Transcript
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From very early in human civilization, humans have tried to explain
the universe.
An early Babylonian idea was that Earth was a flat stationary plate,
and the sky above was like a moving dome, or a roof enclosing earth
as a half-circle.
Later, the ancient Greeks figured out that Earth could not be flat.
As travelers, the Greeks were navigating using the stars for
orientation. One orientation point was the North Star. They noticedthat starting out from Athens, the North star would hover just above
the horizon, but the farther they traveled north, the further it would
raise above the horizon. This could only be explained if the Earth was
round and not flat.
They also experimented with sticks of equal length placed on
different locations on earth, for example, one in Athens and one in
Alexandria. They would place them standing in right angles to theEarth, and measure the shadows they were throwing at one
particular date and time. They now noticed that when one stick at
one date and time threw no shadow in Alexandria, the stick in
Athens at the same date and time would have a shadow. If the earth
were flat, they should throw the same shadow; only if it was curved,
the shadows would be different.
From a Flat to a Spherical EarthBelow: a man, living on the flat earth, under a dome
of sun, moon, and stars, try to break out of the
dome and take a look at the other side. If he goes
any further he falls down.That the earth was flat was obvious from sense
experience: earth is experienced as flat and we
dont fall off. That the sky was moving was also
obvious from experience, since all the objects in the
sky seem to be moving around us in a half-circle
from morning to dusk: the sun, the moon, the stars.
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The Ptolemaic conception of the universe.
To different renditions, First, a Medieval: Earth is not only
in the center of the universe, it is also huge compared to
the sun and the other planets orbiting it. Second, a more
formal model illustrating the centered earth and the eightspheres.
The Greeks had the idea that the most perfect movement had to be
circular.
The circle was the most perfect geometrical form, so if sun, moon,and star revolved around earth, they would do so in perfect circles.
Ptolemy elaborated on Aristotles ideas, and came up with a model
of the universe, that would last for nearly two thousand years.
In the middle we have Earth, and revolving around earth, we have
eight different spheres, that each of them control the movement of
different bodies in the sky. The sphere closest to earth would thus
account for the movement of the moon; the fourth sphere would be
the sphere of the sun; the eighth sphere, farthest away, would be the
sphere of the fixed stars.
The universe was like an onion. In the middle, the earth, from there
you can go out layer by layer, until the eighth sphere. What was
outside the onion, nobody knew or asked about. One assumed that
this was the sphere of God and his heaven.
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Below we still have Ptolemys model. Earth is in
the center, and various planets and stars are
orbiting around it in perfect circles. But we see a
modification of the model, because one has
added to the perfect circles so-called epi-circles.
Observation had shown the early physicist that
the sky was filled with so-called wanderingstars(Planetos), that were wandering forth and back
on the sky. How to solve the problem, without
destroying the idea of perfect circles? By creating
epi-circles!
There was problems with Ptolemys model. Not all bodies on
the sky seemed to move in perfect circles.
Some bodies seemed to wander around in strange patterns,
one therefore gave these bodies the name, planetos, the Greek
for wanderer.
One tried to account for these strange movements by adding
epi-circles to the original circles; one added a circle to the
original circle, such that the second circle had a center moving
with the original circle. Strange movements could now be
explained by epi-circles; and if not by one epi-circle, then by
adding an extra epi-circle, creating an epi-epicircle, etc.
Ptolemys model survived. And when Christianity became the
official religion in Europe, the theologians adopted the model
too, because of its simplicity and perfection. The circle was still regarded as perfect; and could God have
created the universe other than perfect? That the eighth
sphere was a natural boundary of the universe also fit into
Christian thinking. One had created enough space for heaven
and hell. The church liked the model, and regarded every attack
on it as heretic.
