arXiv subjects

Nemanja Kaloper

Publications and source records attributed to Nemanja Kaloper.

At least 37 records · Page 2Linked to original sources

arXiv2022

Pancosmic Relativity and Nature's Hierarchies

We define `third derivative' General Relativity, by promoting the integration measure in Einstein-Hilbert action to be an arbitrary $4$-form field strength. We project out its local fluctuations by coupling it to another $4$-form field strength. This ensures that the gravitational sector contains only the usual massless helicity-2 propagating modes. Adding the charges to these $4$-forms allows for discrete variations of the coupling parameters of conventional General Relativity: $G_N, Λ, H_0$, and even $\langle {\tt Higgs }\rangle$ are all variables which can change by jumps. Hence de Sitter is unstable to membrane nucleation. Using this instability we explain how the cosmological constant problem can be solved. The scenario utilizes the idea behind the irrational axion, but instead of an axion it requires one more $4$-form field strength and corresponding charged membranes. When the membrane charges satisfy the constraint $\frac{2κ_{\tt eff}^2 κ^2 |{\cal Q}_i|}{3{\cal T}^2_i} < 1$, the theory which ensues exponentially favors a huge hierarchy $Λ/\mpl^4 \ll 1$ instead of $Λ/\mpl^4 \simeq 1$. The discharges produce the distribution of the values of $Λ$ described by the saddle point approximation of the Euclidean path integral.

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arXiv2022

Hidden Variables of Gravity and Geometry and the Cosmological Constant Problem

We extend General Relativity by promoting Planck scale and the cosmological constant into integration constants, interpreted as fluxes of $4$-forms hiding in the theory. When we include the charges of the $4$-forms, these `constants' can vary discretely from region to region. We explain how the cosmological constant problem can be solved in this new framework. When the cosmological constant picks up contributions from two different $4$-forms, with an irrational ratio of charges, the spectrum of its values is a very fine discretuum. When the charges are mutually irrational, $\frac{2κ_{\tt eff}^2 κ^2 |{\cal Q}_i|}{3{\cal T}^2_i} < 1$, the discharge processes populating our discretuum will dynamically relax $Λ$, ceasing as $Λ$ approaches zero. Thus the theory exponentially favors a huge hierarchy $Λ/\mpl^4 \ll 1$ instead of $Λ/\mpl^4 \simeq 1$.

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arXiv2022

Rollercoaster Cosmology

(Abridged) Does inflation have to happen all in one go? The answer is a resounding no! All cosmological problems can be solved by a sequence of short bursts of cosmic acceleration, interrupted by short epochs of decelerated expansion. The spectrum of perturbations will still match the CMB and LSS if the earliest stage of the last ${\cal O}(50)-{\cal O}(60)$ efolds is at least ${\cal O}(15)$ efolds long. Other stages can be considerably shorter. But as long as they add up to ${\cal O}(50)-{\cal O}(60)$ efolds and the stages of decelerated expansion in between them are shorter and also overall last less, the ensuing cosmology will pass muster. The presence of the interruptions resets the efold clock of each accelerating stage, and changes its value at the CMB pivot point. This change opens up the theory space, loosening the bounds. In particular some models that seem excluded at ${\cal N}=60$ fit very well as shorter stages with ${\cal N}=30$. Interesting predictions are that both the scalar and tensor spectra of perturbations are rapidly modified at short wavelengths. These features could be tested with future CMB spectroscopy searches and with short wavelength primordial gravity probes. The spatial curvature in these models can be larger than the largest wavelength scalar perturbations, because $Ω_{\tt k}$ evolves differently than the scalar perturbations $\frac{δρ}ρ|_{\tt S}$. Finally, with many short stages of accelerated expansion, the abundance of reheating products from previous accelerated stages does not get completely wiped out. This implies that the universe may contain additional populations of particles, more rare than the visible ones, or even primordial black holes, created during a late decelerated epoch before last reheating, which may be dark matter.

