1_Introduction

1
Please pick up your corrected problem set and
midterm.
Problem Set 4 median score 85 Midterm Exam
median score 72
2
Recap The Story So Far

• Monday, November 3 Next
  planetarium show Thu, Nov 6, 6 pm.

3
History of cosmology
Version 1.0 Superdome model
4
Version 2.0 Geocentric model
spherical Earth at center
5
Version 3.0 Heliocentric model
Sun at center
6
v. 3.1 Infinite heliocentric model
7
v. 4.0 Big Bang model
8
v. 4.1 Big Bang model with space-time curvature.
Mass energy cause space to curve. This
curvature causes an observed bending of the path
of light.
9
Curvature on large scales
Positive curvature gravitational lensing makes
distant objects loom large.
Negative curvature gravitational lensing makes
distant objects appear tiny.
10
Measured curvature on large scales
Observed angular sizes of distant galaxies
consistent with flat space.
If space is curved, its radius of curvature is
bigger than the observable universe.
11
Expansion on large scales
12
As light travels through space, its wavelength
expands along with the expansion of space.
13
Galaxy with the highest known redshift
Name IOK-1 Redshift z 7
14
Redshift z7. What does this mean?
1 nm 1 nanometer 10-9 meters
Hydrogen has an emission line at ?0 122 nm.
In this galaxy, the line is seen at ? 8 122
nm 976 nm.
15
Redshift z7. What does this mean?
Light emitted with wavelength ?0 122 nm has
been stretched to ? 8 122 nm 976 nm.
Universe has expanded from a scale factor a 1/8
(when light was emitted) to a 1
(when light is observed).
16
If we observe a distant galaxy with redshift z,
the scale factor a at the time the galaxys light
was emitted was
Example z 1 implies a1/(11) ½. Lengths
(including wavelengths of light) have doubled
since light was emitted.
17
Photons from distant galaxies arent stamped with
born on dates.
scale factor
However, they are stamped with the amount by
which the universe has expanded since they were
born.
(measurable) redshift
18
When was the light we observe from this galaxy
emitted?
A convenient aspect of a Big Bang universe the
start of expansion gives an absolute zero for
time.
19
Different calendars have a different zero point
(birth of Christ, hijra to Medina, etc.)
For a temperature scale, theres a logical
absolute zero the temperature at which random
motions stop.
For a cosmic time scale, theres also a logical
absolute zero the instant at which expansion
began.
20
t 0 (start of expansion, alias The Big Bang)
t ??? (first galaxies)
t 14 billion years (now)
21
When was the light we observe from this galaxy
emitted?
t 750 million years (when the universe was only
5 of its present age).
22
How far away is this galaxy?
The galaxys light took about 13 billion years to
reach us.
If the universe werent expanding, we could say
its about 13 billion light-years away.
But the universe IS expanding!!!
23
How far away is this galaxy?
Farther away than it used to be!
te time light was emitted to
time now de
distance when light was emitted do distance now
te lt to
de lt do
24
When the light we observe now was emitted
de 1700 megaparsecs
5.5 billion light-years
Now, when we observe the light
do 8 de 8 1700 13,600 megaparsecs
44 billion light-years
25
Point to ponder
5.5 billion light-years (initial distance)
is less than
13 billion light-years (distance if
static)
is less than
44 billion light-years
(current distance)
26
Point to ponder
Current distance to z7 galaxy 44
billion light-years 13,600 megaparsecs more
than 3 Hubble distance!
As z ? infinity,
current distance ? 3.2 Hubble distance
27
The most distant object we can see (in theory) is
one that emitted a photon at t0.
We will see this photon with a huge redshift z,
since the universe has expanded hugely since the
Big Bang.
Photons emitted at t0 come to us from the
cosmological horizon.
28
The cosmological horizon is at a distance of 3.2
the Hubble distance (about 14,000 megaparsecs,
or 46 billion light-years).
Longer than the Hubble distance because of
universal expansion.
29
Wednesdays Lecture
Photons Electrons
Problem Set 5 handed out.
Reading
Chapter 8