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Extragalactic Astronomy


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Title: Extragalactic Astronomy

Extragalactic Astronomy Cosmology
Lecture 2
4246 Physics 316
  • Jane Turner
  • Joint Center for Astrophysics
  • 2003 Spring

Other Early Cosmologies (?)
It should be remembered that our knowledge of
history is solely dependent us on having written
records Also few (if any) of the original
works survive, so we must rely on
later works (true complete reporting ?) Who
knows what ideas have been lost...

One (radical) idea that was not developed
(apparently ignored) is due to Aristarchus
(c280BCE - between Aristotle Ptolemy) a
Heliocentric universe - the Earth orbiting the
Sun (!)

Lecture 2
The following should be remembered
Cosmology one of the oldest
philosophies/sciences Many
ancient cosmologies grappled with some of the
same deep philosophical
questions we still ponder with today.
The Greeks first (we think)
reasoned that Universe was formed by natural
processes which could be
observed, understood/explained by mathematics
Developed the Empirical
Scientific Method Developed
a geocentric system (Pythagoras of Samos,
Aristotle,c.350BCE) culminating with that of
Ptolemy,c.150 involving a
complex arrangement of spheres epicycles.
Reason and beauty/perfection were a
strong influence of their
thoughts. The universe was
reasoned to be finite but eternal/unchanging

Lecture 2

Greeks thought Earth was stationary, if it were
moving, wouldnt we feel a sense of motion
(great winds, loose objects whizzing by us etc)

Lecture 2
Recap (cont)
Mathematics (ie. the Ptolemaic system) seen as a
parameterization, By c.1400, the
Ptolemaic (geocentric) system had remained
essentially unchallenged as the cosmology for
1300 years You should be familiar with
the concept of Parallax the
basics of how the Ptolemaic system works
(how epicycles, deferent etc account for
retrograde motion). the
concept of Ockham's Razor Again, a detailed
knowledge of names, dates and places is not
required However, you should be familiar with at
least the names approximate dates of
Atristotle (c.350BCE) and Ptolemy (c.150).

Lecture 2
Early Summary of Developments
Thales (c.585BCE) universe run by natural
processes Pythagoras (c.530BCE) spherical
Earth Anaxagoras(c.430BCE) Heaven is
knowable Plato (c.420BCE) Geocentric,
planets-circular orbits, stationary
Earth Democritus (c.400BCE) Universe is a
mechanical system Eudoxus (c.340BCE)
Geocentric cosmo, 33 spheres, stationary
Earth Aristotle (c.350BCE) Geocentric cosmo,
55 spheres, stationary Earth Aristarchus
(c.280BCE) Heliocentric cosmo !
(ignored) Hipparchus (c.125BCE) Distance of Moon
(scale on the cosmos) Ptolemy (c.150)
Geocentric cosmo, Epicycles

Lecture 2
Foundatn of Modern Cosmo
Topics The Earth moves from Center
Stage And Then the Apple Dropped
Summary at the beginning of the C20th

Lecture 2
State of the Universe, 1400
By 1400, the geocentric cosmology of Aristotle
Ptolemy (based on concentric spheres, epicycles
etc) had been essentially unchallenged for well
over a thousand years. However the Greek
Empire, under which the studies of
Aristotle,Ptolemy had flourished ( Pythagoras,
Plato, Aristarchus, Hipparchus ... etc !)
long-since collapsed (c.410). An Islamic
Renaissance (refinement of Ptolemic model, but
radical new ideas) had come and gone by c.1100.
However, in the 15th 16th centuries,
following the years of the "Black Death"
centuries of strife, the start of the Renaissance
in W.Europe finally allowed scientific
technological progress.

Lecture 2
Rumblings of Discontent
In c.1430, Nicholas de Cusa published On
Learned Ignorance In which he suggested
the universe is infinite (also
Anaxagoras of Clazomenae (c.430BCE)
and in a poem by Lucreutis (c.100BCE)
but generally ignored),
the universe does not have a center,
the pattern of stars would look the
same at all locations.
all motion is relative,
that the Earth might not be stationary

Homogeneity Relativity
Lecture 2
Earth moves from Center Stage...
The suggestion by Nicholas de Cusa (c.1430) that
the Earth might not be stationary, was
supported by Leonardo da Vinci (c1490), who
amongst many (!) other things also suggested the
Earth moves (rather than the Sun).

