3. The Big Bang Model
A broadly accepted theory for the origin and
evolution of our universe.
12 to 14 billion years ago, the part of the universe
we can see today was hot, dense and only a few
millimeters across.
It has since expanded into a vast and much cooler
cosmos.
Remnants of the original state of the universe can
still be seen as ‘background radiation’ (microwave
radiation)
Friday, 23 July 2010
4. Key idea 1:
General Relativity
2
E = mc
E= energy
m = mass
c = speed of light
Gravity is not really a
‘field’, but a distortion
Albert Einstein’s theories of time and space
Friday, 23 July 2010
5. Key Idea 2:
The cosmological Principle
• Matter is evenly
distributed (on
average)
• The temperature of
the cosmic
microwave
background Distribution of galaxies over a 30°
section of the sky
radiation is very
uniform
Friday, 23 July 2010
6. Evidence - Expansion
• Edwin Hubble
(1929)
• Nearby galaxies are
moving away from
us at a speed
proportional to
their distance from
us.
Expanding raisin bread model
Friday, 23 July 2010
7. Evidence - Light element
abundance
One second after the Big Bang...
Temperature of the universe was ~ 10 billion °C
Universe was filled with a sea of neutrons, protons and
electrons
As the universe cooled, the neutrons either broke down into
protons and electrons or combined with protons to make
deuterium (an isotope of hydrogen)
During the first three minutes of the universe, most of the
deuterium combined to make helium.
Trace amounts of lithium were also produced at this time.
Friday, 23 July 2010
8. Evidence - Background
radiation
The early universe was a very hot place
As it expanded, the gas within it cooled.
Thus the universe is filled with
electromagnetic radiation that is,
literally, the remnant ‘heat’ left over
from the Big Bang.
This is called the “cosmic microwave
background radiation”, or CMB.
Friday, 23 July 2010
9. ‘Heat’?
Expansion =
Originally, cooling = lower
the energy
universe
was very Today, the
hot radiation is
much colder,
High heat = only 2.725°
high above absolute
energy zero (- 273.16°C)
Microwaves
Friday, 23 July 2010
10. Problems with the Big
Bang?
• On average the
universe is very
evenly distributed
but ...
• in local areas, this is
not so (stars and
galaxies exist with
very little between
them)
Friday, 23 July 2010
11. Where will it all end?
• Determined by two competing forces;
momentum and gravity
• therefore, two possibilities....
• Not enough matter = not enough
gravity...Universe expands (and cools) forever
• Sufficient matter ... gravity overcomes
momentum and universe collapses back in on
itself (eventually)...the “Big Crunch”.
• At the moment, the expansion is accelerating!
Friday, 23 July 2010
