Stephen Hawking and the Universe
A fascinating story
The Universe was not in hurry to form the first stars
First atoms formed
MCBR emitted
Dr. Mounib El Eid
AUB, Physics Department
meid@aub.edu.lb
CVSP204: F2014
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Spiral Galaxy like ours
‫كالم جميل‬
The scientists does not study nature because it is useful;
he/she studies it because he/she delights in it, and he/she
delights in it because it is beautiful. If nature were not beautiful,
it would not be worth knowing, and if nature were not worth
knowing, life would not be worth living.
Who? Henry Poincare
My translation
‫العالم اليدرس الطبيعة ألنها مفيدة بل ألنه يجد متعة بجمالها‬
‫لو كانت الطبيعة غير جميلة ليس من متعة في معرفتها‬
‫ومعنى الجمال‬, ‫وأن لم نكن نتوق لمعرفة الطبيعة فما هو معنى الحياة‬2
Short Outline
Part I: Picture of the Universe
Interesting History
Space-Time & Relativity
Part II: Universe
Big Bang
Inflation
Cosmic History
Anthropic Principle
Part III: Relativity & Black Hole (was not included in the lecture)
My aim is three-fold:
not to tell trivial things
Avoid Equations
Initiate interest and thinking
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Part I: Picture of the Universe
1. Interesting history
 Aristotle (340 B.C.) :
Earth spherical. He realized that the Moon is eclipsed by the Earth and
Earth’s shadow was round. He thought: Earth stationary, Sun and Moon
circle the Earth- he believed in the perfection of the circles
 Eratosthenes (276-194 B.C.) : Human beings were always intelligent.
This measurement was done 200 B.C. by this Greek Philosopher
At the time one assumed that the
distance Aswan-Alexandria is 5000
stadia, where one stadium is 0.16
km, or 800 km=5000x0.16 km.
So, 800x50 km=40000 km very close
to the diameter of the Earth
These people had no electric lights
and Iphones, but they were smart.
To be smart, you should have a
functioning brain not only a
functioning cell phone.
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 Ptolemy: (100-170 A.D.) “Almagest”
Geocentric model of the universe
Seven of
these
tortoise
Epicycles and Planetary Motion
This model was suggested to resolve the
retrograde motion
tortus
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Copernicus:
Polish (1473-1543):
Heliocentric model of the Universe, 1514
removed Earth from the center, put the Sun
instead. He was afraid of the church. Hundred
years passed before the idea was take seriously.
Great achievement:
Sun at center (not the Earth) and at rest.
 Only the Moon orbits the Earth.
 Retrograde motion of planets a consequence of the
Earth’s motion relative to a planet.
 All planets revolve around the sun.
Copernicus model represents an exceptional turning point
in the history of science and human thought. He once for
all displaced us from our centric view of ourselves that
Wait for animation
our planet is a focal point.
Believe it or not:
Copernicus writings on the heliocentric universe
were placed on the Church’s index of prohibited
books in 1616. They have been published after 73
years. scientific truth wins always even
against a pope. Well, meanwhile Vatican has
observatory!
Mars
Sun
Earth
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GALILEO GALILEI:
Italian (1564- 1642)
Has used a self-made telescope in 1604
and his observation led to a break- through
of the Greek model. Galileo found:
 Moon shows mountains and craters,
 Dark spots on the surface of the sun
(sunspots). He found that the Sun is
rotating
 Galileo also observed Jupiter. He saw
FOUR small moons orbiting Jupiter
(invisible to the naked eye). This was
another conflict with the Greek’s model:
The Earth is not the only center of rotation
Amazing
Galileo published his results, and this was a play with fire. The Church
(Inquisition) put him under house arrest in 1633 until his death.
Firstly, in 1992, Galileo was rehabilitated by the pope in Rome.
The Church was left alone, but the Copernican/Galileo vision was out of the
bottle for all times!
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JOHANNES KEPLER:
German: (1571-1630)
Found a simple striking structure of the solar system
He formulated three laws
1. he orbits of the planets are ellipses with the Sun at
one focus
2. An imaginary line connecting
the sun to any planet sweeps out
equal areas of the ellipse in equal
intervals of time.
slow
fasr
3.The square of planets orbital period (p)
is proportional to the cube of its semimajor axis (a).
