arXiv subjects

Seokcheon Lee

Publications and source records attributed to Seokcheon Lee.

At least 37 records · Page 2Linked to original sources

arXiv2025

Geometric Interpretation of the Redshift Evolution of H_0(z)

Recent analyses of the Master Type Ia supernova (SN Ia) sample have revealed a mild redshift dependence in the inferred local Hubble parameter, often expressed as tilde{H}_0(z) = H_0 (1+z)^{-α}, where αquantifies possible departures from the standard cosmological time dilation relation. In this work, we show that such an empirical scaling can be interpreted as a purely geometric effect arising from a small, gauge-dependent normalization of cosmic time within the Robertson-Walker metric. This interpretation naturally unifies the observed redshift evolution of tilde{H}_0(z) and the corresponding deviation in SN Ia light-curve durations under a single geometric time-normalization framework. We demonstrate that this mapping leaves all background distances--linked to the Hubble radius in the general-relativistic frame--unchanged, while the apparent evolution in SN Ia luminosity distances arises from the redshift dependence of the Chandrasekhar mass. The result provides a unified and observationally consistent explanation of the mild Hubble-tension trend as a manifestation of the geometric structure of cosmic time rather than a modification of the expansion dynamics.

physics.gen-ph↗

arXiv2025

Revisiting Varying Speed of Light in Cosmology: Insights from the Friedmann-Lemaître-Robertson-Walker Metric

In the Friedmann-Lemaître-Robertson-Walker metric, a varying speed of light (VSL) reflects a change in the clock rate across hypersurfaces, described by the lapse function. This variation is not a dynamical field evolution but a consequence of coordinate choice, as the cosmic time coincides with the proper time of comoving observers due to the Weyl postulate. From an action principle including $\tilde c$, we derive that $\tilde c$ does not have its dynamics but imposes a constraint on the scale factor $a(t)$, indicating that it is not an independent degree of freedom. This insight reframes the VSL concept as a manifestation of gauge freedom in general relativity, wherein physical laws remain invariant under smooth coordinate transformations. Here, gauge refers to the freedom of choosing the temporal coordinate (\textit{e.g.}, setting the lapse $N(t) \neq 1$), which determines how the speed of light appears in the cosmological equations. Recognizing $\tilde c$ as a coordinate-dependent quantity offers a new interpretation of cosmological time and observational tensions, such as the Hubble tension, without invoking new physical fields. This redefinition opens a novel theoretical pathway in interpreting cosmic expansion within a consistent relativistic framework.

physics.gen-ph↗

arXiv2024

3+1 formalism of the minimally extended varying speed of light model

The $3+1$ formalism provides a structured approach to analyzing spacetime by separating it into spatial and temporal components. When applied to the Robertson-Walker metric, it simplifies the analysis of cosmological evolution by dividing the Einstein field equations into constraint and evolution equations. It introduces the lapse function $N$ and the shift vector $N^i$, which control how time and spatial coordinates evolve between hypersurfaces. In standard model cosmology, $N = 1$ and $N^i = 0$ for the Robertson-Walker metric. However, the $N$ becomes a function of time when we apply the metric to the minimally extended varying speed of light model. This approach allows for a more direct examination of the evolution of spatial geometry and offers flexibility in handling scenarios where the lapse function and shift vector vary. In this manuscript, we derive the model's $N$ and $N^i$, along with the constraint and evolution equations, and demonstrate their consistency with the existing Einstein equations. We have shown in a previous paper that the possibility of changes in the speed of light in the Robertson-Walker metric is due to cosmological time dilation. Through the $3+1$ formalism, we can make the physical significance more explicit and demonstrate that it can be interpreted as the lapse function. From this, we show that the minimally extended varying speed of light model is consistent.

gr-qc↗

arXiv2024

The significance of measuring cosmological time dilation in the Dark Energy Survey Supernova Program