Problems with the
Perfect Circles
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The problem with the Ptolemaic model was thepeculiar epi-circular movements. One problem was as
Copernicus understood that every movement could be
explained by means of epi-circles, depending on how bigone made the circle and how many of them one invented
to do the job. Therefore, epi-circles were applied
dogmatically, and did not correspond to observable facts.
Secondly, because of all these epi-cicles, and epi-epi-
circles, etc., the Ptolemaic model had become extremely
complex.
In physics you always seek the simplest explanation.And Copernicus realized that placing the sun in the
center of the universe, and the planets orbiting this
center, would both be simpler, and would explain
observable fact, that before could not be explained.
In this new helio-centric universe, earth was no longerstationary, and it was no longer the center of the
universe.Later Kepler took up the model, and refined it. but itwas not before Galileo, the idea was supported with
observable facts.
At this point, the new model began to concern thechurch, and they deemed the idea heretic, and forced
Galileo to retract his observations.
The Copernican Revolution The two models below look the same, buteverything has changed from one to the other.
The first is the Ptolemaic; it is GEO-CENTRIC:
the Earth in the center, the Sun orbiting
around. The second is the Copernican; it is
HELIO-CENTRIC: Sun in the center, and Earth is
orbiting around
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Although the trinity, Copernicus, Kepler, and Galileo had suggested an
entirely new picture of the universe, much remained unresolved. One for
example did not know why the planets were forced into these orbits around
the sun, and believed it had to be a kind of magnetic forces attracting them.
It was not before Newton, one understood the law of gravity. According to
Newtons law, two bodies will attract each other with a force that is
proportional to their mass, and inverse proportional to their distance. If we
have the bodies, Earth and an apple, the two bodies attract each other, but
because the Earth is enormous compared to an able, we only experience a
pull in one direction, the able falls downwards. Newton extrapolated, the
moon also fallsdownwards, but because of the centrifugal force pulling it in
the other direction, it would be held in a stable orbit around earth.
Since the gravitational force is a weak force, we dont perceive two bodies
of similar mass attracting each other on earth. Two apples on a plate attract
each other, but not visibly.
However, if we talk about massive objects, like moon, planets, sun, then
the attraction is significant. The pull of the gravitational force keep them in
place. Otherwise, they would just be moving in straight lines through space.
In one bodys orbit around another body, there is a balance between the
gravitational pull in one direction, and the centrifugal force in the other.
Thanks to this balance, planets orbit around stars, and moons around
planets.
Newton and Gravity Newtons law for Gravitational
attraction: each body in the
universe is attracted toward
every other body by a force thatis stronger the more massive the
bodies and the closer they are to
each other. Hawking, p. 5
Below: two bodies with the
masses M and m. The force of
the gravitational attraction onefinds by multiplying M and m,
and the gravitational constant, G,
and thereupon divide the result
by the square of the distance
between the bodies.
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During the history of Cosmology our image of the universe has continuously been expanding. The universe not only
expends in actuality, it expands in our imagination. Historically, it was first nothing but a flat earth with a dome on top; then
it became a solar system; then a galaxy; then astronomers started to speculate on the existence of other galaxies; today we
know that our visible universe consists of 1-200 billion galaxies. We also know that this is only our observable universe and
that it must be bigger. Cutting edge cosmology speculates that there may exist an infinite number of alternative universes;that we consequently live in a multiverse.
When we measure distances in the universe, we no longer measure in kilometers, but in light-years. The distance traveled
by light in one year is a light-year. It takes the light of the sun eight minutes to travel to earth, so the sun is eight light
minutes away from us. It takes light from the star closest to our own (Alpha Centauri) about four years to travel to us, so, it
is four light-years away from us.
If we adopt a scale where the Sun is an orange and Earth is a pinhead, the distance between the orange and pinhead is around15 meters.
Distances in the Universe: Planets, Stars, and Galaxies
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Our Address in the Universe
In our galaxy, the Milky Way, there are billions of stars like our sun (approximately 100 billions). We assume that there
are billions of planets orbiting these stars, but we cant detect them easily, because they are small and they dont emit light.
