| MIT’s Max Tegmark: Clusters, Clumps, and the SDSS |
 The
Sloan Digital Sky Survey (SDSS) bills itself on its website as the
"most ambitious astronomical survey ever undertaken," which is
not an overstatement. Using what is probably the largest digital camera
ever built (for civilian use, at least), the 2.5 meter telescope of the
SDSS measures the spectra of 600 galaxies and quasars in a single
observation. By the first phase of operations, completed in June 2005,
the survey had imaged nearly 200 million celestial objects and measured
the spectra and thereby the distance of more than 675,000 galaxies and
90,000 quasars.
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"From the way the galaxies are clustered together, we can extract the fine details of the gravitational pull between them," says Max Tegmark of the Massachusetts Institute of Technology, Cambridge.
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The result has been a series of papers about the nature of the
universe and fundamental issues of cosmology that have parked themselves
in the upper reaches of the most-cited papers in physics. In this issue’s
latest Physics Top Ten, SDSS
papers currently rank at #2 and #3. The paper at #2, a 2004 Physical
Review D report on "Cosmological
parameters from SDSS and WMAP," racked up 45 citations in the
latest two-month period, a small portion of its more than 350 citations
in just over two years since publication (see table
below).
This remarkable run of influential research has also served to put
MIT physicist Max Tegmark, first author on the Phys. Rev. D paper,
among cosmology’s highly cited elite in recent years.
In a special feature for
Thomson Scientific’s Essential
Science Indicators
editorial Web site in-cites.com, this paper ranked #40 among the
"Super
Hot" Papers in Science."
Tegmark was featured in this publication’s latest annual ranking of
hot scientists (see Science Watch, 17[2]: 1-2, March-April
2006) thanks to six Hot Papers published over the last two years. In
the past decade he’s contributed to a dozen papers that have garnered
over 100 citations each.
Tegmark, 39, born in Stockholm, Sweden, earned his B.Sc. in physics
in 1990 from the Royal Institute of Technology in Stockholm (having
received a B.A. in economics in 1989 from the Stockholm School of
Economics). He then turned to the United States and the University of
California, Berkeley, where he obtained his M.A. in physics in 1992 and
his Ph.D. in 1994, working with Joseph Silk. After a post-doctoral year
in Munich at the Max-Planck-Institut für Physik, Tegmark returned to
the U.S. as a Hubble Fellow at the Institute for Advanced Study in
Princeton, New Jersey, where he stayed until 1999. He then became an
assistant professor at the University of Pennsylvania in Philadelphia,
before moving in 2004 to MIT, where he is now an associate professor.
Tegmark
spoke to Science Watch from his MIT office in Cambridge, Massachusetts.
One question we have to ask before getting into questions
about the Sloan Digital Sky Survey and cosmology: You somehow managed to
obtain a bachelor’s degree in economics and a bachelor’s in physics
a year apart and from different universities. We don’t see that kind
of academic record often. How did this come about?
Well, when I was an undergraduate I was basically clueless. So I
started out in economics at the Stockholm School of Economics, because
I had some idealistic notions that I was going to try to make the
world a better place by studying environmental things. Then I got
rather disillusioned. Meanwhile, I had a girlfriend at the time
studying physics, and her textbooks were much more interesting than
mine. The relationship didn’t last, but my love of physics did. And
in Sweden you don’t pay for education, so I was able to secretly
enroll in two different universities at the same time. Although this
meant that occasionally I had would have two exams almost at the same
time. And I would have to go quickly from one school to the other by
bicycle.
You’ve been working with the SDSS since your post-doctoral
years. What did the survey promise that sparked your interest?
The question I was most interested in involved what the
distribution of galaxies could tell us about the universe itself:
about the age of the universe, how the universe expanded over time,
what will happen in the future, how much dark
matter there is, how much dark
energy there is, and a variety of questions that crudely boil down
to measuring six numbers that hadn’t been measured all that well
before.
How do you learn all that from a map of the galaxies
themselves?
The answers to these fundamental questions lie encoded in how the
galaxies are distributed, how they clump together. If we were to fly
through these galaxies with a flight simulator, we’d realize that
they’re social animals. They like to hang out near other galaxies in
clusters, and the clusters in super-clusters. From the way the
galaxies are clustered together, we can extract the fine details of
the gravitational pull between them. In other words, it tells us where
the dark matter is, for instance, and it’s the dark matter that’s
doing most of the pulling. So by studying the details of how these
galaxies are clustered, we can get a direct handle even on the stuff
we can’t see at all—in particular, these dark-matter particles.
