From a theoretical perspective, the fundamental natures of dark matter and dark energy remain two of the greatest mysteries in modern physics. Observationally, an increasing body of cosmological data points to potential deviations from the standard LCDM model, highlighted by the longstanding Hubble tension, the S8 tension, and recent evidence for dynamical dark energy. These cosmological tensions may signal the presence of new physics and provide unique opportunities to deepen our understanding of dark matter, dark energy, and gravity.

Related work:
Early Modified Gravity: the phenomenology of modified gravity before recombination
Acoustic Dark Energy (ADE) model: a simple scalar field solution for the H0 tension
Acoustic Dark Energy with Planck and ACT: test ADE with latest Planck and ACT data
Early Dark Sector and Swampland: a model with string theory motivated Early Dark Energy and Dark matter interaction
Trigger Early Dark Sector: explain the Early Dark Energy coincidence problem with its interaction with dark matter
Dark Energy Tracking Structure Growth model: a phenomenological solution to the S8 tension with late time modification of structure growth at dark energy dominated epoch
Dark Axion-Dark Baryons Interactions: a natural and technically stable particle physics model of DM-DE interactions that shows apparent phantom-crossing DE suggested by DESI BAO and SNa observations

The detection of gravitational waves—both from individual sources and as a stochastic gravitational-wave background (GWB)—has opened a new window for testing fundamental physics, particularly theories of gravity. Ongoing and forthcoming gravitational-wave experiments make this field especially promising for transformative discoveries in the years ahead.

Related work:
Gravitational Wave propagation beyond General Relativity: echoes, waveform distortion, apparent luminosity distance change, polarization birefringence
Degeneracies between GR lensing and Modified Gravity: exact/partial degeneracies, correction to methods in the literature
A test of gravity with Pulsar Timing Arrays: the impact on the angular correlations of PTA observations with a general modified dispersion relation in modified gravity
Probing Parity Violation in the GWB with Astrometry: a null test on the non-vanishing EB correlation signals, the angular correlations, and detectability
Testing Gravity with Realistic Gravitational Waveforms in Pulsar Timing Arrays: the impact on the angular correlations of PTA observations with realistic non-monochromatic waveforms
First astrometric constraints on parity violation in the GWB: constraints from Gaia-DR3 and VLBA data

Following the strong evidence for a stochastic gravitational-wave background (GWB) from Pulsar Timing Arrays, an important next step is to search for its anisotropies. Similar to the case of the Cosmic Microwave Background (CMB), measurements of GWB anisotropies have the potential to yield rich insights into both fundamental physics and astrophysics.

Related work:
Analytical Estimates of GWB anisotropies from Shot Noise and Large-Scale Structure in PTA: The expected shot noise anisotropies from astrophysical sources are comparable to the current NANOGrav constraints and could be detected in the near future