A photon propagator on de Sitter in covariant gauges

We construct a de Sitter invariant photon propagator in general covariant gauges. Our result is a natural generalization of the Allen-Jacobson photon propagator in Feynman gauge. Our propagator reproduces the correct response to a point static charge and the one-loop electromagnetic stress-energy tensor, strongly suggesting that it is suitable for perturbative calculations on de Sitter.Comment: 15 page

Analog geometry in an expanding fluid from AdS/CFT perspective

The dynamics of an expanding hadron fluid at temperatures below the chiral transition is studied in the framework of AdS/CFT correspondence. We establish a correspondence between the asymptotic AdS geometry in the 4+1-dimensional bulk with the analog spacetime geometry on its 3+1 dimensional boundary with the background fluid undergoing a spherical Bjorken type expansion. The analog metric tensor on the boundary depends locally on the soft pion dispersion relation and the four-velocity of the fluid. The AdS/CFT correspondence provides a relation between the pion velocity and the critical temperature of the chiral phase transition.Comment: 16 pages, 2 figures, revised to match the published version, relation to static solutions clarified, the section on de Sitter boundary removed, results unchange

Modifications of gravitational interaction by the effects of quantum fields

Razmatraju se modifikacije gravitacijskog međudjelovanja uzrokovane učincima kvantnih polja. Opisan je način na koji se u dvama teorijskim pristupima javlja ovisnost gravitacijskih parametara o skali kroz regularizaciju i renormalizaciju. Ta ovisnost omogućuje primjenu metoda renormalizacijske grupe. Predviđanja koja time slijede ovise o identifikaciji proizvoljne skale renormalizacijske grupe s fizikalnim energijskim skalama u izučavanom fizikalnom kontekstu. Kako bi se smanjila proizvoljnost i eliminirala potreba za kvalitativnom identifikacijom skale, koristi se sustavna procedura određivanja iste. Ta je procedura u prvom redu vođena zahtjevom očuvanja kovarijantnosti teorije. Procedura se primjenjuje na razini jednadžbi gibanja te na razini djelovanja.Modifications of gravitational interaction by the effects of quantum fields are considered. Dependence of gravitational parameters arising from regularization and renormalization of two chosen field theoretical approaches is described. This allows for use of renormalization group methods. Predictions that follow depend on identification of arbitrary scale with respect to physical scales involved in a given physical context. In order to reduce arbitrariness and eliminate the need for qualitative scale identification, a well defined procedure is used to determine the scale. The procedure is primarily based on preserving the covariance of the theory. Procedure is applied at the level of equations of motion and at the level of the action

Renormalization group scale-setting from the action - a road to modified gravity theories

The renormalization group (RG) corrected gravitational action in Einstein-Hilbert and other truncations is considered. The running scale of the renormalization group is treated as a scalar field at the level of the action and determined in a scale-setting procedure recently introduced by Koch and Ramirez for the Einstein-Hilbert truncation. The scale-setting procedure is elaborated for other truncations of the gravitational action and applied to several phenomenologically interesting cases. It is shown how the logarithmic dependence of the Newton's coupling on the RG scale leads to exponentially suppressed effective cosmological constant and how the scale-setting in particular RG corrected gravitational theories yields the effective $f(R)$ modified gravity theories with negative powers of the Ricci scalar $R$. The scale-setting at the level of the action at the non-gaussian fixed point in Einstein-Hilbert and more general truncations is shown to lead to universal effective action quadratic in Ricci tensor.Comment: v1: 15 pages; v2: shortened to 10 pages, main results unchanged, published in Class. Quant. Gra

Determining the presence and the number of people by using a Wi-Fi signal

We present experimental results and theoretical methods for the precise determination of the presence and the number of people in an observed area by using Wi-Fi signals. Our setup does not require active cooperation of persons present in the Wi-Fi field, and relies only on the received signal strength indicator (RSSI), which is read by the detectors. We first show that the standard deviation of the measured RSSI data can be used as a practical tool to establish the presence of a person (or more persons) with high precision, in particular when the signal source is inside the measurement room. For the more difficult problem of counting the number of persons, we have employed machine learning algorithms to analyze data collected on nine different detectors and up to nine people present in our experiment. We have achieved excellent results (prediction accuracy of 98% and above) for counting already with only few detectors utilized in the analysis.Comment: are welcom

On analytical solutions of f(R) modified gravity theories in FLRW cosmologies

A novel analytical method for f(R) modified theories without matter in Friedmann-Lemaitre-Robertson-Walker spacetimes is introduced. The equation of motion for the scale factor in terms of cosmic time is reduced to the equation for the evolution of the Ricci scalar R with the Hubble parameter H. The solution of equation of motion for actions of the form of power law in Ricci scalar R, is presented with a detailed elaboration of the action quadratic in R. The reverse use of the introduced method is exemplified in finding functional forms f(R) which lead to specified scale factor functions. The analytical solutions are corroborated by numerical calculations with excellent agreement. Possible further applications to the phases of inflationary expansion and late-time acceleration as well as f(R) theories with radiation are outlined.Comment: 16 pages, 6 figures. v2: minor changes, references added. v3: minor changes, more references added. v4: version to appear in IJMPD. v5: DOI and journal reference adde