Drafts by Theodoros PAPANIKOLAOU
In this project, we study the theory of cosmological inflation and its cosmological implications.... more In this project, we study the theory of cosmological inflation and its cosmological implications. Firstly, we present basic elements of the ΛCDM cosmological model as well as the shortcomings of the Big Bang theory which motivated inflationary scenarios in the primordial universe. Then,we adduce the standard single-field theory of inflation and describe the slow-roll conditions. Next, we present the theory of cosmological perturbations related to the inflaton field by analysing theories with massive or massless scalar fields in de Sitter or quasi de Sitter primordial universes. Consequently, we feature the scalar metric fluctuations coupled with the inflaton ones. In addition,we deduce the power spectrum of the curvature perturbations which is directly related to the fluctuations of the gravitational potential. Finally, we establish the relations between the curvature fluctuations and the matter density and temperature fluctuations which can be tested through observation.
In this report, we employ the unitarity of the S matrix in order to constrain several important p... more In this report, we employ the unitarity of the S matrix in order to constrain several important properties of dark matter (DM). The S matrix, a matrix present in Quantum Field Theory (QFT), governs the elastic and inelastic scattering processes. The efficacy of inelastic scattering processes in the early universe regulates the most well meausured DM quantity, the DM density. The DM elastic self-scattering has the the potential to affect the DM gravitational clustering and leave an imprint on the galactic structures we observe. Therefore, the S matrix unitarity provides a tool to contrain these two properties in conjuction, and in a model-independent way. In the context of specific DM particle physics models, these unitarity constraints can be recast into constraints on the fundamental couplings and masses of the theory. In our study, we exploit a primitive version of the optical theorem in QFT, which is an implication of unitarity, and make a partial-wave expansion of the Lorentz invariant matrix elements in Wigner functions. In the end, we obtain an inequality which constrains partial-wave inelastic and elastic scattering cross-sections of 2-to-2 processes. By making use of this theoretical framework we derive some numerical results about the allowed range of scattering cross-sections, imposed by unitarity and the possible values of the unitarity mass upper bound of DM candidates. The theoretical assumptions adopted in the extraction of the inequality relation between the partial-wave inelastic and elastic cross-sections is model-independent, thus rendering our results relevant for large classes of dark matter candidates.
Papers by Theodoros PAPANIKOLAOU
arXiv (Cornell University), Jun 8, 2023
Primordial black holes (PBHs) can answer a plethora of cosmic conundra, among which the origin of... more Primordial black holes (PBHs) can answer a plethora of cosmic conundra, among which the origin of the cosmic magnetic fields. In particular, supermassive PBHs with masses MPBH > 10 10 M⊙ and furnished with a plasma-disk moving around them can generate through the Biermann battery mechanism a seed primordial magnetic field which can later be amplified so as to provide the magnetic field threading the intergalactic medium. In this work, we derive the gravitational wave (GW) signal induced by the magnetic anisotropic stress of such a population of magnetised PBHs. Interestingly enough, by using GW constraints from Big Bang Nucleosynthesis (BBN) and an effective model for the galactic/turbulent dynamo amplification of the magnetic field, we set a conservative upper bound constraint on the abundances of supermassive PBHs at formation time, Ω PBH,f as a function of the their masses, namely that Ω PBH,f ≤ 2.5 × 10 −10 M 10 10 M ⊙ 45/22. Remarkably, these constraints are comparable, and, in some mass ranges, even tighter compared to the constraints on Ω PBH,f from large-scale structure (LSS) probes; hence promoting the portal of magnetically induced GWs as a new probe to explore the enigmatic nature of supermassive PBHs.
Physical Review D
Large-scale primordial magnetic fields (PMFs) threading the intergalactic medium are observed ubi... more Large-scale primordial magnetic fields (PMFs) threading the intergalactic medium are observed ubiquitously in the Universe playing a key role in the cosmic evolution. Their origin is still debated constituting a very active field of research. In the present article, we propose a novel natural ab initio mechanism for the origin of such PMFs through the portal of supermassive primordial black holes (PBHs) forming between the big bang nucleosynthesis and the recombination era. In particular, by considering PBHs furnished with a locally isothermal disk we study the generation of a Biermann battery induced seed magnetic field (MF) due to the vortexlike motion of the primordial plasma around the black hole. Finally, by considering monochromatic PBH mass distributions and deriving the relevant MF power spectrum we make a conservative estimate for the seed PMF in intergalactic scales and at redshift z ¼ 30, when typical galaxies are considered to form, which reads as B ≃ 10 −30 Gð l R 10 6 Þ 2 ð M PBH 10 14 M ⊙ Þ 5=2 , where M PBH is the PBH mass and l R ≡ R d =R ISCO , is the ratio of the radius of the disk, R d over the radius of the innermost stable circular orbit, R ISCO. Interestingly enough, by requiring to seed a PMF of the order of 10 −30 G necessary to give rise to a present day 10 −18 G in intergalactic scales, we find a lower bound on the PBH mass within the range ½10 10-10 16 M ⊙ depending on the radius of the PBH disk.
