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Stephen Hawking and the UniverseA fascinating story

Dr. Mounib El EidAUB, Physics Department

meid@aub.edu.lb

The Universe was not in hurry to form the first stars

Dark age

CVSP204: F2014

First atoms formedMCBR emitted

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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. Henry Poincare

بجمالها متعة يجد ألنه بل مفيدة ألنها الطبيعة اليدرس العالممعرفتها في متعة من ليس جميلة غير الطبيعة كانت لو

الجمال , ومعنى الحياة معنى هو فما الطبيعة لمعرفة نتوق نكن لم وأن

جميل كالم

My translation

1000000000000000000 km=1018 km

100,000 light years

Spiral Galaxy like ours

Who?

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Short OutlinePart I: Picture of the Universe Interesting History Space-Time & Relativity

Part II: Universe Big Bang Inflation Cosmic History Anthropic Principle

My aim is three-fold: not to tell trivial things Avoid Equations Initiate interest and thinking

Part III: Relativity & Black Hole (was not included in the lecture)

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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

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.

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

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Ptolemy: (100-170 A.D.) “Almagest”

Geocentric model of the universe

Epicycles and Planetary Motion

Seven of these tortoise

This model was suggested to resolve the retrograde motion

tortus

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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 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!

Copernicus: Polish (1473-1543):

Wait for animation

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.

3.The square of planets orbital period (p) is proportional to the cube of its semi-major axis (a).

32 aP

slow

fasr

P in Earth's yearsA in unit of 150 million km , Earth-Sun distance

Low of harmony

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Solar System DimensionPlanet 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 39.53 248.6 0.248 1.000

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.

Full harmony

dismissed

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Isaac Newton : British, (1642-1727)

Philosophiae Naturalis Principia Mathematica

Most important single work in physical science

1. Every object remains at rest, or moves at constant speed in a straight line, unless an unbalanced force acts on it.

He invented three laws:

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

2r

MMGF SP

G May be the smartest physicist ever existed

He invented the force of gravity

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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/sin 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

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

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

1

2

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Future

0 0.2 0.4 0.6 0.8 0.9 1.0 1.2 1.4 1.6

Space-Time Diagram: If the Sun disappears you cannot know it immediately

9.08.07.06.05.04.03.02.01.00.0

Time [min]

x/(1011 m) Distance

But wait for 8.33 min

Earth 150 million Km

Light propagation

Sun

Past

Presence

Not accessible

Earth enters the future cone of the Sun

x=ct

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Light conein 2 dimensions

x

yIn case of 2 space dimension, we hav a light cone

time

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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

البرهان مسؤولية لكن التفكير حرية العلم في

برهان إلى التحتاج عقيدة الدين

إرهابين بل إرهابي دين اليوجدالدين في

There is no terror in a religion, but there are terrorists in each religion

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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 3-4%. 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-production3. 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

T=(2.726 0.0013) K

So it is experimentally verified that the MCBR temperature is

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 neededBut how does this work? Gravity wraps space and timeRelativity 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 UniverseHis Answer: he was preparing hell for you when asking this question

Historical RemarksMany 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

You see 3 symmetry breakings to separate the forces

1 2 3

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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 surpriseIt 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?

I hope you enjoyed the lecture. We are ready for your questions

Last question: May be you figure out that at least two conditions must be present for the development of intelligent life in our universe

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 .

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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)

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

2

2

c

GMRsch

The event horizon: orthe entrance to the end of time. Doest the time flow in the hell?

AB

12:00 p.m.

Cross sectionsIn a collapsing star

Singularity

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

Observer B: his time runs normal. However he/she disappears for ever beyond the event horizon

you

astronaut

Schwarzschld’s radius

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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