arXiv subjects

Ido Ben-Dayan

Publications and source records attributed to Ido Ben-Dayan.

At least 19 recordsLinked to original sources

arXiv2026

Slowly Rolling on a Quantum Correction

Recent advances in cosmological measurements such as DESI and ACT may indicate an increase in the preferred value of the spectral tilt $n_s$ such that it disfavors rather popular models such as the Higgs or Starobinski inflation models. We argue that it actually means that the data is now sensitive enough to quantum corrections beyond simple tree-level models. Resurrecting the old theme of Coleman-Weinberg effective potential, we analyze the predictions of such models, as well as likelihood analysis, leading to a more established and interesting predictive framework.

astro-ph.CO↗

arXiv2026

Suppressed Intrinsic Curvature Gravity

We consider various mechanisms of modifying the effect of intrinsic curvature in gravity with respect to General Relativity. Two primary approaches for suppressing intrinsic curvature are studied. First, by considering a Lagrange multiplier or an auxiliary field. Second, by non-minimal coupling between a scalar field and the intrinsic curvature scalar. We promote the foliation to a dynamical field, getting a fully covariant, and foliation independent theory. We reproduce the basic solutions of FLRW cosmology, black hole solutions, Lense-Thirring effect, and gravitational waves. The speed of gravitational waves is modified in comparison to the speed of light. A certain limit of our theory corresponds to the lowest-order Carroll gravity. Hence, our theory is a different UV completion of Carroll gravity, compared to the usual expansion of a small speed of light. Carroll gravity limit also has an enhanced symmetry, making the reduced or vanishing intrinsic curvature technically natural. Finally, our construction defines a one parameter family of theories, that sets the relative strength of the intrinsic and extrinsic curvature, and General Relativity corresponds to a specific value of this parameter.

gr-qc↗

arXiv2026

Towards a Proof of the Improved Quantum Null Energy Condition

The Improved Quantum Null Energy Condition (INEC) was recently derived from the (restricted) quantum focusing conjecture (QFC), and is a statement about the energy-momentum tensor (EMT) of field theories in Minkowski space-time. It is a stronger condition than the quantum null energy condition (QNEC), and includes the possibility of expanding or contracting geodesics. Using the properties of relative entropy and modular Hamiltonian associated with null deformation of the sphere, we show the INEC holds under an additional assumption relating the EMT to the relative entropy. Furthermore, using the QNEC and INEC as a basis, we briefly speculate about a possible modified Quantum Focusing Conjecture.

hep-th↗

arXiv2025

Disentangling the Origins of the NANOGrav Signal: Early Universe Models and $ΔN_{eff}$ Bounds

We investigate whether an Early-Universe stochastic gravitational-wave background (SGWB) can account for the common spectrum process reported by NANOGrav, while also being consistent with current and projected CMB measurements of extra radiation. We compute the contribution of effective number of relativistic species, $ΔN_{eff}$, for a number of Early-Universe models proposed to explain the pulsar timing array (PTA) spectrum. We demonstrate that models predicting $ΔN_{eff}$ above the CMB limit would be firmly excluded, implying that the NANOGrav signal in tension with these bounds must instead arise from astrophysical sources. We find that current NANOGrav 15-year dataset, sensitive up to 60 nHz, gives a negligible contribution to $ΔN_{eff}$ and remains well below the present and future CMB detection threshold. However, when we project future PTA capabilities reaching upto 1 $μ$Hz, even with our conservative estimate we find that Inflation, Scalar Induced Gravitational Waves (SIGW), and metastable cosmic strings can induce a $ΔN_{eff}$ large enough for $>3.5σ$ detection by the Simons Observatory.

