- The
Lopsided Universe
Inflationary cosmology predicts a very specific kind of primordial density perturbations:
nearly scale-free, nearly Gaussian, nearly adiabatic. But that's kind of boring, so it's fun
to look for anomalies that might provide a clue towards what really went on. One such anomaly
is a claimed hemispherical power asymmetry
-- the amplitude of CMB temperature perturbations seems just a bit higher (by about 10%) in
one direction on the sky than in the opposite direction. Adrienne Erickcek, Marc Kamionkowski
and I have taken a stab at explaining this feature of the data by imagining that
a pre-inflationary supermode tilts the universe, as explained in
this blog post.
There are a number of interesting features of the idea, including that it doesn't really
work in simple single-field slow-roll inflation, as that model predicts unacceptably large
temperature anisotropies on very large scales. But we were able to fit everything
by considering a curvaton model, in which the field responsible for inflating ("the inflaton")
is different from the field responsible for the perturbations ("the curvaton"). We're
currently considering whether there are other testable predictions from the model.
- A. Erickcek, M. Kamionkowski, and S.M. Carroll, 2008, "A
Hemispherical Power Asymmetry from Inflation'', arXiv:0806.0377.
[abstract;
citations]
- Preferred
directions and extra dimensions
Start with a vector field that picks out a preferred
direction of space, but add a twist: extra spatial dimension that you don't
see. This opens up a new way to keep the effects of Lorentz violation hidden --
namely, stick them in the extra dimension. Interestingly, the converse is
also true; if other fields couple to the vector field, they can pick up
additional mass associated with their extra-dimensional momentum, making them
harder to detect. Heywood Tam and I wrote a short paper about this idea,
which we labeled Aether
compactification. Sadly, you have to wildly tune some numbers to make very
big dimensions, but the physical effect is still interesting.
I'm currently working on some dynamical issues with Heywood, Tim Dulaney,
and Moira Gresham.
- S.M. Carroll and H. Tam, 2008, "Aether Compactification'', arXiv:0802.0521.
[abstract;
citations]
- Cosmological effects
of Lorentz-violating vector fields
It is easy enough to
violate
Lorentz invariance by positing a tensor
field with nonvanishing expectation value in the vacuum. One way to
test such a possibility is to constrain direct couplings to Standard
Model fields, as I considered in my first published
paper. Another way is to consider effects on gravity. Eugene Lim
and I showed that a single timelike dynamical vector field with fixed norm
would have a very interesting effect: to increase the effective value of
Newton's constant in the Solar System (or the Newtonian limit more generally),
while decreasing it in cosmology. A nice constraint on such a
possibility can therefore be derived from Big-Bang
nucleosynthesis, which is sensitive to the value of the gravitational
constant in cosmology. Subsequently, I worked with Jing Shu on
putting such vector fields to work to help with baryogenesis. Most recently,
I collaborated with Lotty Ackerman and Mark Wise on the effects of a
spacelike vector field during inflation, and we derived
formulas for all the various spherical-harmonic coefficients. This work
was explained in a series of blog posts:
one,
two,
three.
- S.M. Carroll and E.A. Lim, 2004, "Lorentz-Violating Vector Fields Slow
the Universe Down'', hep-th/0407149.
[abstract;
citations]
- S.M. Carroll and J. Shu, 2005, "Models of Baryogenesis via Spontaneous
Lorentz Violation'', hep-ph/0510081.
[abstract;
citations]
- L. Ackerman, S.M. Carroll and M.B. Wise, 2007, "Imprints of a Primordial Preferred Direction on the Microwave Background'', astro-ph/0701357.
