Chapter 1: The Scientific Method
An educated guess based upon observation – sometimes, only one observation
Usually, a hypothesis can be supported or rejected through experimentation or more observation
It cannot be proven to be true
summarizes a hypothesis (or group of hypotheses) that is supported by repeated testing and
considered valid as long as there is no firm evidence to dispute it
theories can also be supported or rejected as we learn more
if evidence accumulates to support a hypothesis, it becomes accepted as a good explanation of some
phenomenon, and becomes a theory
theory is not guaranteed to be true, but it is the best we can formulate based on current evidence
both hypotheses and theories attempt to explain the ‘why’ of some action and theories are
considered to be much better formulated and tested than hypotheses
explains a body of observations
At the time it is made, no exceptions will have been found to that law
Scientific laws explain things, but they do not describe them
A quick way to tell the difference between a theory and a law is to ask if the statement explains
"why" something happened; if it does, it is a theory, not a law
If it describes "how" something happens, it is a law
theory and law go hand-in-hand
o EX. Newton developed a Law of Gravity which predicts the behaviour of an object as it falls
– but Newton’s Law of Gravity does not explain why an object falls.
o EXAMPLE OF THE SCIENTIFIC METHOD: Observation: Every swan I’ve seen this year is white
Hypothesis: All swans must be white Experiment/Test: A random sampling of swans
from each continent where swans are indigenous produces only white swans Publication
of my hypothesis: “My global research has indicated that swans are always white, wherever
they are found” Further testing: Every swan any other scientist has ever observed in any
country has always been white Theory: All swans are white Prediction: The next swan I
see will be white
THE BIG BANG
The Big Bang theory is an effort to explain exactly what happened at the very beginning of the universe.
Singularity: an area in space-time where gravitational force is so high that all known laws of physics break
down and do not apply.
Gigantic Expansion: Not the equivalent of a giant explosion. To have an explosion there has to already be
space into which the explosion spreads. But the Big Bang created space (and time), so perhaps we can think of an infinitesimally small balloon, which in the tiniest fraction of time, suddenly expands – and
keeps on expanding. In this tiny instant, time and space had a finite beginning (at least if you are inside
the balloon).This is really confusing – and there is no pretense otherwise! If this sequence occurred (a
singularity, a Big Bang, and creation of time and space), then can it be true that before that singularity
there was nothing: no space, no time, no energy? That is what the Big Bang theory, as currently
expressed, states. Let us go over the observations that suggest that the universe really was produced by a
Big Bang event (Please review the appendix provided at the end of this chapter, entitled “What
Theories and Hypothesis that Support Big Bang
``The Three Pillars of Proof “
1. Recession of stars/galaxies (as described by Hubble’s Law)
2. The characteristics of cosmic microwave background radiation
3. The abundance of light elements.
1) Recession of stars/galaxies
Edwin Hubble: 1889 to 1953; demonstrated that there were many galaxies in the universe – not just the
one we are in (the Milky Way Galaxy).
He proved that the universe is expanding & showed us how to measure distances in space.
Hubble fought to have astronomy recognized as belonging to the subject of physics. After his death, the
Nobel Prize Committee officially made this recognition (unfortunately, he was never awarded a Nobel
Prize, as they are not awarded posthumously).
In 1990, however, scientists at NASA installed a huge optical telescope into Earth’s orbit, naming it the
Hubble Space Telescope (many of the observations that appear throughout this course came from
evidence collected by the Hubble telescope).
Light’s Redshift and Hubble’s Law
In order to understand the observations that led to
Hubble’s Law, we have to make a comparison (as Edwin
Hubble did) between a property of sound and a property of
light. You all know that if you stand by the side of a railroad
and a train passes blowing its horn, the sound pitch is very
different when the train is approaching (the wavelengths of
sound are compressed and shortened) than when it is moving away (the wavelengths of sound are stretched and lengthened; Fig. 1.3a).
