Astronomy 309R – Spring 2009
The Cosmic Microwave Background Radiation
Define the “Big Bang” by referring to the expansion history of the universe, and to the density
and temperature of matter that fills the universe.
(In the model in which the universe has been
expanding throughout its history, and in which the expansion was much more rapid in the past
than it is now, there appears to be a past moment when the distance between any two locations in
the universe was zero, and equivalently, the density of energy in the universe was infinite.
instance in the history of the universe, which can be interpreted as the cosmic expansion’s
starting point, is referred to as the “Big Bang.”
Based on a variety of cosmological
measurements, we know that the universe has been expanding from the time when it was less
than 1 second old and when distances between points in space were 10
times shorter than
It is possible that at even an
point in its existence, the expansion of the universe
had stagnated, or perhaps, that in some way, the expansion was preceded by a contraction: we
cannot be sure.
Hence the Big Bang is really a statement about the universe when it was about a
fraction of a second old or so; it also refers to our
to even earlier times from which
we have little direct cosmological data.)
What is the only correct answer to the question “What is the direction of the Big Bang on the
(The Big Bang took place in
direction and in
in space in the sense that if
we could see arbitrarily far in some direction on the sky, we would see arbitrarily long into the
past and would eventually see the starting moment.
The Big Bang is
a location in space;
rather, it is an instance in time.)
How do we represent the Big Bang in a space-time diagram?
(See, e.g., Figure 16.1 in Hawley
Please make sure to understand the difference between this type of diagram, that
shows time on the vertical axis, and another common type of diagram, that shows time on the
horizontal axis, and the distance between two points “
” on the vertical axis, e.g., Figures 10.10,
10.12, 11.3, 11.5-10, 13.10, etc.)
What is the farthest distance in the universe that we can possibly “see,” where by seeing we are
referring to our ability to observe a faithful image of the distribution of matter and radiation in
the universe at some past epoch?
Why do we refer to this distance as “the last scattering
What is being scattered, and what are the scatterers?
(The free electrons, that is, the
electrons not bound to protons and helium nuclei in hydrogen and helium atoms, were the agent
that scattered the photons of the cosmic background radiation.
At that time, the photons
outnumbered the electrons by a factor of about a billion to one.
“Scattering” is the microscopic