[abstract;
citations]
- Structure Formation
in Modified Gravity
If the acceleration of the universe is due to modified
gravity rather than dark energy, we may be able to experimentally
detect such a modification by tests of general relativity in the
ultra-low-density regime. The obvious phenomenon to consider in this
regime is the formation of large-scale structure. With Ignacy
Sawicki, I studied perturbation theory in a promising model of modified
gravity proposed by Dvali,
Gabadadze, and Porrati. DGP gravity imagines a brane embedded in an
infinite Minkowski background, but separate Ricci curvature terms on the
brane and in the bulk. We then collaborated with Alessandra Silvestri
and Mark Trodden on another theory, dubbed Modified-Source-Gravity,
in which there are no new propagating degrees of freedom.
- I. Sawicki and S.M. Carroll, 2005,
"Cosmological Structure Evolution and CMB Anisotropies in DGP
Braneworlds,'' astro-ph/0510364.
[abstract;
citations]
- S.M. Carroll, I. Sawicki, A. Silvestri, and M. Trodden, 2006,
"Modified-Source Gravity and Cosmological Structure Formation,''
astro-ph/0607458.
[abstract;
citations]
- Modified gravity as an
alternative to dark energy
The idea that most of the universe is a mysterious form of dark energy provides
an excellent fit to cosmological observations, but seems
unnatural. It is therefore worth pursuing
alternatives, even if they seem equally unpalatable at first. One possibility
is that there is no dark energy, but rather a modification of gravity kicking
in on large scales. Different versions of this idea have been suggested by
Deffayet, Dvali, and
Gabadadze,
Freese and Lewis, and
Dvali and Turner. In
work with Vikram Duvvuri, Mark
Trodden and Michael Turner, we investigated a very simple Lagrangian that
implements this idea: adding a term 1/R to the conventional term
R in the gravitational action, where R is the curvature scalar.
The model has a new tachyonic degree of freedom, but seems to be consistent
with solar-system tests of gravity. (I am not responsible for the goofy
title.) Later we welcomed aboard Antonio DeFelice and Damion
Easson, and investigated cosmological solutions to models with more
baroque curvature modifications.
- S.M. Carroll, V. Duvvuri, M. Trodden, and M.S. Turner, 2003,
"Is Cosmic Speed-Up Due to New Gravitational Physics?''
astro-ph/0306438.
[abstract;
citations]
- S.M. Carroll, A. De Felice, V. Duvvuri, D.A. Easson, M. Trodden,
and M.S. Turner, 2004,
"The Cosmology of Generalized Modified Gravity Models,'' astro-ph/0410031.
[abstract;
citations]
- The arrow of
time
Microscopic laws of physics are essentially time-reversal invariant,
but macroscopic thermodynamics exhibits a profound time-asymmetry;
entropy typically increases in closed systems. This intriguing feature
of the real world has a cosmological origin: the entropy of the
early universe was fantastically small. After a century of effort,
it has been difficult to explain this arrow of time without assuming
time-asymmetric boundary conditions. Jennifer Chen and I have
suggested a simple scenario in which increasing entropy is natural,
based on the idea that the entropy can increase without bound (there is
no equilibrium state) and that the way entropy increases is by creating
universes like our own. In our picture, any generic state first
evolves to an empty de Sitter phase; the small temperature of de Sitter
allows for fluctuations into a proto-inflationary configuration, which
grows and reheats into a conventional Big-Bang spacetime. The same
thing happens in the far past, but with a reversed arrow of time. On
ultra-large scales, therefore, entropy is growing without bound in the
asymptotic future and past.
- S.M. Carroll and J. Chen, 2004, "Spontaneous Inflation and the Origin
of the Arrow of Time'', hep-th/0410270.
[abstract;
citations]
- S.M. Carroll and J. Chen, 2005, "Does Inflation Provide Natural
Initial Conditions for the Universe?'', gr-qc/0505037.
[abstract;
citations]
- The dark-energy equation of
state parameter
One way of characterizing dark energy is through its equation-of-state
parameter w=p/rho, where p is the pressure and rho
is the energy density. For ordinary matter, w = 0; for radiation,
w = 1/3; and for vacuum energy, w = -1.
