RESEARCH INTERESTS
Martin White
Overview
I am a theorist and phenomenologist. While I originally trained in Particle Physics, in the last few years my interest has centered around the question of the formation of galaxies and structure in the universe. How did it originate, what were the forces responsible for making it what it is today and what can it teach us about the universe? I am particularly interested in using cosmology to learn about the nature of the dark energy believed to be causing the expansion of the universe to accelerate, and in the formation and evolution of galaxies and large-scale structure.
Cosmology today is in a state similar to where particle physics was 20 years ago. The big picture is in place, but many of the fundamental questions remain unanswered. With the tremendous growth in computational and observational power, cosmology is a rapidly moving and fairly young field and can also be quite competitive.
I am a member of the Theoretical Cosmology group. My work involves both analytical and numerical components, with some simulations (and data-analysis). The basic framework is (general) relativistic perturbation theory and quantum field theory, plus some simple radiative transfer and fluid mechanics.
While the study of structure formation is a unifying concept, my research falls into three separate categories:
In addition to my purely theoretical work I am a member of several experimental teams. I have a long-standing involvement in the US component of Planck, a European-led satellite mission to study anisotropies in the cosmic microwave background (CMB). I am a member of both funded and proposed experiments to study the polarization of the CMB. I am also involved in the APEX-SZ experiment which will study fine-scale CMB anisotropies and distant clusters of galaxies and a member of the Dark Energy Survey and SNAP satellite team both of which will study the dark energy causing the accelerated expansion of the universe. I am the project scientist for the BOSS and on the science team for WFMOS experiments which aim to study dark energy by the baryon oscillation method, the structure and history of our galaxy, and the formation and evolution of galaxies.
The quickest way to see what I am thinking about is to look at my list of publications, some of which (plus tutorials, movies, pretty pictures etc) are online.
Questions in Physical Cosmology
Since the mid- to late-80s the study of cosmology has changed radically in both the quantity and quality of data available -- it has become a truly quantitative science and even a field of precision measurements. In fact both astrophysics and cosmology have experienced an enormous growth in experimental activity recently, due in part to advances in computing and low-noise detectors, and there is reason to believe that significant progress on the major questions in these fields will occur within the next few years.
We now have an incomplete, but highly predictive, theory which connects the structure we see in the universe today to the exotic high-energy physics of the early universe. Within this paradigm calculations can be carried out with high precision and their predictions compared to the wealth of data we are rapidly accumulating. Our models can simultaneously match the microwave background fluctuations which trace the universe 300,000yr after the big bang, the nearby distribution of galaxies, the distribution of mass revealed by gravitational lensing and the structure seen in the spectra of distant quasars. It reproduces the acceleration of the cosmic expansion rate, the baryon fraction in rich clusters and the baryon abundance fixed by the theory of Big Bang Nucleosynthesis. For the first time we have an apparently complete cosmic census of the major components of the energy density of the universe, but we know almost nothing about most of the matter and most of the energy in our list! Also, we have increasingly good reason to believe that the largest structures that we see in the universe arose from tiny fluctuations in the early universe which were amplified by inflation. But we don't have a good model of inflation which meshes with our knowledge of high energy physics.
To quote a TGSAA report:
The hot big bang cosmological model derived from Einstein's general
theory of relativity provides a framework for understanding the evolution
of the universe from a fraction of a second after its creation until the
present, some 13 billion years later.
The success of this model is an impressive confirmation of our present
understanding of physics, including general relativity itself.
In recent years, dramatic and accelerating progress has been evident in
researchers' pursuit of the central goal of cosmological research: an
understanding of the evolution and content of the universe.
Such understanding rests on knowledge of fundamental physics, which often
can be tested and extended only by observation of phenomena in extreme
conditions in the cosmos.
Questions to which we don't know the answer include:
Part of the reason for our optimism that we will `soon' know the answers
to many of our most basic questions comes from our recent ability to measure
fluctuations in the Cosmic Microwave Background
Radiation (CMBR).
This radiation is a snapshot of the universe when it was only 300,000 years
old, long before the formation of stars and galaxies.
The minute fluctuations in the temperature of the CMBR across the sky can
tell us about the seeds of the structure which we see about us today and
also measure with unprecedented accuracy all of the
major cosmological parameters.
A review of the current state of the cosmos can be found in the web pages
of NASA's Wilkinson Microwave Anisotropy
Probe.
Again from the TGSAA report:
Those words were written before the first results of the
Wilkinson Microwave Anisotropy Probe
were released. The WMAP data, along with nearly 20 ground-based and balloon
borne experiments, have dramatically confirmed our basic theoretical picture.
For more information about current and upcoming CMB experiments (including
satellite experiments), see this
list of CMB experiments.
For more theoretical details, see
The
Cosmic Symphony or
The Cosmic Rosetta
Stone
(which is one of several on-line documents
describing the CMB, large-scale structure and cosmology).
Wayne Hu also has some online
review(s) of the CMB, especially
An
Introduction to the Cosmic Microwave Background for non-experts.
At a slightly higher level, the essay
Echoes of Gravity
by Douglas Scott and myself covers the basic physics of CMB anisotropies
in a few pages.
Excellent reviews of many topics can be found in the web pages of NASA's
Wilkinson Microwave Anisotropy Probe.
The Cosmic Microwave Background gives a
snapshot of the universe (plus some processing) corresponding to
when the universe was about 300,000 years old.
