Khoury, Justin, Steinhardt, Paul J.
The universe can be made flat and smooth by undergoing a phase of ultra-slow (ekpyrotic) contraction with equation of state w >> 1, a condition that is achievable with a single, canonical scalar...
Flat 3-Brane with Tension in Cascading Gravity (2009)
De Rham, Claudia, Khoury, Justin, Tolley, Andrew J.
In the Cascading Gravity brane-world scenario, our 3-brane lies within a succession of lower-codimension branes, each with their own induced gravity term, embedded into each other in a...
We study the cosmology of a galileon scalar-tensor theory, obtained by covariantizing the decoupling lagrangian of the Dvali-Gabadadze-Poratti (DGP) model. Despite being local in 3+1 dimensions, the...
N-Body Simulations of DGP and Degravitation Theories (2009)
We perform N-body simulations of theories with infinite-volume extra dimensions, such as the Dvali-Gabadadze-Porrati (DGP) model and its higher-dimensional generalizations, where 4D gravity is...
Observing the Evolution of the Universe (2009)
Aguirre, James, Amblard, Alexandre, Ashoorioon, Amjad, Baccigalupi, Carlo, Balbi, Amedeo, Bartlett, James, ...
How did the universe evolve? The fine angular scale (l>1000) temperature and polarization anisotropies in the CMB are a Rosetta stone for understanding the evolution of the universe. Through detailed...
Observational Evidence for Cosmological-Scale Extra Dimensions (2008)
Afshordi, Niayesh, Geshnizjani, Ghazal, Khoury, Justin
We present a case that current observations may already indicate new gravitational physics on cosmological scales. The excess of power seen in the Lyman-alpha forest and small-scale CMB experiments,...
CMBPol Mission Concept Study: Probing Inflation with CMB Polarization (2008)
Baumann, Daniel, Jackson, Mark G, Adshead, Peter, Amblard, Alexandre, Ashoorioon, Amjad, Bartolo, Nicola, ...
We summarize the utility of precise cosmic microwave background (CMB) polarization measurements as probes of the physics of inflation. We focus on the prospects for using CMB measurements to...
CMBPol Mission Concept Study: Probing Inflation with CMB Polarization (2008)
Baumann, Daniel, Jackson, Mark G., Adshead, Peter, Amblard, Alexandre, Ashoorioon, Amjad, Bartolo, Nicola, ...
We summarize the utility of precise cosmic microwave background (CMB) polarization measurements as probes of the physics of inflation. We focus on the prospects for using CMB measurements to...
Rapidly-Varying Speed of Sound, Scale Invariance and Non-Gaussian Signatures (2008)
Khoury, Justin, Piazza, Federico
We show that curvature perturbations acquire a scale invariant spectrum for any constant equation of state, provided the fluid has a suitably time-dependent sound speed. In order for modes to exit...
Cascading Gravity and Degravitation (2007)
De Rham, Claudia, Hofmann, Stefan, Khoury, Justin, Tolley, Andrew J.
We construct a cascading brane model of gravity in which the behavior of the gravitational force law interpolates from (n+4)-dimensional to (n+3)-dimensional all the way down to 4-dimensional from...
De Rham, Claudia, Dvali, Gia, Hofmann, Stefan, Khoury, Justin, Pujolas, Oriol, Redi, Michele, ...
We present a higher codimension generalization of the DGP scenario which, unlike previous attempts, is free of ghost instabilities. The 4D propagator is made regular by embedding our visible 3-brane...
Non-Gaussianities in New Ekpyrotic Cosmology (2007)
Buchbinder, Evgeny I., Khoury, Justin, Ovrut, Burt A.
New Ekpyrotic Cosmology is an alternative scenario of the early universe which relies on a phase of slow contraction before the big bang. We calculate the 3-point and 4-point correlation functions of...
On the Initial Conditions in New Ekpyrotic Cosmology (2007)
Buchbinder, Evgeny I., Khoury, Justin, Ovrut, Burt A.