In the context of the dispersion relation $c = λν$ and considering an expanding universe where the observed wavelength today is redshifted from the emitted wavelength by $λ_{0} = λ_{\text{emit}} (1+z)$, to keep $c$ constant, it must be that $ν_{0} = ν_{\text{emit}} /(1+z)$. However, although the theory for wavelength in the RW metric includes the cosmological redshift, the same is not simply deduced for frequency (the inverse of time). Instead, cosmological time dilation $T_{0} = T_{\text{emit}} (1+z)$ is an additional assumption made to uphold the hypothesis of constant speed of light rather than a relation directly derived from the RW metric. Therefore, verifying cosmological time dilation observationally is crucial. The most recent data on supernovae for this purpose was released recently by the Dark Energy Survey. Results from the i-band specifically support variations in the speed of light within 1-$σ$. We used these observations to investigate variations in various physical quantities, including $c$ and $G$, using the minimally extended varying speed of light model. The speed of light was $0.4$\% to $2.2$\% slower, and Newton's constant may have decreased by $1.7$\% to $8.4$\% compared to their current values at redshift $2$. These findings, consistent with previous studies, hint at resolving tensions between different $Λ$CDM cosmological backgrounds but are not yet conclusive evidence of a varying speed of light, as the full-band data aligns with standard model cosmology. However, the data remains valuable for testing variations in fundamental constants over cosmic time. Future analyses, particularly with more refined redshift data, may provide clearer insights into these potential changes.

physics.gen-ph↗

arXiv2024

The minimally extended Varying Speed of Light (meVSL)

Even though there have been the various varying speed of light (VSL) cosmology models, they remain out of the mainstream because of their possible violation of physics laws built into fundamental physics. In order to be the VSL as a viable theory, it should inherit the success of special relativity including Maxwell equations and thermodynamics at least. Thus, we adopt that the speed of light, $\tilde c$ varies for the cosmic time not for the local time, i.e., $\tilde c[z]$ where $z$ is the cosmological redshift. When one describes the background FLRW universe, one can define the constant-time hypersurface by using physical quantities such as temperature, density, and $\tilde c$. It is because they evolve in time, and the homogeneity of the Universe demands that they must equal at the equal cosmic time. The variation of $\tilde c$ accompanies the joint variations of all related physical constants in order to satisfy the Lorentz invariance, thermodynamics, and Bianchi identity. We call this VSL model as a "minimally extended VSL (meVSL)". We derive cosmological observables of meVSL and obtain the constraints on the variation of $\tilde c$ by using the current observations. Interestingly, $z$ and all geometrical distances except the luminosity distance of meVSL are the same as those of general relativity. However, the Hubble parameter of meVSL is rescaled as $H = (1+z)^{-b/4} H^{(\rm GR)}$ which might be used as a solution for the tension of the Hubble parameter measurements. In this manuscript, we provide the main effects of meVSL on various cosmological observations including BBN, CMB, SZE, BAO, SNe, GWs, H, SL, and $Δα$.

astro-ph.CO↗

arXiv2024

Constraints on the minimally extended varying speed of light model using the Pantheon+ dataset

In the context of the minimally extended varying speed of light (meVSL) model, both the absolute magnitude and the luminosity distance of type Ia supernovae (SNe Ia) deviate from those predicted by general relativity (GR). Using data from the Pantheon$+$ survey, we assess the plausibility of various dark energy models within the framework of meVSL. Both the constant equation of state (EoS) of the dark energy model ($ω$CDM) and the Chevallier-Polarski-Linder (CPL) parameterization model ($ω= ω_0 + ω_a (1-a)$) indicate potential variations in the cosmic speed of light at the 1-$σ$ confidence level. For $Ω_{\m0} = 0.30, 0.31$, and 0.32 with $(ω_0 \,, ω_a) = (-1 \,, 0)$, the 1-$σ$ range of $\dot{c}_0/c_0 \, (10^{-13} \, \text{yr}^{-1}) $ is (-8.76 \,, -0.89), (-11.8 \,, 3.93), and (-14.8 \,, -6.98), respectively. Meanwhile, the 1-$σ$ range of $\dot{c}_0/c_0 (10^{-12} \, \text{yr}^{-1}) $ for CPL dark energy models with $-1.05 \leq ω_{0} \leq -0.95$ and $0.28 \leq Ω_{\m0} \leq 0.32$, is (-6.31\,, -2.98). The value of $c$ at $z = 3$ can exceed that of the present by $0.2 \sim 3$ \% for $ω$CDM models and $5 \sim 13$ \% for CPL models. Additionally, for viable models except for the CPL model with $Ω_{\m0} = 0.28$, we find $-25.6 \leq \dot{G}_0/G_0 \, (10^{-12} \, \text{yr}^{-1}) \leq -0.36$. For this particular model, we obtain an increasing rate of the gravitational constant within the range $1.65 \leq \dot{G}_0/G_0 \, (10^{-12} \, \text{yr}^{-1}) \leq 3.79$. We obtained some models that do not require dark matter energy density through statistical interpretation. However, this is merely an effect of the degeneracy between model parameters and energy density and does not imply that dark matter is unnecessary.