However it was not until 1543 when Nicholas
Copernicus publishes his Revolutionibus Orbium
Coelestium (The Revolution of the
Celestial Spheres) that this idea was put of a
more rigorous footing.
Lecture 2
Heliocentric Cosmology
Copernicus suggested the planets rotate (on
circles) around a central Sun .with
"slower" planets being further from the

Heliocentric Cosmology
Copernicus also acknowledged the Earth
rotates on its axis
Lecture 2
A Good (Simpler) Model
The heliocentric model of Copernicus obviously
could be used to make predictions, that could
be compared to observations. It was simpler than
the model of Ptolemy that it replaced. However,
its predictions were not any better than
those of Ptolemys model unless (much smaller)
epicycles were added to quote Alan W. Hirshfeld,
in "Parallax - The Race to Measure
the Cosmos" "Having donned the
Artistolelian straightjacket of uniform
circular motion....Copernicus has each planet
circle at constant speed in a small
epicycle whose center moves uniformly
around the Sun."

Lecture 2
Copernicus gets the credit, but..
to quote Alan W. Hirshfeld again, in
"Parallax - The Race to Measure the
Cosmos "De Revolutionibus may have
triggered the so-called
Copernican Revolution,
but it was not
the revolution itself

Lecture 2
The Cosmological Principles

Cosmological principles are the assumptions which
allow us to deduce the whole of nature on the
basis of the observable to the unobservable. Not
surprisingly, any study of cosmological
principles must combine elements of astronomy,
physics and philosophy.
Lecture 2
The Cosmological Principles

One of the most important aspects of Copernicus
work - took his heliocentric model, went
further and made a model for the cosmos, by
saying, lets assume several things, then use
observations to test whether this is a good model
Lecture 2
The Cosmological Principles

There are several flavours of Cosmological
Principle, all of which are essentially
metaphysical in nature The Copernican
Cosmological Principle This is
sometimes simply referred to as simply
The Cosmological Principle
The other cosmological principles are
extensions of this principle.
The Perfect Cosmological Principle The
Anthropic Cosmological Principle
This principle comes in two "flavours" Weak and
Lecture 2
The Copernican Cosmo Principle

The Copernican Cosmological Principle is a
logical extension of the the Copernican theory
that the Earth is not the center of the universe.
Thus the Earth is not "special", thus the "laws
of nature" on (or around) Earth are not special.
It is essentially a philosophical
requirement/simplification necessary/assumed for
all modern cosmologies - our laws of
physics are otherwise "irrelevant" -
"justified" by Ockham's Razor
Lecture 2
The CCP itself

The Copernican Cosmological Principle is that
On a large scale, the universe is both
homogeneous and isotropic (in 3-D space),
and has/will always be so.
Note that the statement "has/will always be so"
refers to the universe continuing to display the
properties of homogeneity isotropy. The CCP
does not imply that any actual observable
parameter (e.g. the density of matter in the
universe) will remain constant with time. Indeed,
the CCP allows the properties of the universe to
evolve with time, but states that at any given
time the universe will be both homogeneous and
isotropic (in 3-D space).
Lecture 2
The CCP again

Another way of expressing the Copernican
Cosmological Principle is that ... all
observers (in inertial frames) will see
identical properties laws - homogeneity
will NOT see any preferred direction -
We do not occupy a special place in the universe
Lecture 2
homogeneous - same properties everywhere isotropic
- no special direction, uniform in all dirns

homogeneous but not isotropic
isotropic but not homogeneous

The CCP - an analogy
A (small) sentient being living in the center of
a "perfect" loaf of bread! There may be
obvious structure on small scales (air bubbles
etc), but on the large scale the loaf can be
considered uniform
and isotropic The laws of physics (e.g. which
caused the dough to rise)
are the same throughout the loaf. The loaf
might still be rising - but (in this perfect
loaf) this happens uniformly
following then same laws throughout the loaf

Lecture 2
The CCP Evidence for against
The best support for the Copernican Cosmological
Principle is the Cosmic Microwave Background
(CMB), which is isotropic to 1 part in 105 The
obvious observational evidence against the
Copernican Cosmological Principle seems to be the
structure seen in the universe on a variety of
scales (stars, galaxies, clusters,
super-clusters..) This is why the qualifier "On
a large scale.." is required to be added to the
principle. The question them becomes a question
of scale (now large is "large" ?), and whether
the observed structures on large scales are
indeed representative of the universe on these
scales (or are "perturbations" which "happen to
be visible to us).

