arXiv subjects

Recai Erdem

Publications and source records attributed to Recai Erdem.

At least 19 recordsLinked to original sources

arXiv2026

Resolution of the cosmological constant problem by unimodular gravity and signature reversal symmetry

The (old) cosmological constant problem consists of two different problems. The first is the huge discrepancy between the value of the cosmological constant deduced from observations and its value expected from cosmological constant-like theoretical contributions (such as vacuum expectation value of Higgs potential). The second problem is why the value of the cosmological constant has its particular (very small) value. It is well-known that unimodular gravity solves the first problem while it leaves the second problem unsolved. In this paper I show that the second problem may also be resolved in the context of unimodular gravity by letting our 4-dimensional spacetime be a brane in a D = 2(2n + 1) dimensional bulk and imposing the signature reversal symmetry

gr-qc↗

arXiv2025

Gravitational particle production, the cosmological tensions and fast radio bursts

In [26] it had been found that gravitational particle production (to be more specific, gravitational vacuum polarization) results in an effective increase in the directly measured value of the Hubble constant $H_0$ while it does not affect the value of the Hubble constant derived from energy densities $\bar{H}_0$. It had also been pointed out that this may explain why the Hubble constant $H_0$ determined from direct measurements (such as in SN Ia measurements) and the Hubble constant determined from indirect measurements (such as in CMB calculations in the framework of $\Lambda$CDM) are different. In the present study, first I correct a misidentification in \cite{Erdem-Universe}, namely, $\hat{H}_0=\left(\frac{\bar{H}_0}{H_0}\right)\bar{H}_0$ (rather than $\bar{H}_0$) is the value of the Hubble constant measured in CMB and BAO measurements. Then I extend the analysis to the $\sigma_8$ tension, and to determination of the Hubble constant through observations of fast radio bursts. It is observed that inclusion of the effect of gravitational vacuum polarization essentially does not neither mitigate nor exacerbate the $\sigma_8$ tension (while it mitigates or relieves the Hubble tension). This result is significant in the light of the studies in literature that question existence of a true $\sigma_8$ tension. Moreover, the present framework predicts that the value of the Hubble constant measured in fast radio bursts is $\hat{H}_0$ as in CMB and BAO measurements. This may be checked with observations in future after more precise and conclusive measurements of $\hat{H}_0$, $\bar{H}_0$, $H_0$.

gr-qc↗

arXiv2024

Gravitational particle production and the Hubble tension

The effect of gravitational particle production of scalar particles on the total effective cosmic energy density (in the era after photon decoupling till the present) is considered. The effect is significant for heavy particles. It is found that gravitational particle production results in an effective increase in the directly measured value of the Hubble constant $H_0$ while it does not affect the value of the Hubble constant in the calculation of the number density of baryons at present time that is used to calculate recombination redshift. This may explain why the Hubble constant determined by local measurements and non-local measurements (such as CMB) are different. This suggests that gravitational particle production may have a non-negligible impact on the $H_0$ tension.

gr-qc↗

arXiv2021

Curved space and particle physics effects on the formation of Bose-Einstein condensation around a Reissner-Nordström black hole

We consider two scalar fields interacting through a $χ^*χϕ^*ϕ$ term in the presence of a Reissner-Nordstrom black hole. Initially, only $χ$ particles are present. We find that the produced $ϕ$ particles are localized in a region around the black hole and have a tendency towards condensation provided that $ϕ$ particles are much heavier than the $χ$ particles. We also find that such a configuration is phenomenologically viable only if the scalars and the black hole have dark electric charges.

gr-qc↗

arXiv2021

Particle physics processes in cosmology through an effective Minkowski space formulation and the limitations of the method

We introduce a method where particle physics processes in cosmology may be calculated by the usual perturbative flat space quantum field theory through an effective Minkowski space description at small time intervals provided that the running of the effective particle masses are sufficiently slow. We discuss the necessary conditions for the applicability of this method and illustrate the method through a simple example. This method has the advantage of avoiding the effects of gravitational particle creation in the calculation of rates and cross sections i.e. giving directly the rates and the cross sections due to the scatterings or the decay processes.

gr-qc↗

arXiv2021

A metric for gravitational collapse of dust around a Schwarzschild black hole

We consider the problem of gravitational collapse of a fluid under the effect of a Schwarzschild black hole (e.g. a primordial one) that suddenly forms inside the fluid. We assume the fluid initially be a uniform dust. Starting from this configuration we obtain a class of metrics under some assumptions. We find that the metric we obtain includes the dust collapse as a subcase. After discussing some basic properties of the solution, we discuss the case of dust collapse in more detail. We find that the radial and tangential pressures outside the horizon may take positive or negative values depending on the values of the parameters.

gr-qc↗

arXiv2019

A mechanism for formation of Bose-Einstein condensation in cosmology

We introduce a toy model of scalar particles with a trilinear scalar coupling in cosmology. The trilinear coupling $ϕ^2χ$ causes production of non-relativistic $ϕ$ particles through the process $χχ\,\rightarrow\,ϕϕ$ where, initially, only relativistic $χ$ particles are present. We consider the initial times of $χχ\,\rightarrow\,ϕϕ$ and observe that the curved space effects promote formation of Bose-Einstein condensate of $ϕ$ particles.

gr-qc↗

arXiv2019

Black holes, gravitational waves and fundamental physics: a roadmap

The grand challenges of contemporary fundamental physics---dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem---all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress.

gr-qc↗

arXiv2016

Is it possible to obtain cosmic accelerated expansion through energy transfer between different energy densities?