astro-ph.CO↗

arXiv2025

Impact of Low ell's on Large Scale Structure Anomalies

We scrutinize the reported lensing anomaly of the CMB by considering several phenomenological modifications of the lensing consistency parameter, $A_{\rm L}$. Considering Planck spectra alone, we find statisically significant evidence for scale dependence (`running') of $A_{\rm L}$. We then demonstrate that the anomaly is entirely driven by Planck's low multipoles, $\ell \leq 30$. When these data points are excluded a joint analysis with several other datasets clearly favors $Λ$CDM over the extended $Λ\rm CDM+A_L$ model. Not only that the lensing anomaly and low $\ell$ anomaly of the CMB go away in this case, but also the $S_8$ tension is ameliorated, and only the Hubble tension persists.

astro-ph.CO↗

arXiv2025

Islands in Bianchi type-I Universe

We study the conditions for finding an island in an anisotropic universe - Bianchi Type-I filled with radiation. We verify that the existence of islands does not depend on their shape. We then find that islands may form at certain times, near the turnaround point - where the universe turns from contraction to expansion in one of the directions. This is in line with previous analyses regarding cosmological space-times where islands form if one has two energy scales in the problem, such as the typical temperature of the universe and, on top of that, cosmological constant, curvature, anisotropy, or some mass scale.

hep-th↗

arXiv2024

Gravitational-wave background in bouncing models from semi-classical, quantum and string gravity

We study the primordial spectra and the gravitational-wave background (GWB) of three models of semi-classical, quantum or string gravity where the big bang is replaced by a bounce and the primordial tensor spectrum is blue: ekpyrotic universe with fast-rolling Galileons, string-gas cosmology with Atick-Witten conjecture and pre-big-bang cosmology. We find that the ekpyrotic scenario with Galileons does not produce a GWB amplitude detectable by present or third-generation interferometers, while the Atick-Witten-based string-gas model is ruled out in its present form for violating the big-bang-nucleosynthesis bound, contrary to the original string-gas scenario. In contrast, the GWB of the pre-big-bang scenario falls within the sensitivity window of both LISA and Einstein Telescope, where it takes the form of a single or a broken power law depending on the choice of parameters. The latter will be tightly constrained by both detectors.

gr-qc↗

arXiv2024

SYK Model in a Non-Gaussian disorder ensemble and emergent Coleman's mechanism

We consider the case of the SYK model with non-gaussian disorder in the large $N$ limit. After obtaining the effective action, we derive the density of states and the free energy of the modified theory. We show that the non-gaussian disorder corresponds to a non-local Liouville theory, and non-minimally coupled 2D gravity action. It also provides a nice realization of Colemania - Coleman's idea from the 80s of generating a small Cosmological Constant. Finally, we also calculate out of time order correlation functions (OTOC) for the model.

hep-th↗

arXiv2023

Theoretical Priors and the Dark Energy Equation of State

We revisit the theoretical priors used for inferring Dark Energy (DE) parameters. Any DE model must have some form of a tracker mechanism such that it behaved as matter or radiation in the past. Otherwise, the model is fine-tuned. We construct a model-independent parametrization that takes this prior into account and allows for a relatively sudden transition between radiation/matter to DE behavior. We match the parametrization with current data, and deduce that the adiabatic and effective sound speeds of DE play an important role in inferring the cosmological parameters. We find that there is a preferred transition redshift of $1+z\simeq 29-30$, and some reduction in the Hubble and Large Scale Structure tensions.

astro-ph.CO↗

arXiv2023

The Quantum Focusing Conjecture and the Improved Energy Condition

By rearranging its terms, the Quantum Focusing Conjecture (QFC) can be viewed as a quantum energy condition, and we can consider various limits. A recent restricted version is a limiting form where the quantum focusing vanishes $Θ\rightarrow 0$, and has been proven for Braneworld scenario. As a result, we derive an improved quantum null energy condition (INEC) $T_{kk}\geq \frac{\hbar}{2π\mathcal{A}}\left(S''_{out}-\frac{1}{2}θS'_{out}\right)$, that can be proven with field theory techniques. We sketch the beginning of a proof, and briefly discuss possible interpretations in the absence of one.