The change in sound is called the Doppler Shift, or the Doppler Effect.
Doppler Effect: when an object coming toward you makes a sound, the sound waves are compressed by
the motion of the noisy object and sounds differently to you than when the same sound waves are being
carried off away from you
Hubble knew that waves of light would behave somewhat like waves of sound when the light source was
moving toward or away from the observer (as had been proven in a science lab around 1918 by a scientist
called Keeler) (Fig. 1.3b).
If the light source is moving toward the observer the light wavelength appears to shorten (i.e., to move
into the blue spectrum, or “blue-shifted”), and if the light source is moving away from the observer the
light wavelength appears to lengthen (i.e., to move into the red spectrum, or red-shifted) (Fig. 1.3b)
In fact, Hubble realized that the faster the light-emitting object was moving, the greater the shift
The speed of light is fixed and cannot change, so when Hubble observed apparent changes in speed of
light (from a star), it meant the stars had to be moving away from Earth
In fact, applied to everything he could see, the whole universe had to be expanding, and with it the light
waves moving through it.
More distant a galaxy is from us = the longer its light takes to arrive, thus the more ‘red-shifted’ it appears
when it finally arrives
Amount of redshift can be used as a measure of a star or galaxy’s distance from Earth.
Equation for Hubble’s Law: v = H˳d
V= speed expressed in km/second
D= distance of the star/galaxy away from Earth in parsec (1 parsec =3.26 x the distance light travels in one
Ho= hubble constant , the speed of expansion of the universe
You can calculate how far from Earth an object is by dividing its velocity (obtained from its amount of
redshift) by the rate of expansion factor
Always remember your assumption: your calculation assumes that Ho (the expansion factor) is a
constant; if the expansion of the Universe has changed over time, your calculation will be inaccurate
2) Cosmic Microwave Background Radiation
It is estimated that it was extremely hot in the first seconds of the universe and as it expanded, it cooled
The hot light photons, produced in the early period, have since lost energy and dropped from the visible
light energy range into the microwave energy range – and that constitutes the cosmic microwave
background (CMB) that we can still see today
Scientists figure that CMB can be seen from anywhere in the universe because it comes from all
directions, and with nearly the same intensity.
As you can see from the map made of CMB (Fig. 1.5; signal converted to temperature), it shows the same
pattern of distribution throughout all parts of the Universe The only explanation that makes any sense is that this CMB represents the very last remnants of the
light/heat energy of the Big Bang’s initial expansion. This is supported by the fact that while the general
temperature of space should be 0 on the Kelvin scale (which is equal to -273 ° on the Celsius scale), the
actual average temperature is 2.726 K (which would be -270.274 °C)
You can see inhomogeneity in Fig.1.5; this amounts to temperature variation on the order of 1 degree in
100,000 degrees – so it is greatly exaggerated in the figure and in reality is not much at all
3) Abundance of Light Elements
The ratio of all the various atoms of the three lightest elements: hydrogen (75%), helium (25%) and
The observed abundance of all the different atoms of those elements can be explained only if they
originated from one single ratio of the first subatomic particles of matter that can be formed from a
Only way to get this critical ratio is through a unique event like a Big Bang
Those are the primary points that tell us the Big Bang is a fact, not a hoax
THE SHAPE OF THE UNIVERSE
Why is shape significant to the future of the universe?
1. Positive curvature: sphere.
"closed" universe: finite in , no boundary, just like
you could, in principle, fly a spaceship in one
direction and eventually arrive back where you started
closed universes are also closed in time: they
eventually stop expanding, and then contract in a “Big
Crunch”. *Sort of like tossing a ball into the air; for awhile,
the energy of your throw keeps the distance between the
ball and you expanding, but eventually, the energy fades,
and the ball gets pulled back by gravity].
Must be sufficient matter in the universe so that
gravity can eventually pull things back together
Good thing about this model: production of universes is more likely to be a cyclical event: Bang...a
universe, then a crunch collapse; Bang...a universe, then a crunch collapse; and on and on.
