# Research

I am a theoretical physicist, interested in how Nature works. Often I start with cosmology -- the actual behavior of the universe we observe -- and try to use that to learn something about the underlying laws of Nature. But sometimes I start from models of particle physics or gravitation and try to see how those match onto the universe. Some of my favorite topics include dark matter and dark energy, the arrow of time, inflation, extra dimensions, modified gravity, and possible violations of fundamental symmetries.

Recently my focus has moved in the direction of more abstract/foundational/speculative questions -- models of quantum cosmology, the nature of time reversal, making sense of the many-worlds interpretation, self-locating uncertainty in large universes. Another area I'm increasingly interested in is complexity/emergence/evolution. But I still try to find time to write down concrete models for cosmology or field theory now and then.

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

I've tried to group subjects together to lend an appearance of coherence, but more often than not they all run together.

**Time and the Universe**- The arrow of time
- Boltzmann Brains and other cosmological fluctuations
- Cosmological fine-tuning
- Time machines in (2+1)-dimensional gravity
**Extra Dimensions****Dark Matter****Dark Energy and Modified Gravity**- Modified gravity as an alternative to dark energy
- Structure formation in modified gravity
- The dark-energy equation of state parameter
- Quintessence and the rest of the world
**Early-Universe Cosmology**- Testing the Friedmann equation
- Baryogenesis from Lorentz violation
- The origin of primordial magnetic fields
**Late-Universe Cosmology**- Effective Field Theory of Large-Scale Structure
- Special directions and places in the universe
- The lopsided universe
- Polarized radio galaxies and chiral effects on photons
- Gravitational perturbations of the cosmic microwave background
**Quantum Gravity**- Black hole entropy
- Discretized 2D quantum gravity
- Torsion in connection-dynamic theories of gravity
- Canonically quantized supergravity
**Lorentz Violation****Topological Defects**- Domain wall junctions, supersymmetry, and extra dimensions
- Dirichlet topological defects
- Exotic textures
**Miscellaneous**

