The Physical State of the Intergalactic Medium or Can We Measure Y ?

We present an argument for a lower limit to the Compton-y parameter describing spectral distortions of the cosmic microwave background (CMB). The absence of a detectable Gunn-Peterson signal in the spectra of high redshift quasars demands a high ionization state of the intergalactic medium (IGM). Given an ionizing flux at the lower end of the range indicated by the proximity effect, an IGM representing a significant fraction of the nucleosynthesis-predicted baryon density must be heated by sources other than the photon flux to a temperature >∼ few × 10 5 K. Such a gas at the redshift of the highest observed quasars, z ∼ 5, will produce a y >∼ 10 . This lower limit on y rises if the Universe is open, if there is a cosmological constant, or if one adopts an IGM with a density larger than the prediction of standard Big Bang nucleosynthesis.