29:50 Modern Astronomy
Fall 2002
Lecture 34 ...December 9, 2002
Cosmology IV
Exam (quantitative) on Friday. Bring calculators; equations & numbers will be provided.
Remember: Final exam (two exams) on Thursday, December 19, High Noon
Minimum of Algol: Wednesday, December 11 at
7:30 PM. Perseus is about halfway up the sky at 7 PM.
At 6:30 AM, super bright object in southeast is planet Venus. Right next to it is Mars. At that time, Jupiter is high in the sky, Saturn setting.
Geminid meteors. Peak the night of December 13-14 (Friday-Saturday).
(1) History of the Big Bang; lecture notes from last time.
(2) What is the observational evidence for this? See notes from last time.
(3) What is the future? Which of the Friedmann universe curves will we follow? Is the Friedmann model the correct one.
blackboard drawing of R(t) curves.
In a Friedmann universe, if 
the universe is open. If
, the universe is closed. If 
, it is flat. In this,
is the mean density of the universe. For the present estimate of the Hubble constant, 
kg/m 
.
(4) If we calculate the average density of luminous matter (i.e. that tied up in stars & galaxies), we find 
; the visible objects that make up the subject matter of astronomy are way too insubstantial to close the universe.
(5) We know visible matter isn't everything in the universe, there is also dark matter, so there is more to be included. The quantity 
must be larger than
.
(6) As mentioned in the last lecture, the abundances of the light elements tell us what 
is. The result is that 
. The ``B'' subscript means ``Baryons'', which means, protons, neutrons, and other particles that undergo nuclear reactions.
(7) We can count up all matter that exerts a gravitational force in clusters of galaxies (the biggest objects we can use as ``laboratories'', and then count up the number of clusters of galaxies in a big box in space. This gives an estimate of 
.
Title of Turner article in Astrophysical Journal.
(8) The Truly Weird Feature of this result is that most of the matter in the universe is not even Baryonic, and thus is of an unknown form.
(9) Taken at face value, this would seem to indicate that 
, and the universe is open, but many theoretical physicists think the universe must have 
, so there may be more still.
(10) Observations of very distant objects can help us determine how R(t) has varied through the history of the universe.
for open and flat universe.
In ``open case'', light emitted by a distance source will ``spread out'' over a larger volume, and the source will be correspondingly fainter.
(11) Astronomers refer to objects as ``standard candles'', if we believe their luminosity has not changed throughout the history of the universe. A measurement of their apparent magnitude, and their redshift, then tells us how much the light has been ``spread out'', and thus gives us
R(t). This tells us the ``big picture'' with the evolution of the universe. A discussion of this is in the last paragraph of Chapter 26 of the book.
(12) Best candidates for standard candles: Type Ia supernovae. They reach an absolute magnitude (remember what that is?) of about -20, so you can see them a long ways.
Supernova in nearby galaxy
Supernova in distant galaxy
Cosmologically-distant supernovae
Results on Supernovae and the Model of the Universe
