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.

See also my CV, talks, or writings. At the bottom of the page find a list of my collaborators.

Subjects to choose from include:

• Dark Matter and Fifth Forces
• The Lopsided Universe
• Preferred directions and extra dimensions
• Cosmological effects of Lorentz-violating vector fields
• Structure formation in modified gravity
• Modified gravity as an alternative to dark energy
• The arrow of time
• The dark-energy equation of state parameter
• Extra dimensions and the cosmological constant
• Testing the Friedmann equation
• Experimental bounds on non-commuting geometry
• Quintessence and the rest of the world
• Domain wall junctions, supersymmetry, and extra dimensions
• Dirichlet topological defects
• The origin of primordial magnetic fields
• Dark Matter with time-dependent mass
• Discretized 2D quantum gravity
• Polarized radio galaxies and chiral effects on photons
• Exotic textures
• Gravitational perturbations of the cosmic microwave background
• Time machines in (2+1)-dimensional gravity
• Torsion in connection-dynamic theories of gravity
• Canonically quantized supergravity
• Observational studies of variable stars

• Dark Matter and Fifth Forces

A long-range fifth force coupled to dark matter can induce a coupling to ordinary matter if the dark matter interacts with Standard Model fields. We consider constraints on such a scenario from both astrophysical observations and laboratory experiments. We also examine the case where the dark matter is a weakly interacting massive particle, and derive relations between the coupling to dark matter and the coupling to ordinary matter for different models. Currently, this scenario is most tightly constrained by galactic dynamics, but improvements in Eotvos experiments can probe unconstrained regions of parameter space.

• S.M. Carroll, S. Mantry, M.J. Ramsey-Musolf, and C.W. Stubbs, 2008, "Dark-Matter-Induced Weak Equivalence Principle Violation," arxiv:0807.4363. [abstract; citations]
• 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]
• A. Erickcek, S.M. Carroll, and M. Kamionkowski, 2008, "Superhorizon Perturbations and the Cosmic Microwave Background'', arXiv:0808.1570. [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]

Collaborators

• Lotty Ackerman
• Greg Anderson
• Sallie Baliunas
• Padi Boyd
• Jim Bryan
• Jennie Chen
• Antonio DeFelice
• Vikram Duvvuri
• Damien Easson
• Adrienne Erickcek
• Eddie Farhi
• George Field
• Dan Freedman
• Peter Garnavich
• Dan Garretson
• James Geddes
• Monica Guica
• Edward Guinan
• Alan Guth
• Jeff Harvey
• Simeon Hellerman
• Mark Hoffman
• Roman Jackiw
• Marc Kamionkowski
• Manoj Kaplinghat
• Robert Kirshner
• Alan Kostelecky
• Eugene Lim
• Sonny Mantry
• Laura Mersini
• Takemi Okamoto
• Ken Olum
• Miguel Ortiz
• Don Page
• Ted Pyne
• Michael Ramsey-Musolf
• Adam Riess
• Ignacy Sawicki
• Brian Schmidt
• Jing Shu
• Alessandra Silvestri
• Andrew Sornborger
• Chris Stubbs
• Heywood Tam
• Wati Taylor
• Mark Trodden
• Michael Turner
• Bob Wald
• Mark Wise

Genealogy

My Ph.D. advisor was George Field, whose advisor was Lyman Spitzer, whose advisor was Henry Norris Russell, whose advisor was Charles Augustus Young. As far as we know, Young never actually received the Ph.D., so the line stops there. Famous academic relatives include cousin Bob Kirshner (whose advisor was Bev Oke, whose advisor was Spitzer) and grand-uncle Harlow Shapley (whose advisor was Russell).