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The Rapid Expansion
The universe was expected to be decelerating because of the gravitational force applied by its own mass. But in 1998, from the observation of a distant type Ia supernova we came to know that the universe is speeding up its expansion. Type Ia supernovae are identified as the end result of a carbon-oxygen white dwarf star which, as the victim of generosity by its binary companion, grows in mass until it reaches the Chandrasekhar limit (If a white dwarf exceeds 1.4 solar masses, it can't support itself by electron degeneracy pressure. A white dwarf that exceeds this mass must continue its collapse until a new source of pressure, neutron degeneracy pressure, is able to stop the collapse.) At this critical mass, the white dwarf is engulfed by a runaway thermonuclear explosion, which reaches a peak luminosity of about 4 billion times the Sun’s. These supernovae are so bright they can be seen with the Hubble Space Telescope 75% of the way across the Universe and used by astronomers as "standard candles."
Astronomers can measure the distance of supernova and they also know that when it occurred. Though our universe is expanding,so light from the supernova is redshifted. The degree of this `redshift’ provides a direct measure of the change in the size scale of the Universe during the journey of the light.
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The Mysterious Force
The mysterious force accelerating the expansion of our universe is named dark energy. It is also known as anti-gravity or repulsive gravity. But what type of energy is it? Does it exist everywhere? Many questions arise when we discuss dark energy. But we don't have any doubt that it exists and comprises a whopping 73% of our Universe's total mass-energy.
One of the explanations of dark energy is the "cosmological constant" that counters gravity to maintain the balance. The cosmological constant evolved from "general relativity" which was formulated by Albert Einstein in 1915. Einstein assumed that the universe was neither expanding nor contracting, that's why he used it. But after Edwin Hubble's discovery of the expansion of our universe in 1929, Einstein rejected his own idea and called it his greatest scientific blunder. But actually it was not a scientific blunder because our universe is expanding and the positive cosmological constant accelerates the expansion, which agrees with the experimental evidence. Quantum Mechanics has a more physical name for this feature, vacuum energy. Vacuum energy has negative pressure. This property gives rise to repulsive gravity which contributes to the "cosmological constant".
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We don't know exactly what the fate is of our universe. Will the expansion start decelerating? Will gravity come back in the game? Or, will our universe continue expanding in this way? If it keeps expanding in this way I think our future generation will hardly find any stars in the night sky.