  The investigation of emission line regions within active galaxies (AGNs) has
a rich and extensive history, now extending to the use of AGNs and quasars as
"standardizable" cosmological indicators, shedding light on the evolution of
our universe. As we enter the era of advanced observatories, such as the
successful launch of JWST and the forthcoming Vera C. Rubin Observatory's
Legacy Survey of Space and Time (LSST), the landscape of AGN exploration across
cosmic epochs is poised for exciting advancements. In this work, we delve into
recent developments in AGN variability research, anticipating the substantial
influx of data facilitated by LSST. The article highlights recent strides made
by the AGN Polish Consortium in their contributions to LSST. The piece
emphasizes the role of quasars in cosmology, dissecting the intricacies of
their calibration as standard candles. The primary focus centers on the
relationship between the broad-line region size and luminosity, showcasing
recent breakthroughs that enhance our comprehension of this correlation. These
breakthroughs encompass a range of perspectives, including spectroscopic
analyses, photoionization modeling, and collaborative investigations with other
cosmological tools. The study further touches on select studies, underlining
how the synergy of theoretical insights and advancements in observational
capabilities has yielded deeper insights into these captivating cosmic
entities.