The equation of state of an energy density may be significantly modified by coupling it to another energy density. In the light of this observation we check the possibility of producing cosmic accelerated expansion in this way. In particular we consider the case where matter is converted to radiation (or vice versa by particle physics processes). We find that cosmic accelerated expansion can be obtained in this way only if an intermediate state with negative equation of state forms during the conversion.

gr-qc↗

arXiv2014

Single scale factor for the universe from the creation of radiation and matter till the present

A scheme for incorporating the creation of radiation and matter into the cosmological evolution is introduced so that it becomes possible to merge the times before and after the creation of radiation and matter in a single scale factor in Robertson-Walker metric. This scheme is illustrated through a toy model that has the prospect of constituting a basis for a realistic model.

gr-qc↗

arXiv2011

A simple toy model for a unified picture of dark energy, dark matter, and inflation

A specific scale factor in Robertson-Walker metric with the prospect of giving the overall cosmic history in a unified picture roughly is considered. The corresponding energy-momentum tensor is identified as that of two scalar fields where one plays the roles of both inflaton and dark matter while the other accounts for dark energy. A preliminary phenomenological analysis gives an order of magnitude agreement with observational data. The resulting picture may be considered as a first step towards a single model for all epochs of cosmic evolution.

gr-qc↗

arXiv2009

Possibility of automatic Pauli-Villars-like regularization through extra dimensions

Fermions in a space with some extra dimensional reflection symmetries are considered. In this space only one Kaluza-Klein mode is observed at the scales larger than the sizes of the extra dimensions while at the smaller scales all modes become observable. The resulting picture suggests that this model has a built-in Pauli-Villars-like regularization as its inherent characteristic.

hep-th↗

arXiv2009

Finite number of Kaluza-Klein modes, all with zero masses

Kaluza-Klein modes of fermions in a 5-dimensional toy model are considered. The number of Kaluza-Klein modes that survive after integration over extra dimensions is finite in this space. Moreover the extra dimensional piece of the kinetic term induces no mass for the higher Kaluza-Klein modes on contrary to the standard lore.

hep-th↗

arXiv2008

A way to get rid of cosmological constant and zero point energy problems of quantum fields through metric reversal symmetry

In this paper a framework is introduced to remove the huge discrepancy between the empirical value of the cosmological constant and the contribution to the cosmological constant predicted from the vacuum energy of quantum fields. An extra dimensional space with metric reversal symmetry and $R^2$ gravity is considered to this end. The resulting 4-dimensional energy-momentum tensor (obtained after integration over extra dimensions) consists of terms that contain off-diagonally coupled pair of Kaluza-Klein modes. This, in turn, generically results in vanishing of the vacuum expectation value of the energy-momentum tensor for quantum fields, and offers a way to solve the problem of huge contribution of quantum fields to the vacuum energy density.

hep-th↗

arXiv2006

A symmetry for vanishing cosmological constant

Two different realizations of a symmetry principle that impose a zero cosmological constant in an extra-dimensional set-up are studied. The symmetry is identified by multiplication of the metric by minus one. In the first realization of the symmetry this is provided by a symmetry transformation that multiplies the coordinates by the imaginary number i. In the second realization this is accomplished by a symmetry transformation that multiplies the metric tensor by minus one. In both realizations of the symmetry the requirement of the invariance of the gravitational action under the symmetry selects out the dimensions given by D = 2(2n+1), n=0,1,2,... and forbids a bulk cosmological constant. Another attractive aspect of the symmetry is that it seems to be more promising for quantization when compared to the usual scale symmetry. The second realization of the symmetry is more attractive in that it is posible to make a possible brane cosmological constant zero in a simple way by using the same symmetry, and the symmetry may be identified by reflection symmetry in extra dimensions.

gr-qc↗

arXiv2006

Reconsidering extra time-like dimensions

In this study we reconsider the phenomenological problems related to tachyonic modes in the context of extra time-like dimensions. First we reconsider a lower bound on the size of extra time-like dimensions. Next we discuss the issues of spontaneous decay of stable fermions through tachyonic decays and disappearance of fermions due to tachyonic contributions to their self-energies. We find that the tachyonic modes due to extra time-like dimensions are less problematic than the tachyonic modes in the usual 4-dimensional setting because the most troublesome Feynman diagrams are forbidden once the conservation of momentum in the extra time-like dimensions is imposed.

hep-ph↗

arXiv2006

A symmetry for vanishing cosmological constant: Another realization

A more conventional realization of a symmetry which had been proposed towards the solution of cosmological constant problem is considered. In this study the multiplication of the coordinates by the imaginary number $i$ in the literature is replaced by the multiplication of the metric tensor by minus one. This realization of the symmetry as well forbids a bulk cosmological constant and selects out $2(2n+1)$ dimensional spaces. On contrary to its previous realization the symmetry, without any need for its extension, also forbids a possible cosmological constant term which may arise from the extra dimensional curvature scalar provided that the space is taken as the union of two $2(2n+1)$ dimensional spaces where the usual 4-dimensional space lies at the intersection of these spaces. It is shown that this symmetry may be realized through spacetime reflections that change the sign of the volume element. A possible relation of this symmetry to the E-parity symmetry of Linde is also pointed out.

gr-qc↗

arXiv2005

A symmetry for vanishing cosmological constant in an extra dimensional toy model

We introduce a symmetry principle that forbids a bulk cosmological constant in six and ten dimensions. Then the symmetry is extended so that it insures absence of 4-dimensional cosmological constant induced by the six dimensional curvature scalar, at least, for a class of metrics. A small cosmological constant may be induced by breaking of the symmetry by a small amount.

hep-th↗