**The arrow of time**- S.M. Carroll and J. Chen, 2004, "Spontaneous Inflation and the Origin of the Arrow of Time'', hep-th/0410270. [arXiv, SPIRES]
- S.M. Carroll, 2008, "What if Time Really Exists?" arxiv:0811.3772. [abstract, SPIRES]
**Boltzmann Brains and other cosmological fluctuations**- A. Aguirre, M. Johnson, and S.M. Carroll, 2011, "Out of equilibrium: understanding cosmological evolution to lower-entropy states,"
*Journal of Cosmology and Astroparticle Physics***1202**, 024; arxiv:1108.0417. [arXiv, inSPIRE] - K.K. Boddy and S.M. Carroll, 2013, "Can the Higgs Boson Save Us From the Menace of the Boltzmann Brains?", arxiv:1308.4686. [arXiv, inSPIRE]
**Cosmological fine-tuning**- S.M. Carroll and J. Chen, 2005, "Does Inflation Provide Natural Initial Conditions for the Universe?," gr-qc/0505037. [arXiv; SPIRES]
- S.M. Carroll and H. Tam, 2010, "Unitary Evolution and Cosmological Fine-Tuning," arXiv:1007.1417. [arXiv; SPIRES]
- G.N. Remmen and S.M. Carroll, 2013, "Attractor Solutions in Scalar-Field Cosmology,"
*Phys. Rev. D***88**, 083518; arXiv:1309.2611. [arXiv, inSPIRE] **Effective field theory of large-scale structure**- S.M. Carroll, S. Leichenauer, and J. Pollack, "A Consistent Effective Theory of Long-Wavelength Cosmological Perturbations," arXiv:1310.2920. [arXiv, inSpire]
**Dynamical compactification**- S.M. Carroll, M.C. Johnson, and L. Randall, 2009, "Dynamical Compactification," arxiv:0904.3115. [abstract; pdf; SPIRES]
**Black hole entropy**- S.M. Carroll, M.C. Johnson, and L. Randall, 2009, "Extremal Limits and Black Hole Entropy," arxiv:0901.0931. [abstract; pdf; SPIRES]
**Dark matter and new forces**- 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; pdf; SPIRES]
- S.M. Carroll, S. Mantry, and M.J. Ramsey-Musolf, 2009, "Implications of a Scalar Dark Force for Terrestrial Experiments," arxiv:0902.4461. [abstract; pdf; SPIRES]
- L. Ackerman, M.R. Buckley, S.M. Carroll, and M. Kamionkowski, 2008, "Dark Matter and Dark Radiation," arxiv:0807.5126. [abstract; pdf; SPIRES]
**Dark matter with time-dependent properties**- G.W. Anderson and S.M. Carroll, 1997, "Dark Matter with Time-Dependent Mass''; astro-ph/9711288. [abstract; pdf; SPIRES]
- K.K. Boddy, S.M. Carroll, and M. Trodden, 2012,
"Dark Matter with Density-Dependent Interactions,"
*Phys. Rev. D***86**, 123529; arXiv:1208.4376. [arXiv, inSpire] **The lopsided universe**- A. Erickcek, M. Kamionkowski, and S.M. Carroll, 2008, "A Hemispherical Power Asymmetry from Inflation'', arXiv:0806.0377. [abstract; pdf; SPIRES]
- A. Erickcek, S.M. Carroll, and M. Kamionkowski, 2008, "Superhorizon Perturbations and the Cosmic Microwave Background'', arXiv:0808.1570. [abstract; pdf; SPIRES]
**Gravity and Lorentz violation**- S.M. Carroll and E.A. Lim, 2004, "Lorentz-Violating Vector Fields Slow the Universe Down'', hep-th/0407149. [abstract; pdf; SPIRES]
- S.M. Carroll and H. Tam, 2008, "Aether Compactification'', arXiv:0802.0521. [abstract; pdf; SPIRES]
- S.M. Carroll, H. Tam, and I. Wehus, 2009, "Lorentz Violation in Goldstone Gravity,'' arXiv:0904.4680. [abstract; pdf; SPIRES]
**Aether dynamics**- S.M. Carroll, T.R. Dulaney, M.I. Gresham, and H. Tam, 2008, "Instabilities in the Aether'', arXiv:0812.1049. [abstract; pdf; SPIRES]
- S.M. Carroll, T.R. Dulaney, M.I. Gresham, and H. Tam, 2008, "Sigma-Model Aether'', arXiv:0812.1050. [abstract; pdf; SPIRES]
**Special directions and places in the universe**- L. Ackerman, S.M. Carroll, and M.B. Wise, 2007, "Imprints of a Primordial Preferred Direction on the Microwave Background,'' astro-ph/0701357. [abstract; pdf; SPIRES]
- S.M. Carroll, C.-Y. Tseng, and M.B. Wise, 2008, "Translational Invariance and the Anisotropy of the Cosmic Microwave Background,'' arXiv:0811.1086. [abstract; pdf; SPIRES]
**Structure Formation in Modified Gravity**- I. Sawicki and S.M. Carroll, 2005, "Cosmological Structure Evolution and CMB Anisotropies in DGP Braneworlds,'' astro-ph/0510364. [abstract; pdf; SPIRES]
- S.M. Carroll, I. Sawicki, A. Silvestri, and M. Trodden, 2006, "Modified-Source Gravity and Cosmological Structure Formation,'' astro-ph/0607458. [abstract; pdf; SPIRES]
**Modified gravity as an alternative to dark energy**- 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; pdf; SPIRES]
- 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; pdf; SPIRES]
**The dark-energy equation of state parameter**- 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; 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; pdf; SPIRES] - 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; pdf; SPIRES] **Compactifications and branes**- S.M. Carroll and L. Mersini, 2001, "Can We Live in a
Self-Tuning Universe?'',
*Phys. Rev. D***64**, 124008; hep-th/0105007. [abstract; pdf; SPIRES] - 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; pdf; SPIRES] - S.M. Carroll and M.M. Guica, 2003, "Sidestepping the Cosmological Constant with Football-Shaped Extra Dimensions'', hep-th/0302067. [abstract; pdf; SPIRES]
**Testing the Friedmann Equation**- 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; pdf; SPIRES] **Baryogenesis from Lorentz violation**- S.M. Carroll and J. Shu, 2005, "Models of Baryogenesis via Spontaneous Lorentz Violation'', hep-ph/0510081. [abstract; pdf; SPIRES]
**Experimental bounds on non-commuting geometry**- 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; pdf; SPIRES] **Quintessence and the rest of the world**- S.M. Carroll, 1998, "Quintessence and the Rest of the World,''
*Phys. Rev. Lett.***81**, 3067; astro-ph/9806099. [abstract; pdf; SPIRES] **Domain wall junctions, supersymmetry, and extra dimensions**- 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; pdf; SPIRES] - 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; pdf; SPIRES] **Dirichlet Topological Defects**- S.M. Carroll and M. Trodden, "Dirichlet Topological Defects'',
*Phys. Rev. D***57**, 5189; hep-th/9711099. [abstract; pdf; SPIRES] **The origin of primordial magnetic fields**- 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; pdf; SPIRES] - 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; pdf; SPIRES] - G.B. Field and S.M. Carroll, 2000, "Cosmological Magnetic Fields
from Primordial Helicity'',
*Phys. Rev. D***62**, 103008; astro-ph/9811206. [abstract; pdf; SPIRES] **Discretized 2D quantum gravity**- 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; pdf; SPIRES] - 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; pdf; SPIRES] - 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; pdf; SPIRES] - 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; pdf; SPIRES] **Polarized radio galaxies and chiral effects on photons**- 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] - 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] - 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; pdf; SPIRES] **Exotic textures**- J.A. Bryan, S.M. Carroll and T. Pyne, 1994, "A Texture
Bestiary,''
*Phys. Rev. D***50**, 2806; hep-ph/9312254. [abstract; pdf; SPIRES] - A. Sornborger, S.M. Carroll and T. Pyne, 1997,
"The Collapse of Exotic Textures,''
*Phys. Rev. D***55**, 6454; hep-ph/9701351. [abstract; pdf; SPIRES] **Gravitational perturbations of the cosmic microwave background**- 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; pdf; SPIRES] **Time machines in (2+1)-dimensional gravity**- 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] - 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; pdf; SPIRES] **Torsion in connection-dynamic theories of gravity**- 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; pdf; SPIRES] **Canonically quantized supergravity**- 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; pdf; SPIRES] - 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; pdf; SPIRES]
**Observational studies of variable stars**- 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] - S.L. Baliunas
*et al.*, 1995, "Chromospheric Variations in Main-Sequence Stars. II,''*Astrophys. J.***438**, 269. [abstract from ADS; full article from ADS] -
B. Kloppenborg
*et al.*, 2010, "In the Shadow of the Transiting Disk: Imaging epsilon Aurigae in Eclipse,"*Nature***464**, 870-872. [abstract; pdf; SPIRES]