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arXiv2022

Very Hairy Inflation

We revisit the rollercoaster cosmology based on multiple stages of monodromy inflation. Working within the framework of effective flux monodromy field theory, we include the full range of strong coupling corrections to the inflaton sector. We find that flattened potentials $V \sim ϕ^p + \ldots$ with $p \lesssim 1/2$, limited to $ N \lesssim 25 - 40$ efolds in the first stage of inflation, continue to fit the CMB. They yield $0.96 \lesssim n_s \lesssim 0.97$, and produce relic gravity waves with $0.006 \lesssim r \lesssim 0.035$, in full agreement with the most recent bounds from BICEP/{\it Keck}. The nonlinear derivative corrections generated by strong dynamics in EFT also lead to equilateral non-Gaussianity $f_{NL}^{eq} \simeq {\cal O}(1) - {\cal O}(10)$, close to the current observational bounds. Finally, in multi-stage rollercoaster, an inflaton-hidden sector $U(1)$ coupling can produce a tachyonic chiral vector background, which converts rapidly into tensors during the short interruption by matter domination. The produced stochastic gravity waves are chiral, and so they may be clearly identifiable by gravity wave instruments like LISA, Big Bang Observatory, Einstein Telescope, NANOgrav or SKA, depending on the precise model realization. We also point out that the current attempts to resolve the $H_0$ tension using early dark energy generically raise $n_s$. This may significantly alter the impact of BICEP/{\it Keck} data on models of inflation.

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arXiv2022

New Horizons for Fundamental Physics with LISA

The Laser Interferometer Space Antenna (LISA) has the potential to reveal wonders about the fundamental theory of nature at play in the extreme gravity regime, where the gravitational interaction is both strong and dynamical. In this white paper, the Fundamental Physics Working Group of the LISA Consortium summarizes the current topics in fundamental physics where LISA observations of GWs can be expected to provide key input. We provide the briefest of reviews to then delineate avenues for future research directions and to discuss connections between this working group, other working groups and the consortium work package teams. These connections must be developed for LISA to live up to its science potential in these areas.

gr-qc↗

arXiv2021

Double Monodromy Inflation: A Gravity Waves Factory for CMB-S4, LiteBIRD and LISA

We consider a short rollercoaster cosmology based on two stages of monodromy inflation separated by a stage of matter domination, generated after the early inflaton falls out of slow roll. If the first stage is controlled by a flat potential, $V \sim ϕ^p$ with $p < 1$ and lasts ${\cal N} \sim 30 - 40$ efolds, the scalar and tensor perturbations at the largest scales will fit the CMB perfectly, and produce relic gravity waves with $0.02 \lesssim r \lesssim 0.06$, which can be tested by LiteBIRD and CMB-S4 experiments. If in addition the first inflaton is strongly coupled to a hidden sector $U(1)$, there will be an enhanced production of vector fluctuations near the end of the first stage of inflation. These modes convert rapidly to tensors during the short epoch of matter domination, and then get pushed to superhorizon scales by the second stage of inflation, lasting another $20-30$ efolds. This band of gravity waves is chiral, arrives today with wavelengths in the range of $10^8$ km, and with amplitudes greatly enhanced compared to the long wavelength CMB modes by vector sources. It is therefore accessible to LISA. Thus our model presents a rare early universe theory predicting several simultaneous signals testable by a broad range of gravity wave searches in the very near future.

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arXiv2020

Troubles with Global Monopoles in Quantum Gravity

A theory with a global $O(3)$ symmetry broken at a scale $η$ admits topological configurations: global monopoles realized by Goldstone fields winding around the core of false vacuum. One may expect them to behave as heavy, big composite objects, difficult to make and therefore mostly harmless. However, after gravity is turned on, as long as Equivalence Principle holds, one finds that global monopoles have a negative mass, $M \sim - η/\sqrtλ$, where $λ$ is the field theory coupling. This could catalyze an instability in the space of a global monopole, by the production of additional global monopole-antimonopole pairs along with normal particles, leading to energy production {\it ab nihilo}, if the pair energy is dominated by their negative rest masses. In a theory with unbroken local Poincaré symmetry, this could lead to a divergent `decay rate' of the global monopole configurations.

