Introduction to Cosmology

Black Holes 103

Black Hole Creation. Primordial black holes may have been created in the early
universe by initial homogeneities, inflation, phase transitions, bubble collisions, or
the decay of cosmic loops. Small primordial black holes are not expected to exist
because they may already have evaporated.
The neutrons in a neutron star form a cold Fermi gas in which the quantum degen-
eracy pressure of the neutrons prevent the star from collapse. If the mass of the star
exceeds theTolman–Oppenheimer–Volkoff limitof 2.0–3. 0 푀⊙it may not always be sta-
bilized against bounce. They then follow the route of further gravitational collapse to
become a hypotheticalquark staror a black hole.
The fate of a collapsing spherical star can be illustrated schematically by a light
cone diagram. Consider the evolution in time of an event horizon corresponding to the
circular equatorial intersection of the star. With increasing time, vertically upwards
in Figure 5.5, the the equatorial intersection shrinks and in consequence light rays

t

E

D

C

B

a A

b

surface

of star interior

of star

Figure 5.5 A space-time picture of the collapse of a spherical star to form a black hole. For
an observer at the center of the surface (B), the surface (A) is his event horizon and (a) is his
particle horizon. With increasing time, the radius of the star shrinks. For an observer at the
center of the surface (C), the surface (B) is his event horizon and (b) is his particle horizon.
At at the center of the surface (D) the particle horizon coincides with the event horizon (C).
The surface (D) has become a trapped surface. At (E) no future light cone exists, the star has
arrived at the singularity of the black hole.