Classical and Quantum Gravity
Primordial black holes form in the early Universe and constitute one of the most viable candidate... more Primordial black holes form in the early Universe and constitute one of the most viable candidates for dark matter. The study of their formation process requires the determination of a critical energy density perturbation threshold δ c , which in general depends on the underlying gravity theory. Up to now, the majority of analytic and numerical techniques calculate δ c within the framework of general relativity. In this work, using simple physical arguments we estimate semi-analytically the PBH formation threshold within the framework of quantum gravity, working for concreteness within loop quantum cosmology (LQC). In particular, for low mass PBHs formed close to the quantum bounce, we find a reduction in the value of δ c up to 50 % compared to the general relativistic regime quantifying for the first time to the best of our knowledge how quantum effects can influence PBH formation within a quantum gravity framework. Finally, by varying the Barbero–Immirzi parameter γ of loop quantu...
Physical Review D
Primordial black holes (PBHs) forming out of the collapse of enhanced cosmological perturbations ... more Primordial black holes (PBHs) forming out of the collapse of enhanced cosmological perturbations provide access to the early Universe through their associated observational signatures. In particular, enhanced cosmological perturbations collapsing to form PBHs are responsible for the generation of a stochastic gravitational-wave background (SGWB) induced by second-order gravitational interactions, usually called scalar induced gravitational waves (SIGWs). This SGWB is sensitive to the underlying gravitational theory; hence it can be used as a novel tool to test the standard paradigm of gravity and constrain possible deviations from general relativity. In this work, we study the aforementioned GW signal within modified teleparallel gravity theories, developing a formalism for the derivation of the GW spectral abundance within any form of gravitational action. At the end, working within viable fðT; ϕÞ models without matter-gravity couplings, and accounting for the effect of monoparametric fðTÞ gravity at the level of the source and the propagation of the tensor perturbations, we show that the respective GW signal is indistinguishable from that within GR. Interestingly, we find that in order to break the degeneracy between different fðTÞ theories through the portal of SIGWs one should necessarily consider nonminimal mattergravity couplings at the level of the gravitational action.
arXiv (Cornell University), Mar 29, 2023
arXiv (Cornell University), Mar 1, 2023
The Hawking evaporation of ultra-light primordial black holes (PBH) dominating the early Universe... more The Hawking evaporation of ultra-light primordial black holes (PBH) dominating the early Universe before Big Bang Nucleosynthesis can potentially increase the effective number of extra neutrino species Δ eff through the emission of dark radiation degrees of freedom alleviating in this way the 0 tension problem. Interestingly, these light PBHs can form a gas of Poisson distributed compact objects which can induce a gravitational-wave (GW) background due to second order gravitational interactions. Therefore, by considering the contribution to Δ eff due to the production of the aforementioned GW background we revisit in this work the constraints on the relevant parameters at hand, namely the PBH mass, PBH , the initial PBH abundance at PBH formation time, Ω PBH,f and the number of DR radiation degrees of freedom, DR by accounting at the same time for the relevant upper bounds constraints on Δ eff from the Planck collaboration.
arXiv (Cornell University), Jan 24, 2023
Large scale primordial magnetic fields (PMFs) threading the intergalactic medium are observed ubi... more Large scale primordial magnetic fields (PMFs) threading the intergalactic medium are observed ubiquitously in the Universe playing a key role in the cosmic evolution. Their origin is still debated constituting a very active field of research. In the present article, we propose a novel natural ab initio mechanism for the origin of such PMFs through the portal of supermassive primordial black holes (PBHs) forming between the Big Bang Nucleosynthesis and the recombination era. In particular, by considering PBHs furnished with a locally isothermal disk we study the generation of a Biermann battery induced seed magnetic field (MF) due to the vortexlike motion of the primordial plasma around the black hole. Finally, by considering monochromatic PBH mass distributions and deriving the relevant MF power spectrum we make a conservative estimate for the seed PMF in intergalactic scales and at redshift $z=30$, when typical galaxies are considered to form, which reads as $B\simeq 10^{-30}\mathrm{G}\left(\frac{\ell_\mathrm{R}}{10^6}\right)^2\left(\frac{M_\mathrm{PBH}}{10^{14}M_\odot}\right)^{5/2}$, where $M_\mathrm{PBH}$ is the PBH mass and $\ell_\mathrm{R}\equiv R_\mathrm{d}/R_\mathrm{ISCO}$, is the ratio of the radius of the disk, $R_\mathrm{d}$ over the radius of the innermost stable circular orbit, $R_\mathrm{ISCO}$. Interestingly enough, by requiring to seed a PMF of the order of $10^{-30}\mathrm{G}$ necessary to give rise to a present day $10^{-18}\mathrm{G}$ in intergalactic scales, we find a lower bound on the PBH mass within the range $[10^{10}- 10^{16}]M_\odot$ depending on the radius of the PBH disk.
