(ii) Large-Angle CMB Anomalies
The low-multipole (<30\ell < 30) region of the Cosmic Microwave Background (CMB) temperature power spectrum exhibits several statistically anomalous features. These include:
The lack of angular correlation at scales above 6060^\circ [7],
Hemispherical power asymmetry [8],
The alignment of the quadrupole and octupole modes [9].
While each individual anomaly has modest statistical significance (~2--3), the collective probability of all anomalies arising within CDM is estimated to be less than 0.1% [10]. These features suggest a possible breakdown of statistical isotropy and challenge the assumption of Gaussian, scale-invariant primordial perturbations.
Attempts to explain them through cosmic topology [11], inflationary relics [12], or anisotropic expansion models [13] remain inconclusive or insufficiently predictive.
(iii) Early Massive Galaxy Formation
High-redshift observations from JWST and earlier instruments have revealed the existence of galaxies with stellar masses exceeding 1010M10^{10} \, M_\odot at redshifts z10z \gtrsim 10 [14,15]. Such mature structures appear inconsistent with CDM expectations for the timeline of structure formation under standard initial conditions and hierarchical clustering.
Simulations within CDM typically predict significant galaxy assembly only after z6z \sim 6 [16]. Proposed explanations range from enhanced star formation efficiency [17] to primordial non-Gaussianities or modified initial power spectra [18], but these often introduce new degrees of freedom without resolving other cosmological tensions.
Summary of Motivations
