Scientists are looking for the exploding stars in our universe for more clues. When all the energy of this star is spent, the star collapses and becomes White Dwarf. Sometimes other star orbits this White Dwarf. If that star comes too close, it crushes into the White Dwarf and our universe observes spectacular firework display. It's called Supernova.
Scientists are using supernovas to determine how fast our universe is expanding. The brightness of these explosions takes some time to disappear. No matter where these explosions occur it always takes same amount of time. Without Hubble Telescope supernova may never have been discovered.
The distances of supernova are measured by measuring the amount of red light they emit. This is done by analyzing the spectrum of the supernova. The more redder the light appears the faster the star are getting away from us. By measuring how fast the galaxy is getting away from us we can calculate the speed of the expansion of our universe. It is done by comparing the velocities of galaxies with their distances. These data can help to find out when our universe will stop expanding and start shrinking. Or this may lead to some other possibility.
Recently the scientists have discovered shocking news; the Red Shift from distant galaxies is greater than previously measured. This can only mean one thing. Not only our universe is expanding, it is speeding up. Scientists have never predicted that. Scientists had no answer what could have caused this. They could not believe this. But data proves that universe is certainly accelerating. They made up a theory that some sort of invisible energy is working against gravity and causing our universe to accelerate. They call it the DARK ENERGY.
Dark energy is a mysterious energy. Scientists' don't know much about it. It is a negative force because it pushes galaxies away from each other. When our universe was young gravity was dominant force. So galaxies and stars were born. But as our universe continues to expand; dark energy seems to dominate. The result is our Accelerating Universe.