Evidence of the Big Bang

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1. Evidence of the Big Bang
2. The Big Bang
3. Stellar Evolution
4. Astronomical Objects
5. Problems with the Big Bang
6. The Age of the Universe
7. The Future

Edwin Hubble (1889-1953)
I too can see the stars on a desert night, and
feel them. But do I see less or more? The
vastness of the heavens stretched my
imaginationstuck on this carousel my little eye
can catch one-million-year-old light. - Richard
Feynman
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Olbers Paradox
Calculate the total amount of light reaching the
earth from all the stars in the universe.
Let the density of stars be ?? Take it to be
uniform throughout the universe. The number of
stars from R to R?R away will be
The light intensity reaching earthfrom a star
of power P a distance R away will be
IearthP/(4?R2)?
So the total starlight power reaching earth will
be


ò
But this is clearly not the case!
ò

I dn
?
I
R dR P dR
earth
0
0
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19th Century Cosmology
19th-century scientists knew of the solar system,
other stars, and nebulae (interstellar clouds),
but they were unaware of other galaxies, which
they referred to as spiral nebulae.
They also thought the universe was static and
unchanging.
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The cosmological constant
Attempting to achieve a static universe, Einstein
realized that he could modify his Field Equations
by introducing a term proportional to the
metric                                     
The constant ? is called the cosmological
constant. Einsteins effort was unsuccessful
the static universe described by this theory was
unstable, and observations of distant galaxies by
Hubble a decade later confirmed that our universe
is, in fact, not static but expanding. So ? was
abandoned, with Einstein calling it the "biggest
blunder he ever made".
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Standard Candles
Standard candles are celestial objects whose
output power, P, is known. Because intensity
diminishes as 1/r2
measuring the intensity reaching earth of a
standard candle, its possible to measure its
distance, r, away. In the 1920s, Henrietta
Leavitt discovered that Cepheid Variable Stars
can be standard candles. Their power output
period (on the order of days) was related to
their average output power. Also Type Ia
supernovas were found to have a constant peak
power (18). And, once established as
independent entities, entire galaxies have output
powers that could be considered about the same.
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Hubbles Measure-ments

• Hubble measured the recession velocity of
  astronomical objects and found a linear
  relationship with the (redshift) of key spectral
  lines emitted by very distant objects.

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How can the universe be expanding, yet continue
to look the same?

• Proposed in 1948 by Hermann Bondi, Thomas Gold,
  and Sir Fred Hoyle, the Steady-State Theory held
  that the universe is infinite and expanding, and
  matter is continuously created with net constant
  density.
• Only a few atoms per cubic meter per century
  would be required, so no one would ever notice.
• Unfortunately, no mechanism has ever been found
  for matter creation, as required by the
  Steady-State theory.
• Also, the universe should look the same at all
  distances, but it doesnt. We dont see galaxies
  forming nearby. Quasars exist only a billion
  light-years away or more.
• So the universe doesnt actually look the same as
  it expands.

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1. Evidence for the Big Bang Cosmic Microwave
Background Radiation

• Penzias and Wilson observed a cosmic microwave
  background radiation that permeates the universe.
  
• The blackbody radiation of several billion years
  ago has Doppler-shifted to 3 K today.
• Satellite measurements show a nearly isotropic 3
  K radiation background.

Arno Penzias and Robert Wilson and their
microwave antenna in Crawford Hill, NJ
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2. The Big Bang

• The Big Bang model rests on two theoretical
  foundations
• The general theory of relativity
• The cosmological principle, which assumes the
  universe looks roughly the same everywhere and in
  every direction. The universe is both isotropic
  and homogeneous.
• RobertsonWalker metric is the simplest
  space-time geometry consistent with an isotropic,
  homogeneous universe.

