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Cosmology 1

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Henrietta Leavitt. Annie Cannon. Harvard College Observatory. Early Milestones in Cosmology ... Henrietta Leavitt. 1912. L-P Relation of Cepheid Variables. 14 ... – PowerPoint PPT presentation

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Title: Cosmology 1


1
Cosmology1
2
Topics
  • Early Milestones in Cosmology
  • The Expanding Universe
  • Summary

3
Where Do We Come From? What Are We?
Paul Gauguin (1897) Where Are We Going?
Museum of Fine Arts, Boston
4
Early Milestones in Cosmology
  • 1823 Heinrich Olbers
  • An infinite universe of infinite age contains
    infinitely many stars.
  • Therefore, if you look in any direction in the
    sky, eventually, your line of sight would
    intercept the surface of a star.

5
Early Milestones in Cosmology
  • Olbers Paradox
  • Since every direction leads to a star, the sky
    should be as bright as the surface of a star!
  • So why is the sky dark?

6
Early Milestones in Cosmology
  • 1848 Edgar Allan Poe
  • The universe may be infinitely large but be of
    finite age.
  • If so, light has not yet reached us from the most
    distant stars.
  • This would create so many dark gaps between the
    nearer stars that the sky would appear dark.

7
Early Milestones in Cosmology
Henrietta Leavitt
Annie Cannon
Harvard College Observatory
8
Distance Magnitude
luminosity (W) distance (m) flux (W/m2)
L
f
d
For the Sun f 1300 W/m2 L 400 trillion
trillion W
9
Distance Magnitude
Astronomers measure brightness using
the magnitude scale. This is defined so that a
magnitude difference of 5 equals a flux
difference of 100
d
L
m
m -26.8
a 2.54 x 10-8 W/m2
10
Distance Magnitude
The absolute magnitude M of an object is
defined to be its magnitude if it were at a
distance of dM 10 pc 10-5 Mpc
d
L
m
1 parsec (pc) 3.26 ly 3.08 x 1016 m
11
Extra Credit
Eq. (1)
Starting with Eq. (1), derive the
distance modulus Due Friday, 11 April
12
Distance Magnitude
Along with the luminosity distance d, astronomers
also measure distances using the distance
modulus defined as m m M
d
L
m
13
Luminosity-Period Relation
L-P Relation of Cepheid Variables
Henrietta Leavitt 1912
14
Red-Shifts of Nebulae
Vesto M. Slipher (1875 1969) In 1912, he
announced a puzzling discovery the light from
many nebulae was red-shifted, that is, shifted
to the red end of the spectrum
z (lo le ) / le
Lowell Observatory, Flagstaff, Arizona
15
The Nebulae
Drawing by the Earl of Rosse 1840s
16
(No Transcript)
17
Early Milestones in Cosmology
Published 25 November, 1915
Bending of light Prediction 1.74
Karl Schwarzschild
Sir Arthur Eddington Eclipse Expeditions 1919
18
Early Milestones in Cosmology
  • Einstein
  • His new theory of gravity, general relativity,
    predicted that
  • The universe must be either expanding or
    contracting.
  • However, like his contemporaries, Einstein
    rejected this prediction in favor of a static
    universe. Later, he would describe this as the
    biggest mistake of his life.

19
Discovery The Expanding Universe Edwin
Hubble, 1929
Speed (km/s)
Distance (Mpc)
1 Mega-parsec (Mpc) 3.26 x 106 light years (ly)
20
The Expanding Universe
  • 1929 Hubbles Law
  • Hubble assumed that the red shift is caused by
    the motion of galaxies away from us. In this case
  • z v / c
  • His observations are summarized in
  • Hubbles Law
  • v H0 d
  • where H0 is called Hubbles constant.

21
The Expanding Universe
  • Hubbles Law
  • v H0 d
  • The Hubble Time
  • d v t
  • d H0 d t
  • t 1/H0
  • For H0 70 km/s/Mpc
  • t 14 billion years

22
The Expanding Universe
a1 lt 1
d1 a1 d0
l1 a1 lo
z (lo - l1) / l1
1 z 1/a1 1/a(t1) a(t) is called the scale
factor
23
The Expanding Universe
Consider the time derivative of d(t) a(t) d0
Newtons dot notation
The Hubble parameter
This is Hubbles law
24
How Far Is Far ?
t0 t1 is called the look-back time
25
Summary
  • In 1929, Hubble discovered that the red-shifts of
    galaxies increased roughly linearly with
    distance
  • He interpreted these observations as evidence of
    an expanding universe, summarized in the law v
    H0 d
  • This was predicted by Einsteins theory of
    gravity some years earlier. But Einstein did not
    accept the prediction at the time
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