P a
2
3
Low of harmony
P in Earth's years
A in unit of 150 million km ,
Earth-Sun distance
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Full
harmony
Solar System Dimension
Semi-major
axis a [AU]
Orbital period
[Earth years] (p)
e
p2/a3
Mercury
0.387
0.241
0.20
1.002
Venus
0.723
0.615
0.007
1.001
Earth
1.000
1.00
0.017
1.000
Mars
1.524
1.881
0.093
1.000
Jupiter
5.203
11.86
0.048
0.999
Saturn
9.539
29.46
0.056
1.000
Uranus
19.19
84.01
0.046
0.999
Neptune
30.06
164.8
0.010
1.000
Pluto dismissed 39.53
248.6
0.248
1.000
Planet
The data in the table indicate:
1) Except of MERCURY and PLUTO, all orbits are close to circular .
2) The farther the planet is from the sun, the greater its orbital period, as
Kepler’s third law says.
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Isaac Newton :
British, (1642-1727)
Philosophiae Naturalis Principia Mathematica
Most important single work in physical science
He invented three laws:
1. Every object remains at rest, or moves at
constant speed in a straight line, unless an
unbalanced force acts on it.
2. The acceleration (a) produced by an applied force
F on an object of mass m is: a = F/m or F = ma
3. If an object exerts a force on another object, the
second objects exerts an equal and opposite force on
the first: Action = Reaction
He invented the force of gravity
May be the smartest physicist ever existed
M PM S
FG  G
2
r
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Newton” laws in action
The Sun attracts the planet due to the
force of gravity at each position. It forces
into a curved path
The Sun is by far more massive, the
planet tries to go straight (First law),
but it becomes accelerated (Second
law)
Homework: why then the orbit is elliptical?
Conclusions (so far):
The insight through the works by Copernicus, Galileo, Kepler and
Newton led to understand the solar system in a simple way .
The motion of the planets is related to basic principles. The planets are not
independent of each other as the Greeks thought.
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1. Space-Time and special relativity
Aristotle and Newton both believed in absolute time and that time separated
from space. This is also usually our common sense
But our common sense fails when dealing with high speeds significant fraction
of the speed of light, or when dealing with subatomic particles.
We need the theory of relativity and quantum mechanics to hope to
understand the Universe
Why relativity?
It is a striking fact in our universe that the speed of light is
c=2.99792458x108 m/s
in vacuum, independent of the motion of the observer or the source,
experimentally verified (Michelson- Morley experiment)
The theory of special (Einstein 1905, also Poincare’) is based on two
fundamental postulates:
1. universality of the speed of light
2. your description of physical reality is the
same regardless of the constant velocity at which you move
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Implication of the first postulate:
Imagine you are in a spaceship moving toward a flashlight. You could be
moving at 90% of the peed of light, you will measure the speed of light for the
flashlight, as your spaceship would be motionless.
Implication of the second postulate:
Suppose you were inside a railroad car moving due north in a straight line at 100
km/h . Any measurement you make inside the car, for example how long you can
stand on one leg , will be the same as if the car is moving in any other direction
or any other speed or not moving at all.
Striking consequence of relativity
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Equivalence of energy and mass: E=mc2: energy of a moving object adds to
its mass. This means it becomes harder and harder to increase its speed.
Any normal object is forever confined by relativity to move at a speed
less than the speed of light. Only light can travel at that speed
2
Our ideas of space and time received a shock. Relativity put an end to the
idea absolute time, every observer has own clock carried with him. And
moving clocks run slower: Time dilation
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Space-Time Diagram: If the Sun disappears you cannot know it immediately
Time [min]
x=ct
Presence
Not accessible
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
Future
But wait for 8.33 min
Earth
enters the
future
cone of
the Sun
0 0.2 0.4 0.6 0.8 0.9 1.0 1.2 1.4 1.6
x/(1011 m) Distance
Earth
Sun
150 million Km
Past
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Light conein 2 dimensions
time
In case of 2 space
dimension, we hav a
light cone
y
x
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Message to the audience
Science has the freedom of thoughts,
but also the responsibility of proofs
Religion is a believe, does not need a proof
There is no terror in a religion, but
there are terrorists in each religion
‫في العلم حرية التفكير لكن مسؤولية البرهان‬
‫الدين عقيدة التحتاج إلى برهان‬
‫اليوجد دين إرهابي بل إرهابين في الدين‬
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Part II: Universe
This part is challenging as concept. But I am not supposed to tell you
trivial things. Challenging is the best way for learning
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Expanding Universe
Expansion of space does nor affect the size of the material objects such as
galaxies and even apples.