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. You can read more in this
*Scientific American* article.

If you wait long enough, a system in local thermal equilibrium (like the universe may have been in the past, and might be again in the future) can undergo a fluctuation to a lower-entropy state. Anthony Aguirre, Matt Johnson and I looked at the creation of an inflationary universe via up-tunneling from a low-energy vacuum to a high-energy one ("true" to "false" vacua). We found that the most likely trajectory is simply the time-reverse of the ordinary evolution of the universe. Kim Boddy and I looked at the future of the universe, and worried about the creation of Boltzmann Brains. We suggested that BB's might be avoided by decay of the Higgs to a new vacuum at larger field values. However, this only works if either the top quark has a slightly larger mass than most people believe (around 178 GeV), or we choose a particular kind of cosmological measure.

The early universe is in a very special state -- it has an incredibly low entropy, as Roger Penrose has often emphasized. It is often asserted that this fine-tuning can be explained by cosmological inflation. But inflation carries with it its own fine-tuning problems; not just getting the right potential, but getting the right initial conditions for inflation to begin. Heywood Tam and I investigated quantitatively the degree of tuning that is required, using a rigorous measure on cosmological spacetimes invented by Gibbons, Hawking, and Stewart. Building on this work, Grant Remmen and I showed how the informal idea of a "cosmological attractor solution" can be reconciled with the mathematical fact that Hamiltonian systems (like scalar-field cosmology) don't actually have attractors.

The idea behind "effective field theory" is to collect everything that happens at small scales and describe it in terms of an effective theory of large-scale phenomena. This is an attractive approach with which to tackle the problem of cosmological large-scale structure. With Stefan Leichenauer and Jason Pollack, I investigated the underpinnings of this program. We argue that the right cosmological theory isn't really a "field" theory at all, since temporal non-localities are an important part of the description. We also suggest an alternative foundation for the program based on the renormalization group.

Many popular models in physics invoke the existence of extra dimensions of space. A great deal
of work has gone into the subject of "compactification" -- how are these dimensions hidden? Less
work has gone into the question of *why* they are hidden -- what is the dynamical mechanism
that made them that way? Brandenberger
and Vafa imagined that the universe began with all dimensions compact, and proposed a reason why
some of them would start growing. With Matt Johnson and Lisa Randall, I looked at the opposite
possibility: we started with uncompactified dimensions, and some of them curled up. This sounds hard,
but we found that it happens automatically in a very simple theory, six-dimensional Einstein-Maxwell
theory with a positive cosmological constant. Our construction is closely related to the idea of
spontaneous decompactification, but backwards.