The lower (more negative) w is, the more slowly the dark energy
density decreases; for w = -1 it is strictly constant, while for
w < -1 the energy density actually increases as the universe
expands. I have helped out the
High-Z
Supernova Search Team in their exploration of what kinds of
dynamical energy are consistent with their results. My contribution was to
provide a good reason why w < -1 could be ignored -- namely, that it
violates the dominant energy condition, which is what guarantees
stability of the vacuum. This issue was revisted in
work with Mark Hoffman and Mark
Trodden, where we considered models with w < -1, obtained by
giving a negative kinetic energy to a scalar field (as proposed by
Caldwell). In these
models vacuum instability arises because the scalar has negative-energy
excitations, and the vacuum can decay into positive-
and negative-frequency particles. We found that an effective theory might
be phenomenologically acceptable, but only if there is a very low
cutoff on its scale of validity. Mark T. and I then worked with Antonio
De Felice to ask whether we could be tricked into thinking that w
was less than -1 if the Friemann equation were modified. In the specific
context of the scalar-tensor theories we examined, it could only
happen if the scalar potential were extremely fine-tuned.
- P.M. Garnavich, ... and S.M. Carroll
[21 authors], 1998, "Supernova Limits on the Cosmic Equation of
State,'' Astrophys. J. 509, 74; astro-ph/9806396.
[abstract from arxiv.org;
abstract from ADS;
full article from ADS;
citations from spires]
- S.M. Carroll, M. Hoffman, and M. Trodden, 2003,
"Can the dark energy equation-of-state parameter w be less than -1?,''
astro-ph/0301273.
[abstract;
citations]
- S.M. Carroll, A. De Felice, and M. Trodden, 2004,
"Can we be tricked into thinking that w is less than -1?,''
astro-ph/0408081.
[abstract;
citations]
- Extra
dimensions and the cosmological constant
If space has extra compact dimensions (as predicted, for example, by
string theory), the gravitational dynamics of our four-dimensional world
can be altered in startling ways. If the dimensions are large
(as popularized by Arkani-Hamed,
Dimopoulous and Dvali),
we might expect classical general relativity to apply.
James Geddes, Mark
Hoffman, Bob Wald and I studied extra dimensions which were completely
smooth, without including the effects of branes. We found that,
using only positive energy densitites, the extra dimensions should
be positively curved (spherical rather than toroidal or hyperbolic)
in order to be stabilized. Monica Guica and I then studied a similar
problem in the presence of explicit brane sources. We find that
the branes deform a sphere into the shape of an American football,
with the resulting four-dimensional cosmological constant given as
a function of the brane tension and bulk fields. This exact solution
can now be used to study cosmology and particle physics in
factorizable brane models.
A related idea is that of "self-tuning'' branes,
proposed by Arkani-Hamed
et al. and Kachru
et al. In that picture, there is a single extra dimension and a
bulk scalar field, and the geometry becomes insensitive to the brane
tension, but only at the cost of a naked singularity in the extra
dimension.
Laura Mersini and I have shown that, from a cosmology perspective,
the reason this happens is that the scale factor responds to
the combination (energy + pressure) rather than just the energy
density; for a cosmological constant the pressure is minus the
energy density, and the universe is not forced to expand.
- S.M. Carroll and L. Mersini, 2001, "Can We Live in a
Self-Tuning Universe?'', Phys. Rev. D 64,
124008; hep-th/0105007.
[abstract;
citations]
- S.M. Carroll, J. Geddes, M.B. Hoffman, and R.M. Wald, 2002,
"Classical Stabilization of Homogeneous Extra Dimensions'',
Phys. Rev. D 66, 024036; hep-th/0110149.
[abstract;
citations]
- S.M. Carroll and M.M. Guica, 2003,
"Sidestepping the Cosmological Constant with Football-Shaped
Extra Dimensions'', hep-th/0302067.
