Key Concepts: Lecture 33:
The Big Bang!
Age of the Universe and distances from H
0
Closed, Flat and Open Universes
Evidence for the Big Bang (Galactic Evolution; Microwave Background;
Nucleosynthesis)
Modern Theory for Galaxy Formation and Evolution
Hubble’s
Constant
H
o
=70km/sec/Mpc
!
o
=14 billion Years
Also gives us an estimate of the age of the universe
Trace expansion back to size=0
Age = time of expansion =
!
o
= d/v = 1/H
o
Also, once we know H
0
this allows us to measure cosmic
distances by measuring velocity from Redshift (Dopper
shift) of spectral lines:
v = H
o
d
so
d = v / H
0
Gravity slows down the expansion…
•
So the age of the Universe would
be less than we estimated based
on constant velocities, because
velocities were faster in the past.
•
If there is enough mass (in other words if
the density of the Universe is above a
critical value) then perhaps the
expansion can be reversed: the future of
the Universe depends on this. (Like the
escape speed from a planet: do the
galaxies have enough speed to overcome
their gravitational attraction?)
Big Crunch
The Geometry of Space
•
If density > critical density
–
Closed Universe
–
Space curves in on itself
–
Described by spherical geometry
–
No parallel lines
–
Finite (“closed”) in volume, but has no boundaries (no “edge”). Like
surface of a sphere, but in 3 dimensions
–
The Universe will turn around and collapse to “Big Crunch”
•
If density = critical density
–
Spatially Flat Universe (Euclidean geometry)
–
Infinite in volume
–
The universe expands forever, but just barely
•
If density < critical density
–
Open Universe
–
Infinite in volume
–
Easily expands forever
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So what kind of Universe do we live in?
•
The critical density is very small:
about 9x10
27
kg/m
3
.
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 Spring '07
 Dr.HaywoodSmith,Jr
 The Big Bang, General Relativity, Big Bang, Redshift, Cosmic microwave background radiation

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