arXiv subjects

Yalong Nan

Publications and source records attributed to Yalong Nan.

5 recordsLinked to original sources

arXiv2026

Probing generalized equation of state for dark energy with latest cosmological probes

Baryon acoustic oscillation (BAO) measurements from DESI DR2 have strengthened indications of a time-evolving dark energy. In this paper, we investigate observational constraints on the generalized equation of state (GEoS) for dark energy, $w(a) = w_0 - w_β(a^β- 1)/β$, with latest data from UV and X-ray observations of high-redshift quasars, DESI DR2 BAO, Cosmic Microwave Background (CMB) data (including lensing) from Planck and the Atacama Cosmology Telescope (ACT), and Pantheon+ Type Ia Supernovae (SNe Ia). The full Quasar+BAO+CMB+SNe Ia data combination yields stringent constraints on the EoS parameters of $w_0=-0.840^{+0.057}_{-0.084}$, $w_β=-0.90^{+0.88}_{-0.23}$, and $β=2.2^{+2.4}_{-5.2}$. Specially, the $β$ parameter, which describe a broad range of dark energy evolution, is in perfect agreement with the Chevallier--Polarski--Linder model ($β=1$) -- the most widely used two-parameter parametrizations of dynamical dark energy. We also compare GEoS with other dynamical dark-energy parametrizations including CPL, Barboza--Alcaniz (BA) and Jassal--Bagla--Padmanabhan (JBP), and report three key findings. Firstly, we find compelling evidence for dynamical dark energy: all four EoS parametrizations yield lower information criterion (IC) values than $Λ$CDM for the full data combination, indicating a preference for EoS parametrizations under the Akaike information criterion (AIC) and the variance-based deviance information criterion (DIC). Secondly, although the unified phenomenological framework GEoS performs better than JBP, the current data do not strongly discriminate among different EoS parametrizations. Thirdly, the EoS trajectories of GEoS, CPL, and BA exhibit qualitatively similar transitions from the phantom regime at higher redshifts to the quintessence regime at late times, while the JBP parametrization exhibits a nonmonotonic evolution.

physics.gen-ph↗

arXiv2026

Testing General Relativity with Binary Lens Microlensing Events

Einstein's General Relativity (GR), which has been precisely tested on galaxy and solar system scales, is still poorly constrained in extremely weak gravitational fields (with dimensionless Newtonian potentials of $Φ\sim 10^{-8}$). In this Letter, we propose a new method to probe such a weak-field regime of gravity and measure the spatial curvature generated per unit mass. Specifically, binary microlensing events provide a laboratory to measure the parameterized post-Newtonian (PPN) parameter ($γ$), by combining the lensing mass ($M_{\rm lens}^{\rm GR}$) inferred from microlensing with the dynamical mass ($M_{\rm dyn}$) determined from interferometric astrometry. Astrometric tracking of binary lenses was previously hindered by their faintness and minuscule angular separations (typically a few milliarcseconds). This limitation has been alleviated by the deployment of GRAVITY+, which pushes the instrument's sensitivity and astrometric precision to unprecedented heights. Based on astrometric simulations of the binary microlensing event ASASSN-22av, we directly estimate $γ$ at the precision of ~23%. Such test of GR in the extremely weak-field regime can be further improved to much higher precision (~3%), based on more binary microlensing events with precise determination of the dynamical/lens mass.

gr-qc↗

arXiv2026

Predictions on the abundance of primordial black holes: results from the SMILE VLBI milli-lensing sample