hep-th↗

arXiv2023

Probing The Early Universe Cosmology With NANOGrav: Possibilities and Limitations

A stochastic gravitational wave background is a prediction of a number of astrophysical and cosmological phenomena including early Universe Cosmology. Recently, the NANOGrav Collaboration reported conclusive evidence for a stochastic gravitational-wave background. We analyze the NANOGrav signal assuming it is of primordial origin including the reheating phase. We use the latest measurements from NANOGrav to constrain the Universe's reheating equation of state $w_{re}$ the reheating temperature, $T_{re}$, the tensor to scalar ratio $r$, and the tensor tilt $n_t$. Assuming the constant equation of state $w_{re}$ responsible for reheating phase, we find preference for instant reheating, $w_{re} = 0.36^{+0.15}_{-0.28}$, and a very blue tilt $n_t = 1.94^{+0.43}_{-0.88}$. We find a degeneracy between the tensor to scalar ratio $r$ and $T_{re}$ and suggest ways to break this degeneracy. In all cases where the reheating temperature is constrained, it is constrained to be very low with $T_{re}\leq 10^5 GeV$. We further find that a scale-invariant spectrum as suggested by inflation implies a stiff equation of state $w_{re}=19/3$. If extrapolated, the blue-tilted primordial spectrum that agrees with the NANOGrav signal at corresponding frequencies is incompatible with the LIGO bound. This incompatibility is another challenge for connecting NANOGrav with the primordial spectrum. We discuss a number of ways to circumvent this issue. We split the spectrum into a sum of astrophysical and primordial spectra and constrain the astrophysical and primordial components using NANOGrav data and the LIGO bound. In another attempt, we use the same data and constrain the running of the spectrum. Any of these or a combination of such methods can be used to reconcile the NANOGrav data and the LIGO bound with the primordial power spectrum.

astro-ph.CO↗

arXiv2023

Requiem to "Proof of Inflation" or Sourced Fluctuations in a Non-Singular Bounce

Popular wisdom suggests that measuring the tensor to scalar ratio $r$ on CMB scales is a "proof of inflation" since one generic prediction is a scale-invariant tensor spectrum while alternatives predict $r$ that is many orders of magnitude below the sensitivity of future experiments. A bouncing Universe with sourced fluctuations allows for nearly scale-invariant spectra of both scalar and tensor perturbations challenging this point of view. Past works have analyzed the model until the bounce, under the assumption that the bounce will not change the final predictions. In this work, we discard this assumption. We explicitly follow the evolution of the Universe and fluctuations across the bounce until reheating. The evolution is stable, and the existence of the sourced fluctuations does not destroy the bounce. The bounce enhances the scalar spectrum while leaving the tensor spectrum unchanged. The enhancement depends on the duration of the bounce - a shorter bounce implies a larger enhancement. The model matches current observations and predicts any viable tensor-to-scalar ratio $r\lesssim 10^{-2}$, which may be observed in upcoming CMB experiments. Hence, a measurement of $r$ will no longer be a "proof of inflation'', and a Sourced Bounce is a viable paradigm with distinct predictions.

astro-ph.CO↗

arXiv2023

Emergent Unparticles Dark Energy can restore cosmological concordance

Addressing the discrepancy between the late and early time measurements of the Hubble parameter, $H_0$, and the so-called $S_8$ parameter has been a challenge in precision cosmology. Several models are present to address these tensions, but very few of them can do so simultaneously. In the past, we have suggested Banks-Zaks/Unparticles as an emergent Dark Energy model and claimed that it can ameliorate the Hubble tension. In this work, we test this claim and perform a likelihood analysis of the model and its parameters are given current data and compare it to $Λ$CDM. The model offers a possible resolution of Hubble tension and softens the Large Scale Structure (LSS) tension without employing a scalar field or modifying the gravitational sector. Our analysis shows a higher value of $H_0 \sim 70 - 73$ km/sec/Mpc and a slightly lower value of $S_8$ for various combinations of data sets. Consideration of Planck CMB data combined with the Pantheon sample and SH0ES priors lowers the $H_0$ and $S_8$ tension to $0.96 σ$ and $0.94 σ$ respectively with best-fit $Δχ^2 \approx -10$ restoring cosmological concordance. Significant improvement in the likelihood persists for other combinations of data sets as well. Evidence for the model is given by inferring one of its parameters to be $x_0\simeq-4.36$.

