Speaker
Description
Recent cosmological observations continue to challenge the standard $\Lambda$CDM paradigm through persistent tensions in the Hubble constant $H_0$, the clustering amplitude $S_8$, and possible hints of nontrivial late-time dark-energy dynamics. In this talk, we discuss two complementary studies showing how sign transitions in the effective vacuum sector may arise naturally from modified gravity and higher-dimensional geometry. We first present new solution branches in teleparallel $f(T)$ gravity, focusing on the exponential infrared model $f(T)=T e^{T_*/T}$, where $\beta=T_*/T_0$ controls deviations from $\Lambda$CDM. While the positive-$\beta$ branch has been widely explored, the previously overlooked negative-$\beta$ branch exhibits rich and viable cosmological behavior. It predicts an effective dark-energy density that changes sign at a data-driven transition redshift, emerging around $z_\dagger\sim1.5$, rather than being imposed by hand, while remaining consistent with CMB and local gravity constraints through an effective screening mechanism [1]. Interestingly, cosmic acceleration can begin while the effective dark-energy density is still negative, highlighting the nontrivial role of modified gravitational dynamics. Adding a cosmological constant further enlarges the viable parameter space and may allow moderated or transient late-time acceleration. We then discuss, this time within unmodified general relativity, a purely geometric realization of $\Lambda_{\rm s}$CDM using a $(1+3+n)$-dimensional Kaluza--Klein framework with compact extra dimensions of constant intrinsic curvature. For a stabilized internal manifold of radius $s_*$, the internal curvature contributes $\tilde{\chi}=-n(n-1)k_{\rm int}/(2s_*^2)$, which combines with $\tilde{\Lambda}$ to define an effective vacuum term $\tilde{\gamma}=\tilde{\Lambda}+\tilde{\chi}$, yielding a background expansion degenerate with $\Lambda$CDM. A rapid transition $s_{\rm p}\rightarrow s_0$ can shift $\tilde{\gamma}$ abruptly and change its sign. Since $G_{\rm 4D}\propto s^{-n}$, the same event induces a correlated step in gravity strength, providing a key observational discriminator[2]. Together, these results show that sign-changing dark-energy density may emerge from theoretically well-established frameworks, suggesting new insight into cosmic acceleration and current cosmological tensions.
[1]Ö.Akarsu, B.Bulduk, A.De Felice, N.Katırcı and N.M.Uzun,``Unexplored regions in teleparallel $f(T)$ gravity: Sign-changing dark energy density,'' Phys. Rev. D 112 (2025), 083532, arXiv:2410.23068
[2] Ö.Akarsu, B.Bulduk, N.Katırcı, E.Özülker and L.Perivolaropoulos,``Constructing $\Lambda_{\rm s}$CDM via compact, curved extra dimensions: Coupled transitions in the effective $\Lambda_{\rm 4D}$ and $G_{\rm 4D}$,'' in preparation.