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Lecture 23: Cosmology
Astronomy 111
The first three minutes
The First Three Minutes: A Modern View of the Origin of the Universe
by Steven Weinberg
The Big Bang
Bang’s
s hot past
• Today:
– Universe is low-density
low density and very cold (2.7
K)
– Steadilyy expanding
p
g
• ~15 Gyr Ago:
– Universe was smaller
smaller, denser,
denser & hotter
– Expanding at a somewhat faster rate
• How far back can we go?
ASTR111 Lecture 23
Early (expanding) Universe
Average temperature
decreases with expansion
expansion.
ASTR111 Lecture 23
Loosing & binding
• Binding Energy:
– Energy
gy needed to unbind ((break up)
p) matter.
• Binding Temperature:
– Temperature
p
equivalent
q
to the binding
g energy.
gy
– Matter at this temperature “melts” (unbinds)
• Example:
– In massive stars, nuclei melt at T~10 Billion K.
ASTR111 Lecture 23
Typical sizes & binding energies
At
Atoms
Binding
Size Energy
3

10
m 10 K

10
10
K
m
Nuclei
10
p&n
11
10 m 10 K
Quarks 10 m 1013 K
ASTR111 Lecture 23
Fundamental forces of nature
• Gravitation:
– Long-Range
• Electromagnetic Force:
– Long-Range,
g
g 10 stronger
g than g
gravity
y
• Weak Force:
– Range
g <10 meters,, 10 g
gravity
y
• Strong Force:
– Range
a ge <10
0 meters,
ete s, 10
0 g
gravity
a ty
ASTR111 Lecture 23
Unification of the forces
• Electroweak Force:
– EM & Weak forces unifyy at high
g energies
g
(1015K)
– Verified in particle accelerator experiments.
• Grand Unified Theory (GUTs):
– Strong & Electroweak Forces unified.
– Predicted, but no experimental basis (yet?)
ASTR111 Lecture 23
“Dreams
Dreams of a final theory
theory”
• What about Gravity?
– Gravity should unify with the GUTs force at
very high energies.
– Much higher than in any possible
accelerator.
accelerator
– However, these energies could occur in the
early Universe
Universe.
• Problem:
– We have no quantum theory of Gravity!
ASTR111 Lecture 23
The cosmic timeline
• Physics gives us a framework within which
to describe the Big Bang from the earliest
phases to the present.
– Particle accelerators probe matter at states
similar to some of these early phases.
• Large Hadron Collider will soon begin experiments
–A
Astronomers
t
look
l k for
f evidence
id
iin th
the presentt
Universe (e.g., Cosmic Background, primordial
deuterium & helium,, dark energy)
gy)
ASTR111 Lecture 23
LHC
ASTR111 Lecture 23
LHC instrument
ASTR111 Lecture 23
Planck epoch
• Before t=10 sec:
– All 4 forces unified into a single Superforce
– 1 force rules all of physics
• Can’t say much else, since we don’t yet
have a quantum theory of gravity to
guide us.
ASTR111 Lecture 23
Planck epoch
t<10-43 sec
T>1032 K
No theory of
quantum gravity
All forces may
have been unified
ASTR111 Lecture 23
Grand unification epoch
• At t=10 sec, T=1032 K (???):
– Gravity separates from the Superforce
– Strong & Electroweak Forces still unified.
• 2 forces rule physics:
– Gravity & GUTs
• Universe is a soup of quarks, antiquarks
& photons
photons.
ASTR111 Lecture 23
Grand unification epoch
10-43<t<10-38 sec
1032>T>1029 K
Gravity becomes
distinct from other
forces
Era ends when strong
& electroweak forces
decouple; inflation?
ASTR111 Lecture 23
Inflationary epoch
• t=10 sec (?), T=1027 K (?):
– Strong force separates from GUTs force
– EM & Weak forces still unified
• 3 forces rule physics:
– Gravity, Strong, and Electroweak forces
• R
Rapid
id separation
ti ttriggers
i
a rapid
id
“inflation” of the Universe
ASTR111 Lecture 23
Inflationary epoch
10-48<t<10-20 sec
1029>T>1015 K
Gravity, strong, &
electroweak forces are
distinct.
distinct
Era ends when
electrostatic & weak
forces decouple
decouple.
ASTR111 Lecture 23
The inflationary Universe
• Universe grows by a factor of 1050
between 10 and 10 seconds!
• Expansion
p
rate g
greatly
y slows after this
brief burst of inflation.
