A5069009894- Cosmology and Gravitation Theories
- Black Holes and Theoretical Physics
- Galaxies: Formation, Evolution, Phenomena
- Dark Matter and Cosmic Phenomena
- Cold Atom Physics and Bose-Einstein Condensates
- Advanced Thermodynamics and Statistical Mechanics
- Relativity and Gravitational Theory
- Geophysics and Gravity Measurements
- Pulsars and Gravitational Waves Research
- Solar and Space Plasma Dynamics
- Noncommutative and Quantum Gravity Theories
- Quantum, superfluid, helium dynamics
- Astronomy and Astrophysical Research
- Particle physics theoretical and experimental studies
- Advanced Differential Geometry Research
- Quantum Mechanics and Applications
- Atomic and Subatomic Physics Research
- Computational Physics and Python Applications
- Space Science and Extraterrestrial Life
- Quantum Electrodynamics and Casimir Effect
- Theoretical and Computational Physics
- Geophysics and Sensor Technology
- Experimental and Theoretical Physics Studies
- Stellar, planetary, and galactic studies
- Astrophysics and Cosmic Phenomena
California University of Pennsylvania
2024-2025
University of Pennsylvania
2015-2024
William Penn University
2010
Perimeter Institute
2006-2009
Massachusetts Institute of Technology
2005-2006
Columbia University
2003-2004
Princeton University
1999-2004
McGill University
1999
The evidence for the accelerated expansion of Universe and time dependence fine-structure constant suggests existence at least one scalar field with a mass order ${H}_{0}.$ If such exists, then it is generally assumed that its coupling to matter must be tuned unnaturally small values in satisfy tests equivalence principle (EP). In this paper, we present an alternative explanation which allows fields evolve cosmologically while having couplings unity. our scenario, depends on local density:...
We present a novel scenario where scalar field acquires mass which depends on the local matter density: is massive Earth, density high, but essentially free in solar system, low. All existing tests of gravity are satisfied. predict that near-future satellite experiments could measure an effective Newton's constant space different from as well violations equivalence principle stronger than currently allowed by laboratory experiments.
We propose a cosmological scenario in which the hot big bang universe is produced by collision of brane bulk space with bounding orbifold plane, beginning from an otherwise cold, vacuous, static universe. The model addresses horizon, flatness and monopole problems generates nearly scale-invariant spectrum density perturbations without invoking superluminal expansion (inflation). relies, instead, on physical phenomena that arise naturally theories based extra dimensions branes. As example, we...
We present a screening mechanism that allows scalar field to mediate long-range ($\ensuremath{\sim}\mathrm{Mpc}$) force of gravitational strength in the cosmos while satisfying local tests gravity. The hinges on symmetry restoration presence matter. In regions sufficiently high matter density, is drawn towards $\ensuremath{\phi}=0$ where its coupling vanishes and $\ensuremath{\phi}\ensuremath{\rightarrow}\ensuremath{-}\ensuremath{\phi}$ restored. low however, spontaneously broken, couples...
We consider conditions under which a universe contracting towards big crunch can make transition to an expanding bang universe. A promising example is 11-dimensional M theory in the eleventh dimension collapses, bounces, and reexpands. At bounce, model reduce weakly coupled heterotic string and, we conjecture, it may be possible follow from contraction expansion. The possibility opens door new classes of cosmological models. For example, discuss how suggests major simplification modification...
We show that the chameleon scalar field can drive current phase of cosmic acceleration for a large class potentials are also consistent with local tests gravity. These provide explicit realizations quintessence model where couples directly to baryons and dark matter gravitational strength. analyze cosmological evolution existence an attractor solution following minimum its effective potential. For wide range initial conditions, spanning many orders magnitude in energy density, is reached...
In this paper, we present a new scenario of the early universe that contains pre-big bang ekpyrotic phase. By combining with ghost condensate, theory explicitly violates null energy condition without developing any ghostlike instabilities. Thus contracting goes through nonsingular bounce and evolves smoothly into expanding post-big The curvature perturbation acquires scale-invariant spectrum well before in scenario. It is sourced by entropy engendered two scalar fields, mechanism recently...
We study the cosmology of a galileon scalar-tensor theory, obtained by covariantizing decoupling Lagrangian Dvali-Gabadadze-Poratti (DGP) model. Despite being local in $3+1$ dimensions, resulting cosmological evolution is remarkably similar to that full $4+1$-dimensional DGP framework, both for expansion history and density perturbations. As model, covariant theory yields two branches solutions, depending on sign velocity. Perturbations are stable one branch ghostlike other. An interesting...
