Supernova is the cataclysmic explosion of a dying star. It is a brilliant death. With luminosity a few billion times that of the sun, a supernova can be seen across vast cosmic distances. But supernova is not only about the end. It holds the key to understanding the past and future of our universe. One type of supernovae, namely Type Ia, has been remarkably consistent in its light output characteristics throughout the cosmic history. Astronomers thus use Type Ia supernovae as "standard candles" to measure distances across the universe. What they found is the faraway (high redshift) Type Ia supernovae appear to be brighter, whereas the not-so-faraway (low redshift) ones appear to be dimmer, than what we would have expected had the universe been expanding at a constant pace. This tells us our universe underwent an early decelerated expansion phase, and has recently switched to an accelerated expansion phase.
What about our future? Independent measurements of supernovae, the cosmic microwave background, galaxy clusters, and others point to the current makeup of our universe: 5% baryonic matter, 25% dark matter, and 70% dark energy. Baryonic matter is the stuff that makes you, me, planets, and stars. We can see and feel it. We understand it. Dark matter tends to clump in galaxy clusters. We cannot see it, but we can feel its gravitational tug. We do not yet understand what it is. Dark energy, even more mysterious than dark matter, permeates space and pushes space apart. It is the culprit behind the recent onset of the universe's accelerated expansion. Though there are no lack of theoretical ideas on what this dark energy might be, cosmological constant, which once was Einstein's abandoned kid, is currently the most promising candidate. As the universe undergoes accelerated expansion driven by the cosmological constant, matter density gets diluted. The ability for matter to slowdown universe's expansion via gravity, therefore, gets marginalized. As time goes on, the cosmological constant will become more and more dominant, and the universe will continue to expand under its increasing influence. Once the accelerated expansion has begun, there is no way to stop it.
On a side note, it is a non-trivial enterprise to hunt for Type Ia supernovae and to make sure they are indeed "standard candles." For the interested reader, I highly recommend The Extravagant Universe by Robert Kirshner. This book provides an engrossing account on the pursuit of Type Ia supernovae and explains how these supernovae can help us to answer some of the most fundamental questions in cosmology.
Figure: The white dot at the center is SN 1997ff. At a distance of 10 billion light-years away with a redshift of 1.7, SN 1997ff is the furthest Type Ia supernova ever observed. Credit: NASA and A. Riess (STScI) (http://hubblesite.org/newscenter/archive/releases/2001/09/)
