This is a review chapter. Important concepts treated include the nuclear
model of the atom, types of nuclear reactions, wave properties, the
electromagnetic spectrum of light, Doppler shifts, blackbody
radiation, and galaxy classifications. Read through this material to obtain
background for later chapters. Some topics are of particular importance to
understanding cosmology, including:
- Blackbody radiation is a very specific type of spectrum that
corresponds to photons in equilibrium. This radiation is completely
characterized by one parameter, the temperature of the emitter. The cosmic
background radiation is an example of blackbody radiation, where the
emitter is the universe itself. The temperature of the radiation has
dropped throughout the history of the cosmos; currently it is a chilly
2.73 Kelvins, i.e. 2.73 degrees above absolute zero.
- Nuclear Fusion is the phrase for nuclear reactions that combine
light elements into heavier. Stars are powered by fusion. This process also
explains how the elements in the universe are built up from the original
protons and neutrons in the process of nucleosynthesis .
Nuclear reactions that took place in the early universe created helium, whereas
nearly all other elements and isotopes are manufactured in stars at various
stages of their lifetimes, with the heaviest elements created during supernovae.
- Redshifts and Blueshifts are the result of several processes
that produce shifts in an observed wave spectrum. One of the most important is
the Doppler effect, the shifting of the frequency (and thus the wavelength) due
to relative motion between emitter and receiver. The Doppler effect is
essentially classical, but must be modified slightly in order to take special
relativity (see Chapter 7) into account. Doppler shifts are very familiar when
they affect sound waves; this is the reason that an approaching siren seems to
rise in pitch until it reaches the observer, after which it drops in pitch as
the vehicle recedes.
Another important effect is the gravitational shift due
to light moving in a gravitational field; light climbing from a point of
stronger to a point of weaker gravity is redshifted, whereas light falling
in a gravitational field is blueshifted.
Finally, the cosmological redshift due to the expansion of space is one
of the most important overall shift effects in cosmology. The cosmological
redshift causes the temperature of the microwave background to drop as time
passes, and affects the light from distant sources.
- Luminosity distance is a distance to a source as determined
by observing the attenuation of the source's light intensity. Finding the
distances to astronomical objects is one great challenges of
cosmology; The luminosity distance is an important part of this task.
The luminosity distance exploits the simple fact that light intensity is
attenuated as the light travels through space, because the light wave front
spreads out over an ever-increasing area. Thus a comparison of the observed
amount of light received from a given source with some estimate of the
intrinsic brightness of the source should enable us to compute the distance
to the object. In practice, the determination of luminosity distances is
fraught with many potential sources of error, including absorption by
intervening matter of unknown type and density, but the principle is very
simple. Despite the difficulties, luminosity distance is the best means
of measuring the distances to very distant objects.
The chapter concludes with a brief description of the types of
galaxies seen in the universe. Galaxies are assigned to one of three
categories according to their overall shapes and other properties.
Elliptical galaxies are roughly spheroidal or ellipsoidal,
contain mostly old stars, have little dust or gas, and do not contain
well-defined nuclei. Spiral galaxies are flattened disks threaded
by pronounced bright spirals, contain much gas and dust, are usually sites
of young stars and active star formation, and feature reasonably well defined
nuclear bulges. The Milky Way Galaxy is a spiral. The third category,
irregular galaxies, is a catchall for galaxies that do not fit
one of the other two groups. Many irregular galaxies are interacting with
other galaxies in one way or another, either as satellites
or in collisions or near encounters with other galaxies.
The giant elliptical galaxy M87 is a prime example of its type. This huge
galaxy lies at the core of the great cluster of galaxies in Virgo.
M100, a spiral galaxy
The Large Magellanic Cloud, an irregular galaxy, is a satellite of the Milky