arXiv subjects

Ido Ben-Dayan

Publications and source records attributed to Ido Ben-Dayan.

At least 37 records · Page 2Linked to original sources

arXiv2022

Cosmology with the Laser Interferometer Space Antenna

The Laser Interferometer Space Antenna (LISA) has two scientific objectives of cosmological focus: to probe the expansion rate of the universe, and to understand stochastic gravitational-wave backgrounds and their implications for early universe and particle physics, from the MeV to the Planck scale. However, the range of potential cosmological applications of gravitational wave observations extends well beyond these two objectives. This publication presents a summary of the state of the art in LISA cosmology, theory and methods, and identifies new opportunities to use gravitational wave observations by LISA to probe the universe.

astro-ph.CO↗

arXiv2021

Sourced fluctuations in generic slow contraction

We introduce a set of generic conditions for the slow contracting Universe and for a narrowed-down category of models called fast-roll models. We present general conditions for super horizon freeze-out of scalar and tensor perturbations and show that any fast-roll model satisfies them, as in the case of inflation. We are interested in the "Sourced Bounce" scenario, where perturbations are sourced by a $U(1)$ gauge field coupled to a bouncer scalar field. The requirement of a slightly red tilted scalar spectrum greatly restricts the allowed couplings between the scalar and the gauge field. We show that a viable slightly red scalar spectrum is achievable. However, within the fast-roll approximation, the tensor-to-scalar ratio is in general $r\simeq 1/9$, inconsistent with current observations. We demonstrate the general result with an explicit example we dub "Intermediate contraction". We prove that small modifications in fast-roll that do not alter the Green's functions do not result in $r<0.06$ consistent with the data for more than an e-fold. Hence, a successful "Sourced Bounce" requires a different source or a significant deviation from fast-roll.

gr-qc↗

arXiv2021

Emergent dark energy from unparticles

A limiting temperature of a species can cause the Universe to asymptote to it yielding a deSitter (dS) phase due to macroscopic emergent behavior. The limiting temperature is generic for theories slightly shifted from their conformal point. We demonstrate such behavior in the example of unparticles/Banks-Zaks theory. The unparticles behave like radiation at high energies reducing the Hubble tension, and a cosmological constant (CC) at low energies yielding a model that follows closely ΛCDM model but due to collective phenomenon. It is technically natural and avoids the no-dS conjecture. The model is free of the coincidence and initial conditions problems, of scalar fields and of modified gravity.

hep-ph↗

arXiv2021

Trapped Gravitational Waves in Jackiw-Teitelboim Gravity

We discuss the possibility that gravitational fluctuations ("gravitational-waves") are trapped in space by gravitational interactions in two dimensional Jackiw-Teitelboim gravity. In the standard geon (gravitational electromagnetic entity) approach, the effective energy is entirely deposited in a thin layer, the active region, that achieves spatial self-confinement and raises doubts about the geon's stability. In this paper we relinquish the "active region" approach and obtain self-confinement of "gravitational waves" that are trapped by the vacuum geometry and can be stable against the backreaction due to metric fluctuations.

gr-qc↗

arXiv2021

Sourced Scalar Fluctuations in Bouncing Cosmology

We calculate the scalar power spectrum generated by sourced fluctuations due to coupling between the scalar field, which holds most of the energy density of the universe, and a gauge field for a general FLRW metric. For this purpose we calculate the curvature perturbation to second order in the presence of gauge fields, and show that the gauge fields behave like an additional potential term. We then apply the analysis to the case of slow-contraction. Due to the interaction between the scalar field and gauge fields additional 'sourced' tensor and scalar spectra are generated. The resulting spectra are chiral, slightly blue and arbitrarily close to scale invariance. The only difference between the tensor and scalar spectra is the coupling constant with an ${\mathcal O}(1)$ numerical coefficient, and some momentum space polarization vectors. As a result the tilt of the spectra are the same. For the nearly scale invariant case, the momentum integration gives the same leading contribution. Hence, $r\simeq 1/9$ where the deviation from this value is controlled by the deviation from scale invariance, and is not in agreement with CMB observations. Deviating considerably from near scale invariance, and considering a bluer tilt with $n_T>0.12$, the model cannot account for CMB observations, but can be detected by LIGO and/or LISA in the future.

