A
highly detailed computer model has captured the birth of galaxies and giant
black holes. It lets astronomers follow the subsequent growth of these
massive structures in the largest cosmological simulation to date.
The
so-called "Millennium Run" took 28 days of intense computation to
generate its 25 terabytes (25 trillion bytes) of data. The simulation -
named after the 2000-time-frame in which the idea was conceived - tracks the
evolution of matter inside a cube 2 billion light-years on a side.
A light-year is the distance
light travels in a year, about 6 trillion miles (10 trillion kilometers).
"One
of the main advances here is size, which does matter in this business,"
said August Evrard of the University of Michigan.
"We are able to connect the first structures in the universe with the
galaxies we see nearby."
The
simulation starts when the universe was 10 million years old and evolves it all
the way to the present - 13
billion odd years later. The cube contains roughly 10 billion
"particles" - each with the mass of a billion Suns. These
colossal blobs of matter interact gravitationally with each other in
cyberspace.
Gravity
will cause some of the particles to merge. In the center of these matter
clumps, galaxies can form, but exactly what type of galaxy will depend on the
size of the clump and the history of mergers. It would take a clump of a
few thousand particles to house a Milky-Way-sized galaxy.
A
paper describing the Millennium Run appeared in the June 2 issue of Nature.
Invisible skeleton
The
Millennium Run does not actually go into all the messy details of forming stars
and accreting gas. Instead, it essentially provides the framework, or
skeleton, for all that galaxy business by concentrating on the elusive dark
matter, which is the dominant form of matter in our universe.
The
light-emitting stuff - that we are all familiar with - only makes up about a
tenth of the matter. The other 90 percent does not react with
light. This dark matter has yet to be detected directly, but
astrophysicists find it indispensable for explaining the cosmos.
"At
present, cosmologists can simulate dark matter, which we can't see, better than
galaxies and gas, which we can," said Nickolay Gnedin from the University of Colorado in a separate
commentary.
Dark
matter is easier to work with because it does not interact with anything,
except through gravity. Although computing the gravitational interactions
of 10 billion dark matter clumps is no small feat, it becomes significantly
harder when you throw in the radiation and gas dynamics needed to make stars.
In
some sense, then, the Millennium Run is just the first step in creating a
digital universe. Once the dark matter "template" was finished,
the international team of investigators - that calls itself the Virgo
Consortium - was able to tack on a galaxy formation model, which basically told
the computer where to stick bright, shiny things amongst the dark clumps.
Is
it possible to separate the dark matter evolution from galaxy formation? Evrard admitted that there are complications, but
simulations like the Millennium Run have compared favorably
with full hydrodynamical simulations, which
incorporate everything at once but are so computationally expensive that the
represented volumes are considerably smaller.
Quasar lineage
Of
particular interest in the "bright and shiny" category are quasars -
the most luminous objects in the universe. They are believed to be giant
black holes - some of them billions of times more massive than our Sun - which
are gobbling up very hot, glowing material.
Recent
observations by the Sloan Digital Sky Survey (SDSS) have found big
booming quasars so far away that we are seeing them when the universe was
just a tenth of its age. Making black holes this big, this early,
had seemed implausible in the currently favored cosmology.
"Yet
when we tried out our galaxy and quasar formation modeling, we found that a few
massive black holes do form early enough to account for these very rare SDSS
quasars," said lead author Volker Springel of
the Max Planck Institute for Astrophysics.
These
black hole quasar candidates can be traced from when the universe was only a
few 100 million years old, all the way to the present. If the simulation
is correct, the first quasar galaxies later turned into the massive galaxies
that now sit in the center of the biggest galaxy clusters.
This
finding was not surprising, but the Millennium Run allows scientists the
opportunity to watch the entire life cycle of these behemoth structures - as
well as other, more modest galaxy types.
Try out your own pet theory
One
advantage of calculating the cosmic
web of dark matter separately is it allows you the freedom to explore
different ways of building up galaxies.
"The
really cool thing is that in the future, when the data is made public, you can
go in and insert your own rules for galaxy formation," Evrard
said.
This
is seen as a much more efficient use of computer time, as different researchers
- and the ambitious amateur cosmologist - can use the dark matter skeleton from
the Millennium Run to hang their own galaxy models.
"For
this reason, the simulation will have staying power," said Evrard. "Maybe not for a
millennium," he joked, "but for a decade, at least, and perhaps
longer."
This article is part of SPACE.com's weekly
Mystery Monday series.
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