Transcript The measure of Cosmological distances

The measure of
Cosmological distances
Asaf Pe’er
Space Telescope Science
Institute
How far is Jupiter ??
I. Science at ancient times
Greece, c. 300BC
What is the size of earth ?
First measurement of earth’s radius:
Eratosthenes of Cyrene
(276BC- 196 BC)
Syene, Egipt
Eratosthenes:
Earth circumference =40000 km
 Earth radius = 6000 km
Measuring the size of the moon using lunar eclipse
Aristarchus of Samos, 310BC- 230BC
Total eclipse time
~ earth radius
Moon radius ~ ¼ earth radius
Time to full eclipse
~ moon radius
Given moon radius, distance is simple geometry
Distance to the moon ~380.000 km (240.000 mi)
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: 83-161 AD
Circle - “Perfect” shape
Mars motion
according to Ptolemy
Ptolemy model - consistent
with observations !!!
Mars motion according to
Heliocentric theory
II. Rise of Heliocentric Cosmology
Nicolaus Copernicus (1473 - 1543)
Advantages:
1. Correct
2. Simple
Disadvantage:
1. Less accurate than geocentric model
2. Copernicus was unknown
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 !)
Even the sun has spots !
(= sun is not perfect !)
“Smoking gun”: Venus phases
Cannot be explained by Geocentric model
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.)
III. Measuring distance to the stars
William Herschel (1738 - 1822)
Herschel’s 20 foot reflector
Found new planet (Uranus)Discover Infra-Red lightFirst 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
1838: First measurement of distance to a star
Friedrich Bessel (1784 - 1845)
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.
The great debate
Charles Messier
(1730 - 1817):
deep sky catalogue
of Nebulae
M31
“The great debate”:
Are nebulae part of
the milky way galaxy
- or not ?
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 periodicityLuminosity 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
Cepheids in
Globular clusters
Size of the milky way: 100.000 l.y. ;
Thickness = 1.000 l.y
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 !!
IV. Measuring distance to the galaxies
Spectroscopy:
measuring the chemical elements in stars
Fraunhofer, Bunsen, Kirchhoff (1859)
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 !
Hubble’s law
Velocity = Distance  H0
H0 = Hubble’s constant = 70 (km/s) / Mpc
Edwin Hubble (1889 - 1953):
II. 1929 - Discovery of distance
- velocity relation in galaxies
Baade (1952) & Sandage (1954)
corrected the value of H0
Mpc = Mega (Million)-parsec;
Parsec = paralax-arcsecond =~ 3.3 l.y.
V. The big bang theory and beyond
Back in time, all the matter
was concentrated in a very small region
Albert Einstein (1879 - 1955)
1915: General Theory of relativity
-- universe collapse (Gravity);
Cosmological constant 
1922: Alexander Friedman
Universe expands !
1927: Georges Lemaitre
Further proofs for universe expansion & “Big bang”
Ralph Alpher (1921 - 2007):
Universe: 90% H, 9% He
1948: Alpher, Bethe, Gamow H, He production in big bang
Alpher, Gamow & Herman cosmic microwave background (CMB)
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