262 The Poetry of Physics and The Physics of Poetry
Cosmological Speculations
What lies beyond these galaxies that are just barely visible? It is
impossible to observe galaxies further away because they are not bright
enough to be seen but there is no reason to believe that galaxies suddenly
stop existing at 6 billion light years. How far does the universe extend,
however? Is the universe finite in extent as the Greeks imagined or is it
infinite as Bruno and the deists believed? One may also ask whether the
universe will continue to expand forever and if so into what? Or will it
contract eventually and then re-expand as some claim. And what is
causing the acceleration of the expansion? These questions are intriguing
but at the moment are matters of scientific speculation. Various models
of the universe provide different answers to these questions. One of the
more popular approaches introduces the notion of dark matter and dark
energy, which we will say more about presently.
The expansion of the universe provides some clues about the size of
the universe depending on how one regards the origin and nature of this
expansion. If the velocity of the clusters continues to increase with
distances beyond the 6 billion light year boundary for observing
exploding galaxies, then at a distance of 13.7 billion light years the
velocities of the clusters will equal the velocity of light. Light from an
object, no matter how bright it is, could never reach us if its velocity
equals the speed of light because of the Doppler shift. The frequency of
the light emitted by such an object would be shifted all the way to zero.
The observable universe, therefore, would have a finite radius of 13.7
billion light years.
What lies beyond this radius of 13.7 billion light years? Are there
stars and galaxies, which we will never be able to detect because their
light is red shifted to zero? Or does the universe end abruptly at this point
with nothing lying beyond the edge of the 13.7 billion light year radius
but empty space? Or does the universe close back upon itself so that a
space traveler could never come to the edge of the universe? In such a
closed universe if I traveled far enough I would return to the point of my
origin. This is exactly what would happen if I were to walk far enough
along the surface of the Earth. Or do I come to the edge of the universe
and fall off as the sailors who first crossed the Atlantic once feared?
These questions are part of the domain of cosmology, the branch
of astronomy, which tries to understand the universe as a whole.
Cosmologists do not have hard and fast answers to these questions. They