gr-qc↗

arXiv2020

Inflation in supergravity from field redefinitions

Supergravity (SUGRA) theories are specified by a few functions, most notably the real Kähler function denoted by $G(T_i, \bar {T}_i) = K + \log |W|^2$, where K is a real Kähler potential, and W is a holomorphic superpotential. A field redefinition $T_i \rightarrow f_1(T_i)$ does not change neither the theory, nor the Kähler geometry. Similarly, the Kähler transformation, $K \rightarrow K + f_2 + \bar f_2, W \rightarrow e^{-f_2} W$ where $f_2$ is holomorphic also leaves G and hence the theory and the geometry invariant. However, if we perform a field redefinition only in $K(T_i,\bar{T}_i) \rightarrow K(f(T_i),f(\bar{T}_i))$, while keeping the same superpotential $W(T_i)$, we get a different theory, as G is not invariant under such a transformation while maintaining the same Kähler geometry. This freedom of choosing $f(T_i)$ allows constructing an infinite number of new theories given a fixed Kähler geometry and a predetermined superpotential W. Our construction generalizes previous ones that were limited by the holomorphic property of $W$. In particular it allows for novel inflationary SUGRA models and particle phenomenology model building, where the different models correspond to different choices of field redefinitions. We demonstrate this possibility by constructing several prototypes of inflationary models (hilltop, Starobinsky-like, plateau, log-squared and bell-curve) all in flat Kähler geometry and an originally renormalizable superpotential $W$. The models are in accord with current observations and predict $r\in[10^{-6},0.06]$ spanning several decades that can be easily obtained. In the bell-curve model, there also exists a built-in gravitational reheating mechanism with $T_R\sim \mathcal{O}( 10^7 GeV)$.

hep-th↗

arXiv2020

Thermodynamical Interpretation of the Second Law for Cosmology

The area of a future holographic screen increases monotonically. Associating this area with entropy results in a generalized second law for Cosmology (GSLC). Unlike black hole horizons, screens relevant to Cosmology have no thermodynamical interpretation. We suggest relating the entropy of the screens to spacetime degrees of freedom surface density derived from a (seemingly) phase space. This construction enables us to identify the entropy of any holographic screen as the entropy detected by accelerating observers due to their acceleration. Using Unruh's temperature and the equivalence principle, this gives the holographic screens' temperature and yields a thermodynamical interpretation of the GSLC.

hep-th↗

arXiv2020

Banks-Zaks Cosmology, Inflation, and the Big Bang Singularity

We consider the thermodynamical behavior of Banks-Zaks theory close to the conformal point in a cosmological setting. Due to the anomalous dimension, the resulting pressure and energy density deviate from that of radiation and result in various interesting cosmological scenarios. Specifically, for a given range of parameters one avoids the cosmological singularity. We provide a full "phase diagram" of possible Universe evolution for the given parameters. For a certain range of parameters, the thermal averaged Banks-Zaks theory alone results in an exponentially contracting universe followed by a non-singular bounce and an exponentially expanding universe, i.e. \textit{Inflation without a Big Bang singularity}, or shortly termed "dS Bounce". The temperature of such a universe is bounded from above and below. The result is a theory violating the classical Null Energy Condition (NEC). Considering the Banks-Zaks theory with an additional perfect fluid, yields an even richer phase diagram that includes the standard Big Bang model, stable single "normal" bounce, dS Bounce and stable cyclic solutions. The bouncing and cyclic solutions are with no singularities, and the violation of the NEC happens only near the bounce. We also provide simple analytical conditions for the existence of these non-singular solutions. Hence, within effective field theory, we have a new alternative non-singular cosmology based on the anomalous dimension of Bank-Zaks theory that may include inflation and without resorting to scalar fields.

hep-th↗

arXiv2019

Constraints on Scalar and Tensor spectra from $N_{eff}$

At the linear level, the gravitational wave (GW) spectrum predicted by inflation, and many of its alternatives, can have arbitrarily small amplitude and consequently an unconstrained tilt. However, at second order, tensor fluctuations are sourced by scalar fluctuations that have been measured in the cosmic microwave background (CMB). These second order fluctuations generically produce a minimum amount of tensor perturbations corresponding to a tensor-to-scalar ratio of $r\sim 10^{-6}$. Inverting this relationship yields a bound on the tensor tilt sourced by scalar fluctuations. Since this induced GW spectrum depends on the scalar spectrum, we derive a new indirect bound that involves \textit{all scales} of the scalar spectrum based on CMB observations. This bound comes from the constraint on the number of effective relativistic degrees of freedom, $N_{eff}$. We estimate the bound using current data, and the improvements expected by future CMB experiment. The bound forces the running and running of running to conform to standard slow-roll predictions of $α,β\lesssim (n_s-1)^2$, improving on current CMB measurements by an order of magnitude. This bound has further implications for the possibility of primordial black holes as dark matter candidates. Performing a likelihood analysis including this new constraint, we find that positive $α$ and/or $β$ are disfavored at least at $1σ$. Finally, using bounds on the fractional energy density of gravitational waves today obtained by LIGO and the Pulsar Timing Array, we obtain a bound on the primordial scalar spectrum on these scales and give forecast for future measurements.

