What is Missing in the Low-redshift Intergalactic Medium?

 Understanding the intergalactic medium is crucial for comprehending structure formation and galaxy evolution. While our theoretical understanding of the high-redshift (z>2) intergalactic medium aligns well with observations, the low-redshift (z<1) intergalactic medium challenges our current understanding. Observations indicate that over 30% of the gas predicted by the standard model of the universe remains unaccounted for, and the distribution of Doppler widths in the low-redshift Lyman-alpha forest eludes accurate reproduction in all existing simulations. There are still unexplored periods spanning 5 to 10 billion years of cosmic time where measurements of the UV ionizing background and the thermal state of the intergalactic medium are lacking. Additionally, the impact of galaxy formation feedback on the intergalactic medium, particularly at low redshifts, cannot be ignored. This talk will address these pressing issues, focusing on new measurements of the thermal state of the intergalactic medium that suggest something is missing in either simulations or our theoretical understanding of the intergalactic medium.