THE BIGBANG THEORY and MODERN COSMOLOGY

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THE BIG-BANG THEORY and MODERN COSMOLOGY

• COSMOLOGY - THE STUDY OF THE UNIVERSE AS A WHOLE

By Dr Fred Stevenson
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OBJECTIVES

• TO INTRODUCE THE BIG-BANG THEORY (AND CLEAR UP
  SOME MISCONCEPTIONS)
• TO DISCUSS THE EVIDENCE IN SUPPORT OF THE THEORY
  AND HOW IT IS OBTAINED
• TO DISCUSS UNSOLVED PROBLEMS AND QUESTIONS FOR
  THE FUTURE

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HUBBLES KEY OBSERVATION Edwin Hubble (1920s) -
Obtained the first accurate distance to the
Andromeda Nebula (M31) by using Cepheid variable
stars as standard candles Answer 2x106 light
years, placing it way outside our own Milky Way
galaxy - it would take 2,260,000,000,000 years to
reach it at the speed of a jet plane! M31 is
therefore an external galaxy - and hence so were
the other spiral nebulae - a decades old
controversy was solved - the Milky Way galaxy is
just one of countless galaxies spread throughout
the universe
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The Spiral Galaxy, M51, the Whirlpool
Galaxy Distance 15 million light years
M100, Distance 50 million light years
Image from ING La Palma
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Hubble later discovered that the more distant a
galaxy the more redshifted its spectrum
became. redshift ? velocity (the Doppler
effect) so, Velocity ? Distance
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Hubbles original diagrams demonstrating the
expansion of the universe The slope of the graph
gives an estimate of the age of the universe -
about 15 billion years (time distance/velocity)
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Hubbles Great Discovery What is the implication
of these observations?
expanding
The universe is
In the past galaxies were closer together - the
universe was then denser - the present expansion
rate shows that the time since the beginning of
the expansion - THE BIG-BANG - was about 15
billion years ago.
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THEORY - Einstein and General Relativity A theory
of gravity relating space, time, energy and
motion General Relativity in a nutshell Matter
tells space-time how to curve (geometry)
Curvature (geometry) tells matter how to move The
equation naturally contains the possibility of
expansion or contraction - it is very difficult
to keep things still! The universe could have
started in a state of high density - a
big-bang - and has been expanding ever since
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• The predictions of General Relativity have been
  confirmed in experiments to great accuracy -
• Light bends in a gravitational field -
  observations made during the total eclipse of the
  sun in 1919.
• The anomalous precession in the perihelion of
  Mercurys orbit was finally explained.
• Gravitational fields produce an additional
  redshift.
• Accelerating masses produce gravitational waves -
  this has only been confirmed indirectly via
  observations of the binary pulsar. Many
  experiments are currently operating and others
  are being designed to detect them directly.

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COMMON QUESTIONS AND MISCONCEPTIONS What are the
galaxies expanding into? Theyre not!! Space is
expanding pushing the galaxies apart There is no
pre-existing void Hubbles Law is explained as the
uniform expansion of the whole of space Where is
the centre of the expansion? - nowhere! Where is
the edge of the universe? - there isnt one! The
big-bang happened everywhere!
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• Other evidence for the finite age of the universe
  
• The oldest stars known, in globular star
  clusters have ages of 12-15 billion years old

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OTHER EVIDENCE FOR A BIG-BANG Relic radiation
from the hot fireball phase. Because of
expansion the temperature is now only 2.7K -
exhibiting a perfect black-body
spectrum Fluctuations in this temperature in
different directions tell us about 1. Our motion
through the microwaves (the Doppler effect
again) 2. Formation of structure in the universe
at a time corresponding to 300,000yrs after the
big-bang - Remember - looking out looking back
in time
The Microwave Background
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Microwave Background Full Sky Maps (COBE
satellite)
a Dipole anisotropy
b Primordial fluctuations
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The universe at high redshift large 'look-back'
times
Galaxies far away and hence long ago have
different properties to now - in morphology,
colour and clustering properties

• The Hubble and other deep fields.
• Gravitational Lenses - allow us to see galaxies
  to higher redshift than the telescope alone.
• Quasars (QSOs) number-redshift relation - a peak
  in quasar activity at redshift 2-3.
• Galaxy number-magnitude counts - difficult to
  disentangle the effects of cosmology and
  evolution - but interestingly non-evolving models
  are excluded.
• We can directly observe the evolution of galaxies
  and the universe.

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CONCLUSIONS - WHAT WE KNOW 1. WE LIVE IN AN
EVOLVING UNIVERSE IT STARTED OFF 15 Gyrs AGO IN
A HOT DENSE STATE 2. IT HAS BEEN EXPANDING AND
COOLING AND FORMING MORE COMPLEX STRUCTURES EVER
SINCE
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WHAT WE WOULD LIKE TO KNOW 1. THE FAR FUTURE -
ENDLESS EXPANSION OR RE-CONTRACTION? 2. WHAT IS
THE NATURE OF THE DARK MATTER AND THE VACUUM
ENERGY? 3. WHAT HAPPENED IN THE FIRST 10-20
SECONDS? 4. IS OUR UNIVERSE UNIQUE? OR IS IT
JUST PART OF AMULTI-VERSE
These questions are closely related to the holy
grail of modern physics - the theory of
everything - or TOE - the unification of all of
the fundamental forces of nature - this may one
day allow us to understand the origin of the
universe and answer the above questions.