astro-ph.CO↗

arXiv2022

Islands in the Fluid: Islands are Common in Cosmology

We discuss the possibility of entanglement islands in cosmological spacetimes with a general perfect fluid with an equation of state $w$. We find that flat universes with time-symmetric slices where the Hubble parameter vanishes always have islands on that slice. We then move away from such slices, considering still universes with a general perfect fluid. Under the local thermal equilibrium assumption, the comoving entropy density $s_c$ is constant. As a result, the conditions for an island become an inequality between the energy density (or Hubble parameter) and the temperature at some time of normalization. The consequences are that islands can exist for practically all fluids that are not radiation, i.e. $w\neq 1/3$. We also discuss the ramifications of our results for universes with spatial curvature. Finally, we show that islands occur in the Simple Harmonic Universe model which has no classical singularity at the background level, in contrast to all previous examples where islands occurred only in space-times with singularities.

hep-th↗

arXiv2022

The Grand Canonical Multiverse and the Small Cosmological Constant

We consider the Multiverse as an ensemble of universes. Using standard statistical physics analysis we get that the Cosmological Constant (CC) is exponentially small. The small and finite CC is achieved without any anthropic reasoning. We then quantize the CC. The quantization allows a precise summation of the possible contributions and using the measured value of the CC yields a prediction on the temperature of the Multiverse that we define. Furthermore, quantization allows the interpretation of a single Universe as a superposition of different eigenstates with different energy levels rather than the existence of an actual Multiverse.

hep-th↗

arXiv2022

Small Field models with ACTPol and BICEP3 data -- Likelihood analysis

We perform a Bayesian analysis for small field models of inflation, using the most recent datasets produced by Planck`18, ACTPol, and BICEP3. We employ Artificial Neural Networks (ANN) to perform analyses with model coefficients, instead of their proxy slow-roll parameters. The ANN connects the models with their projected scalar index $n_s$ and index running $α$, in lieu of the less accurate Lyth-Riotto expressions. We recover the most likely coefficients for a sixth degree polynomial inflationary potential, which yields a tensor-to-scalar ratio $r\lesssim 0.03$. We do so for the case of joint Planck and ACTPol datasets, and for each dataset alone. The BICEP3 data is included in all three analyses. We show that these models are likely, with coefficients that are tuned to about $Δ\gtrsim 1/60$. Curiously, we also find a significant tension between ACTPol and Planck datasets, which we try to account for.

astro-ph.CO↗

arXiv2022

More on Emergent Dark Energy from Unparticles

In a recent paper \cite{Artymowski:2020zwy} we suggested the possibility that the present acceleration of the Universe is due to thermodynamical behavior of unparticles. The model is free of scalar fields, modified gravity, a Cosmological Constant (CC), the coincidence problem, initial conditions problem and possesses interesting distinct predictions regarding the equation of state of Dark Energy, the growth rate and the number of relativistic degrees of freedom at BBN and CMB decoupling. In this work, we relate to a recent paper \cite{Abchouyeh:2021wey}, which discusses a similar setup of unparticles with and without a CC as an external source of late-time acceleration. The authors have shown how such a model is inconsistent with the data. We show that these claims are viable only in a particular part of the parameter space and that model \cite{Artymowski:2020zwy} stands tall. We further suggest a consistency condition in terms of observables. We then fit publicly available supernovae data to derive the expected Hubble parameter and constrain the parameters of the model.

astro-ph.CO↗

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↗