Einstein's Theory of General
Relativity implies that the size of the Universe should change
with time. Imagine the Universe as grid patterns on a balloon. The
patterns will become larger when the balloon is inflating and
smaller when contracting. However, even Einstein himself could not
accept that our Universe is ever-changing. He therefore
constructed a "Steady State Universe" model by
introducing a bizarre term, cosmological constant, into his
theory. It was not until 1929 did astronomer Edwin P. Hubble
discovered the red shift of distant galaxies and thereby proved
that the Universe is expanding in all directions. When Einstein
heard and fully appreciated these observations, he declared that
the inclusion of the cosmological constant was his "biggest
Hubble Space Telescope images pinpoint distant supernova, which
exploded and died billions of years ago. Scientists are using
these faraway light sources to estimate if the universe was
expanding at a faster rate long ago and is now slowing down.
How the Universe evolves is affected by
three factors, namely the mean density of mass-energy, the space-time
curvature and a mysterious factor, the cosmological constant. The future
of the Universe is mainly determined by the mean density of mass-energy
and the space-time curvature, while cosmological constant does not carry
any weight among popular cosmological theories. Since substances attract
each other under gravitational pull, it will slow down the expansion of
the Universe. Hence, if the mean density of mass-energy is too high, it
will ultimately stop the expansion and lead to a closed Universe.
Contraction will follow until the whole Universe is squeezed into an
infinitely small region. On the other hand, if the mean density of
mass-energy is too low, the Universe will keep on expanding forever,
resulting in an open Universe. It is very difficult to measure directly
the total mass of the Universe because most of the matter in the
Universe is dark matter, which do not give out light. However,
physicists inclined to believe that the total mass of the Universe will
be "just right", that means it is enough to stop the expansion
of the Universe at last but is not enough to pull it back. Hence, the
Universe is neither closed nor open but a flat Universe.ˇ@
New observational findings give us new insights. Astronomers found that
certain kind of supernova outbursts could be used as a cosmic yardstick.
By measuring the luminosity of these supernovae, accurate distances
between the Earth and the supernovae together with their host galaxies
can be worked out. Two different groups of American scientists resorted
to this new method and discovered separately that the Universe is
expanding in an accelerating speed.
Other astronomers also reached the same conclusion when they were
studying the fluctuations of the microwave background radiation. This
finding revealed that cosmological constant factor cannot be neglected.
It represents a repelling force, which fuels the Universe's expansion.
Does this imply that the Universe is open and will expand endlessly? By
now we know too little about cosmology constant to give a definite
answer. It seems that the cosmology constant has something to do with
the energy density of empty space, and it may change with time, or may
even do the opposite one day - compel the Universe to contract.
New discoveries always reassure us a plain fact - every time we explore
deeper into the Universe, more unexpected and interesting phenomenon
will pop out; the more we understand the Universe, the more mysteries we
have to solve. Looking up the sky, we can't help but to be awed when we
realize how little we actually know. No one knows what we will believe
tomorrow, but one thing is clear, astronomy and cosmology will continue
to thrill our minds.