astro-ph.CO↗

arXiv2019

Generalized Clockwork Theory

We generalize the clockwork theory in several directions. First, we consider beyond nearest neighbors interactions. Considering such interactions keeps a larger subgroup of the original $U(1)^{N+1}$ unbroken and can allow for different symmetry breaking patterns. We recover the original clockwork scenario in the presence of these additional interactions. In such case, the masses of the massive modes change, but a single massless mode remains intact. Such interactions are naturally interpreted as higher derivative terms from the point of view of extra dimensions. Second, we generalize the clockwork shift symmetry to non-abelian global groups. Third, trivial embedding of the clockwork scenario in supergravity, yields an AdS minimum as big as the clockwork interaction. Specifically, the clockwork is connected to the cosmological constant. We analyze the different ways in which a Minkowski supersymmetric minimum can be constructed, and demonstrate simple SUSY breaking mechanisms that preserve or break the clockwork symmetry. We show that the clockwork direction is actually a special SUSY breaking direction, that does not require the inclusion of additional fields or parameters. Fourth, we review the extra-dimensional origin of the mechanism and interpretation, in the case of conformal coupling to gravity.

hep-ph↗

arXiv2019

Draining the Swampland

We discuss some implications of the recently suggested Swampland conjecture $\frac{|\nabla V|}{V}\gtrsim c\sim 1$, together with a previous one $Δϕ\lesssim 1$. We list some implications for particle phenomenology and the Early Universe. The most intriguing implication of the conjecture could be a significant shift in allowed inflationary models, if not ruling out slow-roll (single field) inflation altogether. The tension of inflation and the conjecture does not only regard the amplitude of the tensor spectrum, but also its tilt, as $c\gtrsim 1$ implies both a yet unobserved tensor to scalar ratio, and an enhancement of the observed scalar power spectrum on large scales in discord with current data that favors a suppression on these scales. Scalar fields are abundant in theories of quantum gravity. Considering a second scalar field, its dynamics are dictated by the relation between its mass, $m$, and the Hubble parameter, $H$, at different epochs in the history of the Universe. This scalar field, a drainon, fulfills the conjecture draining up the swampland. For Inflation, this drainon requires a modest hierarchy compared to the inflaton. For the rest of the thermal history of the Universe, the drainon can be a coherently oscillating scalar field strengthening the case of Dark Matter candidates of that sort.

hep-th↗

arXiv2019

f(R) and Brans-Dicke Theories and the Swampland

We discuss the viability of $f(R)$ and Brans-Dicke theories of gravity in light of the recent Swampland conjectures. We show that in the case of perfect fluid domination the swampland conjecture conditions may be easily satisfied and therefore we focus on the constraints in the vacuum theory. We derive the conditions for the swampland conjectures in $f(R)$ and Brans-Dicke framework, and find that a large portion of the parameter space is not allowed. Furthermore, we analyze particular $f(R)$ models of inflation and dark energy and in their case we set the allowed range of $R$.

gr-qc↗

arXiv2016

Gravitational Waves in Bouncing Cosmologies from Gauge Field Production

We calculate the gravitational waves (GW) spectrum produced in various Early Universe scenarios from gauge field sources, thus generalizing earlier inflationary calculations to bouncing cosmologies. We consider generic couplings between the gauge fields and the scalar field dominating the energy density of the Universe. We analyze the requirements needed to avoid a backreaction that will spoil the background evolution. When the scalar is coupled only to $F \tilde F$ term, the sourced GW spectrum is exponentially enhanced and parametrically the square of the vacuum fluctuations spectrum, ${\cal P}^s_T\sim ({\cal P}^v_T)^2$, giving an even bluer spectrum than the standard vacuum one. When the scalar field is also coupled to $F^2$ term, the amplitude is still exponentially enhanced, but the spectrum can be arbitrarily close to scale invariant (still slightly blue), $n_T\gtrsim 0$, that is distinguishable form the slightly red inflationary one. Hence, we have a proof of concept of observable GW on CMB scales in a bouncing cosmology.

astro-ph.CO↗

arXiv2016

Vacuum energy sequestering and conformal symmetry

In a series of recent papers Kaloper and Padilla proposed a mechanism to sequester standard model vacuum contributions to the cosmological constant. We study the consequences of embedding their proposal into a fully local quantum theory. In the original work, the bare cosmological constant $Λ$ and a scaling parameter $λ$ are introduced as global fields. We find that in the local case the resulting Lagrangian is that of a spontaneously broken conformal field theory where $λ$ plays the role of the dilaton. A vanishing or a small cosmological constant is thus a consequence of the underlying conformal field theory structure.

