Time&Eternity

Time in the Formulation of Scientific Theory 161
The boundary of a black hole is characterized as an absolute event horizon
or Schwarzschild radius. It is assumed that in the center of a black hole,
the curvature of space-time becomes infinite, i.e., that space and time come
“to an end.” This end must remain hidden to every external observer, however,
because the “naked” singularity always lies on the other side of the
event horizon, over which no light can penetrate to the outside. A black
hole can be observed only indirectly, namely, by means of the gravitational
force of attraction that it exerts on its environment. A person approaching a
Schwarzschild radius would instantly fall into the black hole (according to
the person’s reference system), but an observer on earth would see the same
person for an infinite period of time just in front of the Schwarzschild radius;
from her perspective, the person would never cross this boundary. If
time dilatation becomes infinite, as it does on the event horizon of a black
hole, then a microsecond for one is infinite time for another. 261
According to the time symmetry of the theory of relativity, there must
also be a phenomenon that demonstrates a directly opposite behavior of
time, i.e., infinitely dense matter in a singularity that explodes into a cascade
of light. In physics, however, such “white holes” are generally thought
to be improbable. Due to their untenable physical consequences, Roger
Penrose simply excluded them by using a hypothesis called “cosmic censorship.”
Something similar also applies to so-called time travel into the past;
physical laws, at least according to Hawking, appear to favor a cosmic
“Chronology Protection Agency,” 262 which does not permit such things to
happen. 263
One should distinguish between the “end” of time in the singularity of a
black hole and a possible end singularity of the entire universe. Currently,
nothing definitive can be said about the latter, for what one can expect for
the distant future of our expanding universe is critically dependent upon
the mean density of matter. Up until now, however, the density of the universe
has not been determined in any precise way.
If the density is so low that gravity cannot curb the expansion, then the
universe will continue to expand. In this case, we are living in an open universe
that is becoming infinitely larger and larger. If enough mass exists,
however, so that its gravitation stops the expansion and ultimately causes
the universe to collapse, then we live in a closed, finite universe.
The third possibility has a special appeal: The mass density of the universe
could possess precisely that critical level above which it would collapse
and below which it would expand forever. In this case, the expansion velocity
of the universe would become slower and slower without ever reaching
the zero point. In this case, we would also live in an open universe. This
third model is called the Einstein–de Sitter universe.