astro-ph.CO↗

arXiv2024

Cosmography of the minimally extended Varying Speed of Light Model

Cosmography, as an integral branch of cosmology, strives to characterize the Universe without relying on pre-determined cosmological models. This model-independent approach utilizes Taylor series expansions around the current epoch, providing a direct correlation with cosmological observations and the potential to constrain theoretical models. Cosmologists can describe many measurable aspects of cosmology by using various combinations of cosmographic parameters. The varying speed of light model can be naturally implemented, provided that we do not make any further assumptions from the Robertson-Walker metric for cosmological time dilation. Therefore, we apply cosmography to the so-called minimally extended varying speed of light model. In this case, other cosmographic parameters can be used to construct the Hubble parameter for both the standard model and the varying speed-of-light model. On the other hand, distinct combinations of cosmographic values for the luminosity distance indicate the two models. Hence, luminosity distance might provide a method to constrain the parameters in varying speed-of-light models.

physics.gen-ph↗

arXiv2024

Review on the minimally extended varying speed of light model

It has been known that dimensional constants such as $\hbar$, $c$, $G$, $e$, and $k$ are merely human constructs whose values and units vary depending on the chosen system of measurement. Therefore, the time variation of dimensional constants lacks operational significance due to their dependence on them. It is well-structured and represents a valid discussion. However, this fact only becomes a meaningful debate within the context of a static or present universe. As well-established theoretically and observationally, the current universe is undergoing accelerated expansion, wherein dimensional quantities, like the wavelength of light, also experience redshift phenomena elongating over cosmic time. In other words, in an expanding universe, dimensional quantities of physical parameters vary with cosmic time. From this perspective, there exists the possibility that dimensional constants, such as the speed of light, could vary with the expansion of the universe. In this review paper, we contemplate under what circumstances the speed of light may change or remain constant over cosmic time, and discuss the potential for distinguishing these cases observationally.

physics.gen-ph↗

arXiv2023

The cosmological evolution condition of the Planck constant in the varying speed of light models through adiabatic expansion

There have been various varying speed of light (VSL) models with one free parameter, $b$, to characterize the time variation of the speed of light as a function of a scale factor, $c = c_0a^{b/4}$, based on the expanding universe. One needs to induce cosmological evolutions of other physical constants and quantities having different powers of scale factor as a function of $b$ to satisfy all known local physics laws, including special relativity, thermodynamics, and electromagnetic force. These models should be based on the Friedmann-Lemaître-Robertson-Walker metric satisfying the isotropic and homogeneous three-space known as the cosmological principle. Adiabaticity is a necessary condition to keep homogeneity and isotropy because a net energy flux would falsify the isotropy if there is a preferential energy flow direction. It also might forge homogeneity if the outward (inward) flow is isotropic. Thus, any VSL model based on the expanding universe should preserve an adiabatic expansion condition to be a viable model. We show that this condition specifies the cosmological evolution of the Planck constant as $\hbar = \hbar_0 a^{-b/4}$.