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arXiv2020

On Black Hole Echoes

We consider a very simple model for gravitational wave echoes from black hole merger ringdowns which may arise from local Lorentz symmetry violations that modify graviton dispersion relations. If the corrections are sufficiently soft so they do not remove the horizon, the reflection of the infalling waves which trigger the echoes is very weak. As an example, we look at the dispersion relation of a test scalar field corrected by roton-like operators depending only on spatial momenta, in Gullstrand-Painlevé coordinates. The near-horizon regions of a black hole do become reflective, but only very weakly. The resulting ``bounces" of infalling waves can yield repetitive gravity wave emissions but their power is very small. This implies that to see any echoes from black holes we really need an egregious departure from either standard GR or effective field theory, or both. One possibility to realize such strong echoes is the recently proposed classical firewalls which replace black hole horizons with material shells surrounding timelike singularities.

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arXiv2020

On Hybrid Monodromy Inflation (Hic Sunt Dracones)

We revisit two-field hybrid inflation as an effective field theory for low-scale inflation with sub-Planckian scalar field ranges. We focus on a prototype model by Stewart because it allows for a red spectral tilt, which still fits the current data. We describe the constraints on this model imposed by current CMB measurements. We then explore the stability of this model to quantum corrections. We find that for relevant, marginal, and at least a finite set of irrelevant operators, some additional mechanism is required to render the model stable to corrections from both quantum field theory and quantum gravity. We outline a possible mechanism by realizing the scalars as compact axions dual to massive $4$-form field strengths, and outline how natural hybrid inflation may be supported by strong dynamics in the dual theory.

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arXiv2019

Irrational Monodromies of Vacuum Energy

We present a theory with axion flux monodromies coupled to gravity, that reduces to the local vacuum energy sequester below the axion mass scales. If the axion potentials include a term generated by nonperturbative couplings to gauge sectors, with a decay constant incommensurate with monodromy periods, the low energy potential germinates a landscape of irrational axion vacua, with arbitrarily small cosmological constants. The sensitivity of the values of cosmological constants to unknown UV physics can be greatly reduced. The variation of the cosmological constant in each vacuum, from one order in perturbation theory to the next, can be much smaller than the naïve cutoff. The nonperturbative transitions in the early universe between the vacua populate this landscape, similar to the case of irrational axion. In such a landscape of vacua a small cosmological constant can naturally emerge.

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arXiv2019

Dark Energy, $H_0$ and Weak Gravity Conjecture

We point out that the physics at the extreme IR---cosmology---might provide tests of the physics of the extreme UV---the Weak Gravity Conjecture. The current discrepancies in the determination of $H_0$ may hint at a modification of $Λ$CDM. An extension which may fit better comprises of an early contribution to dark energy which `decays' into relativistic matter. On the other hand the discourse on WGC to date suggests that fields which support cosmic acceleration may produce relativistic matter after they traverse a $\sim$ Planckian distance in field space. We explain how this offers a simple realization of the requisite cosmic phenomenology. Thus if the resolution of $H_0$ discrepancies is really early dark energy that ends with a shower of relativistic matter and the current ideas on WGC are indicative, this may be a rare opportunity to link the two extreme limits of quantum field theory.

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arXiv2019

Neutrino Masses from Outer Space

Neutrinos can gain mass from coupling to an ultralight field in slow roll. When such a field is displaced from its minimum, its vev acts just like the Higgs vev in spontaneous symmetry breaking. Although these masses may eventually vanish, they do it over a very long time. The theory is technically natural, with the ultralight field-dependent part being the right-handed Majorana mass. The mass variation induced by the field correlates with the cosmological evolution. The change of the mass term changes the mixing matrix, and therefore suppresses the fraction of sterile neutrinos at earlier times and increases it at later times. Since the issue of quantum gravity corrections to field theories with large field variations remains open, this framework may give an observational handle on the Weak Gravity Conjecture.

hep-ph↗

arXiv2019

A Goldilocks Higgs

The Higgs could couple to a topological 4-form sector which yields a complex vacuum structure. In general such couplings could lead to direct CP violation in the Higgs sector. In many of the Higgs vacua electroweak symmetry is unbroken. In just as many it breaks when the 4-form flux is large enough. For a fixed value of flux, the symmetry breaking vacua have a smaller vacuum energy than the symmetric ones, where the difference is quantized because it is set by the $4$-form flux. This leads to the possibility that there is a value of the 4-form flux for any UV contributions to the Higgs {\it vev} that automatically cancels it down to the right value, $\sim$ TeV, if the 4-form charges are quantized in the units of the electroweak scale. This would still leave the cosmological constant which could be selected anthropically.

