World View of the Universe
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Reading: Chapter 26: Cosmology
Cosmological Facts
The primary observational facts are the
first three results. The other results
will be discussed when appropriate. Before we move on to
the main topics, let us consider something known as
Olbers's Paradox
Models of the Universe
Based on Hubble's law, the CMBR, and the chemical composition
of the Universe (90 % hydrogen, 10 % helium),
our general world view of the Universe is rather
firm. On the assumption that Hubble's Law is due
to an uniform expansion of the Universe, models have been constructed
based on Einstein's General Theory of Relativity
(GTR)
and the Cosmological Principle
(the Universe is homogeneous
and isotropic on large scales). The basic model used is the
Big Bang Theory wherein it is posited
that the Universe is expanding and that it was much hotter and smaller in
the past.
In the early part of the twentieth century,
Friedman developed his mathematical
models that describe such universes (the open, closed, and flat universes).
In the middle of the twentieth century, Alpher and Gamow
made the first calculations of the properties of the
Big Bang Theory in which they predicted the general
chemical composition of the Universe and the CMBR! Later, Dicke and Peebles
performed more detailed calculations and were in fact preparing to perform
an experiment to detect the CMBR (when Penzias and Wilson made their
discovery). These successes of the Big Bang Theory
places it on a very strong footing. The general theory is likely to
be correct; the data I discuss later constrain things within the Big Bang Theory not the overall theory itself.
Einstein also made models for the Universe (circa
1910-1920's). Unfortunately, he erred because he
believed in an homogeneous, isotropic, and
static Universe (on philosphical grounds).
This led Einstein to the following argument
- In a gravitating Universe, a static solution is not possible because
gravity is always attractive ---> it always tries to pull things closer
together
- To get around this problem, Einstein postulated the existence of a
large-scale repulsive force which could counteract gravity
- This force is parameterized by what is referred to as
The Cosmological Constant
(represented by the Greek letter capital
Lambda)
When Hubble's result became known (1929), Einstein immediately withdrew his
suggestion of a Cosmological Constant as there was no need for
a large-scale repulsive force in a non-static Universe. Today,
however, the idea of a Cosmological Constant has been revived. This does not
mean we believe in steady-state universes though; the basic models for the
Universe all consider models which are expanding. We will now consider the
properties of the proposed models.
I. I will first view the Universe from a theoretical slant:
II. I will next look at the observations relevant
to theory
III. Finally, I will consider our place in the Universe