And what are the six numbers that all this boils down to?
They fall into two groups. One group tells us what the universe is
made of, and the other group tells us how it started out. The what-it’s-made-of
group, you might call the "matter budget." If you average
over everything there is, what percent is what? As it turns out, about
4% of all the stuff are atoms; about 20% of the stuff is dark matter,
and the rest is almost all dark energy. That’s the remaining 75 or
76%. Then we have a little bit of neutrinos and other things mixed in.
So these numbers constitute new information about the universe?
|
Highly Cited Papers by Max Tegmark and
Colleagues, Published Since 1996
(Ranked by total citations)
| Rank |
Paper |
Citations |
| 1 |
M.
Tegmark, et al., "Cosmological
parameters from SDSS and WMAP," Phys. Rev.
D, 69(10): 103501, May 2004.
[see
also] |
368 |
| 2 |
K.
Abazajian, et al., "The first data release
of the Sloan Digital Sky Survey", Astron. J.
126(4): 2081-6, 2003. |
280 |
| 3 |
M.
Tegmark, et al., "How small were the first
cosmological objects?," Astrophys. J.,
474(1): 1-12, 1997. |
205 |
| 4 |
M.
Tegmark, et al., "The three-dimensional
power spectrum of galaxies from the Sloan Digital Sky
Survey," Astrophys.
J., 606(2): 702-40, 2004. |
185 |
| 5 |
K.
Abazajian, et al., "The second data release
of the Sloan Digital Sky Survey," Astronom. J.,
128(1): 502-12, 2004. |
174 |
|
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That’s right. We didn’t have these numbers when I was in
graduate school. A lot of people back then thought that there was 95%
dark matter, for example, rather than 20%. Most people thought the
dark energy percentage was zero, rather than 75%.
And what’s the other group of numbers?
The other numbers tell you about how things started out. So our
universe—everything we can see out there—is moving apart,
expanding, and that means it was much denser in the past. All the
black you see in the sky between galaxies isn’t just vacuum; it’s
hydrogen gas. If that hydrogen gas was denser in the past, it must
have been hotter. Gradually this universal expansion dilutes and
cools. But another thing happened, too. We’ve also gone from being a
boring universe to being an interesting one, from uniform to clumpy.
Now the universe is very clumpy, when it used to be almost exactly the
same everywhere.
I realize we’re getting away from the other numbers, but the
obvious question here is, how did the clumps come about?
If you look around the room, you’ll see air has almost exactly
the same density. It’s not exactly the same, though, because you can
hear me. And so there are sound waves that cause the air to be a
little bit denser in some places than others. In the universe, early
on, we had a bunch of hydrogen gas that was about 10-5 more
dense in some places than others. In other words, a thousandth of a
percent more here, a thousandth of a percent less there. Gravity then
makes this all clumpier. Small clumps become bigger clumps, eventually
turning into galaxies.
So that’s the other number: the amplitude of this initial
clumpiness, and it’s actually 2x10-5. There’s
also a second number that tells you whether there were more large
clumps or small clumps or, more or less, an equal number at all
scales. And then those numbers give you clues about what it was that
made those initial seed fluctuations.
And what was responsible for these initial fluctuations?
We believe that they really have an amazing origin—mainly, that
those initial seed fluctuations were caused by the Heisenberg
Uncertainty Principal of quantum mechanics, which says you can’t
have something really uniform, even if you wanted to. It sounds stupid
because quantum physics should be about the microscopic, and galaxies
are not exactly small. But the beauty is that because space expanded,
you had these quantum fluctuations originally on wavelengths smaller
than atoms, and they eventually got stretched out to be wavelengths
longer than galaxies. This is the most successful theory we have for
what did this, and it’s hanging together very, very nicely.
How much of what we learned from the SDSS was new to us, and
how much was confirmation of what we had learned elsewhere?
Well, regarding that one paper with the crazy, high number of
citations: basically, if you were inclined, you could say the paper
concluded that there’s nothing new. It said that the emerging
Standard Model works great and, by the way, here are some slightly
more accurate measurements of those six numbers.