2. Negative curvature: saddle-shaped
"open," universe: infinite in size and no boundaries
Parallel lines eventually diverge expands forever, with the expansion rate never approaching zero (so the ball you tossed up in the air just
kept going and going).
3. Flat universe
infinite in spatial extent, no boundaries. Parallel lines are always parallel
imagine a flat universe by cutting out a piece of your balloon material and stretching it with your hands
The surface of the material is flat, not curved, but you can expand and contract it by tugging on either
Like saddle-shaped universes, flat universes also expand forever, but the expansion rate approaches zero
(the ball goes and goes, but eventually just appears to hang there, the movement outward is so slow)
Why do we think the real shape is nearly flat?
To all three of our models, a density parameter is critical
If space has a negative curvature, there is insufficient matter around to allow gravity to act and stop the
expansion; let us say the density parameter is less than 1 (in Fig. 1.6 that is signified by Ωo<1)
if space has a positive curvature, there is more than enough matter around to allow gravity to pull
everything back together (we can express that as Ωo>1)
If space is exactly flat, we can say there is exactly the ‘critical’ value of matter around that will prevent
the universe from pulling back together or from expanding indefinitely to oblivion (Ωo=1)
So what is out there that affects gravity?
o conventional matter: stars, planets, asteroids, comets, etc. a bit less than 0.4% of the universe
(Fig. 1.7). If that is all there is, then we might as well just turn out the lights; pretty soon, the
distances between stars will be so great, we will not be able to see any others than Sun.
o dark matter: matter we’ve never seen b/c it gives off no (electromagnetic) energy, we know it
exists because we can detect its gravitational attraction to conventional matter. Constitutes
about 23% of universe
o dark energy: 70-73% of the universe – acts opposite to gravity: it repels matter, causing
expansion of space to increase
When it exactly counterbalances the kinetic energy of the Big Bang expansion, we are at
the ‘critical’ value of 1 for a density parameter (i.e. a flat Universe).
Right now, with increasing expansion rate we seem to be accepting that the universe is
almost perfectly flat – but has just the slightest negative curvature. For the purpose of
this course, we will accept ‘flat’.
AGE OF THE UNIVERSE The best evidence comes from cosmic microwave radiation
There are several lines of investigation we can use to determine the age of things – even the universe
Radioactivity is used to interpret the age of rocks on Earth by:
a) observing the compositions of gases around old stars
b) knowing the exact radioactive processes requires to produce these gas compositions from the
very first element created in the Big Bang
c) knowing all the time factors involved in breaking down one component to yield others
This information gives us universe age estimates ranging between 11.5 and 17.5 billion years. The search
has been on to find a star (one of the most original from the earliest times) that could be radioactively
dated - In May, 2007 one was found and dated at 13.2 billion years. So the Universe has to be older than
Hubble’s Expansion Constant
In order for Hubble to develop his equation that connected redshift with distance/velocity of light sources
in the Universe, he had to propose a ‘constant’ for the expansion rate (Ho).
this equation can be used to find age of the most distant light sources we can find
In 2002, the Hubble Space Telescope found some white dwarf stars – these are the remnants of stars that
have consumed all their ‘fuel’ and are sitting around cooling off
dwarf is very, very old –prime candidate for dating: all of these stars were 12-13 billion years old
Would have taken something less than 1 billion years for the cosmos to cool sufficiently (from the Big
Bang event) to form a star, the age of the Universe had to fall between 13 and 14 billion years.
