Transcript The Big Bang • The Big Bang • Tempature and density history of the Universe • Cosmic Background Radiation • Contents of the Universe.

The Big Bang
• The Big Bang
• Tempature and density history of the
Universe
• Cosmic Background Radiation
• Contents of the Universe
Big Bang
• Our conclusion that the Universe actually
began at some point in time is based on
extrapolating back the observed Hubble
expansion of galaxies
• Is there any other evidence?
Big Bang
If the Universe
was smaller in
the past, but had
roughly the same
amount of matter
and energy, then
the density of
matter and
energy must
have been higher
in the past.
The Universe was hotter in the past
lower T
higher T
• Temperature is proportional to the average kinetic energy
per molecule
1 2 3
K  mv  kT
2
2
k = Boltzmann constant =
1.3810-23 J/K = 8.6210-5 eV/K
Big Bang
Big Bang
Big Bang
Big Bang
First protons and neutrons at about 1 second.
Helium nuclei formed at about 100 seconds.
Observed ratio of Helium/Hydrogen matches Big
Bang prediction.
Universe is opaque.
Opaque
Big Bang
At one million years, electrons combine with
nuclei and atoms form.
Universe becomes transparent.
Transparent
Transition occurs at around T = 3740 K.
Cosmic Microwave Background
The Universe glows at 2.7 K in every direction.
CMB
Discovered by Arno
Penzias and Robert
Wilson in 1960-65
while employed by
AT&T’s Bell Labs and
attempting to find the
source of noise in an
antenna used to bounce
telephone signals
bounced off metallic
balloons high in the
atmosphere.
They won the Nobel
prize in 1978.
CMB
Radiation is a blackbody spectrum originally emitted
at 3000 K but red shifted by a factor of 1000.
Three pieces of evidence for the
Big Bang model
• Hubble expansion: galaxies are moving away
from us with speed proportional to distance.
• The cosmic microwave background: a 2.7 K
glow seen in all directions.
• The ratio of Helium to Hydrogen in gas clouds
unaffected by stars.
Cosmic Microwave Background
Small fluctuations are due to sound waves at
recombination.
Fluctuations in
CMB
• Properties of the CMB are set by physics of ionized plasma at
~4000 K – well known physics.
• Temperature variations in the Cosmic Microwave Background
(CMB) are observed to be about 0.0003 K, agrees with calculations
of sound waves propagating through plasma.
• The expected physical size of the hot/cold regions can be
calculated. What can we do with this?
Angular size in curved space

D
D

R  sin(d / R) d

D
d

D
D

R  sinh(d / R) d
• We can draw triangles!
• Angles in triangles in curve space do not equal 180°.
• This affects the angular size
Angular size of CMB fluctuations
• In positively curved Universe, fluctuations appear larger than
calculated
• In negatively curved Universe, fluctuations appear smaller
• In flat Universe, fluctuations appear at expected size
Curvature of the Universe
The curvature of the Universe is determined
by the density parameter 0

0 
C
0 < 1  negative curvature
0 > 1  positive curvature
Measurement of CMB fluctuations gives
0  1.02  0.02
Contents of the Universe
• Normal matter
– Stars
– hot gas
– anything made of atoms
• Total is 4% of C
Rotation curve of Milky Way
Mass of the Milky Way
Dark Matter
• Dark – it doesn’t produce light (any kind)
• Does have mass, produces gravity
• Nature is unknown
• Most likely it is elementary particles
Contents of the Universe
• Normal matter is 4% of C
• Dark matter is 23% of C
• Total of normal and dark matter is M = 0.3
• But, we need 100% of C since Universe is flat
• Remainder, 73%, is “Cosmological constant”
or “dark energy”  = 0.7
Contents of the Universe
Why can't we see radiation produced during
the first 300,000 years after the Big Bang?
1.
2.
3.
4.
It was absorbed soon after it was emitted.
It hasn't reached us yet.
It has been deflected by black holes.
It passed by our part of the universe a few
billion years ago.
Review Questions
• Give three pieces of evidence for the Big
Bang model.
• What will happen to the Universe if the
density is less than the critical density?
• A few seconds after the big bang did
hydrogen atoms exists? Why or why not?