hep-th↗

arXiv2016

Hypercharge Axion and the Diphoton $750$ GeV Resonance

The CMS and ATLAS reports on a possible excess of diphoton events at $750$ GeV are the cause of great excitement and hope. We show that a pseudoscalar axion coupled to the topological density of hypercharge, suggested in the past by Brustein and Oaknin as a candidate for inducing baryogenesis, can explain the signal in the diphoton channel. The hypercharge axion (HCA) can also decay to $Zγ$ and $ZZ$. The expected number of events in these channels is too small to have been detected, but such decays should be observed in the future. The HCA can be produced via vector boson fusion (VBF) or via associated production (AP). The latter should be manifested by a characteristic decay to 3 photons which should be observed soon. We adapt the previous analysis of possible detection of the HCA at the LHC by Brustein and Oaknin and by Elfgren to the case that the mass of the HCA is 750 GeV and find the expected cross section and decay width in terms of the HCA coupling. We find that the expected cross-section fits well the observed number of excess events and that there is tension between the measured cross-section and the reported partial decay width. The tension is caused because for both VBF and AP the production cross-section and the decay width depend linearly on each other. We therefore expect that if the new resonance is indeed the HCA, either its width should be significantly smaller than the reported width or the production cross-section should be significantly higher than the reported one.

hep-ph↗

arXiv2015

Constraints on small-scale cosmological fluctuations from SNe lensing dispersion

We provide predictions on small-scale cosmological density power spectrum from supernova lensing dispersion. Parameterizing the primordial power spectrum with running $α$ and running of running $β$ of the spectral index, we exclude large positive $α$ and $β$ parameters which induce too large lensing dispersions over current observational upper bound. We ran cosmological N-body simulations of collisionless dark matter particles to investigate non-linear evolution of the primordial power spectrum with positive running parameters. The initial small-scale enhancement of the power spectrum is largely erased when entering into the non-linear regime. For example, even if the linear power spectrum at $k>10h {\rm Mpc}^{-1}$ is enhanced by $1-2$ orders of magnitude, the enhancement much decreases to a factor of $2-3$ at late time ($z \leq 1.5$). Therefore, the lensing dispersion induced by the dark matter fluctuations weakly constrains the running parameters. When including baryon-cooling effects (which strongly enhance the small-scale clustering), the constraint is comparable or tighter than the PLANCK constraint, depending on the UV cut-off. Further investigations of the non-linear matter spectrum with baryonic processes is needed to reach a firm constraint.

astro-ph.CO↗

arXiv2015

On Soft Limits of Large-Scale Structure Correlation Functions

We study soft limits of correlation functions for the density and velocity fields in the theory of structure formation. First, we re-derive the (resummed) consistency conditions at unequal times using the eikonal approximation. These are solely based on symmetry arguments and are therefore universal. Then, we explore the existence of equal-time relations in the soft limit which, on the other hand, depend on the interplay between soft and hard modes. We scrutinize two approaches in the literature: The time-flow formalism, and a background method where the soft mode is absorbed into a locally curved cosmology. The latter has been recently used to set up (angular averaged) `equal-time consistency relations'. We explicitly demonstrate that the time-flow relations and `equal-time consistency conditions' are only fulfilled at the linear level, and fail at next-to-leading order for an Einstein de-Sitter universe. While applied to the velocities both proposals break down beyond leading order, we find that the `equal-time consistency conditions' quantitatively approximates the perturbative results for the density contrast. Thus, we generalize the background method to properly incorporate the effect of curvature in the density and velocity fluctuations on short scales, and discuss the reasons behind this discrepancy. We conclude with a few comments on practical implementations and future directions.

astro-ph.CO↗

arXiv2014

Constraining the primordial power spectrum from SNIa lensing dispersion

The (absence of detecting) lensing dispersion of Supernovae type Ia (SNIa) can be used as a novel and extremely efficient probe of cosmology. In this preliminary example we analyze its consequences for the primordial power spectrum. The main setback is the knowledge of the power spectrum in the non-linear regime, 1 Mpc^{-1} < k < 10^2-10^3 Mpc^{-1} up to redshift of about unity. By using the lensing dispersion and conservative estimates in this regime of wavenumbers, we show how the current upper bound sigma_mu(z=1) < 0.12on existing data gives strong indirect constraints on the primordial power spectrum. The probe extends our handle on the spectrum to a total of 12-15 inflation e-folds. These constraints are so strong that they are already ruling out a large portion of the parameter space allowed by PLANCK for running alpha = d n_s / d ln k and running of running beta = d^2 n_s / d ln k^2. The bounds follow a linear relation to a very good accuracy. A conservative bound disfavors any enhancement above the line beta(k_0)=0.036-0.42 alpha(k_0) and a realistic estimate disfavors any enhancement above the line beta(k_0)=0.022-0.44 alpha(k_0).

astro-ph.CO↗