We summarize the utility of precise cosmic microwave background (CMB) polarization measurements as probes physics inflation. focus on prospects for using CMB to differentiate various inflationary mechanisms. In particular, a detection primordial B‐mode would demonstrate that inflation occurred at very high energy scale, and inflaton traversed super‐Planckian distance in field space. explain how such or constraint illuminate aspects Planck scale. Moreover, can constrain scale‐dependence...
We propose a novel theory of dark matter (DM) superfluidity that matches the successes LambdaCDM model on cosmological scales while simultaneously reproducing MOdified Newtonian Dynamics (MOND) phenomenology galactic scales. The DM and MOND components have common origin, representing different phases single underlying substance. consists axion-like particles with mass order eV strong self-interactions. condensate has polytropic equation state P~rho^3 giving rise to superfluid core within...
The symmetron is a scalar field associated with the dark sector whose coupling to matter depends on ambient density. decoupled and screened in regions of high density, thereby satisfying local constraints from tests gravity, but couples gravitational strength low such as cosmos. In this paper we derive cosmological expansion history presence field, tracking evolution through inflationary, radiation- matter-dominated epochs, using combination analytical approximations numerical integration....
Early dark energy (EDE) offers a particularly interesting theoretical approach to the Hubble tension, albeit one that introduces its own set of challenges, including new ``why then'' problem related EDE injection time at matter-radiation equality, and mild worsening large-scale structure (LSS) tension. Both these challenges center on properties matter, which becomes dominant component Universe is also responsible for seeding LSS. Motivated by this, we present potential couplings between...
We study the generation of density perturbations in ekpyrotic scenario for early universe, including gravitational backreaction. expose interesting subtleties that apply to both inflationary and models. Our analysis includes a detailed proposal how generated contracting phase may be matched across `bounce' those an expanding hot big bang phase. For physical conditions relevant scenario, we re-obtain our earlier result nearly scale-invariant spectrum energy perturbations. find perturbation...
We show that an interaction between dark matter and energy generically results in effective dark-energy equation of state $w<\ensuremath{-}1$. This arises because the alters redshift dependence density. An observer who fits data treating as noninteracting will infer fluid with argue model is consistent all current observations, tightest constraint coming from estimates density at different redshifts. Comparing luminosity angular-diameter distance relations $\ensuremath{\Lambda}\mathrm{CDM}$...
We study the possibility of decoupling gravity from vacuum energy. This is effectively equivalent to promoting Newton's constant a high-pass filter that degravitates sources characteristic wavelength larger than certain macroscopic (super) horizon scale $L$. underlying physics and consistency this phenomenon. In particular, absence ghosts, already at linear level, implies in any such theory graviton should either have mass $1/L$, or be resonance similar width. has profound physical...
The phenomenological constraints on the scalar field potential in cyclic models of Universe are presented. We show that require a comparable degree tuning to needed for inflationary models. reduced set simple design rules including "fast-roll" parameters analogous "slow-roll" inflation.
We show that the correlation functions of any single-field cosmological model with constant growing-modes are constrained by an infinite number novel consistency relations, which relateN+1-point a soft-momentum scalar or tensor mode to symmetry transformation on N-point hard-momentum modes. derive these relations from Ward identities for tower non-linearly realized global symmetries governing and perturbations. These can be labeled integer n. At each order n, constrain — completely n = 0,1,...
If dark energy --- which drives the accelerated expansion of universe consists a light scalar field, it might be detectable as "fifth force" between normal-matter objects, in potential conflict with precision tests gravity. Chameleon fields and other theories screening mechanisms, however, can evade these by suppressing forces regions high density, such laboratory. Using cesium matter-wave interferometer near spherical mass an ultra-high vacuum chamber, we reduce mechanism probing field...
We show that curvature perturbations acquire a scale invariant spectrum for any constant equation of state, provided the fluid has suitably time-dependent sound speed. In order modes to exit physical horizon, and in solve usual problems standard big bang cosmology, we argue only allowed possibilities are inflationary (albeit not necessarily slow-roll) expansion or ekpyrotic contraction. Non-Gaussianities offer many distinguish features. As with small speed, non-Gaussianity can be relatively...
We present a generalization of the Dvali-Gabadadze-Porrati scenario to higher codimensions which, unlike previous attempts, is free ghost instabilities. The 4D propagator made regular by embedding our visible 3-brane within 4-brane, each with their own induced gravity terms, in flat 6D bulk. model ghost-free if tension on larger than certain critical value, while metric remains flat. gravitational force law ``cascades'' from behavior at largest distances followed 5D and finally regime...
The chameleon, or generalizations thereof, is a light scalar that couple to matter with gravitational strength, but whose manifestation depends on the ambient density. A key feature screening mechanism suppressing its effects in high-density environments determined by local field value. Under very general conditions, we prove two theorems limiting cosmological impact: i) Compton wavelength of such can be at most Mpc present cosmic density, which restricts impact non-linear scales; ii)...