The Big Bang model describes the origin and evolution of our universe. It postulates that approximately 13.8 billion years ago, the entire observable universe was only a few millimeters in size and extremely hot and dense. Since then, the universe has been expanding and cooling. Evidence for the Big Bang includes the expansion of the universe, the cosmic microwave background radiation, and the relative abundance of light elements like hydrogen and helium. The Doppler effect and redshift help astronomers measure the speeds at which distant galaxies are receding from Earth, leading to the discovery that the expansion of the universe is accelerating. Dark matter and dark energy are hypothesized to explain discrepancies in measurements of the density and expansion rate of the universe.

1. ASTROPHYSICS
D3 Cosmology

2. THE BIG BANG
MODEL

3. The Big Bang
You are required to know the basic theory
behind the stages of development of the
Universe.
Detailed accounts are not necessary.
Just a general explanation.

4. The Big Bang
At the start, the Universe would have
been very hot.
As it expanded, the Universe cooled to a
temperature where atoms could be
formed.

6. Background radiation
In 1960 two physicists, Dicke and Peebles, realising that
there was more He than it could be produced by stars,
proposed that in the beginning of the Universe it was at a
sufficiently high temperature to produce He by fusion.
In this process a great amount of highly energetic radiation
was produced. However, as the Universe expanded and
cooled, the energy of that radiation decreased as well
(wavelength increased). It was predicted that the actual
photons would have an maximum λ corresponding to a black
body spectrum of 3K.
So, we would be looking for microwave radiation.

7. Background radiation
Shortly after this
prediction, Penzias and
Wilson were working with
a microwave aerial and
found that no matter in
what direction they
pointed the aerial it
picked up a steady,
continuous background
radiation.

8. Background radiation
In every direction, there is a very low energy and very
uniform radiation that we see filling the Universe. This is called
the 3 Degree Kelvin Background Radiation, or the Cosmic
Background Radiation, or the Microwave Background.
These names come about
because this radiation is
essentially a black body with
temperature slightly less
than 3 degrees Kelvin
(about 2.76 K), which peaks
in the microwave portion of
the spectrum.

9. Background radiation
Why is the background radiation an
evidence for the Big Bang?
The cosmic background radiation (sometimes called the
CBR), is the afterglow of the big bang, cooled to a faint
whisper in the microwave spectrum by the expansion of the
Universe for 15 billion years (which causes the radiation
originally produced in the big bang to redshift to longer
wavelengths).

10. Big Bang
The Big Bang Model is a broadly accepted theory for the
origin and evolution of our universe.
It postulates that 12 to 14 billion years ago, the portion of
the universe we can see today was only a few millimetres
across.
It has since expanded from this hot dense state into the
vast and much cooler cosmos we currently inhabit.
We can see remnants of this hot dense matter as the now
very cold cosmic microwave background radiation which still
pervades the universe and is visible to microwave detectors as
a uniform glow across the entire sky.

11. Big Bang
The singular point at which space, time, matter
and energy were created. The Universe has been
expanding ever since.
Main evidence:
Expansion of the Universe – the Universe is expanding
(redshift)  it was once smaller  it must have started
expanding sometime  “explosion”
Background radiation  evidence of an hot Universe that
cooled as it expanded
He abundance  He produced by stars is little  there is
no other explanation for the abundance of He in the Universe
than the Big Bang model.

12. Galactic Motion
We learned earlier that most Galaxies are
moving away from each other.
Expansion of the Universe.
Evidence- Red Shift of stellar spectra.
Further away… greater the Red Shift.

13. Galactic Motion
A method of determining the recessional speed of a
galaxy away from Earth is determined using the
equation…
Where  = the difference between the spectral line
from a stationary source and the spectral line from the
receding galaxy.
 = spectral line of the stationary source.



c

14. Example
 A characteristic absorption line often
seen in stars is due to ionized helium. It
occurs at 468.6 nm. If the spectrum of
a star has this line at a measured
wavelength of 499.3 nm, what is the
recession speed of the star?

