Livermore astrophysicist Peter Anninos works on stockpile stewardship
projects. Many astrophysicists at Lawrence Livermore work primarily
in the weapons program to safeguard the reliability of the nuclear
stockpile because the two fields have so much in common: the fusion
process that powers stars is the same one that unleashes the deadly
energy of a thermonuclear weapon. Astrophysics is about as
close to Weapons 101 as you can get in college, he says.
Anninoss first love is cosmology, the evolution of the universe.
In his spare time, he began to develop a new computer code he called
COSMOS to simulate an unprecedented variety of astrophysical events
in one, two, or three dimensions.
had only bits and pieces of the COSMOS code finished until about
a year ago, when astrophysicist Stephen Murray instigated the hiring
of Chris Fragile as a postdoctoral fellow to perform astrophysics
research full time. Fragile, who transformed a fascination with
black holes into a career in astrophysics, has fleshed out the code
with Anninos and performs most computer runs.
virtually complete, COSMOS can model almost anything from a small
black hole to the entire universe. (See the box below.) It has already
been applied to an ambitious array of astrophysical problems: the
evolution of the very early universe, accretion of matter by black
holes, star formation, and the evolution of dwarf galaxies.
Given Peter Anninoss interest in cosmology, COSMOS
is an appropriate name for the code he developed. When
Anninos first began work on it, he was hoping to simulate
such isolated phenomena as black holes and neutron stars
in three dimensions. These all require modeling nonlinear
interactions between different sources of matter and highly
relativistic gravitational fields.
COSMOS is unusual
in being easily adaptable to either relativistic or Newtonian
astrophysical phenomena. The exceedingly strong gravitational
fields in effect immediately after the big bang, during
the initial inflation of the universe, and near black
holes are governed by the laws of relativity. In contrast,
classical Newtonian physics governs cosmological and astrophysical
events that occur in the presence of much weaker gravitational
fields. Examples include the creation of stars and galaxies
during which gravity behaves in a fashion more familiar
The general and
special theories of relativity provide a unified description
of space and time as a single continuous fabric called
spacetime. The general theory also describes gravity
through the notion of curved spacetime
and governs the motion of all objects in the presence
of this curvature. For instance, general relativity predicts
that when a large enough mass is concentrated in a small
enough volume, that mass
distorts the space around it so much that a part of space
wraps itself up and leaves the rest of normal space behind.
This is a black hole. Anything that falls into the black
holeincluding lightcan never get out.
Codes that can
simulate relativistic flows in the presence of ultrastrong
gravitational fields have been around for some time, but
each has been tuned to a particular purpose,
such as modeling black hole dynamics, cosmological gravitational
waves, or binary neutron stars. COSMOS, in contrast, is
designed for generic applications so that with only minor
modifications, it can simulate a variety of events.
are very computationally intensive. COSMOS could have
only been developed at an institution such as Lawrence
Livermore with its massively parallel terascale computers.
To date, the code has run successfully on several different
problems are inherently multidimensional, notes
Anninos, so he designed COSMOS to run in up to three dimensions.
It currently uses a uniform mesh, composed of quadrilateral-shaped
zones in two dimensions and hexagonal-shaped zones in
three. Although all calculations that have been run to
date use Cartesian coordinate systems, the code can be
adapted to curvilinear meshes as well.
The Universe in Transition
The young universe was a hot, dense foam
of quantum fields until moments after the big bang, when it began
to expand. About 300,000 years later, hydrogen atoms first appeared
when temperatures had cooled enough for electrons and nuclei to
join. Hundreds of millions of years later, matter began to come
together to form stars and galaxies. (See the box below.)
Anninos recently used COSMOS
to model one of the phase transitions that took place just after
expansion began, when the universe was about a hundred-thousandth
of a second old. During this transition, the most fundamental forms
of matterquarks and gluonsjoined to become protons and
neutrons. Because particles that contain three quarks, including
protons and neutrons, are known collectively as hadrons, this event
is known as the quarkhadron phase transition. While a few
other researchers have examined early cosmology in one spatial dimension,
Livermores simulations of the quarkhadron transition
are the first in multiple dimensions.
In the universe of today,
matter is spread haphazardly in clumps with vast areas of some unknown
substanceperhaps dark matterin between. Astrophysicists
surmise that the manner in which the early phase transitions took
place may be the cause of this unevenness. These phase transitions
may also have given rise to a population of primordial black holes
and may have set the foundation for the production of galactic and
extragalactic magnetic fields.
If the quarkhadron
phase transition were turbulent, says Anninos, the universe
would be more homogeneous today. We wanted to find out how stableor
unstablethe transition boundaries were to flow perturbations
and shock collisions. We also wanted to determine whether destabilizing
mechanisms play a role in how hadrons evolve and mix.
