Gravitational
fields are only homogeneous over small volumes of space and show
different degrees and fluctuations of intensity over large
cosmological distances. The curvature of space is increasingly
effected by the mass and consequent gravitational intensity of
larger astrophysical bodies and as light propagates along this
spacial curvature, the apparent position and actual position of
objects in space differs. Planets, stars, galaxies and galactic
clusters all have an effect on curving light that either emanates
from them, or that passes through their gravitational influence
in a line of sight propagation from the original source to the
observer.
Gravitational effects not only determine the apparent position of
objects but also their appearance as far as apparent shape and
size are concerned. These effects were first predicted by
Einstein's relativity theory and proven by the inadequacy of the
earlier Newtonian theory in the observation of the planet
Mercury's perihelion (closest point in its orbit to the Sun) also
in the now proven bending of starlight as it passes close to the
limb of the Sun and displaces the starfield's apparent position
more, the closer to the limb this light passes.
We are also used to seeing objects reduce in apparent size with
the effects of perspective - this phenomenon may actually be
reversed at very large cosmological distances, predicted to occur
at about 5000 MPc (one Parsec = 3,26 Light Years, one LY =
9,4605e+12 km, therefore about 4.7e+22 km). Past these distances
further objects may actually begin to appear larger depending on
the actual curvature of space about which there is still some
conjecture.
When a distant light emitting source lies in exact line of sight
with a sufficiently large gravitational body such as a galaxy,
the closer object's field may cause a gravitational lens to be
formed which can split the further object's image into multiple
images - this is the effect seen above.
The first actual gravitational lens was discovered in 1979 by the
British astronomer Dennis Walsh. This lens is a giant elliptical
galaxy which produces a double image of a vastly more distant
quasar (quasi-stellar object - some of the most energetic and
bright objects, all lying at vast distances.)
An even stranger phenomenon is the "Einstein Lens"
which causes the further object's image to be projected as a
perfect ring, this first observed in 1988.
The actual curvature of the Universe depends solely on the
matter-density per cubic volume of space. Einstein and De Sitter
worked extensively on models of future cosmic evolution and
universes that were respectively open, closed, bounded and
unbounded were postulated. In a universe where the density factor
is exactly Unity, the universe will have a Euclidean geometry. A
density factor of more than unity will cause the universe to
reach an equilibrium phase in the far future after which it will
gravitationally collapse in on itself, a "Big Crunch"
which in itself might be a cyclically transitional phase for
another Big Bang creative phase.
If the density factor is less than unity the universe will
essentially continue to expand forever. The density factor can
empirically be determined by measuring the distribution of matter
in the visible universe. When this is done by advanced
astrophysical measurement techniques, the amount of visible
matter in the universe falls far short of a unity distribution.
It is however strongly suspected that so-called "dark
matter", interstellar gas and dust clouds that consist of
non-emitting material, contributes vastly more matter than
attributed only to the visible component, thereby making up for
the "missing matter". The latest observational evidence
places the density distribution extremely close to a unity
factor, a stable everlasting universe. The evidence for a
Euclidean universe comes mainly from observations of the cosmic
microwave background radiation at a telescope in Saskatoon,
Canada, although this has also been exmined by NASA's Cosmic
Background Explorer (COBE). Gravitational lenses are however
regarded as counter-evidence for the Euclidean geometry of space
as it is argued that such lensing effects should be common in
such a universe whereas in reality they are rare.
| Another type of gravitational light distortion, sweeping arcs of light, such as seen here in the nebula Abell 2218, should be infrequently observed whereas they are far more common. This image was photographed by the Hubble Space Telescope, HST's wide field planetary camera. |  |
The case for the open or closed universe is therefore not yet solidly resolved and the intriguing mystery continues...
