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 blunder".

These 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.　


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.


Photo courtesy:NASA