It is possible that bubbles
and droplets of varying phases may have coexisted for a time, resulting
in an uneven production and distribution of hadrons. The simulation
shown in the figure below illustrates how these bubbles may have
behaved as a wall of hadronic material collides with an isolated
bubble of hadrons. The background is initially composed of supercooled
quark material. But the expanding hadronic regions quickly convert
quarks into hadrons immediately behind a detonation or shock front.
However, as shocks pass through the cooled hadron regions, they
reheat the hadrons. Reheating may either decompose the hadrons back
into their quark constituents or simply impart a spectrum of thermal
fluctuations to the hadrons. The simulations provide clear evidence
of the formation of quark nuggets that may still survive
today in the form of dark matter.
Although the teams
simulations to date show complex behavior, there is no evidence
of hydrodynamic turbulence during this transition period, at least
for the cases they investigated.
In this simulation, a wall
of cold hadronic material collides with an isolated bubble
of hadrons. Although the background is initially composed
of supercooled quarks, the expanding hadronic regions quickly
convert quarks into hadrons immediately behind the detonation
or shock front. There is also clear evidence of the formation
of quark nuggets (the dark spots) that potentially
may have survived to the present as dark matter.
the Quicktime simulation.
Tug of Black Hole
Black holes tugged at Chris
Fragiles imagination when he was younger. Black holes also
tug madly at anything that comes within their gravitational field,
sucking in dust, gas, and even stars. According to the general theory
of relativity, black holes drag spacetime around them in a
Although a black hole itself
is not visible, hot matter that orbits around it is. The gases closest
to the black hole are very hot and emit x rays. Further away from
the black hole, cooler material emits visible radiation.
Black holes are suspected
to exist in the center of all galaxies. In October 2002, scientists
reported having found strong evidence for a dense black hole more
than two million times as massive as our Sun in the center of the
Milky Way Galaxy. After tracking the paths of several stars in the
vicinity of the presumed black hole for 10 years, they discovered
at least one star in an orbit that may send it to its death in the
black hole in about 15 years. Another indicator of the presence
of a black hole is the speed with which things move. In our galaxy,
most objects move at about 100 kilometers per second. Near black
holes, however, they may move as fast as 9,000 kilometers per second.
Fragile is modeling single
rotating black holes with a disk or torus of gas being sucked into
them, a process known as accretion. The topmost figure below shows
a model of an accreting gas torus around a black hole. In one version,
a black hole has its spin axis aligned with the angular momentum
axis of the torus. Although they are spinning in opposite directions
(180 degrees from each other), the black hole accretes the most
material from the torus in this position.
In another version, the spin
axis is tilted 30 degrees relative to the angular momentum axis
of the same accreting gas torus. No one has modeled such a tilted
black hole torus before, although they are expected to exist in
The tilt angle produces important
differences in the geometry of the accretion pattern, particularly
close to the black hole. Here, spiraling spacetime produces
what is known as frame dragging, a general relativistic effect,
which causes the shape of the torus to warp. Frame dragging also
affects how much of the gas can eventually be captured by the black
hole, which in turn will affect how bright the systems x rays
For constructing the model,
COSMOS currently uses a three-dimensional Cartesian mesh, which
has a uniform grid of zones that describe discrete elements of the
model. To have enough zones to get far out in the torus, we
end up with just a few around the black hole, Fragile says.
That means the resolution around the black hole isnt
as good as we would like.
One solution is to add zones
at the black hole without adding them further out in the torus,
creating a nonuniform grid. The other is to create a spherical grid,
which would resemble a globe with the black hole at the center.
But modeling in this way is difficult, especially near the boundaries
of the black hole where gravity is strongest. The challenges never
|(a) Simulation of the spin
axis of a black hole aligned with the angular momentum axis
of the torus. (b) The black hole spin axis is tilted 30 degrees
relative to the angular momentum axis of the same accreting
gas torus. No one had modeled a tilted black hole torus such
as this before. The tilt produces major differences in how the
black hole accretes matter.
|A radio telescope reveals
this image of high-energy radio jets coming from a massive black
holes dont just pull cosmic junk in. They also spit it out.
For decades, astronomers have observed massive rotating black holes
in the centers of some huge elliptical galaxies that spew high-energy
jets, as shown in the figure at left. These narrow streams of high-velocity
particles emit radiation in the form of radio waves, but their exact
nature and how they interact with their surroundings remain a mystery.