Would you have preferred something a little more radical?
Personally, I think it’s actually quite interesting when there’s
nothing new. When we have something and we’re able to measure it
much better and it still agrees with everything else we know, that’s
interesting. People like to joke about how cosmologists are often
wrong but are never in doubt. I’m the kind of guy who never really
believes in something until we measure it in more than one way and get
the same answer. That’s what happened here.
What’s the next step in this research? Is the SDSS still
gathering data? And is there a next generation of surveys coming along
to put it out of business?
Both. Buoyed by the success we had, people are planning even more
ambitious sky surveys. The SDSS is still taking data, but when it’s
completed, it will still have done only about a quarter of the sky. So
we need to do the other three quarters. Part of the sky has a huge
amount of light and obstruction from the plane of the Milky Way
galaxy, but we still have the southern sky, which is at least the same
area again, that we can do really well. Then, more importantly, we
have bigger telescopes coming, and they can see much fainter galaxies;
they can map things out in three dimensions and with maybe ten million
or even a hundred million galaxies. It will certainly blow away what
we’ve done, and it will also allow us to ask a lot of other
questions that we can’t ask right now. We’ll be studying galaxies
so far away that we’ll be seeing the universe when it was much, much
younger. And we also have some really cool stuff that we’re working
on now with the galaxies we have. The papers we’ve been discussing
were done using plain-vanilla galaxies, but there’s a special kind
of galaxy—luminous red galaxies—that are larger. They shine
brighter and therefore we can see them at much larger distances. So we
can make a map that has fewer galaxies but extends over a much greater
volume. That will already blow away the stuff we published, in terms
of power for doing cosmology.
If you had carte blanche to do one single experiment—your
dream experiment—what would you do and what would it tell you?
I’d build a satellite that measures the polarization of the
cosmic microwave background really, really accurately. Because if we
can do that, we can see back to when the universe was ridiculously
dense, perhaps 10-35 seconds old, which would be the
farthest leap ever back in time and in our understanding. This would
allow us to figure out what made those density fluctuations in the
early universe—the ultimate origins. The theory in which these
quantum fluctuations happened is called "inflation." It’s
the best theory we have for what put the bang into the Big Bang. And
this satellite would allow us to see the smoking gun of this
characteristic signal in the cosmic microwave background.
One way to think of this, very crudely, is that when you look into
space, all the hydrogen between the galaxies is transparent, which is
very convenient. You can see what’s happening far away. But when the
universe was less than a few hundred thousand years old, the hydrogen
was so hot it was a plasma. It was opaque. So when you look far into
space, you’re staring into this opaque wall of hydrogen plasma, and
it’s censoring from view what happened earlier. So the best way to
see into this opaque region is with so-called gravitational waves;
these are ripples in the fabric of space-time. That’s what this
supreme experiment is going to do. It will detect their presence by
studying the polarization of the microwave background and it will tell
us what happened when everything that we see in space was once in a
region smaller than a grapefruit. Basically the know-how is out there
in the community to do it. We could start working on it tomorrow. We’d
build detectors, fly them from balloons and other things first, test
out the technology, tweak a few things, and then put them in a
satellite. It wouldn’t be cheap, though. Maybe a billion dollars, or
perhaps half that. You said I should be ambitious so I would do the
billion-dollar version. We could really nail it.
- Fast Research Front map on SLOAN
DIGITAL SKY SURVEY.
- Fast Breaking Paper comment by coauthor
Professor Michael Strauss regarding: M. Tegmark,
et al., "The three-dimensional power spectrum of galaxies from the Sloan Digital Sky Survey,"
(ESI Special Topics, May 2005).
- Fast Breaking Paper comment by Uros
Seljak regarding: U. Seljak,
et al., "Cosmological parameter analysis including SDSS Ly alpha forest and galaxy bias: Constraints on the primordial spectrum of fluctuations, neutrino mass, and dark energy,"
(ESI Special Topics, February 2006). Max Tegmark is also
the coauthor of this paper.
- Special Topic: Dark
Matter and Dark Energy - An
interview with Dr. Jeremiah Ostriker.
Science
Watch®, July/August 2006, Vol. 17, No. 4
Citing URL:
http://www.sciencewatch.com/july-aug2006/sw_july-aug2006_page3.htm |
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