Science at its Best: The Universe is Expanding AND Accelerating

The 2011 Nobel Prize in Physics was recently awarded to three scientists: Adam Riess, Saul Perlmutter and Brian Schmidt. Their computations and observations of a very specific type of exploding star upended decades of commonly accepted beliefs of our universe. Namely, that the expansion of the universe is accelerating.

Prior to their observations, first publicly articulated in 1998, general scientific consensus held that the universe would expand at a steady rate forever or slow, and eventually fold back in on itself in a cosmic Big Crunch.

The discovery by Riess, Perlmutter and Schmidt laid the groundwork for the idea that a mysterious force called “dark energy” is fueling the acceleration. This dark energy is now believed to make up 75 percent of the universe. Direct evidence of dark energy is lacking, but most cosmologists now accept that universal expansion is indeed accelerating.

Re-published here are the notes and a page scan from Riess’s logbook that led to this year’s Nobel Prize, which show the value of the scientific process:

[div class=attrib]The original article is courtesy of Symmetry Breaking:[end-div]

In the fall of 1997, I was leading the calibration and analysis of data gathered by the High-z Supernova Search Team, one of two teams of scientists—the other was the Supernova Cosmology Project—trying to determine 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 ?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.

Adam Riess, Johns Hopkins University

The discovery, and many others like it both great and small, show the true power of the scientific process. Scientific results are open for constant refinement, or re-evaluation or refutation and re-interpretation. The process leads to inexorable progress towards greater and greater knowledge and understanding, and eventually to truth that most skeptics can embrace. That is, until the next and better theory and corresponding results come along.

[div class=attrib]Image courtesy of Symmetry Breaking, Adam Riess.[end-div]