[abstract;
citations]
- Testing the Friedmann Equation
Given that the best-fit model for our universe requires so much fine tuning,
it is natural to wonder whether we aren't missing some truly profound
difference between our conventional cosmological model and the real
world. For example, in general relativity the expansion rate is related
to the energy density of the universe by the Friedmann equation; but
in alternative models, including those with extra dimensions, this crucial
equation might be modified. So
Manoj Kaplinghat and I began
to wonder about the empirical evidence in favor of this equation. If
you have some well-defined alternative theory of gravity, there are all
sorts of tests to which you can subject it; however, the only
model-independent test of the expansion rate comes from Big-Bang
Nucleosynthesis. When the universe was about a minute old, free protons
and neutrons combined into light elements (mostly helium [and hydrogen,
of course, for those protons which didn't combine], but also deuterium
and lithium). The amounts produced depend sensitively on the expansion
history, and so probe the Friedmann equation. Introducing a simple
two-parameter family of possible expansion histories, we find that a
one-dimensional space of possibilities is consistent with the data.
Thus, a generic modification of general relativity will be ruled out,
but there is still some room for very different universes.
- S.M. Carroll and M. Kaplinghat, 2002, "Testing the Friedmann
Equation: The Expansion of the Universe During Big-Bang
Nucleosynthesis'', Phys. Rev. D 65,
063507; astro-ph/0108002.
[abstract;
citations]
- Experimental bounds on
non-commuting geometry
The idea that spacetime may be intrinsically noncommutative ---
that a product of functions f(x)g(x) may not equal
g(x)f(x) [here's a
review]
--- has been around for a while, and has enjoyed
a resurgence in popularity following a big paper by
Seiberg and Witten
on the connection to string theory (essentially, that gauge theories
on branes in a background antisymmetric tensor field are automatically
non-commuting). It is natural to ask whether the real world might
be noncommuting, and in particular what bounds we can place on the
noncommutativity parameter (an antisymmetric two-index tensor).
Jeff Harvey,
Alan Kostelecky,
Charles Lane,
Takemi
Okamoto and I have been considering how to obtain bounds from the
fact that non-commutativity necessarily violates Lorentz
invariance, and we already have good bounds on the various ways
that Lorentz violation can be manifested in particle and atomic
physics (Alan has a nice
FAQ if you
want to know more about such things). We find that the mass scale
characteristic of non-commutativity must be larger than
about 10 TeV. It has been subsequently claimed that
infrared effects
allow for a much more stringent bound.
- S.M. Carroll, J.A. Harvey, V.A. Kostelecky, C.D. Lane, and
T. Okamoto, 2001,
"Noncommutative Field Theory and Lorentz Violation'',
Phys. Rev. Lett. 87, 141601;
hep-th/0105082.
[abstract;
citations]
- Quintessence and the rest of the world
A popular model for dynamical dark energy is a slowly-rolling scalar
field, sometimes called "quintesence." Scalar-field models are able
to reproduce all of the empirical successes of a standard cosmological
constant, but introducing dynamics also introduces new ways to constrain
such fields. For example, the field can couple directly to standard-model
particles, even if only through nonrenormalizable higher-order terms.
Such a field would induce a long-range "fifth force", as well as
make the constants of nature appear time-dependent. The absence
of such couplings requires additional fine tunings in quintessence
models. (Although we should keep in mind recent claims that the
fine-structure constant actually is
changing.)
We can suppress these couplings by introducing an approximate
global symmetry; this mechanism leaves open a possible pseudoscalar
coupling, which might be detectable in polarization
measurements. This turns quintessence into an axion, such as
the ones predicted by string theory; see comments by
Witten and models
by Choi and
Kim and Nilles.
- S.M. Carroll, 1998, "Quintessence and the Rest of the World,''
Phys. Rev. Lett. 81, 3067; astro-ph/9806099.