• Helps to explain why the universe is so
very smooth on large scales.
ASTR111 Lecture 23
Four forces at last
• t=10 sec, T=1015 K:
– Electroweak separates
p
into EM & Weak forces
– All forces are now separate
• 4 forces rule physics:
p y
– Gravity, Strong, Weak & Electromagnetic
• Conditions becoming right for free matter
to begin to exist separate from photons.
ASTR111 Lecture 23
Four forces at last
ASTR111 Lecture 23
Quark freeze-out
• At t=10
t 1066 sec, T=10
T 1013 K:
K
– Free quarks combine into hadrons
(primarily protons & neutrons)
– particle-antiparticle pairs & photons in
equilibrium:
q
pp  
nn 
ASTR111 Lecture 23
Nucleon freeze-out
• At t=0.01 sec, T=1011 K
– protons & neutrons decouple from photons
and exist as free particles.
– electrons & p
positrons in equilibrium
q
with
photons
– neutrinos & nucleons in equilibrium
• Free neutrons are stable during this
epoch.
h
ASTR111 Lecture 23
Freeze-out
As universe cools, particles and anti-particles
anti particles annihilate,
but fortunately for us there are slightly more particles:
ASTR111 Lecture 23
Freeze-out
10-10<t<0.001 sec
1015>T>1012 K
Amount of matter &
antimatter nearly equal
About 1 extra proton
for every 109 protonantiproton pairs.
Inequality very
important!
ASTR111 Lecture 23
Neutrino decoupling
• At t=1 sec, T=1010 K
– neutrinos decouple from matter
– stream out into space freely
– cosmic neutrino background (not yet
observed)
• Free neutrons are no longer
g stable:
– Decay into protons, electrons & neutrinos
– Left with about 1 neutron for every 7 protons
ASTR111 Lecture 23
Epoch of nucleosynthesis
• t~3 min, T=109 K:
• Fusion of protons & remaining free
neutrons:
– Formation of 2H ((Deuterium)) & 4He
– End up with ~75% H, 25% He
– Traces of D, Li, Be, B
• We cannot observe this directly, but we
products of these events.
can look for the p
ASTR111 Lecture 23
Epoch of nucleosynthesis
0.001 s<t<5 min
1012>T>109 K
Begins when matter
annihilates remaining
antimatter.
antimatter
Nuclei begin to fuse
ASTR111 Lecture 23
Helium nuclei formed
at ~3 minutes
Epoch of nuclei
• 3 min<t<300,000 yrs, 109>T>3000 K
• Universe cools to leave Hydrogen and
Helium nuclei
ASTR111 Lecture 23
Epoch of nuclei
5 min<t<380
min<t<380,000
000 yrs
109>T>3000 K
Universe becomes too
cool for photons to
break helium apart
(f
(free-streaming)
t
i )
ASTR111 Lecture 23
Epoch of recombination
• t=300,000 yr T=3000 K:
• Electrons & nuclei combine into neutral
atoms
– Universe becomes transparent
– Photons stream out into space
– Origin of the Cosmic Background Radiation
• Earliest we can see back directly using
light.
ASTR111 Lecture 23
Recombination
ASTR111 Lecture 23
Epoch of recombination
380,000
,
yrs
y <t<109 yyrs
3000>T>30 K
Atoms formed.
formed
Cosmic background
radiation released
released,
Universe becomes
transparent to
electrons
(Surface of last
scattering)
ASTR111 Lecture 23
ASTR111 Lecture 23
Epoch of galaxies
• Galaxy formation: t=109 yrs, T~30 K
– Quasars
– First generation of stars.
– First metals from first supernovae.
supernovae
• Present: tt=10
1010 yrs, T
T=2.726
2.726 K
– Galaxies, stars, planets, us...
– Metals from supernovae of massive stars
stars.
ASTR111 Lecture 23
Cosmic timeline
ASTR111 Lecture 23
What about the beginning?
• Our physics can not yet probe earlier than
the end of the Planck epoch (t=10 sec).
• Some would say we have problems back
before the Electroweak epoch
p
((t=10 sec).
)
• This will be the astrophysics of the 21st
Century (or maybe the 22nd…)
ASTR111 Lecture 23
Summary:
• Physics of the early Universe
– Informed by experimental & theoretical
physics
• The cosmic timeline:
– Observations go back to t~3 minutes
– Reasonably firm physics back to tt~10
10 sec
– Speculative back before t~10 sec
 sec
– Present theories stop at t~10
ASTR111 Lecture 23
ASTR111 Lecture 23