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arXiv2019

Quantum Cosmic No-Hair Theorem and Inflation

We consider implications of the quantum extension of the inflationary no hair theorem. We show that when the quantum state of inflation is picked to ensure the validity of the EFT of fluctuations, it takes only ${\cal O}(10)$ efolds of inflation to erase the effects of the initial distortions on the inflationary observables. Thus the Bunch-Davies vacuum is a very strong quantum attractor during inflation. We also consider bouncing universes, where the initial conditions seem to linger much longer and the quantum `balding' by evolution appears to be less efficient.

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arXiv2019

de Sitter Branes in a Flat Bulk of Massive Gravity

We construct de Sitter branes in a flat bulk of massive gravity in $5D$. We find two branches of solutions, reminiscent of the normal and self-accelerating branches in DGP, but with rather different properties. Neither branch has a self-accelerating limit: the background geometry requires having a nonvanishing tension. On the other hand, on both branches there are sub-branches where the leading order contributions of the tension to the curvature cancel. In these cases it turns out that larger tensions curve the background less. Further, both branches support a localized $4D$ massless graviton for a special choice of bulk mass terms. This choice may be protected by enhanced gauge symmetry at least at the linearized level. Finally, we generalize the solutions to the case of bigravity in a flat $5D$ bulk.

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arXiv2018

Strongly Coupled Quintessence

We present a family of consistent quantum field theories of monodromy quintessence in strong coupling, which can serve as benchmarks in modeling dark energy different from cosmological constant. These theories have discrete gauge symmetries which can protect them from quantum field theory and quantum gravity corrections, both perturbative and nonperturbative. The strong coupling effects, at scales $\ga {\rm mm}^{-1}$, flatten the potential and activate operators with higher powers of derivatives. The predicted equation of state is close to, but not exactly equal to $-1$, thus being within reach of the (near!) future programs to explore the nature of dark energy.

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arXiv2017

Landscaping the Strong CP Problem

One often hears that the strong $CP$ problem is the one problem which cannot be solved by anthropic reasoning. We argue that this is not so. Due to nonperturbative dynamics, states with a different $CP$ violating paramenter $θ$ acquire different vacuum energies after the QCD phase transition. These add to the total variation of the cosmological constant in the putative landscape of Universes. An interesting possibility arises when the cosmological constant is mostly cancelled by the membrane nucleation mechanism. If the step size in the resulting discretuum of cosmological constants, $ΔΛ$, is in the interval $({\rm meV})^4 < ΔΛ< (100 \, {\rm MeV})^4$, the cancellation of vacuum energy can be assisted by the scanning of $θ$. For $({\rm meV})^4 < ΔΛ< ({\rm keV})^4$ this yields $θ< 10^{-10}$, meeting the observational limits. This scenario opens up 24 orders of magnitude of acceptable parameter space for $ΔΛ$ compared to membrane nucleation acting alone. In such a Universe one may not need a light axion to solve the strong $CP$ problem.

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arXiv2017

Monodromy inflation at strong coupling: $4π$ in the sky

We present a simple effective field theory formulation of a general family of single field flux monodromy models for which strong coupling effects at large field values can flatten the potential and activate operators with higher powers of derivatives. These models are radiatively and non-perturbatively stable and can easily sustain $\ga 60$ efolds of inflation. The dynamics combines features of both large field chaotic inflation and $k$-inflation, both of which can suppress the tensor amplitude. Reducing the tensor-scalar ratio below the observational bound $r \lesssim 0.1$ while keeping the scalar spectral index $n_s$ within experimental bounds either yields equilateral nongaussianity $f_{NL}^{eq} \simeq {\cal O}(1)$, close to the current observational bounds, or ultimately gives very small $r$.

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