[abstract;
citations]
- Domain wall junctions, supersymmetry, and
extra dimensions
Extended objects play an important role in string theory (where
they are known as "branes") and in field theory (where they are
known as "topological defects" or "solitons"). Exact solutions
representing such objects can be hard to come by, but there are
sometimes a special set of static solutions, known as "Bogomolny"
or "BPS" depending on context, which minimize the energy giving
certain boundary conditions and often have other nice properties
(such as preserving supersymmetry).
Simeon Hellerman,
Mark
Trodden and I have shown that one example
of such a BPS state is a junction of domain walls.
We've looked at N=1 supersymmetric theories in four
dimensions with a finite number of discrete vacua, and argue that
wall-junction configurations exist which preserve precisely
one supercharge. The same conclusion was reached independently
(and a couple of days earlier, if you want the truth) by
Gibbons
and Townsend.
One application of such configurations is to the suggestion by
Randall
and Sundrum that our world could be a domain wall embedded
in a (noncompact) five-dimensional space. Their scenario seems
to work most straightforwardly if there is a single extra dimension,
but Arkani-Hamed,
Dimopoulos, Dvali and Kaloper pointed out that more large
dimensions could be accommodated when one considers wall junctions.
We therefore derived equations describing such junctions
in a supersymmetry-inspired theory of scalars coupled to gravity,
in the spirit of similar work on single walls by
Behrndt and
Cvetic,
Skenderis
and Townsend, and
DeWolfe, Freedman,
Gubser and Karch.
- S.M. Carroll, S. Hellerman, and M. Trodden, 1999, "Domain Wall Junctions
are 1/4-BPS States'',
Phys. Rev. D 61, 65001; hep-th/9905217.
[abstract;
citations]
- S.M. Carroll, S. Hellerman, and M. Trodden, 1999, "BPS Domain Wall
Junctions in Infinitely Large Extra Dimensions'',
Phys. Rev. D, 61, 044049; hep-th/9911083.
[abstract;
citations]
- Dirichlet Topological Defects
A prominent role in recent developments in string theory has
been played by Dirichlet branes
(D-branes
for short), which are
higher-dimensional membranes on which fundamental strings can
end. Given the similarities between fundamental strings and
stringlike solitons (vortices or cosmic strings) in field theories,
it is natural to ask whether models exist of scalar fields with
configurations in which one soliton can end on another of equal
or higher dimensionality --- a Dirichlet topological defect.
Mark Trodden
and I have succeeded in constructing a set of
such models in 3+1 dimensions: walls ending on walls, strings
ending on walls, and strings ending on strings. Here is an
example. Our strings ending on walls are
conceivably related to supersymmetric QCD strings, which
can end on QCD walls (as discussed by
Witten).
- S.M. Carroll and M. Trodden, "Dirichlet Topological Defects'',
Phys. Rev. D 57, 5189; hep-th/9711099.
[abstract;
citations]
- The origin of primordial magnetic
fields
One of the lesser-known unsolved problems of cosmology is the origin
of magnetic fields in galaxies. There is no consensus on how such
fields evolve with time, but there are reasons to believe that the
fields observed today may have originated in the early universe.
A primary concern in this game is how to take the small-scale fields
from the early universe and stretch them to cosmologically interesting
lengths. In the context of magnetohydrodyamics, this can happen
via an inverse cascade, but only if the fields have a large amount of
magnetic
helicity (or Chern-Simons number, to you particle
theorists).
George
Field and I have been examining how much
inverse cascade can occur in the presence of helicity; the
results are intriguing but not definitive. (See
George's
site for further discussion.)
Our work builds on
earlier investigations by
Cornwall and
Son.
One encouraging point is that there are mechanisms to get primordial
fields with substantial helicity: George and I considered one in
collaboration with Dan Garretson, and a more effective scenario has
been suggested by
Joyce and
Shaposhnikov.