**Cosmic Microwave Background Radiation**
These signals offer the most accurate view to date of conditions in the early Universe
Based upon all the best physics available, a sophisticated model of the universe (from the time
represented by that CMB signal map back to the time when the Big Bang produced those first photons)
has been produced
Many assumptions have been made in order to get a coherent model. Assuming the right model has
been developed (another assumption!), the universe is exactly 13.72 ± 0.12 billion years. This is the
number nearly everyone uses these days – and we just hope it is close to correct
Neil Turok & The Big Bang
proposes a universe consisting of two infinitely extensive sheets, separated by a very thin layer of energy
(you could call it dark energy, it does not really matter)
This is a ‘sandwich’ model, every once in a while, the intermediate layer becomes unstable at some point,
gravity starts pulling things together, the layers bounce together, and – from the point of the contact –
sufficient energy is generated to produce another Big Bang
From then, until the next period of instability the universe continues its life of expansion. Of course,
there is much more to it than that but the theory is still under construction, so we will leave the topic at
a light-year is the distance that light travels in one year -used to measure distances in space light travels at about 300,000 km per second; there are 31,500,000 seconds in a year. Multiply that and
you will see that light travels 9.4608 x 1012km per year. That is way too big a number to work with, so
we measure distance in terms of numbers of light years (Table 2.1).
Relies on an object appearing to be at a different place relative to the
background, depending on your viewpoint
o Ex. The nearby tree, viewed against a more distant field, appears to
move slowly behind you as you pass it by
o hold right forefinger out at arm's length in front of you; keep it in that
o close your left eye and look at your finger against a distant background
o open your left eye but close your right and look at your finger - finger
appears to jump to the right. That’s parallax.
If we know the distance between your two eyes and the angle between the two
lines of sight to your finger, we can calculate the distance to your finger
Astronomers use this simple technique to measure distances up to almost 500
light years away, except they use the orbit of Earth about Sun as a measurement base
(rather than the distance between their eyes). (fig.2.1)
Star X, measured against the background of stars, appears at X1 in January (at A
on the orbit) and at X2 six months later (at B on the orbit). The angle AXS can be found
because we know the distance AS (1 astronomical unit = distance from Earth to Sun =
150 million km), we can also calculate the distance XS
Hertzsprung-Russell diagram (be able to sketch the diagram, know what the horizontal
and vertical axes are, where to find the Main Sequence, giants and dwarfs)
1. The first technique for stars in this range deals with the brightness of stars. Astronomers use a chart,
called a Hertzprung-Russell Diagram, relating the luminosity of stars. (sun falls in mainstream) o Using a calibrated colour chart, we determine the exact colour of the star we are studying (which
allows us to know the exact temperature of the star since the two are tightly related –and we
have already defined that relationship)
o Now we draw a vertical line from that colour determined temperature to intersect our Main
Sequence average best-fit line on the H-R diagram, and measure (using a horizontal line), on the
brightness scale (left side of the diagram) what the ‘true
o brightness’ (called ‘intrinsic’ brightness) of that star must be. However, a star’s brightness dims
with distance, so the brightness you see from Earth (called the ‘apparent’ brightness) is rather
less than true.
o That is all you need! To get distance, you use the following equation:
o Apparent brightness (which you see and measure) = Intrinsic brightness (graph) / (distance)2
o This method of determining distance from colour is called mainsequence fitting, and it is good for
distances up to about 150,000 light years away, which is beyond the Milky Way.
2. The second technique for determining distance in this middle range makes use of ‘marker stars’ that have
a special property: they have a pulsing brightness that peaks with absolute regularity (its ‘‘period’), which
is completely related to the star’s brightness. We call these Cepheids, after the first one discovered
Here is how it works: We find a Cepheid and carefully measure the time between one brightness
peak and the next (that is, we determine its period) – and that gives us the intrinsic brightness
value (from a calibrated chart).