With the expected increase in data over the next few years, these early
conditions, and a variety of cosmological parameters, will become ever
better determined.
In order to study how the structure in the universe evolves from these
small fluctuations at early times into the
large scale structure we observe in many other ways,
numerical simulations have become crucial.
This is because gravity acts non-linearly -- a large
amount of mass will attract other mass around it, increasing the amount of
mass at that position. This enhances the attraction to other nearby
mass and so on. Beyond a certain stage analytic calculation becomes
intractable. The heuristic picture is understood:
gravitational collapse takes the initial pattern of small density
fluctuations in the early universe and transforms them into regions which
are either more and more dense or more and more underdense, with overdense
regions eventually forming stars, galaxies, galaxy clusters, etc.
The resulting "cosmic web" has highly dense regions embedded as nodes in an
interconnected filamentary structure.
More details of this process are becoming accessible to both observational
and numerical investigation.
Due to advances in computational power, numerical simulations can now
reliably capture many properties of this evolution, providing copies of
mock universes whose statistical properties can be compared to our own
and upon which we can experiment.
These simulations can either be used to characterize features of the standard
paradigm (e.g. with
Cold Dark Matter
dominating this collapse process) or to test how changes in this paradigm
(for instance varying initial conditions, cosmological parameters or properties
of the dark matter) might affect various observations.
These simulations can have
dark matter
alone (which is collisionless and so just involves gravitational interactions),
include dark matter and other matter (and thus collisions and hydrodynamic
effects), or go even further.
Although simulations cannot yet describe all processes and scales of interest,
in certain cases the necessary physics can be included and compared with
observation.
This can be used to confront the new and exciting data with both standard
theory and its extensions.
In my work I have been using numerical simulations to study
many of these cases, including
galaxy clusters (some simulation results are
here),
weak
gravitational lensing by large scale structure or galaxy clusters, the
Lyman alpha forest,
and the
Sunyaev-
Zel'dovich effect (scattering of CMB photons off of ionized electrons).
There are transparencies from an introductory talk on numerical
simulations in cosmology
here and some movies linked from here.
The study of
the large-scale structure of the universe
is also undergoing a tremendous growth spurt.
In the early 1980s there were less than 5,000 galaxy redshifts known.
We now have over 50,000 and will soon have more than 1,000,000.
Several new surveys have recently been
completed (notably the Anglo-Australian Telescope 2-Degree Field
[AAT 2dF] project)
and a new very large survey is underway: the
Sloan Digital Sky Survey.
Within a few years the 3D distribution of galaxies in our local universe will
be mapped, and that structure can be matched
to the early-time snapshot provided by the CMBR.
Since any model of structure formation must explain
both the tiny ripples in the Cosmic Microwave Background temperature across
the sky, and the large-scale structures we see in the universe today, the
combination of these two probes is especially powerful.
Together they enable us to probe the spectrum of fluctuations over about 4
decades in length scale and its evolution over almost the entire age of the
universe.
A key role in the formation of large-scale structure is played by
dark matter, whose gravitational influence
dominates that of the other components for most of the history of the
universe. Information on the properties, distribution and amount of dark
matter in the universe is thus crucial to understanding the formation of
large-scale structure and provides a compelling link to high energy physics.
Recently it has become possible to map directly the dark matter distribution
through its effect on the propagation of light through the universe.
The gravitational
deflection of light provides a mapping from the source plane to the
observed (image) plane which contains information about the distribution of
the deflecting mass.
Since the deflecting potentials are dominated by the dark matter on
cosmological scales this provides a means of mapping the large-scale
distribution of dark matter as a function of time directly, rather than
by luminous tracers. This weak lensing technique is fast becoming
an important tool in cosmology, with numerous groups having preliminary
detections of cosmic shear and several large surveys getting underway.
While gravitational lensing provides us with an "integrated" view of
structure projected along the line-of-sight, we can also watch the growth
of large-scale structure in action. By measuring fluctuations in the
absorption of light from distant quasars (the so-called
Lyman-alpha
forest) we obtain a map of the universe when it was 10% of its current age.
By mapping galaxy clusters
(the mountains of the cosmos) we gain an insight into the process of
structure formation as it is occuring today -- a special epoch when the
universal expansion is beginning to accelerate.
The Microwave Background
Observations by NASA's Cosmic Background Explorer (COBE) satellite have
led to major advances in current understanding of the most important
cosmological fossil, the CMBR.
COBE measured its temperature to four significant digits, it is
2.728 +/- 0.002 K, and showed that it is the most
perfect black body ever studied.
These findings established beyond any doubt that the CMBR is the "echo" of
the big bang. By mapping the dipolar variation in the CMBR's temperature
across the sky, COBE determined Earth's velocity with respect to the cosmic
rest frame to a precision of 1%.
COBE also detected small variations (about 0.001%) in the intensity of the
CMBR coming from different directions separated by angles of about 10
degrees and larger. This discovery provided the first evidence for the
primeval lumpiness that under gravitational attraction grew into all the
structure seen today, and it represented the first step toward establishing
that all structure arose from subatomic quantum fluctuations.
Since the COBE discovery, more than 10 other experiments, some ground-based,
others on balloons, have also detected variations in the intensity of the
CMBR. These measurements have confirmed the COBE result and are beginning
to map out the inhomogeneities in the distribution of matter that gave rise
to present structure.
Numerical simulations of structure formation
Large-Scale Structure