- W.D. Garretson, G.B. Field and S.M. Carroll, 1992,
"Primordial Magnetic Fields from Pseudo-Goldstone Bosons,''
Phys. Rev. D 46, 5346; hep-ph/9209238.
[abstract;
citations]
- S.M. Carroll and G.B. Field, 1998, "Primordial Magnetic Fields
that Last?'', in 33rd Rencontres de Moriond:
Fundamental Parameters in Cosmology, 17-24 January 1998, Les Arcs,
France; astro-ph/9807159.
[abstract;
citations]
- G.B. Field and S.M. Carroll, 2000, "Cosmological Magnetic Fields
from Primordial Helicity'', Phys. Rev. D 62,
103008; astro-ph/9811206.
[abstract;
citations]
- Dark Matter with Time-Dependent
Mass
The cosmological constant seems fine-tuned or unnatural from a variety
of viewpoints. This has led a number
of people to consider dynamical sources of dark energy, usually a
slowly-rolling scalar field.
Greg Anderson
and I are proposing a new alternative, in which the dark matter
consists of particles whose mass increases as the universe expands ---
variable-mass particles, or "vamps". In such a model the age of the
universe is older than in standard CDM, and the evolution of structure
is changed in a dramatic fashion whose details are currently under
investigation. The mechanism for the time dependence of the masses
is a slowly evolving scalar field phi with a potential energy of the
form U(phi) = 1/phi. Although this seems unusual, in fact it can
arise naturally in the context of the nonperturbative lifting of flat
directions in supersymmetric gauge theories. We are in the process of
attempting to construct realistic particle physics models of the
type that would lead to vamps.
- G.W. Anderson and S.M. Carroll, 1997,
"Dark Matter with Time-Dependent Mass''; astro-ph/9711288.
[abstract;
citations]
- Discretized 2D quantum gravity
For a while I worked on using
triangles
to quantize 2-dimensional Euclidean gravity,
with Miguel Ortiz
and Wati Taylor.
Four papers are now in the public domain:
Paper One deals
with some general formalism, while
Paper Two applies it all
to duality of the Ising model coupled to gravity. The exciting
Paper Three and
Paper Four
consider the Ising model with a boundary magnetic field, and compute
the magnetization on the boundary and in the bulk. The behavior of the
magnetization as a function of the boundary field leads to some
insights about the behavior of the geometry in 2D gravity.
So far the best response has been
this.
- S.M. Carroll, M.E. Ortiz and W. Taylor IV, 1996,
"A Geometric Approach to Free Variable Loop Equations in Discretized
Theories of 2D Gravity,'' Nucl. Phys. B468, 383;
hep-th/9510199.
[abstract;
citations]
- S.M. Carroll, M.E. Ortiz and W. Taylor IV, 1996,
"Spin/Disorder Correlations and Duality in the c=1/2 String,''
Nucl. Phys. B468, 420; hep-th/9510208.
[abstract;
citations]
- S.M. Carroll, M.E. Ortiz and W. Taylor IV, 1996,
"The Ising Model with a Boundary Magnetic Field on a Random Surface,''
Phys. Rev. Lett. 77, 3947; hep-th/9605169.
[abstract;
citations]
- S.M. Carroll, M.E. Ortiz and W. Taylor IV, 1998,
"Boundary Fields and Renormalization Group Flow in the
Two-Matrix Model,'' Phys. Rev. D 58,
046006; hep-th/9711008.
[abstract;
citations]
- Polarized radio galaxies and chiral effects on
photons
A paper by Nodland
and Ralston claimed to find evidence for anisotropic effects in the
propagation of polarized radio waves through the universe. The data
used to support this claim are the same as those investigated years
ago by George Field, Roman Jackiw and me, in a study which placed
constraints on similar types of effects. George and I were therefore
moved to look at the data ourselves, to see if we agreed with this
provocative new result. Unfortunately, we do not; I've set up a
web page which summarizes what has been going
on, including some entertaining links.
