H 2 , D / H and the CMBR Temperature at z = 3 . 025 Toward QSO 0347 – 3819

A new molecular hydrogen cloud has been identified at z = 3.025 in the absorption-line spectrum of the quasar 0347–3819 observed with the UVES spectrograph at the VLT/Kueyen telescope. At the same redshift numerous metal lines and the D i Ly-5, Ly-8, Ly-10, and Ly-12 lines were detected. The simultaneous analysis of metal and hydrogen lines yielded D/H = (3.75± 0.25)× 10. This value is consistent with SBBN if the baryon-to-photon ratio, η, lies within the range 4.4 × 10 ≤ η ≤ 5.3 × 10, implying Ωb h 2 100 = 0.018 ± 0.002 (1 σ c.l.). The population of the ground state rotational levels of H2 from J = 0 to 5 revealed a Galactic-type UV radiation field in the H2-bearing cloud ruling out UV pumping as an important mechanism for C. Accounting for two other important excitation mechanisms such as collisions and FIR photon absorption, we were able to measure for the first time the temperature of the cosmic background radiation at z > 3, TCMBR = 12.1 +1.7 −3.2 K, from the analysis of the C + fine-structure lines. This result supports the value of TCMBR = 10.968 ± 0.004 K predicted at z = 3.025 by the hot Big Bang cosmology.