astro-ph.CO↗

arXiv2023

Constraint on the minimally extended varying speed of light using time dilations in type Ia supernovae

The Friedmann-Lemaître-Robertson-Walker model establishes the correlation between redshifts and distances. It has a metric expansion of space. As a result, the wavelength of photons propagating through the expanding space is stretched, creating the cosmological redshift, $z$. It also relates the frequency of light detected by a local observer to that emitted from a distant source. In standard cosmology (\textit{i.e.}, a constant speed light model), this relation is given by a factor $1/(1+z)$. However, this ratio is modified in the minimally extended varying speed of light model (meVSL, $c = c_0 a^{b/4}$) as $1/(1+z)^{1-b/4}$. This time dilation effect is detected as the observed rate of the time variation in the intensity of emitted radiation. The spectra of type Ia supernovae (SNe Ia) provide a reliable way to measure the apparent aging rate of distant objects. We use data on 13 high-redshift ($0.28 \leq z \leq 0.62$) SNe Ia to obtain $b = 0.198 \pm 0.415$ at the $1$-$σ$ confidence interval. The current data is too sparse to give meaningful constrain on the meVSL and cannot distinguish the meVSL model from the standard model.

astro-ph.CO↗

arXiv2023

Constraining minimally extended varying speed of light by cosmological chronometers

At least one dimensionless physical constant (i.e., a physically observable) must change for the cosmic time to make the varying speed of light (VSL) models phenomenologically feasible. Various physical constants and quantities also should be functions of cosmic time to satisfy all known local laws of physics, including special relativity, thermodynamics, and electromagnetism. Adiabatic condition is another necessary condition to keep the homogeneity and isotropy of three-dimensional space. To be a self-consistent theory, one should consider cosmic evolutions of physical constants and quantities when one derives Einstein's field equations and their solutions. All these conditions are well satisfied in the so-called minimally extended varying speed of light (meVSL) model. Unlike other VSL models, we show that the redshift-drift formula of the meVSL model is the same as a standard model. Therefore, we cannot use this as an experimental tool to verify the meVSL. Instead, one can still use the cosmological chronometers (CC) as a model-independent test of the meVSL. The current CC data cannot distinguish meVSL from the standard model (SM) when we adopt the best-fit values (or Gaussian prior) of the present values of $H_0$ and of $Ω_{m0}$ from the Planck mission. However, the CC data prefer the meVSL when we choose Pantheon 22 data.

astro-ph.CO↗

arXiv2023

A viable varying speed of light model in the RW metric

The Robertson-Walker (RW) metric allows us to apply general relativity to model the behavior of the Universe as a whole (i.e., cosmology). We can properly interpret various cosmological observations, like the cosmological redshift, the Hubble parameter, geometrical distances, and so on, if we identify fundamental observers with individual galaxies. That is to say that the interpretation of observations of modern cosmology relies on the RW metric. The RW model satisfies the cosmological principle in which the 3-space always remains isotropic and homogeneous. One can derive the cosmological redshift relation from this condition. We show that it is still possible for us to obtain consistent results in a specific time-varying speed-of-light model without spoiling the success of the standard model. The validity of this model needs to be determined by observations.

physics.gen-ph↗

arXiv2022

GMM Clustering for In-depth Food Accessibility Pattern Exploration and Prediction Model of Food Demand Behavior

Understanding the dynamics of food banks' demand from food insecurity is essential in optimizing operational costs and equitable distribution of food, especially when demand is uncertain. Hence, Gaussian Mixture Model (GMM) clustering is selected to extract patterns. The novelty is that GMM clustering is applied to identify the possible causes of food insecurity in a given region, understanding the characteristics and structure of the food assistance network in a particular region, and the clustering result is further utilized to explore the patterns of uncertain food demand behavior and its significant importance in inventory management and redistribution of surplus food thereby developing a two-stage hybrid food demand estimation model. Data obtained from a food bank network in Cleveland, Ohio, is used, and the clusters developed are studied and visualized. The results reveal that this proposed framework can make an in-depth identification of food accessibility and assistance patterns and provides better prediction accuracies of the leveraged statistical and machine learning algorithms by utilizing the GMM clustering results. Also, implementing the proposed framework for case studies based on different levels of planning led to practical results with remarkable ease and comfort intended for the respective planning team.