- S.M. Carroll, G.B. Field and R. Jackiw, 1990, "Limits on
A Lorentz and Parity-Violating Modification of Electrodynamics,''
Phys. Rev. D 41, 1231.
[pdf file;
spires entry;
citations]
- S.M. Carroll and G.B. Field, 1991, "The Einstein Equivalence
Principle and Polarization of Radio Galaxies,''
Phys. Rev. D 43, 3789.
[pdf file;
spires entry;
citations]
- S.M. Carroll and G.B. Field, 1997, "Is There Evidence
for Cosmic Anisotropy in the Polarization of Distant Radio Sources?'',
Phys. Rev. Lett. 79, 2397; astro-ph/9704263.
[abstract;
citations]
- Exotic textures
Textures are configurations in scalar field theories which are
topologically nontrivial but unstable to collapse. Their evolution
may lead to seeds for the formation of large-scale structure in the
universe. I've been involved in the classification and dynamics
of "exotic'' textures, in collaboration with
Jim Bryan,
Andrew Sornborger
and Ted
Pyne. You can get the low-down on this work at the
fascinating exotic
textures web page.
- J.A. Bryan, S.M. Carroll and T. Pyne, 1994, "A Texture
Bestiary,'' Phys. Rev. D 50, 2806;
hep-ph/9312254.
[abstract;
citations]
- A. Sornborger, S.M. Carroll and T. Pyne, 1997,
"The Collapse of Exotic Textures,'' Phys. Rev. D
55, 6454; hep-ph/9701351.
[abstract;
citations]
- Gravitational perturbations of the cosmic microwave
background
The celebrated Sachs-Wolfe effect is the imprinting of temperature
fluctuations on the cosmic microwave background radiation by
gravitational perturbations in the universe.
In this work, Ted Pyne and I considered the Sachs-Wolfe effect at
second order in the perturbations, deriving complete formulae for
the anisotropies induced by arbitrary metric fluctuations. Ahead of our
time as usual, our work has only recently been rediscovered
(by Komatsu
and Spergel and
Hu and Cooray,
among others). To learn about the CMB, hurry to
Wayne Hu's site.
- T. Pyne and S.M. Carroll, 1996, "Higher-Order
Gravitational Perturbations of the Cosmic Microwave Background,''
Phys. Rev. D 53, 2920; astro-ph/9510041.
[abstract;
citations]
- Time machines in (2+1)-dimensional
gravity
Is it possible to travel backwards in time?
Embarassingly, we don't
know the answer to that question nearly as well as we should. I
worked with Edward Farhi, Alan Guth and
Ken Olum
on obstacles to constructing time machines in (2+1) dimensional
gravity, a possibility first suggested by Richard Gott
(PRL
server). We showed that (2+1) dimensional open
universes could be classified into two types: those that inevitably
contained a Gott time machine, and those that could never contain one.
The case of closed universes was solved by 't Hooft
(spires
listing), who showed that any attempt to build a Gott time
machine in a closed universe would be foiled by collapse to a
singularity before the time machine could arise.
Our work was briefly considered
newsworthy.
For general information on the sticky subject of time travel in
the context of general relativity, see the
sci.physics
faq, Scientific American's
ask the
experts, or some thoughts from
John
Gribbin.
- S.M. Carroll, E. Farhi and A.H. Guth, 1992, "An Obstacle to
Building a Time Machine,'' Phys. Rev. Lett. 68, 263;
Erratum: 68, 3368.
[pdf file;
spires entry;
citations]
- S.M. Carroll, E. Farhi, A.H. Guth and K.D. Olum,
1994, "Energy-Momentum Restrictions on the Creation of Gott Time
Machines,'' Phys. Rev. D 50, 6190; gr-qc/9404065.
