The measure of Cosmological distances

Asaf Pe’erSpace Telescope Science

Institute

How far is Jupiter??

Greece, c. 300BCWhat is the size of earth ?

I. Science at ancient times

Syene, EgiptFirst 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 radiusTotal eclipse time

~ earth radius

Moon radius ~ ¼ earth radius

Aristarchus of Samos, 310BC- 230BC

Distance to the moon ~380.000 km (240.000 mi)

Given moon radius, distance is simple geometry

Geocentric (=earth at the center) vs. Heliocentric (=sun at the center)

universeAristotle (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 universe4. 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: 83-161 AD Circle - “Perfect” shape

Mars motion according to Ptolemy

Mars motion according to Heliocentric theory

Ptolemy model - consistent with observations!!!

Nicolaus Copernicus (1473 - 1543)

Advantages:1. Correct

2 .Simple

Disadvantage:1 .Less accurate than geocentric model

2. Copernicus was unknown

II. Rise of Heliocentric Cosmology

Tycho Brahe (1546 - 1601):Accurate measurements of planet orbits

Johannes Kepler (1571 - 1630):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 (1564 - 1642):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 (1625 - 1712)

1672 -Cassini & Richer measure the distance to Mars

Using Kepler’s laws, Cassini deduce the distance to the sun:150.000.000 km = 1 Astronomical unit (1 A.U.)

William Herschel (1738 - 1822)

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 (1784 - 1845)

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 = 10.000 l.y,.(today: 100.000 l.y ).Thickness = 1.000 l.y.

Charles Messier (1730 - 1817):deep sky catalogueof Nebulae

“The great debate”:Are nebulae part of the milky way galaxy

- or not?

M31

The great debate

John Goodricke (1764 - 1786):Discovery of variable stars & Cepheids

Mechanism: Envelope contains opaque He2+ - heated - pressure increases- expansion - radiation escape - cooling

Henrietta Leavitt (1868 - 1921):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: 100.000 l.y. ; Thickness = 1.000 l.y

Cepheids in Globular clusters

What about the nebulae?

Edwin Hubble (1889 - 1953):I. 1923 - Discovery of Cepheids in Andromeda galaxy

Distance to M31: 900.000 light years >> Milky

way!!

Spectroscopy: measuring the chemical elements in starsFraunhofer, Bunsen, Kirchhoff (1859)

IV. Measuring distance to the galaxies

Spectrum of the sun

1868 - Lockyer & Janssen discover a new element in the sun (He)1860’s - Huggins: stars contain the same elements as the earth.

1868 - William Huggins finds red shift of Sirius,determine its velocity: 45 km/s

Red Shift

1912 :Vesto Slipher measures red shift of galaxies

V ~ 300-1000 km/s

Strangley, most of the galaxies

are receding from us !

Edwin Hubble (1889 - 1953):II. 1929 - Discovery of distance - velocity relation in galaxies

Hubble’s law Velocity = Distance H0

H0 = 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 H0

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 (1879 - 1955)

V. The big bang theory and beyond

Further proofs for universe expansion & “Big bang”

Ralph Alpher (1921 - 2007):

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