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By Linda Copman
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| Photo: Saul Perlmutter. |
“We live at an unusual time in history in that our rapid
advance in technology has given us the opportunity to address
truly fundamental questions about the world we live in, by
making measurements and not just philosophizing. These are
the kinds of questions that bring out the curious child in
all of us — and thus bring people together from all backgrounds.” — Saul
Perlmutter, co-winner of the 2006 Shaw Prize for Astronomy.
When Saul Perlmutter was a graduate student, he worked very
near the Lawrence Berkeley Lab offices of Jerry Nelson and
Terry Mast, who designed and engineered the Keck I telescope. “Each
week I would hear about their latest successes and setbacks,
and I watched as they realized this amazing new segmented-mirror
telescope concept. At the time I didn’t know how much my research
was going to depend on what they were doing,” recalls Perlmutter.
A few years later, Perlmutter and his team demonstrated a technique
which enabled astronomers to find “batches,” on demand, of
supernovae at cosmologically useful distances. This capability
was critical to the study of cosmology, or the study of the
origin, structure, and evolution of the universe. Here’s why:
supernovae, or exploding stars, can be a few billion times
more powerful than our Sun, and therefore very bright objects
in the sky. One kind of supernova, “Type Ia,” is caused by
the explosion of a white dwarf star and the characteristics
of these supernovae are fairly consistent. They are very luminous,
and they all have nearly the same power, “like car headlights,” explains
Alex Filippenko, High-Z Supernova Search Team member.
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| Figure: Strategy to guarantee supernovae
discovery. Courtesy of Saul Perlmutter. |
By comparing the brightness of nearby supernovae with the brightness
of distant supernovae, astronomers are able to determine the
distance of the fainter objects. To do this, astronomers need
to verify that they are observing the right kind of supernovae,
or Type Ia supernovae. “With hours and hours of observations
on the best previous telescope, we could begin to make a difficult
case for a single supernova, but with the new much-larger Keck
telescope we could quickly identify supernova after supernova,
several per hour,” explains Perlmutter.
Astronomers obtained “spectra” of distant Type Ia supernovae
using the Keck telescopes (which means that they separated
the light from these objects into its component colors), in
order to learn more about these objects, including their velocity
in space. What they found was that very distant supernovae,
viewed when the universe was two-thirds of its present age,
are receding away from our planet more slowly than they had
anticipated. Yet nearer supernovae, viewed at essentially the
present time, are receding more rapidly. Thus, as the universe
has expanded following the Big Bang, its expansion rate has
accelerated over time.
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| Image: Courtesy of Saul Perlmutter. |
Here’s what Brian Schmidt, leader of the High-Z
Supernova Search Team, has to say about this unexpected
discovery:
“Rather than measuring deceleration, we discovered acceleration — the
universe was speeding up over time, and this meant it was made
up of something we didn’t know about — something everyone now
calls ‘dark energy.’ The big question now is — what is dark
energy? — and we still haven’t a clue. Possibly it is what
Einstein concocted back in 1917 to keep his equations from
having the universe expand (or contract) - the ‘cosmological
constant.’ This is an energy that Einstein’s equations allow
to exist — and it is the energy of empty space. It would cause
the universe to accelerate, but what we do not understand is
why the energy of space would be what we measure today. So
figuring out what dark energy is has become one of the big
questions for astronomers and physicists today.”
Saul Perlmutter, leader of the competing Supernova
Cosmology Project, adds:
“We set out to make a measurement of the expansion history
of the universe, expecting to find out how much it has been
slowing down (due to gravity) since the Big Bang. We thought
that we might discover the ‘fate of the universe’ (an exciting
goal!), since it was possible that the universe was slowing
enough to come to a halt, collapse, and end in a ‘Big Crunch.’ What
we found was a complete surprise, the universe was not slowing
at all — its expansion has been speeding up.
This result has posed a new mystery: is there a new previously
unknown component of our universe, a ‘dark energy’ that is
accelerating the expansion? If so, it makes up most of the
universe (about 75 percent). Alternatively, do we need a major revision
to our understanding of gravity (Einstein’s General Relativity)?
These exciting questions have led the physics and astrophysics
communities to start entirely new projects, including a proposed
dedicated space experiment specially designed to make the unusually
precise cosmology measurements needed to explore the nature
of the dark energy or the corrections to Einstein’s theory
of gravity.”
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| Image: In 1916 Vesto Slipher observed about
50 nearby galaxies, spreading their light out using a
prism, and recording the results onto film. The results
confounded him and the other astronomers of the day.
Almost every object he observed had its light stretched
to redder colors, indicating essentially everything in
the universe was moving away from us. Here we show the
spectrum of a galaxy as Slipher would have seen it. The
light is stretched in the bottom spectrum, so that the
dark lines (the colors where elements such as sodium
absorb light), are stretched to redder colors. Image
and caption courtesy of Brian Schmidt. |
Perlmutter’s and Schmidt’s teams made these groundbreaking
discoveries about the history and fate of the universe simultaneously,
working at different institutions in different parts of the
world. For a list of Perlmutter’s team members, see
here, and for a list of Schmidt’s team members, please click
here. Both teams relied on the Keck telescopes for two
things:
- To identify the right type of supernovae, Type Ia, and
- To determine how much the universe had expanded since
the light from the supernova being studied was emitted.