From there on, we just use the same procedure as for the first technique
Best of all, Cepheids are all over the place so they tell us how far away galaxies are up to 500
million light years away
Use of Hubble’s Law
Hubble was the first person to prove that the Milky Way Galaxy was not alone – and he used this Cepheid
October 6, 1923, at the Mount Wilson Observatory in California, he photographed a fuzzy, spiral-shaped
clump of stars known as M31, or Andromeda, which most astronomers assumed was part of the Milky
He soon realized that within the clump he had found a tiny jewel: a Cepheid star. He was able to
determine the distance between Earth and that star and discovered that the star and the cloud, or nebula,
in which it resided were a million light years away -three times the then-estimated diameter of the entire
* Studying the image he took that night (Fig. 2.3), he soon recognized the shape of a galaxy. In fact, this
Andromeda Galaxy was found to be our closest neighbouring galaxy, to look quite a bit like our own Milky
Way Galaxy (although it is much bigger), and to resemble a Frisbee (or a very large fried egg with a big
Once again, it is Edwin Hubble to the rescue!
Around the 1920s or so, the primary question astronomers wanted desperately to answer was: “Is the
Universe expanding?” In 1929, Edwin Hubble became the very first astronomer to answer the question by
working out a relationship between the distance of a star/galaxy from Earth and the ‘redshift’ of the light
from that star/galaxy. This is the Hubble Law that we considered in Chapter 1 when we needed some
proof of the existence of the Big Bang. As we learned already, Hubble formulated an equation known as
the Hubble Law:v = Hod, where v is the speed expressed in kilometres per second; d is the distance of the
star/galaxy away from Earth in parsecs (1 parsec = 3.26 light years), and Ho is the ‘Hubble constant’. Ho is
actually the speed of expansion of the Universe. So, in simplest terms, according to the Hubble equation, you can tell how far away a galaxy (or a star) is by dividing its velocity (obtained from its degree of
redshift) by the rate of
expansion. That is what we use for really distant objects.
SCALE OF SPACE
Galaxy :collection of stars, gas and dust held together by gravitational attraction) – and there are about 100 billion
3 largest galaxies in the Local Group:
1) Andromeda Galaxy (by far the largest; this is the one Hubble photographed as he
worked on his Hubble Law)
2) Milky Way Galaxy (home to our solar system)
3) Triangulum Galaxy (the smallest of the ‘big three’)
**The solar system developed on the Orion Arm of the Milky Way Galaxy**
Sitting out a bit more than 2/3 from the core is our solar system, included as one of the many millions of
star-planet systems on the Orion Arm of the Milky Way
Element: a substance that cannot be broken down to anything simpler by any chemical means.
Atom: a particle of matter that has the unique properties of an element (the essential particles of elements.
consist of a central nucleus that contains one or more particles called protons (+) and may or may not
contain particles called neutrons (neutral, or no charge); the nucleus as a whole is thus positively charged.
nucleus is surrounded by one or more electrons (-). This offsets or balances the positive charge of the
nucleus, making most atoms neutral (i.e., most atoms have the same number of protons and electrons
Ions: an atom with either a negative charge (i.e., it has extra electrons) or an atom with a positive charge. (i.e. it
has a deficiency of electrons). Positive ions usually combine with negative ions to form neutral compounds.
Isotopes: all atoms of a particular element have the same number of protons (by definition) but may contain
different numbers of neutrons within their nuclei. These atoms of the same element with differing amounts of
neutrons are called isotopes. This very slightly affects their mass (i.e., an isotope with 1 more neutron than
another will weigh more). We identify each isotope by the sum of the protons and neutrons in its nuclei; this sum
is called the mass number.
Radioactivity: Radioactivity: the spontaneous breakdown of unstable atoms (i.e., unstable isotopes) of an
element, with the production of energy and other particles
Supernovae: the explosive death of a massive star. While the explosion is virtually instantaneous, the bright effect
can sometimes be seen for weeks, both day and night. On July 4th, 1054, a massive star exploded to produce what
we now call the Crab Nebula; records in China say the light from it allowed people to read newspapers at midnight. PERIOIDC TABLE OF ELEMENTS
The Periodic Table of the Elements: an arrangement of all the known chemical elements in a table according to a
elements are listed lig