Challenges for $\Lambda$CDM: An update
  A number of challenges to the standard $\Lambda$CDM model have been emerging
during the past few years as the accuracy of cosmological observations
improves. In this review we discuss in a unified manner many existing signals
in cosmological and astrophysical data that appear to be in some tension
($2\sigma$ or larger) with the standard $\Lambda$CDM model as specified by the
Cosmological Principle, General Relativity and the Planck18 parameter values.
In addition to the well-studied $5\sigma$ challenge of $\Lambda$CDM (the Hubble
$H_0$ tension) and other well known tensions (the growth tension, and the
lensing amplitude $A_L$ anomaly), we discuss a wide range of other less
discussed less-standard signals which appear at a lower statistical
significance level than the $H_0$ tension some of them known as 'curiosities'
in the data) which may also constitute hints towards new physics. For example
such signals include cosmic dipoles (the fine structure constant $\alpha$,
velocity and quasar dipoles), CMB asymmetries, BAO Ly$\alpha$ tension, age of
the Universe issues, the Lithium problem, small scale curiosities like the
core-cusp and missing satellite problems, quasars Hubble diagram, oscillating
short range gravity signals etc. The goal of this pedagogical review is to
collectively present the current status (2022 update) of these signals and
their level of significance, with emphasis on the Hubble tension and refer to
recent resources where more details can be found for each signal. We also
briefly discuss theoretical approaches that can potentially explain some of
these signals.