Lecture 3
The Perfect Cosmo Principle
The Perfect Cosmological Principle is an
extension of the Copernican Cosmological
Principle and is that
not only on a large scale, the
universe is both homogeneous and isotropic (in
3-D space), and has/will always be so
but also that it
is also true for all times This principle is a
fundamental assumption of the Steady-State
Cosmological Theory the universe should present
a similar aspect when viewed from any point in
space AND time.

Lecture 2
The PCP - an analogy
A (small) sentient being living in the center of
a "perfect" loaf of bread! There may be
obvious structure on small scales (air bubbles
etc), but on the large scale the loaf can be
considered uniform
and isotropic The laws of physics (e.g. which
caused the dough to rise)
are the same throughout the loaf. However
(contrary to the case for the CCP loaf) the loaf
has always ( will always) exist with the same
characteristics as a function of time. So if
the loaf appears still be rising, (which in this
perfect loaf) this happens uniformly
following the same laws throughout the loaf),
then the density of the loaf must remain
constant, thus bread-particles must spontaneously
appear to compensate for the expansion.

Lecture 2
We are up to 1400 We briefly mentioned the ideas
of Nicholas de Cusa Homogeneity
Relativity We discussed the Heliocentric system
of Nicholas Copernicus We then discussed the
Copernican Cosmological Principle On
a large scale, the universe is both
homogeneous and isotropic (in 3-D space) and
the Perfect Cosmological Principle On
a large scale, the universe is both
homogeneous and isotropic (in space AND time)

Lecture 2
The Anthropic Cosmo Principle
The Anthropic Cosmological Principle is an
extension of the Copernican Cosmological
Principle and is that not only on a
large scale, the universe is both homogeneous
and isotropic (in 3-D space), and
has/will always be so
but also that by our very
being here, we are viewing "our universe"
at a "privileged" location in spacetime The
rationale behind the first part is as for the
CCP. The implication is that the same
laws of physics hold throughout the universe.
The rationale behind the second part is as an
explanation as to why the laws of physics ( the
universe itself) are the way they are (at least
as seen by us).

Lecture 2
The Weak ACP
The Weak Anthropic Cosmological Principle states
that the conditions necessary for the development
of sentient beings (capable of asking the
question why is the universe the way it is ?)
will only exist in a universe where the laws of
physics are the way they are as they are
experienced by us. i.e. sentient
beings can only evolve and exist in a universe
-that "happens" to have a density close to that
observed (by us), -that "happens" to be about as
old as ours, -where the charge of an electron
"happens" to have the value observed (by
us). ... etc.

Lecture 4
The Weak ACP - an analogy
Back to our sentient being in the center of a
"perfect" loaf of bread As before, on
the large scale the loaf is uniform and
isotropic The laws of physics are the
same throughout the loaf. However (as an
extension to the CCP loaf) the sentient being
reasons that out of all the possible loaves
(universes) (ingredients, proportions,
open temperatures, baking times etc),
they exist in the loaf/universe they do since the
conditions were just
to bake such a loaf. Had
they not been (no yeast added, baking
time not long enough etc), then it would
not have been possible for them to reach the
level they have (if they could even
exist at all). This is the Weak flavor of the

Lecture 2
The Strong ACP
The Strong Anthropic Cosmological Principle takes
the Weak ACP one step further to state that
there could be many different universes (or
regions in a single universe) where the laws of
physics are different There are yet more
flavors of the Strong ACP dealing with whether or
not sentient beings might be able to have evolved
in these other "universes". But
such metaphysical questions are beyond the scope
of the current course

Lecture 2
The Strong ACP - yet more loaves

In the Strong flavor of the ACP, the sentient
beings go on to allow for the
possibility of the existence of
other loaves
having been in the oven (or other ovens),
with different mixtures of ingredients
etc. Whether the conditions may have
been right for other sentient beings
to evolve in (a very small number of) these
other loaves (most likely in a form
v.different to themselves) is a
matter of debate. However (most
of) the sentient beings see
no possiblity of loaf-to-loaf travel in
any case...
Lecture 2
The ACPs - just games in logic ?!
Not really ! There appear to have to have been a
large number of coincidences for life
(as we know it) to exist..