?s2 a(t)2 ?x2 ?y2 ?z2 c2?t2
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Microwave background fluctuations in the
different universe shapes
microwave background intensity vs. ? and ?.
Recent measurements of the angular variation in
the microwave background by the Wilkinson
Microwave Anisotropy Probe (WMAP) indicate a flat
universe.
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The Missing Mass and Dark Matter
Luminous matter only accounts for 3 to 4 of the
critical density of the universe. What about
dark matter that we cant see?
Using this simple relation, we can estimate the
total mass of the galaxy, M.
The dark matter halo covers the space between
100,000 to 300,000 light-years from the galactic
center. About 70 of the Galaxy is composed of
dark matter.
100,000 light-years
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The Missing Mass and Dark Matter
One possibility for dark matter is MAssive
Compact Halo Objects (MACHOs).
Example brown dwarves (objects whose mass is
between twice that of Jupiter and the lower mass
limit for nuclear reactions (8 of the mass of
our sun). Brown dwarfs are failed stars with
insufficient density to start nuclear fusion.
Brown dwarves (artists rendition)
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The Missing Mass and Dark Matter
Another possibility for dark matter is much
smaller particles Weakly Interacting Massive
Particles (WIMPs).
WIMPs are elementary particles with very tiny
interactions with ordinary matter (likely only
the weak force). They don't emit or absorb
photons. With hardly any interactions, they would
be very hard to detect. But they have gravity.
Neutrinos almost fill the bill But their mass
is too low. Maybe the WIMPS needed are a kind of
particle that hasn't been discovered yet.
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The cosmological constant rules!

• Visible matter is only 4 of the total mass in
  the universe. Dark matter accounts for 23, and
  73 is mysterious dark energy.
• The star cluster and galaxy data are consistent
  with a low density universe.
• The cosmic microwave background radiation is
  consistent with a flat universe.
• Distance determinations based on Type Ia
  supernovae data are consistent with an
  accelerating universe.
• These sets of data constrain the universe mass
  parameters to the values Ok 0, O???????, and
  O? 0.7.
• The cosmological constant appears to be the
  dominant effect! But why? And what is it really?

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The Inflationary Universe

• A variation of the Big Bang model proposes that
  the universe suddenly expanded by a factor of
  1050 during the time 10-35 to 10-31 seconds after
  the Big Bang. This is called the inflationary
  epoch. It is due to the separation of the nuclear
  and electroweak forces.
• After the inflationary period, it resumed its
  evolution from the Big Bang.
• The inflationary theory requires that the mass
  density be near the critical density.
• The universe reached equilibrium before the
  inflationary period began.
• This explains the homogeneous universe.
• Magnetic monopoles would have to occur along the
  boundaries or walls of different domains. So this
  explains why we dont see them.

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Other Lingering Problems

• Formation of Stars Galaxies
• The universe is clumpy. The distribution of stars
  and galaxies is not uniform.
• The cosmic background radiation indicates
  fluctuations that may have led to galaxy
  formation.
• But which formed first, stars or galaxies?

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7. The Future

• The Demise of the Sun
• The sun is about halfway through its life as a
  star, which started 4.5 billion years ago. As its
  hydrogen fuel is exhausted, the sun will
  contract, but then heat up even more as it next
  burns helium.
• The heat will cause it to expand and even consume
  the Earth.
• The sun will become a red giant and the surface
  will cool from 5500 K to 4000 K.
• Eventually the light elements in the outer layers
  will boil off and the sun will contract to the
  size of the Earth with a final mass that will be
  half its current mass.
• The sun will cool down to become a white dwarf
  and then a cold black dwarf.

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The Long-Term Future of the Universe

• The universe is flat, but it is expanding. And
  the expansion is accelerating.
• Eventually all the stars in our galaxy will die
  as well as those in all other galaxies. Black
  holes will consume them and eventually consume
  all available mass.
• It seems that the universe will evolve to a cold,
  dark place. And thats even before protons begin
  to decay
• But theres so much we dont know. We await the
  next revolution