stat.AP↗

arXiv2021

Cosmic distance duality as a probe of minimally extended varying speed of light

We obtain the current constraint on the minimally extended varying speed of light (meVSL) model by analyzing cosmic distance duality relation (CCDDR) of it, $D_{L}/D_{A}(1+z)^{-2} = (1+z)^{b/8}$. We use the Pantheon type Ia supernova (SNIa) data, the Hubble parameter $H(z)$ using the cosmic chronometers approach, and the cosmic microwave background (CMB) distance priors from the latest Planck data. We find that the current data show the 1-$σ$ deviation from the standard CCDDR. Thus, this provides the constraint on meVSL model and future precision observations might be able to put stronger constraints on it.

astro-ph.CO↗

arXiv2021

Constraints on the time variation of the speed of light using Strong lensing

Due to the recent growth of discoveries of strong gravitational lensing (SGL) systems, one can statistically study both lens properties and cosmological parameters from 161 galactic scale SGL systems. We analyze meVSL model with the velocity dispersion of lenses by adopting the redshift and surface mass density depending power-law mass model. Analysis shows that meVSL models with various dark energy models including $Λ$CDM, $ω$CDM, and CPL provide the negative values of meVSL parameter, $b$ when we put the prior to the $Ω_{m 0}$ value from Planck. These indicate the faster speed of light and the stronger gravitational force in the past. However, if we adopt the WMAP prior on $Ω_{m0}$, then we obtain the null results on $b$ within 1-$σ$ CL for the different dark energy models.

astro-ph.CO↗

arXiv2020

The viable f(G) gravity models via reconstruction from the observations

We reconstruct the viable f(G) gravity models from the observations and provide the analytic solutions that well describe our numerical results. In order to avoid unphysical challenges that occur during the numerical reconstruction, we generalize f(G) models into f(GA), which is the simple extension of f(G) models with the introduction of a constant A parameter. We employ several observational data together with the stability condition, which reads d2f/dG2 > 0 and must be satisfied in the late-time evolution of the universe, to give proper initial conditions for solving the perturbation equation. As a result, we obtain the analytic functions that match the numerical solutions. Furthermore, it might be interesting if one can find the physical origin of those analytic solutions and its cosmological implications.

astro-ph.CO↗

arXiv2019

Dark Ultra-Light Scalars and Cosmic Parity Violation

If the dark sector of the Universe consists of ultra-light scalars, their coupling to photon via a Chern-Simons term would induce a rotation of the polarization plane of the cosmic microwave background (CMB). This rotation would convert E -mode polarization into B -mode polarization, resulting in new CMB BB correlation and parity-violating TB and EB cross correlations. We review the subject giving details about the derivation of the rotational effects and summarizing the possible signals in current and future CMB B-mode experiments.

astro-ph.CO↗

arXiv2019

New Policy Design for Food Accessibility to the People in Need

Food insecurity is a term used to measure hunger and food deprivation of a large population. As per the 2015 statistics provided by Feeding America - one of the largest domestic hunger-relief organizations in the United States, 42.2 million Americans live in food insecure households, including 29.1 million adults and 13.1 million children. This constitutes about 13.1% of households that are food insecure. Food Banks have been developed to improve food security for the needy. We have developed a novel food distribution policy using suitable welfare and poverty indices and functions. In this work, we propose an equitable and fair distribution of donated foods as per the demands and requirements of the people, thus ensuring minimum wastage of food (perishable and non-perishable) with focus towards nutrition. We present results and analysis based on the application of the proposed policy using the information of a local food bank as a case study. The results show that the new policy performs better than the current methods in terms of population being covered and reduction of food wastage obtaining suitable levels of nutrition.

econ.GN↗