(you will NOT be tested on these)
for those interested, see...
Lecture 2
Tycho Brahe (c.1570))
Also famous for having lost his nose in a

Tycho Brahe (c.1570) did accept that the
(other) Planets move around the
Sun but did not accept that the Earth
Stars move around the Sun
Why ? Falling bodies fall towards the Earth ...
The lack of Stellar Parallax
Lecture 2
What was his problem ?)
Falling Bodies fall towards the Earth Indeed
if you throw something vertically upwards,
it falls vertically downwards (to the same spot)
Tycho Brahe reasoned this surely
meant the Earth was the center
of the universe

Tycho Brahe was unable to detect (by naked-eye)
Stellar Parallax and reasoned that in a
Copernican system this would require the
Stars to be so far away they would have to be
"unreasonably" large/bright.
Lecture 2
Tycho Brahe the observer
Tycho Brahe was primarily an observer made
observations that strengthened the
rejection of the cosmology of Aristotle
Ptolemy a Supernova
position did not change (so it was not a comet or
meteor), - lack of Parallax must
be in one of the outer spheres
brightness changed - outer sphere of star does
change! a Comet position
did not change significantly throughout the
night. - lack of Parallax, must
lay beyond the orbit of the Venus
positions of Mars twice-daily
which implied its orbit intersects that of the
Sun. - apparent crossing/smashing
of the crystalline spheres
there are no solid spheres "holding" the
celestial bodies

Lecture 2
Tycho Brahe - his contribution
So, even though Tycho Brahes
(geocentric) cosmological model was wrong
his observations did play a major role in the
rejection of the notions of
Aristotle/Ptolemy that the
celestial bodies are carried by crystalline
spheres, with everything beyond
the Moon eternal unchanging. In addition
Tycho Brahe also actually published his data !
In particular his twice-daily measurements
of the position of Mars provided Johannes
Kepler with a crucial database a few years later.

Lecture 2
Johannes Kepler - Laws 1 2
In 1609 Johaness Kepler publishes
Astronomia Nova (New Astronomy) 1st two laws
Planets move in ellipses with the Sun at one
focus (not the perfect circular orbits
previously assumed)
Planets "sweep out" equal
"areas" in equal times during their motion
around the Sun
Lecture 2
Johannes Kepler - Law 3
In 1619 Johaness Kepler publishes Harmonices
Mundi (Harmonies of the World) 3rd law The
period P (of a Planet's orbit) squared is
proportional to the (average) distance R cubed
P2 K x R3 (where K
is a constant)
Cosmology finally escapes the "Artistolelian
straightjacket of uniform circular motion
Keplers laws are "parameterizations" of motion
(rather than how/why ) e.g. the physics
"hidden" in the constant of proportionality K was
not understood but for the 1st time, laws
provide the ability to measure the
relative size of the Solar System.
Lecture 2
Sun at the center - still somewhat radical
Following the publication of Revolutionibus
Orbium Coelestium by Nicholas Copernicus (1543),
a heliocentric solar system slowly slipped into
the scientific mainstream. Many still
considered the Copernican system radical
... Others took the ideas even further.
Thomas Digges (1572) - universe is infinite (in
space) and
populated by innumerable suns
worlds. Giordane Bruno (1584) -
universe is infinite in both time space
contains many worlds
with intelligent beings.

Lecture 2
Orbits and Keplers Laws ht
  • For the Earth, we know that
  • P1 year
  • R150 million km (1 Astronomical Unit, A.U.)

Orbits and Keplers Laws ht
Read Chapters 12 of Hawley Holcomb Answer
chapter 2, question 10 (should take 10 mins),
email response to turner_at_lucretia.gsfc.nasa.gov
work through web-tutorial www.aw.com/astronomyp
lace 1 Scales of the Universe
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