Way back in the 1970's, Jacob Bekenstein and Stephen Hawking showed that black holes have an entropy proportional to the
area of their event horizon. As one of the high points of the Second Superstring Revolution,
Strominger and Vafa showed that string theory
offered a microscopic understanding of the space of states implied by that entropy, at least in
certain special cases. But there are still unanswered questions, including how information is
encoded in the outgoing radiation. In 1994, Hawking,
Horowitz and Ross used Euclidean quantum gravity to make a surprising claim: the entropy
of an *extremal* black hole, one with a charge equal to its mass, is exactly zero, despite
the fact that the area does not vanish. Most people simply think this result is not right, but
there remain some puzzles about how to reconcile the various approaches. Matt Johnson, and
Lisa Randall and I argued that the extremal limit of a non-extremal black hole is discontinuous in
an interesting way: the region of spacetime in between the inner and outer event horizons does
not shrink to zero size, but blows up into a completely separate spacetime (anti-de Sitter space
times a two-sphere). We suggest that the approaches to calculating the entropy of extremal
black holes can be reconciled if the entropy is associated with that spacetime, rather than with
the black hole.

In a universe where 96% of the energy density is in a dark sector (dark matter and dark
energy), it's worth keeping an open mind about what kinds of physics may be lurking
therein. One possibility is a long-range fifth force coupled to dark matter.
If a massless scalar field is responsible for such a force,
and the dark matter couples to the SU(2)_{L} weak
interactions of the Standard Model, quantum effects will induce a fifth force between
ordinary particles. With Sonny Mantry, Michael Ramsey-Musolf, and Chris Stubbs, I
considered constraints on such
a scenario from both astrophysical observations and laboratory experiments.
If instead the force is mediated by a new U(1) gauge boson -- the "dark photon" --
the coupling to ordinary matter can be negligible, but there are interesting
new effects in cosmological dark-matter dynamics. With Lotty Ackerman, Matt Buckley,
and Marc Kamionkowski, I explored the constraints on such models from relic abundance
calculations and primordial nucleosynthesis, and found limits on the strength of
dark electromagnetism from the requirement that the dark matter be nearly collisionless.
More on scalar forces here,
and on dark electromagnetism here.

The existence of dark matter is well-established, but its properties remain largely unknown. Various aspects of dark-matter physics and cosmology could be very different if the dark matter's mass and coupling were different in the early universe than they are today, for example due to the evolution of a cosmological scalar field. Not long before the acceleration of the universe was discovered, Greg Anderson and I proposed a model in which the dark matter consists of particles whose mass increases as the universe expands -- variable-mass particles, or "vamps". The underlying physics of our idea later served crucial roles in some cool ideas like chameleon fields and mass-varying neutrinos. More recently, Kim Boddy and Mark Trodden and I worked out a model in which the dark matter's self-interaction cross-section varied with time, allowing us to obtain the right thermal relic abundance even for particles that interact relatively strongly today.

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"). Subsequent
research has shown that it's hard to make a model like this consistent with large-scale structure
observations, but it's not impossible, and it's still arguably the best model on the market.

A simple way to violate Lorentz invariance is to imagine a tensor field with a nonzero vacuum expectation value. There has been a great deal of investigation of particle-physics theories coupled to such fields, but less on their gravitational effects. Eugene Lim and I studied the simplest possible case of a timelike "aether" vector field in two contexts: Robertson-Walker cosmology, and the static Newtonian limit (applicable to the Solar System). We found that in both cases the primary effect of the aether was to renormalize the value of Newton's gravitational constant, but in different ways; the observable consequence is that the universe expands more slowly than you would otherwise expect. Heywood Tam and I studied a spacelike aether field, with a twist: pointing into an extra dimension. 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. Unfortunately you have to wildly tune some numbers to make very big dimensions, but the physical effect is still interesting. Heywood and I later collaborated with Ingunn Wehus on the possibility of emergent gravity from Lorentz violation, in which the graviton is a Goldstone boson associated with a fixed-norm tensor field.

Since Lorentz-violating aether fields have so many fun uses, it's important to verify that the theories are well-behaved. With Tim Dulaney, Moira Gresham, and Heywood Tam, we investigated perturbations in the aether. We found that the results of a naive stability analysis were sensitively dependent on what Lorentz frame you do are looking in -- in a boosted frame, a purportedly stable model begins to look unstable. One exception was what we called "sigma-model aether," so we looked at the empirical constraints on that model. Our stability results have subsequently been challenged by Donnelly and Jacobson, who argue that everything can be fixed if you choose boundary conditions carefully.

