Download Cosmology Basic Assumptions of Cosmology

Cosmology AST2002 Prof. Voss
Page 1
Cosmology
the study of the universe as a whole
Geometry
finite or infinite? center? edge?
static or dynamic?
Olbers' Paradox Why does it get dark at night?
If the universe is
static, infinite, eternal,
uniformly filled with stars,
then every line of sight
will eventually hit a star.
Although light from each star
decreases as 1/R2,
the number of stars in a given
direction increases as 1/R2,
so each direction will be as
bright as the Sun!
Sagittarius star field
Olbers assumed dust
blocks distant stars
but eventually heats
to same T, glows like sun.
universe not infinitely old
Edgar Allen Poe (1848)
light from distant stars
has not reached us yet
universe is expanding
light from distant stars
red-shifted to very low freq. (cold)
18-01b
observable universe - that part that we can see
Basic Assumptions of Cosmology
Homogeneity
matter uniformly spread through space
at sufficiently large scales
not actually observed
Isotropy
looks the same in all directions
at sufficiently large scales
Cosmological Principle
any observer in any galaxy
sees the same general features
Copernican Principle - Earth is not a "special" place
12/3/2001
Cosmology AST2002 Prof. Voss
Page 2
no edge or center
Universality - physical laws are the same everywhere
Newton - gravity the same for apples on Earth and the Moon
Fundamental Observations of Cosmology
1. It gets dark at night.
2. The universe is expanding
light from distant galaxies
has red shifts ∝ distance
no center
18-03a
18-03b
Hubble constant 70 km/s/Mpc ⇒ age 14 billion year
Geometry of Space-Time
Einstein's
General Relativity
matter
⇒ local distortion
Black Holes
Large Scales
ultimate fate
determined by
density or
total mass
closed
universe
positive
curvature
flat
universe
zero
curvature
open
universe
negative
curvature
Critical Density
4×10-30gm/cm3
if density is
greater ⇒
closed
expand then contract
equal ⇒
flat
expansion slows
less than ⇒
open
expansion continues
Search for Dark Matter
to find fate
12/3/2001
Cosmology AST2002 Prof. Voss
Page 3
WIMPs
exotic weakly interacting
unknown in laboratories
MACHOs
Massive Compact
Halo Objects
low mass stars or planets
gravitational lens
have been observed
but too few
18-13
The Big Bang
Early History
no distinction
different forces
radiation dominated
nearly equal
12/3/2001
Cosmology AST2002 Prof. Voss
Page 4
matter - antimatter
gamma ray ⇔
particle+antiparticle
as it cooled
the composition
changed
differentiated
segregated
18-16
initially gamma rays
in 4 sec cooled:
neutrons, protons,
electrons complete
2 minutes
more cooling
produced
hydrogen 75%
deuterium
helium 25%
but little heavier
no stable isotopes
of atomic mass 5 or 8
18-12
106 years - 3000 K
neutral atoms formed
universe became transparent
18-11
Cosmic Background Radiation
red shift remnant
of 3000 K
blackbody radiation
from universe
at age 1 million
corresponds to cooler
temperature 2.7 K
18-09a
18-09b
18-09c
COsmic Background Explorer
COBE satellite 1992
showed 2.7 K radiation
(from age 1 million)
uniform
with slight variations
inflationary universe theory
12/3/2001
Cosmology AST2002 Prof. Voss
Page 5
quantum mechanics + general relativity + cosmology
⇒ very rapid expansion around 10-35 sec
uniformity of cosmic background radiation
slight variations in COBE ⇒ flat universe
Gravity causes matter to clump together
amplifies small variations over time giving
Large-Scale structure in the universe
maps of most distant galaxies
N and S from Milky Way
show a Great Wall
largest known structure
plus filaments and voids
also found in simulations
of gravitational attraction
12/3/2001