Cosmological tensions are one of the central topics of modern cosmology. The most prominent is the Hubble tension in which the measured expansion rate of the Universe from distance ladder is more than $5 \sigma$ larger than that predicted by $\Lambda$CDM based on CMB. Recently, BAO observation from DESI also reports a $2 - 3 \sigma$ tension with $\Lambda$CDM which might point to evolving dark...
3-form dark energy shows a similar behavior to but distinguishable one from that of a scalar field in an expanding universe, and its tensorial structure allows for unique couplings that cannot be experienced by canonical scalars.
We here explore the possibility that 3-form dark energy can explain the cosmic birefringence angle β~0.3°, as suggested by observations of the CMB polarization.
We...
The discrepancy between early- and late-Universe determinations of the Hubble constant may point to physics beyond ΛCDM or to unaccounted-for systematics. Numerous late-time modifications to the expansion history have been proposed to alleviate this discrepancy, with limited success. Recent works have shown that, when the sound-horizon and supernova calibrations are held fixed,any purely...
The nearly scale-invariant primordial power spectrum provides the standard initial conditions for cosmological perturbations. However, the largest observable scales remain only weakly constrained by current CMB measurements, leaving open the possibility of deviations from scale invariance, such as an infrared (IR) cutoff in the primordial spectrum. This possibility is further motivated by the...
In modern cosmological research, using cosmological numerical simulations to study the evolution of the large-scale structure of the Universe has proven to be a highly effective technique. However, cosmological numerical simulations beyond the standard cosmological model require numerous modifications to existing cosmological simulation frameworks. They are technically complex and have a high...
Observations of the cosmic microwave background (CMB) and radio wavelengths can probe multiple evolutionary stages spanning the early to late Universe, which feature large-scale structures and long-timescale processes. These observations are therefore of fundamental importance for investigating core physical problems concerning dark matter, dark energy, and gravitational waves. Future CMB...
The detection of GW170817 and its electromagnetic counterpart placed a stringent bound on the difference between the speeds of gravitational waves and light, severely restricting viable scalar-tensor models of dark energy. In particular, large parts of Horndeski and beyond Horndeski theories appeared to be ruled out as a description of late-time cosmic acceleration. In this talk we revisit...
Dark matter may leave observable signatures in gravitational-wave detectors through a remarkably broad range of physical mechanisms. In this talk, I will discuss recent searches using data from the fourth observing run of the LIGO–Virgo–KAGRA detector network. First, I will present a direct search for ultralight dark matter, in which coherently oscillating scalar, vector, and tensor fields can...
The axion solution to the strong CP problem is only as robust as the Peccei-Quinn symmetry it relies on, and quantum gravity is expected to break it. We quantify the resulting axion quality problem using principled measures of fine-tuning, and show that the cost is severe. For the QCD axion to account for the observed dark matter, Planck-suppressed symmetry-breaking operators must be absent up...
I will discuss a new mechanism for generating primordial large-scale curvature perturbations and gravitational wave (GW) anisotropies via a first-order phase transition modulated by a light spectator field. The spectator acquires fluctuations during inflation, which come into play at a post-inflation phase transition by modulating the bubble nucleation rate. Such a modulation induces spatial...
Cosmological correlators are calculated through the in-in formalism in (quasi-)de Sitter background. Their effective description is best represented by a reduced density matrix. In this talk, I will first review the differences between the usual EFT used in scattering amplitude calculations and the Schwinger-Keldysh EFT for correlators. I will then discuss the reduced density matrix for a...
A thermal medium modifies the propagation of gauge fields, producing collective excitations with a plasmon gap. Does it affect primordial gravitational waves in the same way? For gravity, the thermal soft scale coincides with the Hubble rate, making cosmic expansion essential to answering this question. In this talk, I compute the linear response of thermal radiation, modeled by a conformal...
Primordial gravitational waves generate radiation density perturbations at second order, providing a possible connection between small-scale tensor power and primordial black hole (PBH) formation. In this talk, we revisit this connection using a finite-time semianalytic solution for arbitrary source momenta. The solution retains the regular homogeneous contribution fixed by the initial spatial...
Chiral gravitational wave backgrounds are predicted in many well-motivated mechanisms. We investigate the detection of Chiral GWBs using a network of detectors under arbitrary configurations. We also propose a dipole pulsar timing array, which demonstrates sensitivity to Chiral gravitational wave backgrounds from the nanohertz to microhertz regime.
Recent PTA analyses reporting evidence for a nanohertz common-spectrum process motivate targeted tests of whether any anisotropic component of the stochastic gravitational-wave background (SGWB) is correlated with the nearby large-scale structure (LSS), as anticipated for an astrophysical background dominated by supermassive black hole binaries. We present the first Bayesian PTA likelihood...
Although the standard model of Big Bang nucleosynthesis successfully describes the formation of light elements, it faces a well-known challenge: the observed abundance of lithium-7 is approximately three times lower than the theoretically predicted value. This work proposes a mechanism to resolve this primordial lithium deficit through the emission of baryons by evaporating primordial black...
I will introduce a neat mathematical framework that can be utilized to study, in a unified manner, different aspects of $f(R),\,f(T),\,f(G)$ or $f(Q)$ gravity. As an example of the application of this framework, I will show its applicability for a comparative analysis between $f(R)$ gravity and the trivial FLRW connection branch of $f(Q)$ gravity, with respect to the two following questions:...
Gravity from entropy proposes quantum relative entropy between the spacetime metric and the metric induced by geometry and matter fields as the fundamental action for gravity. The resulting modified Einstein equations reduce to general relativity in the weak-coupling, low-curvature limit, while departing from it at high curvature. In this talk, I derive the Friedmann-equation analogues of GfE...
The Big Bang singularity marks a fundamental breakdown of General Relativity, yet all proposed quantum gravitational remedies invoke microphysical assumptions beyond the observational reach. We show that a minimal, purely classical modification of the gravitational potential --- preserving the inverse-square law where tested, but free to depart at unmeasured scales --- is sufficient to produce...
We study the late-time expansion history of the Universe within the VCDM model, a Type-II MMG realization that preserves the successes of General Relativity while extending beyond constant vacuum energy through a minimal Hamiltonian modification, generating a time-dependent vacuum sector without introducing additional degrees of freedom. We investigate this framework within a cosmographic...