In 1985, before he came to
Livermore, astronomer Wil Van Breugel led a team studying whether
a radio jet emanating from elliptical galaxy NGC 541 was interacting
with a cooler cloud of gas and causing the formation of stars. Such
an event is known as a jet-induced starburst. His team used radio
and optical imaging as well as optical spectroscopy to compare emissions
from NGC 541 with emissions from a confirmed prototypical starburst
The idea of a jet triggering
a starburst in a cloud made news, says Van Breugel. But
many scientists didnt believe it at the time. It seemed too
much like science fiction.
At the time, many thought
the gas might be part of a preexisting galaxy that happened to be
nearby. Furthermore, it was unclear if it was even possible for
a jet to trigger the collapse of a gas cloud.
More recently, much more
sensitive observations by Van Breugel and others using the Hubble
Space Telescope and the Keck Observatory on Mauna Kea, Hawaii, indicate
that jet-induced star formations do indeed occur and may even have
been a common phenomenon in the early universe when galaxies were
forming. In young galaxies, much of the gas has yet to form into
stars. Jets may help this process by pushing gas clouds to higher
densities, forming stars a bit sooner than they would if only gravitational
forces acted. In powerful jets, star formation is probably initiated
by shocks that move sideways, along the edge of the jets.
process of jet-induced star formation requires numerical simulations
with complex, multidimensional computer codes such as COSMOS,
says Van Breugel. A COSMOS simulation of jetcloud interactions,
using temperature, density, and velocity data estimated from observed
systems such as NGC 541, is shown in the leftmost figure below.
Livermores simulations are the first ever to incorporate cooling,
a critical component of the process of star formation.
Van Breugel wants to use
COSMOS to help answer a number of questions: What is the range of
jet and shock velocities that allows the clouds to collapse rather
than heat up and disperse? What are the required densities and temperatures
of the gas in the star-forming clouds? What is the chemical composition
of the clouds, that is, how important is the cooling efficiency
of the gas? These answers will provide valuable insight into the
nature of the jets themselves and the physical conditions in galaxies
as they form.
These new calculations may
also help to determine whether feedback from active jets, emanating
from the vicinity of black holes, helps or hinders the growth of
galaxies. A few years ago, scientists discovered that the masses
of black holes and their parent galaxies are closely related, making
the feedback mechanism an important issue in astrophysics today.
the Universe Started
The Standard Model of cosmology says that the big bang
happened about 15 billion years ago. In the first moments
after that cataclysmic event, the universe was still a
single hot, dense entity in which all the forces we know
todaythe strong, electromagnetic, weak, and gravitationalwere
unified. In the first hundred-billionths of a second after
the big bang, the four forces came into being, one by
one, in a series of rapidly occurring phase transitions.
The most elementary
particlesquarks and gluonsbriefly floated
freely. But during the final phase transition of the early
universe, at about a hundred-thousandth of a second after
the big bang, they became bound together to form the protons
and neutrons that make up ordinary matter today. Three
minutes after the big bang, protons and neutrons first
formed nuclei of hydrogen and helium in a process known
as nucleosynthesis. The universe was
300,000 years old before electrons and nuclei joined to
form any atoms heavier than a simple protonneutron
hydrogen atom. All heavier elementsnitrogen, oxygen,
and so onwere created much later in stars, which
began to develop when the universe was 100 million to
about 1 billion years old.
At the time of
electronnuclei combination, the radiation temperature
of the universe was about 3,000 kelvins. The universe
has expanded and cooled since then such that its radiation
temperature today is just 3 kelvins. This temperature
corresponds to that of the microwave radiation that rains
down upon us today from all directions, radiation that
has been traveling through the universe since it decoupled
from matter. This radiation is just one of many clues
that have allowed scientists to solve the puzzle of how
the universe got started.
are reasonably certain about events back to
0.000 000 000 000 000 000 000 000 000 000 001 (10-33)
second after the big bang.
Astrophysicist Stephen Murray,
like Anninos, is primarily a weapons physicist. But, together with
colleagues at the University of California at Santa Cruz, he obtained
funding from the National Aeronautics and Space Administration to
study dwarf spheroidal galaxies, which orbit much more massive galaxies.
Dwarf galaxies were
most likely the first galaxies to form in the early universe,
Murray says. They are likely the building blocks of larger
galaxies. So the number of dwarf galaxies we observe today are probably
the remnants of a much larger initial population, most of which
went to form our own galaxy. These survivors make excellent laboratories
for studying how stars form in the early universe and may tell us
something about the early evolution of more massive galaxies.
Because dwarf spheroidal
galaxies are the smallest type of dwarf galaxy, one might expect
them to be simple laboratories for studying star formation. But
astronomers find that they exhibit a variety of histories. Some
show evidence of only a single burst of star formation while others
show signs of having had multiple bursts. Yet others appear to have
had more or less continuous star formation over their lifetimes.