New Ekpyrotic Cosmology is an alternative scenario of early universe cosmology in which the universe existed before the big bang. The simplest model relies on two scalar fields, whose entropy...
Degravitation of the Cosmological Constant and Graviton Width (2007)
Dvali, Gia, Hofmann, Stefan, Khoury, Justin
We study the possibility of decoupling gravity from the vacuum energy. This is effectively equivalent to promoting Newton's constant to a high-pass filter that degravitates sources of characteristic...
New Ekpyrotic Cosmology (2007)
Buchbinder, Evgeny I., Khoury, Justin, Ovrut, Burt A.
In this paper, we present a new scenario of the early Universe that contains a pre big bang Ekpyrotic phase. By combining this with a ghost condensate, the theory explicitly violates the null energy...
Mach's Holographic Principle (2006)
Khoury, Justin, Parikh, Maulik
Mach's principle is the concept that inertial frames are determined by matter. We propose and implement a precise formulation of Mach's principle in which matter and geometry are in one-to-one...
Fading Gravity and Self-Inflation (2006)
We study the cosmology of a toy modified theory of gravity in which gravity shuts off at short distances, as in the fat graviton scenario of Sundrum. In the weak-field limit, the theory is...
More on the Spectrum of Perturbations in String Gas Cosmology (2006)
Brandenberger, Robert H., Kanno, Sugumi, Soda, Jiro, Easson, Damien A., Khoury, Justin, Martineau, Patrick, ...
String gas cosmology is rewritten in the Einstein frame. In an effective theory in which a gas of closed strings is coupled to a dilaton gravity background without any potential for the dilaton, the...
Unveiling Chameleons in Tests of Gravitational Inverse-Square Law (2006)
Upadhye, Amol, Gubser, Steven S., Khoury, Justin
Scalar self interactions are known to weaken considerably the current constraints on scalar-mediated fifth forces. We consider a scalar field with a quartic self interaction and gravitation-strength...
Super-acceleration as Signature of Dark Sector Interaction (2005)
Das, Subinoy, Corasaniti, Pier Stefano, Khoury, Justin
We show that an interaction between dark matter and dark energy generically results in an effective dark energy equation of state of w
Weighing Neutrinos with Galaxy Cluster Surveys (2005)
Wang, Sheng, Haiman, Zoltan, Hu, Wayne, Khoury, Justin, May, Morgan
Large future galaxy cluster surveys, combined with cosmic microwave background observations, can achieve a high sensitivity to the masses of cosmologically important neutrinos. We show that a weak...
Detecting Dark Energy in Orbit -- the Cosmological Chameleon (2004)
Brax, Philippe, Van De Bruck, Carsten, Davis, Anne-Christine, Khoury, Justin, Weltman, Amanda
We show that the chameleon scalar field can drive the current phase of cosmic acceleration for a large class of scalar potentials that are also consistent with local tests of gravity. These provide...
Constraining the Evolution of Dark Energy with a Combination of Galaxy Cluster Observables (2004)
Wang, Sheng, Khoury, Justin, Haiman, Zoltan, May, Morgan
We show that the abundance and redshift distribution ($dN/dz$) of galaxy clusters in future high--yield cluster surveys, combined with the spatial power spectrum ($P_c(k)$) of the same clusters, can...
Scalar self-interactions loosen constraints from fifth force searches (2004)
Gubser, Steven S., Khoury, Justin
The mass of a scalar field mediating a fifth force is tightly constrained by experiments. We show, however, that adding a quartic self-interaction for such a scalar makes most tests much less...
Tuning Locked Inflation: Supergravity versus Phenomenology (2004)
Easther, Richard, Khoury, Justin, Schalm, Koenraad
We analyze the cosmological consequences of locked inflation, a model recently proposed by Dvali and Kachru that can produce significant amounts of inflation without requiring slow-roll. We pay...
A Briefing on the Ekpyrotic/Cyclic Universe (2004)
This is a short overview of the ekpyrotic/cyclic model of the universe, an alternative to the standard big bang inflationary paradigm.