The European Physical Journal C
Primordial black hole (PBH) fluctuations can induce a stochastic gravitational wave background at... more Primordial black hole (PBH) fluctuations can induce a stochastic gravitational wave background at second order, and since this procedure is sensitive to the underlying gravitational theory it can be used as a novel tool to test general relativity and extract constraints on possible modified gravity deviations. We apply this formalism in the framework of f(T) gravity, considering three viable mono-parametric models. In particular, we investigate the induced modifications at the level of the gravitational-wave source, which is encoded in terms of the power spectrum of the PBH gravitational potential, as well as at the level of their propagation, described in terms of the Green function which quantifies the propagator of the tensor perturbations. We find that, within the observationally allowed range of the f(T) model-parameters, the obtained deviations from general relativity, both at the levels of source and propagation, are practically negligible. Hence, we conclude that realistic a...
Physical Review D
The phenomenology of primordial black holes (PBHs) physics and the associated PBH abundance const... more The phenomenology of primordial black holes (PBHs) physics and the associated PBH abundance constraints, can be used in order to probe the physics of the early Universe. In this work, we investigate the PBH formation during the standard radiation-dominated era by studying the effect of an early F (R) modified gravity phase with a bouncing behavior which is introduced to avoid the initial spacetime singularity problem. In particular, we calculate the energy density power spectrum at horizon crossing time and then we extract the PBH abundance in the context of peak theory as a function of the parameter α of our F (R) gravity bouncing model at hand. Interestingly, we find that in order to avoid PBH overproduction at formation time, namely Ω PBH,f < 1, α should lie within the range α ≤ 10 −17 M 2 Pl. This constraint can be translated to a constraint on the energy scale at the onset of the Hot Big Bang (HBB) phase, H RD ∼ √ α /2 which can be recast as H RD ≤ 2 × 10 −8 M Pl .
arXiv (Cornell University), Sep 9, 2022
The Laser Interferometer Space Antenna (LISA) mission, scheduled for launch in the mid-2030s, is ... more The Laser Interferometer Space Antenna (LISA) mission, scheduled for launch in the mid-2030s, is a gravitational wave observatory in space designed to detect sources emitting in the millihertz band. LISA is an ESA flagship mission, currently entering the Phase B development phase. It is expected to help us improve our understanding about our Universe by measuring gravitational wave sources of different types, with some of the sources being at very high redshifts z ∼ 20. On the 23rd of February 2022 we organized the 1 st LISA in Greece Workshop. This workshop aimed to inform the Greek scientific and tech industry community about the possibilities of participating in LISA science and LISA mission, with the support of the Hellenic Space Center (HSC). In this white paper, we summarize the outcome of the workshop, the most important aspect of it being the inclusion of 15 Greek researchers to the LISA Consortium, raising our total number to 22. At the same time, we present a road-map with the future steps and actions of the Greek Gravitational Wave community with respect to the future LISA mission. https://indico.physics.auth.gr/event/11/ a. The shape and organization of this document, have been inspired by the work of our Canadian Colleagues.
arXiv (Cornell University), May 12, 2022
Primordial black hole (PBH) fluctuations can induce a stochastic gravitational wave background at... more Primordial black hole (PBH) fluctuations can induce a stochastic gravitational wave background at second order, and since this procedure is sensitive to the underlying gravitational theory it can be used as a novel tool to test general relativity and extract constraints on possible modified gravity deviations. We apply this formalism in the framework of f (T) gravity, considering three viable mono-parametric models. In particular, we investigate the induced modifications at the level of the gravitational-wave source, which is encoded in terms of the power spectrum of the PBH gravitational potential, as well as at the level of their propagation, described in terms of the Green function which quantifies the propagator of the tensor perturbations. We find that, within the observationally allowed range of the f (T) model-parameters, the obtained deviations from general relativity, both at the levels of source and propagation, are practically negligible. Hence, we conclude that realistic and viable f (T) theories can safely pass the primordial black hole constraints, which may offer an additional argument in their favor.
