*"for the discovery of the accelerating expansion of the Universe through observations of distant supernovae"*.

Btw, same three people won 2006 Shaw prize for Astronomy (from Honkong) .

As Adam Reiss puts it ,

I started with the thirst of determining the fate of our universe: Will it expand forever, or will it halt and contract, resulting in the Big Crunch?To find the answer, we had to determine the mass of the universe. It can be calculated by measuring how much the expansion of the universe is slowing.

First, we had to find cosmic candles—distant objects of known brightness—and use them as yardsticks. On this page, I checked the reliability of the supernovae, or exploding stars, that we had collected to serve as our candles. I found that the results they yielded for the present expansion rate of the universe (known as the Hubble constant) did not appear to be affected by the age or dustiness of their host galaxies.

Next, I used the data to calculate Ω

_{M}, the relative mass of the universe.It was significantly negative!

The result, if correct, meant that the assumption of my analysis was wrong. The expansion of the universe was not slowing. It was speeding up! How could that be?

I spent the next few days checking my calculation. I found one could explain the acceleration by introducing a vacuum energy, also called the cosmological constant, that pushes the universe apart. In March 1998, we submitted these results, which were published in September 1998.

Today, we know that 74 percent of the universe consists of this dark energy. Understanding its nature remains one of the most pressing tasks for physicists and astronomers alike.

1. Hubble's Law : Current expansion of the universe. This can be easily understood by this equation , v = H * D .

v = Recession velocity.

D = proper distance of the far galaxy receding away from us.

H = The Hubble's constant , roughly around 70 (Km/s) / Mpc

2.

Most astronomers had expected that the Universe's rapid growth following the Big Bang would gradually slow down as gravity pulled distant galaxies towards each other. Yet the discovery was accepted almost immediately by the astronomical community — in part because the idea of a cosmic pressure pushing the Universe outwards had already been mooted by Albert Einstein. - Nature news.So now, from those lines, we begin to think about gravity pulling back and hence slowing down the velocity of Expansion.

As always in Physics, bottom line is, "We're confident about being not confident about what's going on" - Reiss

For thirsty souls, here are those 1998 papers:

http://dx.doi.org/10.1086/307221

http://dx.doi.org/10.1086/300499

-Pv

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