Some contain differing amounts of heavy elements, while others show
little variation from star to star. Understanding the reasons for
such variety requires looking at the many factors that affect dwarf
spheroidal galaxies. Murray and Fragile have used COSMOS to simulate
two phenomena in these galaxies that relate to their evolution:
enrichment and tidal stripping.
For star formation to occur,
a cloud of gas must first cool enough for hydrogen molecules
to begin to form. As the cloud cools, it will also become
denser. This simulation shows some of the early steps in the
cooling and condensing process caused by the interaction of
a radio jet with a cloud of gas. The jet is not visible because
it is larger than the cloud and covers the whole grid. (a)
Over a span of 1.4 million years, the cloud of gas increases
in density by a factor of 1,000. (b) At the same time, the
temperature of the cloud of gas cools from 5,000 kelvins to
less than 800 kelvins.
the Quicktime simulation of (a).
the Quicktime simulation of (b).
|Dwarf spheroidal galaxies
are not very photogenic. Here, the Pegasus dwarf spheroidal
galaxy is hiding among brighter stars. (Keck Observatory image,
courtesy of P. Guhathakurta, University of California at Santa
Enrichment is the process
by which elements heavier than helium are created and dispersed
into the universe. Most such elements are formed by nuclear fusion
within the cores of stars much more massive than the Sun. When these
stars die, they explode as supernovas, dispersing the elements they
have created into space. The heavy elements are then mixed into
the galactic gas from which a subsequent generation of stars may
form. Only through countless stellar deaths over the millennia have
there come to be elements heavier than helium in amounts comparable
to those seen in the Sun.
Murray and Fragile examined
the ability of dwarf spheroidal galaxies to retain gas expelled
by supernovas. Simulations with COSMOS examined the effects of multiple
supernova explosions at random locations throughout the cores of
dwarf galaxies. A three-dimensional code was essential because no
assumptions could be made about the symmetry of the system.
The simulations showed for
the first time how supernova gases can chimney their
way out of the galaxy without mixing with galactic material. This
effect may explain why some dwarf spheroidal galaxies have less
heavy-element enrichment than would be expected from their history
of star formation. The material ejected from supernovas in dwarf
galaxies would almost certainly be captured by massive galaxies
that form after the dwarfs. Such preenrichment may help to explain
why astronomers studying our own Milky Way Galaxy find very few
stars lacking in heavy elements.
Factors external to the dwarf
galaxy may also explain their evolution. Tidal stripping is a process
whereby massive galaxies strip material from their smaller neighbors.
This transfer of mass to the larger galaxy is a galactic-scale version
of black hole accretion. Big galaxies make bad neighbors,
says Murray with a smile.
His team modeled a dwarf
galaxy under the influence of a larger nearby galaxy. If its gas
is ionized and heated, either by external or internal sources, then
the gas may be rapidly lost from the system, preventing the formation
of subsequent generations of stars. The result, in the presence
of a massive galaxy, is to limit the ability of the dwarf system
to form multiple generations of stars or, in extreme cases, to form
any stars at all.
(a) Plots reveal the density
of gas along slices through the center of an initially undisturbed
dwarf galaxy. Following a fairly rapid burst of several supernovas
near the center of the galaxy, substantial disturbance of
the gas is visible. (b) The concentration of enriched material
from the first supernova chimneys its way out
of the galaxy. This chimneying effect is a new discovery in
dwarf spheroidal galaxies and may explain why some such galaxies
have less heavy-element enrichment than might be expected.
the Quicktime simulation of (a).
the Quicktime simulation of (b).
Over the course of 190 million
years, a nearby massive galaxy steals material from a dwarf
spheroidal galaxy in a process known as tidal stripping.
observational data continually improve, astrophysics codes must
be able to keep up and include as many physical processes as possible.
The code is mostly
done, says Anninos. Now were concentrating on
actually using it. But in the next year or two, well be adding
more physics to it.
He and Fragile plan to add
photon transport and neutron diffusion to the code, and they will
modify the existing system of equations for radiation chemohydrodynamics
to include magnetic fields. To improve the codes accuracy
and efficiency for problems requiring varying degrees of spatial
resolutionsuch as the black hole torus simulationsthey
will add some form of adaptive grid technology to the code.
COSMOS is just beginning
to give us the why and how of astronomers
observations, says Anninos. As the codes capabilities
are expanded, it will bring observations into ever clearer view.
astrophysics, black hole accretion, COSMOS code, dwarf galaxies,
early cosmology, radio jetcloud interactions, star formation.
For further information contact Peter Anninos (925) 423-6045 (firstname.lastname@example.org).
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