By Don Lincoln, Fermilab
The ability to measure the absolute light emitted by supernovae and their red shift to determine velocity is not always possible using a single telescope. Accordingly, astronomers need to combine observations from many instruments to make their measurements.
Supernova Cosmology Project (SCP)
In the 1980s and early 1990s, a collaboration of astronomers, led by researchers Saul Perlmutter and Carl Pennypacker, both associated with both the Lawrence Berkeley National Lab and the University of California at Berkeley, worked to validate the idea that Type1a supernovae were a reliable method to determine cosmic distances.
While the idea had been bandied about before their work, there was some skepticism as to whether it would work. The experimental collaboration was called the Supernova Cosmology Project, or SCP.
Forming the High-Z Supernova Search Team
The SCP used several telescopes to do their searches from some of the premier astronomy sites on the planet, from the Canary Islands, just off Africa, to Cerro-Tololo, Chile, to the Keck Observatory in Hawaii, to the Hubble Space Telescope. They combined the red shift and the brightness measurements of supernovae to make their measurement.
Meanwhile, another experimental group had joined the hunt. In 1994, a group of astronomers led by Australian Brian Schmidt and American Nicholas Suntzeff proposed what is now called the High-Z Supernova Search Team. Over the course of the next four years, they took a similar set of data. By 1998, they were led by American astronomer Adam Riess.
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Measuring the Expansion History of Universe
On January 8, 1998, at a press conference during the American Astronomical Society’s annual meeting, Saul Perlmutter announced that the SCP collaboration had measured the expansion history of the universe. The High-Z Supernova Search Team independently confirmed the measurement shortly thereafter.
So, by this standard, the SCP collaboration got there first. However, the first paper that was accepted and published by the scientific periodical, Astronomical Journal, was published by the High-Z team. There was some embarrassing and predictable academic bickering as to which group had precedent.
At some level, this bickering was entirely understandable. There were reputations to be made or lost depending on how the scientific community assigned precedence. Perlmutter, Riess, and Schmidt shared the 2011 Nobel Prize in physics, which gives one a sense of what was at stake, academically speaking.
Unexpected Observations of the SCP and High-Z Team
So, what did the SCP collaboration and the High-Z team observe? One needs to remember that they were trying to understand the history and fate of the universe. While researchers were performing their measurements, there were thought to be three distinct possibilities, which we already know about.
- One was that the universe would expand and collapse under the force of gravity.
- Two was that the universe would expand and slow, but never stop expanding.
- Three was that the universe would expand and slow, eventually stopping at infinity.
So, what did they see? Which outcome due to the slowing of the expansion of gravity was the one that described the universe, was it one, two, or three? The answer is: What they saw was, wait for it, number four. The expansion of the universe wasn’t slowing down at all. It was speeding up.
Looking at Velocity
The research teams looked at very distant supernovae, or really the galaxies that hosted the supernovae. Some might have been a billion light years away; some were seven or eight billion. A couple were more distant.
Though not possible, but if we try to ignore the expansion and look at a galaxy a billion light years away, we’ll measure its velocity a billion years ago. If the exact same night we turn our telescope to a galaxy that is eight billion light years away, we’ll be measuring the velocity of that galaxy eight billion years ago. Two measurements, the same night, will determine the velocities of two galaxies separated by large distances and seven billion years.
Thus, when you do this sort of astronomy, you need to keep such things in mind. When you see a particular celestial body, you always need to remember when the light you are seeing was emitted. And, when you’re trying to understand the history of the universe itself, this sort of thing is a big deal.
Understanding the Observations of SCP
It’s often said that the researchers saw that the expansion of the universe is accelerating, and that’s true to a degree, but it’s not the complete story their data was telling them.
What they observed was that for the first nine or so billion years of the history of the universe, the expansion was constantly and predictably slowing down, just as predicted by old-fashioned big bang theory. However, about five billion years ago (just about the time our sun began to coalesce out of interstellar gas) something changed, and the expansion rate of the universe changed from slowing down to speeding up.
Common Questions about the Observations of the Supernova Cosmology Project
In the 1980s and early 1990s, a collaboration of astronomers, led by researchers Saul Perlmutter and Carl Pennypacker, both associated with both the Lawrence Berkeley National Lab and the University of California at Berkeley, worked to validate the idea that Type1a supernovae were a reliable method to determine cosmic distances. The experimental collaboration was called the Supernova Cosmology Project, or SCP.
Besides SCP, another experimental group had joined the expedition to measure cosmic distances. In 1994, a group of astronomers led by Australian Brian Schmidt and American Nicholas Suntzeff proposed what is now called the High-Z Supernova Search Team. By 1998, they were led by American astronomer Adam Riess.
In a bid to understand the history and fate of the universe, the SCP used several telescopes to do their searches from some of the premier astronomy sites on the planet. Based on their observations, it was found that contrary to all expectations, the expansion of the universe wasn’t slowing down at all. It was speeding up.
