The paper discusses the expansion history of the universe, focusing on the last ten E-foldings and their implications for cosmic microwave background (CMB) observations. It highlights how small historical perturbations, such as those caused by sound and temperature variations, can influence the CMB, effectively retaining a memory of these events. The sensitivity to the microphysics of components, including their sound speed, is crucial for understanding these perturbations. This analysis contributes to a deeper comprehension of the universe's expansion dynamics. The discussion emphasizes the importance of CMB data in revealing the universe's history.