15. Solution
499.3468.6
468.6
 
3 x 108
v1.97 x107
ms-1



c

16. Hubble’s Red Shift Law
We have just seen how we can determine
a galaxies recessional velocity from its
Redshift.
We can also determine the galaxies
distance.

17. Hubble’s Red Shift Law
Hubble discovered a relationship
between a galaxies distance and its
recessional velocity.

18. Hubble’s Red Shift Law
Distant galaxies were receeding very fast.
This fits with the expanding Universe
Theory.

19. Hubble’s Red Shift Law
 v ≈ d
 v = Hd
 Where H is
Hubbles
Constant
 It is the
slope of the graph.

20. Hubble’s Red Shift Law
Hubble’s Constant is estimated to be
around 65 km s-1Mpc-1
Much debate on the accuracy of this value.
Try this Raisin Bread analogy animation

21. Redshift z and cosmic scale
factor R

22. Fate of the Universe
Universe
Closed Open
Enough matter 
density is large enough
to prevent an infinite
expansion  gravity will
stop the Universe
expansion and cause it
to contract (Big Crunch)
Not enough matter
 density is such
that gravity is too
weak to stop the
Universe
expanding forever
Flat
Critical density 
Universe will only
start to contract
after an infinite
amount of time

25. Doppler effect
In astronomy, the Doppler effect was originally studied in the
visible part of the electromagnetic spectrum. Today, the
Doppler shift, as it is also known, applies to electromagnetic
waves in all portions of the spectrum.
Also, because of the inverse
relationship between frequency
and wavelength, we can
describe the Doppler shift in
terms of wavelength. Radiation
is redshifted when its
wavelength increases, and is
blueshifted when its wavelength
decreases.

26. Doppler effect
Astronomers
use Doppler shifts
to calculate
precisely how fast
stars and other
astronomical
objects move
toward or away
from Earth.

27. Doppler effect
Why is Doppler effect so
important?
In 1920’s Edwin Hubble and Milton Humanson realised
that the spectra of distant galaxies showed a redshift, which
means that they are moving away from Earth. So, if galaxies
are moving away from each other then it they may have
been much closer together in the past
Matter was concentrated in one point and some
“explosion” may have thrown the matter apart.

28. Critical density
The density of the Universe that separates a universe that
will expand forever (open universe) and one that will re-
collapse (closed universe).
A universe with a density equal to the critical density is
called flat and it will expand forever at a slowing rate.
So, how do we measure the density of the
Universe?

29. Critical density
If we take in account all the matter (stars) that we can see
then the total mass would not be enough to keep the galaxies
orbiting about a cluster centre.
So, there must be some matter that can not be seen – dark
matter. This dark matter cannot be seen because it is too cold
to irradiate.
According to the present theories dark matter consists in
MACHO’s and WIMPS

31. MACHO’s
WIMP’s
Massive compact halo objects – brown and
black dwarfs or similar cold objects and
even black holes.
Non-barionic weakly interacting massive
particles (neutrinos among other particles
predicted by physics of elementary
particles)
It seems that there is also what is called “dark energy”…

33. The type Ia supernova evidence for an accelerated
universe has been discussed by Perlmutter and the
diagrams below follows his illustration in Physics
Today.

34. One of the foundations for the big bang model
was the observed expansion of the universe.
Measurement of the expansion rate has found
that the expansion rate is very nearly "flat".
That is, the universe is very close to the critical
density, above which it would slow down and
collapse inward toward a future "big crunch".
One of the great challenges of astronomy and
astrophysics is distance measurement over the
vast distances of the universe.

35. Since the 1990s it has become apparent
that type Ia supernovae offer a unique
opportunity for the consistent measurement of
distance out to perhaps 1000 Mpc.
Measurement at these great distances suggests
that the expansion rate of the universe is
actually accelerating. That acceleration implies
an energy density that acts in opposition to
gravity which would cause the expansion to
accelerate. This is an energy density which we
have not directly detected observationally and it
has been given the name "dark energy".

36. TOK
 Scientists claim our knowledge of the
universe is based upon 5% of what is in
the universe. Can we claim to know
anything about the universe?
 Are there other ways besides Science to
explain the universe? What happens
when these alternatives meet? Is one
right and the other wrong?