Since there are billions of galaxies in the universe, and these billions of galaxies are composed of billions of stars, around
which trillions of planets must be orbiting, astronomers seriously believe given the huge numberthat life must exist on
some of these other planets. Our location in the Milky Way is somewhat in the outskirts of the galaxy. It is fortunate that we are not too close to the
center, because it consists of a super-massive black hole, that ribs everything apart coming too close.
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Einstein discovered that light has a speed, and it is invariable. It takes time for light
to travel. A thousand kilometers takes around 0.0033356 second, and a million
kilometers 3.3356 seconds. In other words, light travels at a speed of approximately
300,000 kilometers per second. Since light has an invariable speed, and we know its value, we can measure
distances by measuring the time it takes for light to travel from one event to another
(therefore, we no longer measure distances in a metrical system). In Hawkings
illustration to the right, one sends a pulse of radio-waves (traveling with the same
speed as light) out to an object, and measure the time it takes for it to be reflected
back. Time is measured on the vertical y-axis; while distance is measured on the
horizontal x-axis. To know the distance to the object, we take the time for the pulse
to be reflected back to us, divide it by two (since it took a round-trip), and multiply
time with the speed of light. Consequently, we have the distance to the object.
We have said that the distance from the sun to Earth is about 8 Light-minutes. This
means that the sunlight we see now is eight minutes old, or, it was emitted eight
minutes ago. It also implies that we can know nothing about the sunlight that is
being emitted in our present now. And since nothing travels faster than light, there is
no way we can know the present
On Hawkings illustration to the left, we have the Sun and the Earth lined up attime 0 on the x-axis. Time 0 represents the absolute present. At time 0, we imagine
that the sun implodes and disappears; however, Earth is still unaffected, because it is
outside the light cone of the sun; it is in what Hawking calls the Elsewhere. As the
clock ticks, we move up along the vertical time axis, and after 8 minutes, we enter
the light cone of the sun, and experience what happened 8 minutes ago, that the
sun has vanished.
Speed of Light
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When we enter the suns light-cone, we experience the implosion, although it iseight minutes old. It is in the so-called absolute past that is, from our point ofview. There are light cones for as well absolute futureand absolute past. They
are relative to what we decide is the present observation point. The light cone inthe figure illustrates the relations between absolute past, present, and absolutefuture.
The Light Cone
The Future andPast Light Cone
When we observegalaxies and galaxyclusters, we only seewhat is past. The fatheraway the object is, thelonger it takes for its lightto reach us, and the
further we look back intothe Past Light Cone. What happens in ourpresent, we cannot know;we cannot know whatspace is on the so-calledHyper-Surface of thePresent. Relative to the FutureLight Cone of an object,we are in the Elsewhere;only as time passes, wemove into the light
emitted from the object.
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In order to understand the next discovery about our universe, we
must understand the doppler effect.
The doppler effect is well-known in our experience of sound.When for example a car comes toward us, the sound of the car
increases in pitch (the sound waves are compressed), and when it
moves away, the sound decreases in pitch (the sound waves are
stretched out). As with the police-car in the example. When the
sound moves toward us (when sound-waves are compressed), the
waves are blue-shifted. When the sound moves away from us, the
waves are red-shifted.
The same is the case with light. When observing light emitted by a
star, one breaks it up into a spectrum spanning from the deep red
to the deep blue. Depending of the chemical composition of the
star, the spectrum reveal patterns of absorptions lines (i.e., patternsof dark lines in the spectrum that indicates the presence of various
elements, such as Helium, Hydrogen, Carbon, etc.) From the
laboratory we know these patterns well enough, and from looking
at them the astronomer can quickly determine the stars chemical
composition.
However, as in sound, there is a difference in the light spectrum
according to whether the object moves away from us, or toward
us. If it is moving toward us, the pattern of absorption lines is
shiftedtoward blue. If it is moving away from us, the pattern is
shiftedtoward red.