The cosmic microwave background provides a wealth of information, all of which can be accounted for by a fairly simple underlying model: isotropic, nearly scale-free, Gaussian, adiabatic perturbations. Studying deviations from those assumptions is of crucial importance in verifying that we are on the right track, not to mention a potential avenue for making big discoveries. The one assumption that has rarely been loosened is that of statistical anisotropy: the statistics of CMB perturbations should be the same in every direction. With Lotty Ackerman and Mark Wise, I studied the simplest possible deviation: a violation of rotational invariance, which we argued would show up first in a quadrupole power asymmetry. Mark and I later worked with Chien-Yao Tseng on violations of translational invariance in addition to rotational invariance. My paper with Lotty and Mark was explained in a series of blog posts: one, two, three.

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.

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
four-dimensional theory 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, and unfortunately seems inconsistent
with solar-system tests of gravity. (I am not responsible for the goofy
title.) Later work by others was able to develop more sophisticated models
of "*f*(*R*) gravity" that are consistent with observation;
see for example Hu and Sawicki.
Later we welcomed aboard Antonio DeFelice and Damion
Easson, and investigated cosmological solutions to models with more
baroque curvature modifications.

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.

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.

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.

I worked with graduate student Jing Shu on putting Lorentz-violationg vector fields to work to help with baryogenesis. The standard Sakharov conditions for baryogenesis include a departure from thermal equilibrium as well as violations of baryon number, C, and CP. However, once we violate Lorentz invariance, a chemical potential can arise even in equilibrium, loosening one of the most stringent requirements of baryogenesis scenarios. We had to fool around a bit to get the Lorentz-violating field to eventually go away, but otherwise the models are fairly robust.

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 considered 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 found 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.

A popular model for dynamical dark energy is a slowly-rolling scalar field, sometimes called "quintessence." 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. 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.

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.

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).

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.

For a while I worked on quantizing 2-dimensional Euclidean gravity via dynamical triangulations, with Miguel Ortiz and Wati Taylor. Four papers made it into the public domain: Paper One deals with some general formalism for free-variable loop euqations, 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.

My first ever paper was about violating Lorentz invariance -- long before it was cool. Roman Jackiw had helped pioneer the idea of Chern-Simons electromagnetism in 3 spacetime dimensions. My graduate advisor George Field, being a practical sort, wondered how it might work in four dimensions. It wouldn't, Roman replied, because it would violate Lorentz invariance. But LI is something that should be tested, not simply assumed -- so we figured out how to constrain it observationally. A four-dimensional Chern-Simons coupling to electromagnetism causes "cosmological birefringence." That means that a photon traveling through empty space will have its polarization vector gently rotated along the way; we were able to put stringent limits on such an effect. Years later, 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 our earlier paper. George and I were therefore moved to look at the data ourselves, to see if we agreed with this provocative new result. Unfortunately, we did not.

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.

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 was eventually rediscovered (by Komatsu and Spergel and Hu and Cooray, among others). To learn about the CMB, hurry to Wayne Hu's site.

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.

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.

Supersymmetry is a 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.

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. In 2010 Epsilon Aur went into eclipse again; Brian Kloppenborg, Bob Stencel and others led an effort to use interferometry to obtain some amazing images of the star in eclipse. 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.

## Collaborators

- Lotty Ackerman
- Anthony Aguirre
- Greg Anderson
- Sallie Baliunas
- Kim Boddy
- Jim Bryan
- Matt Buckley
- Jennie Chen
- Chien-Yao Tseng
- Antonio DeFelice
- Tim Dulaney
- Vikram Duvvuri
- Damien Easson
- Adrienne Erickcek
- Eddie Farhi
- George Field
- Dan Freedman
- Peter Garnavich
- Dan Garretson
- James Geddes
- Moira Gresham
- Monica Guica
- Edward Guinan
- Alan Guth
- Jeff Harvey
- Simeon Hellerman
- Mark Hoffman
- Roman Jackiw
- Matt Johnson
- Marc Kamionkowski
- Manoj Kaplinghat
- Robert Kirshner
- Alan Kostelecky
- Stefan Leichenauer
- Eugene Lim
- Sonny Mantry
- Laura Mersini
- Takemi Okamoto
- Ken Olum
- Miguel Ortiz
- Don Page
- Jason Pollack
- Ted Pyne
- Michael Ramsey-Musolf
- Lisa Randall
- Grant Remmen
- Adam Riess
- Ignacy Sawicki
- Brian Schmidt
- Jing Shu
- Alessandra Silvestri
- Andrew Sornborger
- Chris Stubbs
- Chien-Yao Tseng
- Heywood Tam
- Wati Taylor
- Mark Trodden
- Michael Turner
- Bob Wald
- Ingunn Wehus
- 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).