The Early Universe

1
The Early Universe

• Thursday, January 24
  (planetarium show tonight 7 pm, 5th floor Smith
  Lab)

2
Hot, dense, opaque objects emit light.
Color (or wavelength) of light depends on
objects temperature.
3
Temperature is inversely proportional to
wavelength.
red-hot iron 1000 K
lightbulb filament 2900 K
Suns surface 5800 K
4
Early universe was hot, dense, and opaque it
emitted light.
In 1965, two astronomers (Penzias Wilson)
discovered faint static in their microwave
antenna.
5
This static was the leftover light from hot,
dense, opaque early universe.
Its spectrum peaks at ? 1 mm this is microwave
radiation.
Scientists call the leftover light the
Cosmic Microwave Background (CMB).
6
(No Transcript)
7
The universe became transparent at a temperature
T 3000 K.
8
How did the cosmic background change from visible
infrared light (? 0.001 mm) to microwave
light (? 1 mm)?
How did its temperature drop from 3000 K to 3 K?
9
The universe is expanding.
Distance between galaxies increases.
Wavelength of light (distance between wave
crests) increases.
10
Wavelength of cosmic background light has
increased by a factor of 1000.
Why? Because the universe has expanded by a
factor of 1000 since the time it became
transparent.
11
We now have two ways to think about a galaxys
redshift.
1) The redshift is the result of a Doppler shift.
2) The redshift is the result of expansion
stretching the wavelength.
12
Example a galaxy has a redshift z (?-?0)/?0
0.01.
Doppler explanation

• Radial velocity of the galaxy is 1 the speed of
  light
• v 0.01 c 3000 km/sec d
  v/H0 42.9 Mpc.

13
Expansion explanation
2) The distance to the galaxy now is 1 greater
than it was when the light we observe was
emitted dnow 42.9 Mpc
dthen 42.9 Mpc / 1.01 42.5 Mpc
14
So, which way of thinking about redshift (Doppler
or expansion) is The Right Way??
In the limit of small redshift (v ltlt c) they are
identical.
Lets see why!!
15
Light from galaxy has traveled
daverage 42.7 Mpc 139 million
light-years in a time t 139 million years.
During that time, distance to the galaxy has
expanded by 0.01 daverage
1.39 million light-years.
Average radial velocity
1.39 million light-years / 139 million years
0.01 c.
16
Some galaxies have very high redshift.
The arrowed galaxy at left has z
(?-?0)/?0 5.7
For very distant galaxies, its best to think
of redshift as being due to expansion
(no guarantee of constant radial velocity!)
17
Astronomers are fascinated by galaxies at high
redshift.
A telescope is a time machine.
Larger redshift ? larger distance ? longer light
travel time.
18
Astronomers
the Cosmic Microwave Background
19
The CMB has highest redshift of anything we can
see (z 1000).
When we look at the CMB, we look at the surface
of the glowing fog that filled the early
universe!
20
When we look at the CMB, we see a message direct
from the early universe.
What is this message telling us?
Messages are often (1) hard to read (2) hard to
interpret.
21
1) Reading the CMB
COBE
WMAP
Water vapor in Earths atmosphere absorbs
microwaves go above the atmosphere!
22
WMAP (Wilkinson Microwave Anisotropy Probe) is at
the L2 point, beyond the Moons orbit.
23
2) Interpreting the CMB
Observation Temperature of CMB is nearly
isotropic (the same in all directions).
Interpretation early universe was nearly
homogeneous (the same in all locations).
24
? hotter
cooler ?
Observation Temperature of CMB is slightly
hotter toward Leo, cooler toward Aquarius (on
opposite side of sky).
Temperature fluctuation 1 part per 1000.
25
Interpretation difference in temperature results
from a Doppler shift.
Earth orbits Sun
(v 30 km/sec)
Sun orbits center of our galaxy
(v 220 km/sec)
Galaxy falls toward center of
Local Group (v 50 km/sec)
Local Group falls toward Virgo
Cluster (v 200 km/sec)
26
Net motion toward Leo, with a speed v 300
km/sec 0.001 c.
Cosmic light from direction of Leo is slightly
blueshifted (shorter wavelength, higher
temperature).
27
Observation After subtracting the effect of our
motion through space, CMB still shows hot cold
spots, about 1 across.
Temperature fluctuation 1 part per 105.
28
Interpretation observed temperature fluctuations
result from density fluctuations in the early
universe.
Regions that were compressed had higher density,
but also higher temperature (gases heat up as
they are compressed).
29
Hot spots in the CMB are higher in temperature
than cold spots by only 1 part per 100,000.
Implication the density fluctuations in the
early universe were also small (about 1 part per
100,000).
30
The Rich Get Richer, the Poor Get Poorer.
A region that was only slightly denser than
average will eventually become much denser than
average its compressed by its own gravity.
31
Great Oaks from Tiny Acorns Grow.
A dense region that initially has a small mass
will become more massive with time its gravity
attracts surrounding matter.
32
Computer simulations of the growth of density
fluctuations
33
(No Transcript)
34
Gravity sucks.
Since gravity sucks (and doesnt spew), tiny
density enhancements become dense, massive
clusters of galaxies.
35
Tuesdays Lecture
The VERY Early Universe
Problem Set 3 due!!
Reading
Chapter 4