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| Image: This plot shows the High-Z Supernova
Search Team’s data. Across the bottom axis is distance
(or equivalently time since the supernova exploded).
The side axis shows change in the expansion rate from
its current rate. As you can see, the points do not show
the universe slowing down enough to eventually collapse.
Furthermore, they do not even seem to show the universe
coasting; rather they show that the universe is accelerating!
Image and caption courtesy of Brian Schmidt. |
“Keck — as the most powerful spectroscopic facility of its era — allowed us,
for the first time, to take spectra of these distant exploding stars we were
discovering, and thereby do 1) and 2) above. Without it, we just wouldn’t have
had the information we needed to discover the accelerating Universe,” explains
Schmidt.
The next steps are to quantify the expansion history of the
universe and to determine how much the universe has been accelerating
recently, compared with 2, 4, 6, or 8 billion years ago. These
measurements will allow astronomers to better understand the
nature of the "dark energy" that is driving the accelerated
expansion.
And what of the competition among the scientific teams? This
is a good thing, says Brian Schmidt:
“Saul’s team was our competition — and Saul played a similar
role to me in his team. Having two teams work competitively
on this research raised the whole standard. Everything we did
we knew was going to be publicly scrutinized and judged in
competition with Saul’s group. Both teams learned from the
other’s successes, and we adopted a fairly similar strategy
in the end. The competition also made us work more quickly — we
had to ensure we were not scooped. We now work a bit more together
toward our common goals, but not completely.
To solve a problem you need a group of people with a variety
of skills. I believe in making collaborations to get the required
skills needed to solve the problem, but I think science also
needs to be competitive, or it drags to a standstill. I should
say, by ‘competitive’ I mean supporting a diversity of teams
and efforts to reach similar goals. I do not mean that software,
data, etc., should be kept proprietary in the interests of
competition. This is a terrible thing that I see microbiology
falling into due to intellectual property issues. Eventually,
this restriction of knowledge will slow progress down as everyone
either reinvents the wheel or is prohibited from using the
wheel due to intellectual property concerns. I have no prescription
on how to work collaboratively. I just want to preserve diversity
and keep science as open as possible, without barriers.”
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| Animation: Here is a toy model of the universe.
Imagine if we expand it by five percent, and overlay the two images,
centered on a galaxy near the center of the two pictures.
As you can see, every galaxy appears to have moved away
from the galaxy that we have centered the images on.
Furthermore, the farther a galaxy is away from the center
object, the farther it has moved in the expansion. This
is exactly what Hubble saw. Another good part of this
explanation is that everyone in the universe sees the
same thing. Here we have centered the two pictures on
a different galaxy. From this galaxy's perspective everything
is moving away from it - it sees exactly the same thing
as the previous galaxy. Original image
and caption courtesy of Brian Schmidt. |
UC Berkeley Astronomy professor Alex Filippenko was a member
of both teams at one time. He first worked as part of Perlmutter’s
team, and is now a member of Schmidt’s team. Filippenko was
recently awarded the 2007 Richtmyer Memorial Award from the American
Association of Physics Teachers for his research (including
the discovery of the accelerating universe) and his ability
to explain it to the general public. He also received the 2006
National Professor of the Year Award from the Carnegie
Foundation for the Advancement of Teaching and the Council
for Advancement and Support of Education in Washington, D.C.
Filippenko is one of the most published and cited astronomers
in the world, and he has also been recognized as one of its
great teachers. When asked what qualities make a great astronomer,
Filippenko replied as follows:
“Astronomers are driven by a deep desire to understand
the structure and evolution of the universe, and the physical
nature of its contents. We want to know how things work, and
we want to determine our origins. Of course, we also want to
do all this first, and best. The competition, and a genuine
interest in figuring things out, compel us to excel.”
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| Image: This diagram reveals changes in
the rate of expansion since the universe's birth 14 billion
years ago. The more shallow the curve, the faster the
rate of expansion. The curve changes noticeably about
5 billion years ago, when objects in the universe began
flying apart as a faster rate. Astronomers theorize that
the faster expansion rate is due to a mysterious, dark
energy that is pushing galaxies apart. Credit for image:
NASA/STSci/Ann Field. |
But why should ordinary mortals care about the frontiers of
cosmological research? Here’s why, explains Brian Schmidt: “Making
discoveries in astronomy provides us as human beings a perspective
of our place in the universe. The accelerating universe is
especially profound, and it might be that there is a glaring
error in our theories of gravity and how gravity interacts
with things at the smallest scale via quantum mechanics. So
it is just possible that understanding the accelerating universe
might open up a window into how gravity and things on the atomic
scale work together — and who knows where that might lead.
It was the theory of quantum mechanics that opened up the possibilities
of computers and the digital era . . .”
Read Brian Schmidt’s
lecture, which he drafted upon acceptance of the 2006
Shaw Prize for Astronomy. The Shaw Prize honors individuals
who have achieved a “significant breakthrough in academic and
scientific research or application, and whose work has resulted
in a positive and profound impact on humankind.” 
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