Circle a galaxy on a balloon, the circle will not expand as the balloon is inflating
Important to know this
We can detect the expansion only if our measuring instruments have fixed sizes.
Otherwise if everything changes size we would not notice the difference
interesting
Even before Hubble’s discovery of the expanding universe. The Russian Alexander
Friedman solving the Einstein's equations proposed that the galaxies are moving
away from each other,
He proposed:
The universe looks the same in each direction and for all observers whereever.
How do we understand this?
Analogy:
If you look at a forest. Nearby, you see the space between the trees. But from far
away every square meter is similar to another.
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Friedman’s model begins with zero size followed by expansion. But with what
fate? Expanding forever, or crushing?
The idea of the Big Bang was born, but not many believed it
Fred Hoyle (Scottish) proposed a steady-state universe.
But in 1965, a great discovery was made, the discovery of the cosmic
Microwave Background Radiation (CMBR)
It is like in the microwave in your kitchen, but less powerful , and it is partially on
your TV when no picture appears
It was accidental discovery by Penzias and Wilson (1965), It is a relic of
hot Big Bang (BB),
But, why do we say “hot”? How do we know?
Fingerprint
At about 3 min after BB, the universe was a fusion reactor. The percentage of
Helium in the Sun is 28%, But Helium cannot be produced in stars except of 34%. Thus most of the helium was made during the first minutes and this needs at
least 100 million degrees, really hot.
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Interesting
The relativity theory of Einstein does not provide a true picture about the origin of
the Universe.
Why?
Einstein’s theory of general relativity predicts infinite temperature, density and
curvature, or singularity in mathematical sense. It is like dividing by zero.
Reflection
We are not saying that the general relativity is wrong, on the opposite it is
beautiful. However, it cannot be applied to the beginnig. It is after all a classical
field theory.
First Phase of expansion
It is called “cosmological INFLATION”:
The Universe expanded by a of 1000 ………….0000 (30 zeros) between a time
10-32 and 10-30 seconds
See Figure next page
Project for you: can you talk about MCBR with single temperature without inflation?
This is called horiozon problem
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We have a trouble!
It looks as the Universe was
expanding faster than light
We calm down:
The expansion of the
universe is the expansion of
space itself not the motion of
objects through space.
Concept of limited speed to
the speed of light does not
apply here
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Three indicators in favor of the Big Bang
1. MCBR
2. He-production
3. Hubble Expansion (receding Galaxies)
Now, the theory inflation is not well understood, it needs quantum gravity. But the
inflation was not completely uniform. This means very small irregularities in
temperature may exist.
Indeed, the COBE (Cosmic Background Explorer) satellite, and also the WMAP
(Wilkinson Microwave Anisotropy Probe) both discover this fluctuation
So it is experimentally verified that the MCBR temperature is
T=(2.726  0.0013) K
This will not heat your food in the microwave more than -270 K
The Universe is really cool in every respect
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Reflection:
A uniform Universe would be boring.
We are lucky to have these irregularities.
The Universe having some irregularities mean that some regions would
have slightly higher densities. Gravity would help the regions to collapse
to form galaxies, stars and planets and finally we came out!
If you think about this you could say we are product of quantum fluctuations in
the very early Universe.
WE ARE PTODUCT OF THE GREAT DESIGN
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Most Challenging:
The universe was a quantum event, because of the extremely small scales.
To have the ambition of understanding the beginning a combination of general
relativity and quantum mechanics is needed
But how does this work?
Gravity wraps space and time
Relativity easy to wrap space, but how to wrap time?
Let us think:
If you speak about space and time separately, you can do that in case of
low speed and weak gravity
But in general, space and time are intertwined. In case of space, you have no
problem to go around like around the Earth, since it is not flat. But time is like a
railway track, isn’t ?
If it has a beginning, then someone has to set the train going. But does this go on
forever?
When quantum mechanics is added, time becomes like another dimension of
space, which means that the early inverse is at least four-dimensional.
