RADIATION. This “sea” of electromagnetic radiation fills the universe
at a temperature of about 2.7K (2.7 degrees C above absolute zero).
Background radiation had been proposed by general relativity as the
remaining trace of an early, hot phase of the universe following the big
bang. The observed cosmic abundance of helium (20 to 30 percent by
weight) is also a required result of the big-bang conditions predicted
by relativity theory. In addition, general relativity has suggested
various kinds of celestial phenomena that could exist, including neutron
stars, black holes, gravitational lenses, and gravitational waves.
According to relativistic theory, neutron stars would be small but
extremely dense stellar bodies. A neutron star with a mass equal to
that of the Sun, for example, would have a radius of only 10 km (6 mi).
Stars of this nature have been so compressed by gravitational forces
that their density is comparable to densities within the nuclei of
atoms, and they are composed primarily of neutrons. Such stars are
thought to occur as a by-product of violent celestial events such as
supernovae and other gravitational implosions of stars. Since neutron
stars were first proposed in the 1930s, numerous celestial objects that
exhibit characteristics of this sort have been identified. In 1967 the
first of many objects now called pulsars was also detected. These
stars, which emit rapid regular pulses of radiation, are now taken to be
rapidly spinning neutron stars, with the pulse period represent the
period of rotation. Black holes are among the most exotic of the
predictions of general relativity, although the concept itself dates
from long before the 20th century. These theorized objects are
celestial bodies with so strong a gravitational field that no particles
or radiation can escape from them, not even light–hence the name. Black
holes most likely would be produced by the implosions of extremely
massive stars, and they could continue to grow as other material entered
their field of attraction. Some theorists have speculated that
supermassive black holes may exist at the centers of some clusters of
stars and of some galaxies, including our own. While the existence of
such black holes has not been proven beyond all doubt, evidence for
their presence at a number of known sites is very strong. In theory,
even a relatively small mass could become a black hole. The mass would
have to be compressed to higher and higher densities until it diminished
to a certain critical radius, the so-called “event horizon,” named the
SCHWARZSCHILD RADIUS because it was first calculated in 1916 by German
astronomer Karl Schwarzschild. (His calculations apply to a nonrotating
object. The figures for a rotating object were developed in 1963 by New
Zealand mathematician Roy Kerr.) For an object having the mass of the
Sun the event horizon would be approximately 3 km (2 mi). Scientists
such as the English theoretical physicist Stephen HAWKING have
speculated that tiny black holes may indeed exist. The concept of
gravitational lenses is based on the already discussed and proven
relativistic prediction that when light from a celestial object passes
near a massive body such as a star, its path is deflected. The amount
of deflection depends on the massiveness of the intervening body. From
this came the notion that very massive celestial objects such as
galaxies could act as the equivalent of crude optical lenses for light
coming from still more distant objects beyond them. An actual
gravitational lens was first identified in 1979. One phenomenon
predicted by general relativity has not yet been substantially verified,
however: the existence of gravitational waves. Gravitational waves
would be produced by changes in gravitational fields. They would travel
at the speed of light, transport energy, and induce relative motion
between pairs of particles in their path (or produce strains in more
massive objects). Astrophysicists think that gravitational waves should
be emitted by dynamic sources such as supernovae, massive binary (or
multiple-star) systems, and black holes or collisions between black
holes. Various attempts, unsuccessful thus far, have been made to
observe such waves. A more fundamental matter confronting general
relativity is that of the attempt being made by physicists to unite
gravitation with QUANTUM MECHANICS, the other paradigm of modern
physics. This search for some UNIFIED FIELD THEORY is the major task of
workers in QUANTUM COSMOLOGY.