Sitting in a small, non-descript room in the basement of the Lawrence Berkeley National Lab in Berkeley, physics graduate student Hannah Swift and physicist Saul Perlmutter are searching for supernovae, stars destroyed in huge explosions millions or billions of years ago. (They're looking for ones that exploded billions of years ago). Through a computer hooked up to Hawaii's Keck 2 telescope -– one of the largest in the world –- they are able to follow along as UC Berkeley post-doc Rahman Amanullah, a team member who had traveled to Hawaii a few days earlier, supervises the night's observation.

Their goal is to use a device called a spectrograph to get a spectra of five to eight supernovae. This spectrum is a "photograph" of the light emitted by the supernova and it allows scientists to determine whether they are the type Ia supernovae that are useful for their research. By figuring out how far away these type Ia supernovae are (or "were," since by the time their light reaches us, they have long since vanished) and observing how much the light traveling towards the Earth has shifted towards red wavelengths, they can determine how long ago the star exploded. With this information, they're building a history of the expansion of the universe. The weather in Hawaii is good, and the researchers are forecasting a good "seeing."

It's encouraging and moving to see how young these researchers are. Swift is a twenty-something from Kansas. She's so young that she has always understood the universe to be accelerating, that is, expanding faster and faster. She has never thought that the universe is decelerating, which is what the scientific community believed before 1998, when Perlmutter's team at Lawrence Berkeley National Lab and another team simultaneously reported their findings that the universe is accelerating. When this announcement was made, Swift was in the 8th grade. Perlmutter himself is only in his 40s. When he started his post-doc, he and his colleagues had to beg for time on the big telescopes. That wasn't that long ago: our understanding of the universe has shifted very fast.

A few days later, Swift invites us back to the lab to show us some of the data the researchers gathered during the observation. They obtained spectra for five supernovae – a good night's work. Swift is now in the process of figuring out which of the supernovae are type Ia's. To do this, she's comparing their spectrograms to the spectra of a standard type Ia. Light is a very useful tool because different elements emit light of different wavelengths. Swift shows me the spectrogram for a standard type Ia. It has telltale peaks and valleys that reveal what exploding type Ia supernovae eject when they explode – silicon and cobalt, among other elements.

I wonder what big changes in the way we understand our universe will be commonplace when my one-year-old niece is in her 20s. What new discoveries will we use as points of reference in the coming decades? (Assuming we're hip enough to use astronomical discoveries as points of reference!) Will scientists know by then the nature of dark energy, the now-mysterious "something" that is making the universe accelerate, pushing its fabric apart? Will I be able to preface a comment to my niece with, "When you were little, before we knew what dark energy is…"? And will she be able to reply, "Aunt Gabi, you're so ooold!" I suspect she'll do the latter no matter what.

Watch the "Dark Energy" TV Story online, as well as find additional links and resources. Also don't miss our online photo set.

The project was headed by a group at a university in Toronto and a group at a university in Paris. Canada and France sponsor the 4-meter telescope that is used to discover and observe the supernovae from the point of explosion to the final days when the supernova fades from view. We call this the imaging part of the program. This data constrains the apparent brightness and life cycle of the supernova, and eventually the absolute distance to the supernova.

Our contribution to the project was primarily through our affiliation with Keck Observatory. We were typically awarded four nights a year to observe recently discovered supernovae spectroscopically. The data is used to determine the redshift and the kind of supernova explosion.

The supernovae are used to study the rate of expansion of the universe. It was this type of experiment that was first used to discover that the universe is actually dominated by dark energy.

No one really suspected the presence of dark energy for almost the entirety of the 20^th century. Now, we not only know it exists but are actually trying to understand it in the same way we understand gravity, protons, and electrons. That is where projects like the Supernova Legacy Survey come in. With projects like this, we work to collect enormous samples of well-studied supernovae that can improve our understanding of dark energy.

We use a certain type of supernova as yardsticks to measure distances in the universe. We then model the affects of dark energy on the expansion history of the universe by comparing distances and rates of expansion. This comparison is typically represented in a Hubble Diagram.

The Supernova Legacy Survey has been very successful in its attempts thus far. On the right, I show the Hubble Diagram from the first year of data. This is less than 20% of the full sample. The dotted line outlines the expectations of the 1990's cosmology crowd. The solid line shows the prediction from the more sophisticated cosmologists of the 21^st century. As you can see, the original expectations were pretty far off the mark – the supernovae just don't lie on top of the dotted line.

Now that this program is finishing up, we should be seeing similar figures that are teeming with supernovae. Future programs should do an even better job of making these measurements. Someday we may actually understand this dark energy thing, it may turn out to be something else completely new and unexpected!

Kyle S. Dawson is engaged in post-doctorate studies of distant supernovae and development of a proposed space-based telescope at Lawrence Berkeley National Laboratory.

Julien Guy: supernova cosmologist

I spent the last week at a conference in the Italian Alps with about 200 skier/cosmologists. Mornings were spent in the conference hall watching 15 or 25 minute presentations. Afternoons were for the slopes. Evenings were back in the conference hall.

The conference started with supernova talks – I was fourth on the list. Being in the field, I had heard most of the results that were presented in the other talks. Ditto the other attendees' perspectives on my talk. However, there were some new and very promising results from the Supernova Factory.

The supernova factory is a LBNL-based research group that focuses on "nearby supernovae". By nearby, I mean only a few hundred million light years away. These supernovae occur in galaxies that are distant enough to be free of the gravity of the Milky Way and our neighboring galaxies but close enough to observe with smaller telescopes.

The supernovae observed by the SN factory are very bright compared to the supernovae I observe with the Hubble Space Telescope. The supernovae are bright enough to make very precise measurements at each wavelength of the supernova spectrum. Just like my earlier post on spectroscopy, the supernova light is imaged after passing through a prism. These images provide very detailed information about the molecules and atoms that are present in the supernova explosion.

The spectroscopic observations also tell us how one supernova may differ from another. The small variations in type Ia supernovae have been a mystery for quite some time. If we can learn the causes of these variations, these supernovae could be come even more useful for measuring distances in space.

There are several models and theories to explain the differences, but none has been extensively tested. A large number of bright nearby supernovae is required to test these models. Hopefully, a project like the supernova factory will provide that sample. In this conference, they only showed a handful of supernovae. All but one of these supernovae was well-behaved, fitting our current models. The last one differed enormously from the others, but the detailed spectroscopic observations lent evidence as to why this may be the case. The data is still being examined, but I am encouraged by the progress necessary if supernovae are to be used to explain the cosmology of our universe.

The presentations over the next five days covered a very large range of topics. Some conference attendees presented ideas that had never occurred to me. One that I found very interesting was an experiment to model the orbital paths of stars around the black hole at the center of the Milky Way. For those patient enough to watch these stars for 15 years, it should be possible to measure the properties of gravity and the black hole itself by looking for deviations in the stars orbits from our current models.

While the talks were very interesting and well-attended, I can't help but comment on the other important side of this conference. That would of course be the skiing. The Europeans really have it right – they chose the site and the schedule with the perfect balance for leisure time. We were only ten miles from the tallest mountain in Europe, within site of the Matterhorn, had perfect snow all week, and had just enough time to enjoy it. I even had a chance to practice my amateur photography on the slopes. Now the next challenge will be to organize a conference in Tahiti!

Kyle S. Dawson is engaged in post-doctorate studies of distant supernovae and development of a proposed space-based telescope at Lawrence Berkeley National Laboratory.