1
Cosmology
The Study of the Universe as a Whole
Including the beginning of the Universe
What is the largest kind of structure in the universe?
The
~100Mpc
flaments,
shells and voids?
On larger scales, things look more uniform.
600 Mpc
The Variety of Galaxy Morphologies
Spectra of galaxies
in clusters of
increasing distance
prominent
pair of absorption
lines
In 1920's, Hubble used Cepheids to find distances to
some of these receding galaxies.
Showed that redshift
or recessional velocity
is proportional to distance
:
V
=
H
0
x
D
(Hubble's Law)
velocity (km / sec)
Distance (Mpc)
Hubble's Constant (km / sec / Mpc)
Or graphically. . .
Current estimate:
H
0
= 73 +/ 2 km/sec/Mpc
If H
0
= 75 km/sec/Mpc, a
galaxy at 1 Mpc moves
away from us at 75 km/sec,
etc.
The Cosmological Principle
On the largest scales, the universe is roughly homogeneous (same
at all places) and isotropic (same in all directions).
Given no evidence of further structure, assume:
Hubble's Law might suggest that everything is expanding away
from us, putting us at center of expansion.
Is this necessarily true?
(assumes
H
0
= 65
km/sec/Mpc)
If we were at center of expansion, universe would be isotropic (but
only from our location) but not homogeneous:
Us
Galaxies expanding away
from us into empty space
But if we were not
at center, universe would be neither
isotropic nor
homogeneous:
Us
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So if the CP is correct, there is no center, and no edge
to the Universe!
Best evidence for CP comes from Cosmic Microwave Background
Radiation (later).
The Big Bang
All galaxies moving away from each other.
If twice as far away from
us, moving twice as fast (Hubble's Law).
So, reversing the Hubble
expansion, everything must have been together once.
How long ago?
Hubble’s Law:
v = H
0
d
where
H
0
gives rate of expansion.
Assume H
0
= 75 km / sec / Mpc.
So
galaxy at 1 Mpc from us moves away at 75 km/sec.
How long did it
take to move 1 Mpc from us?
time
=
=
=
=
13 billion years
distance
velocity
1 Mpc
75 km/sec
The faster the expansion (the greater H
0
), the shorter the time to get to
the present separation.
1
H
0
Big Bang
: we assume that at time zero, all separations were infinitely
small.
Universe then expanded in all directions.
Galaxies formed as
expansion continued.
But this is not galaxies expanding through a preexisting, static space.
That would be an explosion with a center and an expanding edge.
If CP is correct, space itself
is expanding, and galaxies are taken
along for the ride.
There is no center or edge, but the distance
between any two points is increasing.
A raisin bread analogy provides some insight:
But the cake has a center and edge.
Easier to imagine having no center or
edge by analogy of universe as a 2d expanding balloon surface:
To understand what it would be like in a 2d universe, read Flatland
by Edwin Abbott
DEMO
 Expansion in 1D
Now take this analogy "up one dimension".
The Big Bang occurred
everywhere at once, but "everywhere" was a small place.
If all distances increase, so do wavelengths of photons as they travel and
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 Fall '08
 GregTaylor
 Big Bang, Redshift, Physical cosmology, Cosmic microwave background radiation

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