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In 1929, the astronomer Edwin Hubble applied the notion ofthe Doppler effect to his observations of galaxies. He
expected to see a random distribution of blue-shifted and
red-shifted galaxies, but observed instead that distant
galaxies are moving rapidly away from us; they were all red-
shifted.
Furthermore, not only were galaxies moving away, but there
was a correlation between their distance from us and their
velocity. In other words, the father away they are from us, the
faster they move away.
In the model to the right, the x-axis represents the distance
from the observer; the y-axis represents the velocity (speed)
of the galaxy. Galaxies close to us, moves away from us with
slower speeds. Distant galaxies moves away with higherspeeds. The father away a galaxy is, the faster does it also
move away. The relation is a constant (diagonal line; Hubbles
Constant: H0 = 72)
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This observation could only imply that the universe is expanding, and more
rapidly so, the deeper we go into the universe.
The universe expands, like if you blow up a balloon, or set a raisin bread to
rise. In the form of the raisins, you have certain points representing stars or
galaxies. We choose one of the raisins to be our observation point, and in the
vicinity of that point, we follow the expansion of other raisins. When the
dough starts to raise, the raisins start to expand away from our observations
point, and the further away they are, the more they expand. On the model,
the point nearest us expands from 5 cm to 10 cm; the point farthest away
from us expands from 10 cm to 20 cm.
Understanding this logic, it did not much thinking to figure out that if galaxies
are expanding, they must at some point have been closer together than they
are now. If they are expanding today, they must have been closer together
yesterday, and still closer the day before yesterday, and so on until we find a
beginning of the expansion.
Hubble was able to calculate the rate by which they expand, that is, the
velocity of expanding galaxies. This made it possible to estimate the time it
has taken the universe to expand this far, and estimate the distant beginning
of the universe; a point called a singularity, or better known as the big bang.
The universe was no longer eternal and unchanging. It had a beginning, and it
was, and is still, constantly changing. The universe had a birth, a creation
from where it came into being.
The Universe is Expanding
from a Singularity
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A Model of the Universe Expanding from an
Inflationary Hot Big Bang Event
A model of the expandinguniverse. It starts in a super-hot Big
Bang explosion in the first fractionof a second (T = 10-43). At this point,there are no particle formation andno physical forces; consequently nophysical laws.
At T = 10-32 seconds, this bigbang singularity starts to inflate(with a doubling time of 1picosecond). The model illustrateshow the different forces starts toform, first strong, then weak, theelectromagnetic, finallygravitational, and how the firstparticles start to form.
Not before at around one secondafter the explosion, light elementsstart to form like He, D, Li. Theexploding universe continues toproduce clouds of mass, until thegravitational effect takes over, andmass starts to collapse into starsand galaxies. After around 13.7
billions years, the universe reachesis current state.
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A Model of the Universe Expanding from an
Inflationary Hot Big Bang Event
A model of the expandinguniverse. It starts in a super-hot Big
Bang explosion in the first fractionof a second (T = 10-43). At this point,there are no particle formation andno physical forces; consequently nophysical laws.
At T = 10-32 seconds, this bigbang singularity starts to inflate(with a doubling time of 1picosecond). The model illustrateshow the different forces starts toform, first strong, then weak, theelectromagnetic, finallygravitational, and how the firstparticles start to form.
Not before at around one secondafter the explosion, light elementsstart to form like He, D, Li. Theexploding universe continues toproduce clouds of mass, until thegravitational effect takes over, andmass starts to collapse into starsand galaxies. After around 13.7
billions years, the universe reachesis current state.
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Cosmic Expansion:
Look-back Distance versus Real DistanceEleven Billion years ago, a
photon of light departs froma distant galaxy toward the
Milky Way. Lets say that the
two galaxies are separated by
4 billion light years
However, the universe keeps
expanding, and the photon
does not reach the Milky Wayafter 4 billion years as one
would suppose. After 8, 9,
and 10 billion years, it is still
traveling.