Khoury, Justin, Weltman, Amanda
The evidence for the accelerated expansion of the universe and the time-dependence of the fine-structure constant suggests the existence of at least one scalar field with a mass of order H_0. If such...
Chameleon Fields: Awaiting Surprises for Tests of Gravity in Space (2003)
Khoury, Justin, Weltman, Amanda
We present a novel scenario where a scalar field acquires a mass which depends on the local matter density: the field is massive on Earth, where the density is high, but is essentially free in the...
Designing Cyclic Universe Models (2003)
Khoury, Justin, Steinhardt, Paul J., Turok, Neil
Recent advances in understanding the propagation of perturbations through the transition from big crunch to big bang (esp. Tolley et al. hep-th/0306109) make it possible for the first time to...
Inflation versus Cyclic Predictions for Spectral Tilt (2003)
Khoury, Justin, Steinhardt, Paul J., Turok, Neil
We present a simple, nearly model-independent estimate that yields the predictions of the simplest inflationary and ekpyrotic/cyclic models for the spectral tilt of the primordial density...
Conditions for Generating Scale-Invariant Density Perturbations (2003)
Gratton, Steven, Khoury, Justin, Steinhardt, Paul J., Turok, Neil
We analyze the general conditions on the equation of state $w$ required for quantum fluctuations of a scalar field to produce a scale-invariant spectrum of density perturbations, including models...
Density Perturbations in the Ekpyrotic Scenario (2001)
Khoury, Justin, Ovrut, Burt A., Steinhardt, Paul J., Turok, Neil
We study the generation of density perturbations in the ekpyrotic scenario for the early universe, including gravitational backreaction. We expose interesting subtleties that apply to both...
From Big Crunch to Big Bang (2001)
Khoury, Justin, Ovrut, Burt A., Seiberg, Nathan, Steinhardt, Paul J., Turok, Neil
We consider conditions under which a universe contracting towards a big crunch can make a transition to an expanding big bang universe. A promising example is 11-dimensional M-theory in which the...
A Brief Comment on "The Pyrotechnic Universe" (2001)
Khoury, Justin, Ovrut, Burt A., Steinhardt, Paul J., Turok, Neil
We respond to the criticisms by Kallosh, Kofman and Linde concerning our proposal of the ekpyrotic universe scenario. We point out a number of errors in their considerations and argue that, at this...
Visible Branes with Negative Tension in Heterotic M-Theory (2001)
Donagi, Ron Y., Khoury, Justin, Ovrut, Burt A., Steinhardt, Paul J., Turok, Neil
It is shown that there exist large classes of BPS vacua in heterotic M-theory which have negative tension on the visible orbifold plane, positive tension on the hidden plane and positive tension,...
The Ekpyrotic Universe: Colliding Branes and the Origin of the Hot Big Bang (2001)
Khoury, Justin, Ovrut, Burt A., Steinhardt, Paul J., Turok, Neil
We propose a cosmological scenario in which the hot big bang universe is produced by the collision of a brane in the bulk space with a bounding orbifold plane, beginning from an otherwise cold,...
Inflationary solutions in the brane-world and their geometrical interpretation (2000)
Khoury, Justin, Steinhardt, Paul J., Waldram, Daniel
We consider the cosmology of a pair of domain walls bounding a five-dimensional bulk space-time with negative cosmological constant, in which the distance between the branes is not fixed in time....
Sylvestre, Julien, Khoury, Justin, Lovejoy, Shaun, Bastien, Pierre, Schertzer, Daniel
We present a study of the scaling properties of bipolar outflows, generic to the circumstellar environment of young stellar objects, and which are generally believed to be magnetohydrodynamically...
Dib, Rim, Khoury, Justin, Lam, C. S.
The problem of restoring Froissart bound to the BFKL-Pomeron is studied in an extended leading-log approximation of QCD. We consider parton-parton scattering amplitude and show that the sum of all...