering the expansion rate? That is, what is the expansion history of the Universe? Like the marathon runner who tires at the end…

36 Universe Doesn’t Instant Message! Reality: Message is delayed by Time Delay=D/c delay~1 billion yrs, we measure H 1 delay~2 billion yrs, we measure H 2 delay~3 billion yrs, we measure H 3 Powerful way to see if expansion rate is slowing, is H 0

37 Expansion History To address this question cosmologists have to measure ever more distant galaxies, use the “time delay effect” (finite travel time of light) to discover changes in the expanding Universe. We will show what was found (I was part of the first team to successfully do this!) in a few more lectures.

38 Epilogue: Hubble from “The Realm of the Nebulae” Hubble did not live to receive the Nobel he deserved nor to witness the present age of “precision cosmology” “With increasing distance, our knowledge fades, and fades rapidly. Eventually, we reach the dim boundary--the utmost limits of our telescopes. There, we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial. The search will continue. Not until the empirical resources are exhausted, need we pass on to the dreamy realms of speculation.”

39 The END


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