The measure of Cosmological distances Asaf Pe’er Space Telescope Science Institute August 2008

How far is Jupiter ??

Greece, c. 300BC What is the size of earth ? I. Science at ancient times

Syene, Egipt First measurement of earth’s radius: Eratosthenes of Cyrene (276BC- 196 BC) ‏

Eratosthenes: Earth circumference =40000 km  Earth radius = 6000 km

Measuring the size of the moon using lunar eclipse Time to full eclipse ~ moon radius Total eclipse time ~ earth radius Moon radius ~ ¼ earth radius Aristarchus of Samos, 310BC- 230BC

Distance to the moon ~ km ( mi) ‏ Given moon radius, distance is simple geometry

Geocentric (=earth at the center) vs. Heliocentric (=sun at the center) universe Aristotle (384BC - 322BC) ‏ Aristarchus of Samos (310BC - 230BC) ‏ Why Geocentric ? 1. "we see" 2. if the earth moves, where is the wind ? 3. Gravity – everything is attracted to the center of the universe 4. Parallax: stars don't move !

Parallax Parsec = paralax-arcsecond =~ 3.3 l.y.

But some stars do move.. Retrograde motion of Mars

The universe according to Ptolemy Ptolemy: AD Circle - “Perfect” shape

Mars motion according to Ptolemy Mars motion according to Heliocentric theory Ptolemy model - consistent with observations !!!

Nicolaus Copernicus ( ) Advantages: 1. Correct 2. Simple Disadvantage: 1. Less accurate than geocentric model 2. Copernicus was unknown II. Rise of Heliocentric Cosmology

Tycho Brahe ( ): Accurate measurements of planet orbits Johannes Kepler ( ): Planets move in ellipses (not circles) around the sun Kepler’s laws of planetary motion helped Neuton to develop the theory of Gravity

Next breakthrough: the Telescope (1608)

Galileo Galilei ( ): First astronomer to use a telescope Moon has craters ! (= not perfect sphere !) Jupiter has moons ! (= not everything rotates around earth !)

“Smoking gun”: Venus phases Cannot be explained by Geocentric model Even the sun has spots ! (= sun is not perfect !)

Measuring the distance to the sun Giovanni Cassini ( ) Cassini & Richer measure the distance to Mars Using Kepler’s laws, Cassini deduce the distance to the sun: km = 1 Astronomical unit (1 A.U.)

William Herschel ( ) Herschel’s 20 foot reflector III. Measuring distance to the stars -Found new planet (Uranus) -Discover Infra-Red light -First map of the sky: Idea: All the stars are the same. Therefore, bright stars are closer.

Herschel’s model of the milky way: -Stars are ordered in space. We are part of the Galaxy. - But he could not scale the size of the galaxy

Friedrich Bessel ( ) 1838: First measurement of distance to a star 61 signi Distance = 100,000,000,000,000 km (= 11 light years)  Scaling the milky way: width = l.y., (today: l.y.) Thickness = l.y.

Charles Messier ( ): deep sky catalogue of Nebulae “The great debate”: Are nebulae part of the milky way galaxy - or not ? M31 The great debate

John Goodricke ( ): Discovery of variable stars & Cepheids Mechanism: Envelope contains opaque He 2+ - heated - pressure increases- expansion - radiation escape - cooling

Henrietta Leavitt ( ): 1908: Discovery of periodicity- Luminosity relation in Cepheids 25 Cepheids at the small Magellanic cloud  All at same distance from us

1917: Shapley & Hertzsprung measured the distance to a Cepheid - allow the use of Cepheids as “Standard candles” 1918: Harlow Shapely measures the milky way Size of the milky way: l.y. ; Thickness = l.y Cepheids in Globular clusters

What about the nebulae ? Edwin Hubble ( ): I Discovery of Cepheids in Andromeda galaxy Distance to M31: light years >> Milky way !!

Spectroscopy: measuring the chemical elements in stars Fraunhofer, Bunsen, Kirchhoff (1859) IV. Measuring distance to the galaxies

Spectrum of the sun Lockyer & Janssen discover a new element in the sun (He) 1860’s - Huggins: stars contain the same elements as the earth.

William Huggins finds red shift of Sirius, determine its velocity: 45 km/s Red Shift

1912: Vesto Slipher measures red shift of galaxies V ~ km/s Strangley, most of the galaxies are receding from us !

Edwin Hubble ( ): II Discovery of distance - velocity relation in galaxies Hubble’s law Velocity = Distance  H 0 H 0 = Hubble’s constant = 70 (km/s) / Mpc Mpc = Mega (Million)-parsec; Parsec = paralax-arcsecond =~ 3.3 l.y. Baade (1952) & Sandage (1954) corrected the value of H 0

Back in time, all the matter was concentrated in a very small region 1915: General Theory of relativity -- universe collapse (Gravity); Cosmological constant  1922: Alexander Friedman Universe expands ! 1927: Georges Lemaitre Albert Einstein ( ) V. The big bang theory and beyond

Further proofs for universe expansion & “Big bang” Ralph Alpher ( ): 1948: Alpher, Bethe, Gamow - H, He production in big bang Alpher, Gamow & Herman - cosmic microwave background (CMB) Universe: 90% H, 9% He 1964: Penzias & Wilson discover the CMB

1991- Fluctuations in the CMB (COBE satellite): “embryos” of galaxies Mather & Smoot, 2006 Nobel prize

The future 1998: A surprising twist Astronomers led by Adam Riess (STScI), Saul Perlmutter (Berkeley) - The universe accelerates !!!

The universe, 2008 Wmap