arXiv (Cornell University), Jan 26, 2023
Primordial black holes form in the early Universe and constitute one of the most viable candidate... more Primordial black holes form in the early Universe and constitute one of the most viable candidates for dark matter. The study of their formation process requires the determination of a critical energy density perturbation threshold δ c , which in general depends on the underlying gravity theory. Up to now, the majority of analytic and numerical techniques calculate δ c within the framework of general relativity. In this work, using simple physical arguments we estimate semi-analytically the PBH formation threshold within the framework of quantum gravity, working for concreteness within loop quantum gravity (LQG), which constitutes a non-perturbative and backgroundindependent quantization of general relativity. In particular, for low mass PBHs formed close to the quantum bounce, we find a reduction in the value of δ c up to 50% compared to the general relativistic regime quantifying for the first time to the best of our knowledge how quantum effects can influence PBH formation within a quantum gravity framework. Finally, by varying the Barbero-Immirzi parameter γ of LQG we show its effect on the value of δ c while using the observational/phenomenological signatures associated to ultra-light PBHs, namely the ones affected by LQG effects, we propose the PBH portal as a novel probe to constrain the potential quantum nature of gravity.
The phenomenology of primordial black holes (PBHs) physics and the associated PBH abundance const... more The phenomenology of primordial black holes (PBHs) physics and the associated PBH abundance constraints, can be used in order to probe the physics of the early Universe. In this work, we investigate the PBH formation during the standard radiation-dominated era by studying the effect of an early F (R) modified gravity phase with a bouncing behavior which is introduced to avoid the initial spacetime singularity problem. In particular, we calculate the energy density power spectrum at horizon crossing time and then we extract the PBH abundance in the context of peak theory as a function of the parameter α of our F (R) gravity bouncing model at hand. Interestingly, we find that in order to avoid PBH overproduction at formation time, namely Ω PBH,f < 1, α should lie within the range α ≤ 10 −17 M 2 Pl. This constraint can be translated to a constraint on the energy scale at the onset of the Hot Big Bang (HBB) phase, H RD ∼ √ α /2 which can be recast as H RD ≤ 2 × 10 −8 M Pl .
The phenomenology of primordial black holes (PBHs) physics and the associated PBH abundance const... more The phenomenology of primordial black holes (PBHs) physics and the associated PBH abundance constraints, can be used in order to probe the physics of the early Universe. In this work, we investigate the PBH formation during the standard radiation-dominated era by studying the effect of an early F (R) modified gravity phase with a bouncing behavior which is introduced to avoid the initial spacetime singularity problem. In particular, we calculate the energy density power spectrum at horizon crossing time and then we extract the PBH abundance in the context of peak theory as a function of the parameter α of our F (R) gravity bouncing model at hand. Interestingly, we find that in order to avoid PBH overproduction at formation time, namely Ω PBH,f < 1, α should lie within the range α ≤ 10 −17 M 2 Pl. This constraint can be translated to a constraint on the energy scale at the onset of the Hot Big Bang (HBB) phase, H RD ∼ √ α /2 which can be recast as H RD ≤ 2 × 10 −8 M Pl .
The gravitational potential of initially Poisson distributed primordial black holes (PBH) can ind... more The gravitational potential of initially Poisson distributed primordial black holes (PBH) can induce a stochastic gravitational-wave background (SGWB) at second order in cosmological perturbation theory. This SGWB was previously studied in the context of general relativity (GR) and modified gravity setups by assuming a monochromatic PBH mass function. Here we extend the previous analysis in the context of GR by studying the aforementioned SGWB within more physically realistic regimes where PBHs have different masses. In particular, starting from a power-law cosmologically motivated primordial curvature power spectrum we extract the extended PBH mass function and the associated to it PBH gravitational potential which acts as the source of the scalar induced SGWB. At the end, by taking into account the dynamical evolution of the PBH gravitational potential during the transition from the matter era driven by PBHs to the radiation era we extract the respective GW signal today. Interestingly, in order to trigger an early PBH-dominated era and avoid the GW constraints at BBN we find that the spectral index n s of our primordial curvature power spectrum should be within the narrow range n s ∈ [1.1482, 1, 1493] while at the same time the GW signal is found to be potentially detectable by LISA.
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Drafts by Theodoros PAPANIKOLAOU
Papers by Theodoros PAPANIKOLAOU