First after 11 billion years, it
reaches the Milky Way, and
we see it as it was emitted 11
billions years ago; this is theso-called look-back distance.
The real distance to the other
galaxy is different, because
with the expansion of space it
has kept expanding away
from the original emission
point. It is now 18 billion
light-years away from us.
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To the left, we have a model that depicts us in the middle, as observing the universe. The universe is
consequently all around us, and the deeper we look into the universe with advanced telescopes like Hubble,
the earlier an universe we see. With the current telescopes with can see until the blue line, the so-called
Hubble Ultra Deep Field, which approximately corresponds to the formation of the first galaxies. Further outwe have the radiation era, the Cosmic Microwave Background, and the outer periphery would represent the
Big Bang, 13.7 billion years ago. In this perspective the Big Bang is all around us. We notice that the universe
expands from the inner core, i.e., from the center of the circle, pressing the periphery outwards
To the right, we have a model that depicts the Big Bang in the middle, therefore a realistic model where
the universe starts in the Big Bang, then expands up till the outer periphery, which represents the present
state of the universe.
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The Cosmic Microwave BackgroundThe white noise one picks up in radios and television sets is coming from the Cosmic Microwave Background,the universe as it had formed 400.000 thousand years after the big bang. The Microwave Background is aplasma of high-density matter. Stars and galaxies have as yet not formed. The minute differences temperatures(in the order of 1/10,000) can be picked up, and be depicted as in the colored map as below. These differencewill eventually end up as the differences in galaxy concentrations in our universe. The Microwave Background is
like the embryo of the universe. The existence of the Microwave Background is proof of the Big Bang theory andthe expansion of the universe.
iff ibl f h i
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Different Possible Fates of the Universe:
Old View: Decelerating Expansion was accepted as the behavior of the universe, out of
three possibilities: the universe re-collapses in a big crunch; the expansion of the
universe flattens out; the universe keeps expanding, but with decelerating velocity.New View: The universe keeps expanding, but with accelerating velocity.
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Fine Tuning and Anthropic Principle
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Fine Tuning and Anthropic Principle
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Fine Tuning and Anthropic Principle
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if a star before it starts to collapse on itself, is sufficiently
heavy (about 2-3 times the mass of our sun), then the
exclusion principle can no longer support it. The repulsive
forces from nuclear reactions in its core cannot counteract its
collapse. That is, without anything to stop the process, itcontinues to collapse. It collapses into a singularity, or a black
hole.
A black hole is a star that has collapsed into a mass that is so
dense that light can no longer escape its gravitation. It is
relatively easy to escape the gravitational field of earth. If one
can make an object travel with a certain speed, one can send
it out in space. The speed it needs in order to escape thegravitation of Earth is called its escape velocity. On a planet
with higher mass than earth, it would be harder to escape,
and one would need a higher escape velocity for the object
to escape. On a neutron star, an object would need an
incredible high speed.
Now, since we know that there is a limit to speed, since
nothing can travel faster than light, it can now be calculatedthat some objects would have masses so dense that not even
light would have an escape velocity sufficiently high to
escape from the objects gravitation. Such objects would be
black holes. They must obviously appear black, since they
emit no light. Light would be dragged back into the black
hole. And if light can not escape, nothing else can
everything is pulled back into the black hole.
A star with large mass has eventually burned out
of fuel. Without sufficient repulsion of nuclear
reactions, it continues to collapse into
a singularity. This singularity is infinitely dense,
and everything that comes within a certain
boundary is pulled in and destroyed. This
boundary, which is called the event horizon, is
the distance within which everything, also light,
is pulled in.
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The two giants of modern physics: Newton and Einstein.
Newton with his fixed and flat space, and Einstein with his
dynamic and curved.
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