[abstract;
citations]
- Torsion in connection-dynamic theories of
gravity
General relativity is a theory of the dynamics of geometry, as
described by the metric tensor. In addition to the metric, there
is another important geometric object, the connection, which in GR
is defined in terms of the metric. So-called connection-dynamic
theories of gravity take the connection as an independent variable,
and give rise to a set of fields called the torsion tensor. George
Field and I studied what happens when you allow these extra fields
to propagate, and described the experimental constraints on such
theories. They turn out not to be very good, since there is nothing
to stop the torsion fields from having very large masses: large
enough to remove them from the possibility of observational constraint.
A brief
introduction to torsion is part of John Baez's
general relativity
tutorial.
- S.M. Carroll and G.B. Field, 1994, "Consequences of
Propagating Torsion in Connection Dynamic Theories of Gravity,''
Phys. Rev. D 50, 3867; gr-qc/9403058.
[abstract;
citations]
- Canonically quantized supergravity
Supersymmetry
is a very popular hypothesized (but as yet unobserved) relationship
between particles of different spin. Supergravity, then, adds to the
usual spin-2 graviton of general relativity a new particle, the spin-3/2
gravitino. There was a claim by Peter D'Eath
(hep-th/9304084)
that physical solutions in quantum supergravity could be found that
involved only the graviton, without the gravitino. This would have
had important consequences for quantizing the theory, but
Dan Freedman,
Miguel Ortiz, Don Page
and I argued that the result was not correct,
and in fact any physical state would have to have an infinite
gravitino number. Csordas and Graham went on to suggest an exact
solution to the constraints of quantum N=1 supergravity
(gr-qc/9507008).
Paulo Vargas Moniz has a nice summary of
supersymmetry
and supergravity, and one of
supersymmetric
quantum cosmology.
- S.M. Carroll, D.Z. Freedman, M.E. Ortiz, and D.N. Page,
1994, "Physical States in Canonically Quantized Supergravity,''
Nucl. Phys. B423, 661; hep-th/9401155.
[abstract;
citations]
- S.M. Carroll, D.Z. Freedman, M.E. Ortiz, and D.N. Page,
1994, "Bosonic physical states in N=1 supergravity?''
Talk given at 7th Marcel Grossmann Meeting on General Relativity
(MG 7), Stanford, CA, 24-30 Jul 1994; gr-qc/9410005.
[abstract;
citations]
- Observational studies of variable stars
As an undergraduate I became involved in a number of projects involving
variable stars. At
Villanova I worked
on observations and modelling of the well-known eclipsing binary
Epsilon
Aurigae. We found that the invisible companion in this system is most
likely a large semi-transparent disk, possibly a protoplanetary system.
(Here's an artist's rendering of the system,
by Mark Garlick.)
At the CfA I played a small role in
the HK Project,
a long-term effort to track the chromospheric activity on a large number
of stars. This activity is related to starspot cyles, and has implications
for the behavior of the Sun.
- S.M. Carroll, E.F. Guinan, G.P. McCook and R.A. Donahue, 1991,
"Interpreting Epsilon Aurigae," Astrophys. J.
367, 278.
[abstract
from ADS;
full article
from ADS;
citations
from ADS]
- S.L. Baliunas, ... S.M. Carroll et al.
[27 authors], 1995, "Chromospheric Variations in Main-Sequence
Stars. II,'' Astrophys. J. 438, 269.
[abstract
from ADS;
full article
from ADS;
citations
from ADS]
My research interests include a variety of topics in theoretical
physics, especially including cosmology, field theory, and
gravitation, or elementary physics
more broadly. This is an especially exciting time for this kind
of science; a flood of data and suprising observational results
are revolutionizing cosmology, new experiments (from accelerators
and elsewhere) are invigorating particle physics, and advances
in string theory have brought it into closer contact with
low-energy physics and gravitation. We live in a
preposterous universe, and it's
our job to make sense of it.