Primordial black holes (PBHs) formed in the early universe represent a well-motivated dark matter candidate. However, their abundance in the intermediate-to-high mass range ($10^6$-$10^9\,M_\odot$) remains relatively less constrained. In this mass regime, the corresponding Einstein radii on the milliarcsecond scale make Very Long Baseline Interferometry (VLBI) the most direct observational probe via milli-lensing of compact radio sources. In this paper, we present predictions for the abundance of PBH dark matter ($f_{\rm PBH}$) using the large SMILE sample of $\sim 5000$ flat-spectrum compact radio sources. Under the optimistic observational conditions (with an angular resolution of $1.5\,\mathrm{mas}$, a maximum image separation of $100\,\mathrm{mas}$, and a maximum detectable flux ratio of $40$), assuming that no milli-lensing events are confirmed, we obtain the projected 95% upper limit of $f_{\rm PBH}\lesssim0.12%$ in the intermediate-to-high mass range, which would be approximately an order of magnitude tighter than previous VLBI milli-lensing limits. Broader lognormal mass functions yield correspondingly weaker projected limits. Generalising the optical depth to account for magnification bias and finite-core resolvability changes these projected limits by only about $5$%. Moreover, we show that the predicted limits are highly sensitive to a small number of confirmed events, i.e., even one confirmed milli-lensing event would relax the projected upper limit to $0.20$%, highlighting the critical importance of robust candidate confirmation in the SMILE sample. Finally, under the optimistic baryonic accretion scenario, the PBH mass growth changes the projected limits by $\lesssim 1$% within the VLBI-sensitive mass range, confirming the robustness of our results.

astro-ph.CO↗

arXiv2026

Hierarchical cosmological constraints through strong lensing distance ratio

Strong gravitational lensing provides an independent and powerful probe of cosmic expansion by directly linking observables to cosmological distances. Upcoming surveys such as LSST will discover large number of galaxy-galaxy strong lensing systems, offering a new route to precise cosmological constraints. In this paper, we propose a Fisher-like sensitivity factor to map how the cosmological information of strong-lensing distances changes across the lens-source redshift plane. Applying such factor to the distance ratio $D_{ls}/D_s$, the time-delay distance $D_{Δt}$, and the double-source-plane ratio, we determine the ``sensitivity valleys'' where an observable becomes insensitive to a given parameter. The realistically simulated LSST lens population, which largely lies outside the distance-ratio valleys, covers the most sensitive region for $(w_0,w_a)$ parameter space. We then develop a new hierarchical framework, which could calibrate the redshift evolution of lens mass-density slopes and constrain cosmological parameters simultaneously. Focusing on the LSST mock data, we demonstrate that ignoring mass-profile evolution can bias $Ω_m$ by up to $\sim 10σ$, while modeling the lens evolution could perfectly recovers the fiducial cosmology and yield stringent cosmological constraints (e.g., $ΔΩ_m \simeq 0.01$ and $Δw \simeq 0.1$ for $\sim 10^4$ lenses).

astro-ph.CO↗

arXiv2024

Multiple measurements on the cosmic curvature using Gaussian process regression without calibration and a cosmological model

In this letter, we propose an improved cosmological model-independent method to measure cosmic curvature, combining the recent measurements of transverse and line-of-sight directions in the baryon acoustic oscillations (BAO) with cosmic chronometers (CC) datasets. Considering that the CC dataset is discrete and includes only 32 $H(z)$ measurements, we apply Gaussian process (GP) regression to fit the CC dataset and reconstruct them. Our methodology, which does not need the calibration or selection of any cosmological model, provide multiple measurements of spatial curvature ($Ω_K$) at different redshifts (depending on the redshift coverage of BAO dataset). For combination of all BAO data, we find that the constraint result on cosmic curvature is $Ω_K=-0.096^{+0.190}_{-0.195}$ with $1σ$ observational uncertainty. Although the measured $Ω_K$ is in good agreement with zero cosmic curvature within 1$σ$ confidence level, our result revels the fact of a closed universe. More importantly, our results show that the obtained $Ω_K$ measurements are almost unaffected by different priors of the Hubble constant. This could help solve the issue of the Hubble tension that may be caused by inconsistencies in the spatial curvature between the early and late universes.

astro-ph.CO↗