When we say “beginning”, we look backward to a time beyond our
experience, but not beyond our imagination, it is still subject to
Mathematics (I like this)
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Project:
what is Mathematics? More imagination than experience? Poor physicists they have to
go down with their imagination to the level of experience
Very strange
If time is another direction of space (say: spacetime), then we get rid of talking
about the beginning of time. Simply, you cannot bring it into you experience!
Question:
Why do religions can speak of beginning? Is it because the are not asked to bring
it into the experience?
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Back to physics
If we can combine general relativity with quantum mechanics, we stop asking
about what was before the beginning
Asking St Augustine: what was God doing before creating the Universe
His Answer: he was preparing hell for you when asking this question
Historical Remarks
Many believed including Aristotle: Universe must have always existed in order to
avoid the problem of
how it was set up
Others believed: Universe had a beginning and used that to indicate
existence of God
Any alternative?
Hawking: Universe governed by the laws of scinces and does not need to be
set in motion by some God. This touches your asnd my believe !
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Cosmic History
1
2
3
You see 3 symmetry breakings to separate the forces
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Comments
Einstein’s general relativity predicts that space-time begin at Big Bang singularity.
But this is a classical field theory. Then, what is the role of quantum mechanics?
The problem is: very strong gravitational field
Questions:
 Why the Early Universe so hot?
 Why the Universe so uniform on large scale?
 Why does it look the same in all directions and having the same temperature?
 we have Galaxies and stars owing to density fluctuations. What is their origin?
Challenging thoughts
How were the initial conditions been chosen? By God the omnipotent?
But why God did choose to let the Universe evolve according to laws we undestand?
There is a clear order in the Universe.
Is that order divinely inspired?
diveinly
We arrive to talk about the Anthropic principle
We see the Universe the way it is, because we exist- a problematic statement
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Two versions: weak an strong
Weak version:
In a Universe that is large or infinite in space and/or time the conditions
necessary for the development of intelligent life will occur in certain regions
limited in space & time. No surprise
It is like a rich person living in a wealthy area not seeing poverty
Strong version:
Either many different universes, or many different regions in a single universe.
Our own universe would have the right conditions for the development of
intelligent life (But what about Dahesh ??)
The strong statement is:
Asking: Why the universe is the way we see it?
Answer: if is were different we would not exist.
Well, we know that the fundamental constants (strength of the forces) are fine-tuned.
Still there are objections to the strong version.
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In what sense can many universes exist? We cannot know what happened in
them. We tend to remove them fro the theory
If they are different regions of the same universe, the law of sciences would be
the same in each region, so that we can move from one to the other. But the
initial conditions would be different, and we are back to the weak version
Another argument against the strong version is:
It runs against the tide of the history of science. All what we have discovered
from the geocentric model to the modern picture and all what is existing,
hundred billion of galaxies is for our sake?
Last question: May be you figure out that at least two conditions must be
present for the development of intelligent life in our universe
I hope you enjoyed the lecture. We are ready for your questions
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Part III: Relativity and Black Holes
The most dramatic prediction of general theory of relativity is the existence of black
holes.
As the matter is compressed to extreme densities the strength of gravity at the
surface of the sphere increases dramatically.
Consequence .
Gravitational bending of light (experimentally tested)
How come that this bending happens at all? Newtonian description of gravity
cannot be applied to light, because it has no mass (it is pure energy). But
Einstein’s description of gravity gives the answer. (see next page)
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Curved spacetime around a black hole: no escape even for light.
Gravity in the Einstein’s theory is affecting the geometry of space, it
corves the space in the vicinity of the mass
Black hole in general relativity
Singularity
The event horizon:
or
the entrance to the
end of time. Doest the
time flow in the hell?
Schwarzschld’s radius
Rsch
2GM

c2
12:00 p.m.
Cross sections
In a collapsing star
you
astronaut
Observer A: Never sees
the light emitted at 12:00
p.m., when observer B
enters the event horizon.
His light signal propagates
along the edge of the
cylinder
B
A
Observer B: his time runs normal.
However he/she disappears for ever
beyond the event horizon
Some references:
1. A Brief History of Time
S. Hawking 1988
Check chap. 1,2,3 and 8 (not easy to read)
2. The Grand Design
S. Hawking and L. Mlodinow, 2010
3. Universe
Freedman, Keller and Kaufmann
9th edition, Freeman 2011
end chapters in the book
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