Measurements of the deuterium abundance in the early Universe provide a sensitive test of the standard Big Bang cosmology. The probable detection of deuterium absorption by a gas cloud between us and a distant quasar suggests an abundance much greater than estimated from observations in the Milky Way, and consistent with the amount of presently observed luminous matter comprising all the baryons in the Universe. The same spectra imply a cosmic background temperature for the early Universe which is consistent with our expectations from standard Big Bang cosmology.

Primordial abundances can be measured by analyzing absorption spectra of high-redshift quasars. Radiation from quasars encounters many foreground gas clouds at various redshifts, leading to absorption lines of a variety of species which provide a sensitive probe of cloud composition. This technique allows the Astronomer to discover directly the value of D/H in a variety of very distant and fairly chemically unevolved environments, where much less time has elasped since the Big Bang and where one can verify that little stellar enrichment has occurred, hence little deuterium destruction. Furthermore, some high-column-density quasar absorption clouds (the "LLSs", which are optically thick to photoelectric absorption by hydrogen) are good sites for measuring D/H accurately. The H column density of a cloud can be measured very precisely from the optical depth at the Lyman continuum limit or from Voigt profile fitting of multiple lines of the series. A simple model of a multicomponent gas cloud, consisting of a series of velocity components, each one characterized by a velocity, a column density and spectral 'b-parameter' (b=(2kT/m)^1/2 where m is the atomic mass and T the temperature) can thereby be overconstrained by simultaneously fitting many lines of the Lyman series. Deuterium is measured by its isotopically shifted absorption lines, displaced towards the blue from the hydrogen lines by an apparent velocity of 82 km/s. The much smaller D column density is detectable and accurately measured in the lowest-order lines.

Ref: A. Songaila, L. L. Cowie, C. J. Hogan & M. Rugers
Nature Vol 368. 14 April 1994.

The prediction of the author's cloud model for deuterium-alpha (D I) absorption in Q0014+813. Crosses mark data values, and the three solid lines show model fits for D/H=10^-4 (top), 3x10^-4 (middle) and 10^-3 (bottom).

Observations obtained with the recently refurbished Hubble Space Telescope reveal strong absorption arising from single ionized helium along the line of sight to a high-redshift quasar. The strength of the absorption suggests that it may arise in a diffuse ionized IGM. The detection also confirms that substantial amounts of helium existed in the early Universe, as predicted by Big Bang nucleosynthesis theory.

Flux and wavelength calibrated FOC far-UV prism spectrum (solid line) of Q0302-003. The thin solid line gives the 1-sigma uncertainty per 10 Angstroms wavelength bin due to photon statistics. The absolute flux calibration should be accurate to within a factor of about 2. The position of the He II (304 Angstrom) line in the quasar rest frame is marked.

The intervening He II ions responsible for the detected He II (304 Angstroms) absorption are most likely located either in the Lyman forest clouds or in the ambient IGM (i.e., in the two most likely examples of primordial matter in the early Universe). Since the helium has been detected in the gas at very early epochs corresponding to redshifts of about 3.0-3.3, important information (qualitative) lending support to Big Bang nucleosynthesis theory has been obtained. Previous attempts at detecting He Gunn-Peterson absorption in the He I (584 Angstrom) line of neutral helium have been unsuccessful; helium absorption in quasars has only previously been detected in the form of weak He I lines in the case of three metal-line systems in the UV-bright object HS1700+6414 (z=2.72) during the Astro-2 mission.

That the intervening helium reveals itself strongly in the form of singly-ionized He II also lends support to the notion that the hydrogen contained in the Lyman forest clouds (and by implication the ambient IGM) is highly ionized, as implied by the proximity effect and the forest line widths.

The present lack of knowledge of the precise ionization level of the detected He II makes an accurate determination of the helium abundance of the absorbing gas impossible at present. Also, the low-resolution (about 20 Angstroms) of the author's data prevents one from being able to distinguish between the He II absorption due to line blanketing in the discrete He II lines corresponding to the Lyman forest clouds and the redshift-smeared He II Gunn-Peterson trough of a more smoothly distributed IGM.

The inference that the Universe is opaque in He II at redshifts of about 3.3 raises the possibility that the observed high value of [He II/H I] is simply a result of the IGM not yet having become re-ionized in He II at this epoch. In the conventional model, where the ionizing flux from quasars is responsible for re-ionizing the IGM through photoionization, complete re-ionization and overlapping of the individual H II (and He II) Stromgren spheres surrounding quasars is expected to occur at z>5. But because of the lower output of He II ionizing photons compared to H I and He I ionizing photons, the re-ionization of the Universe in He II could possibly lag behind that of H I and He I by a significant interval in redshift. To test this hypothesis one could carry out similar observations of quasars at lower redshifts (z=2-3) with space-borne instrumentation covering shorter UV wavelengths. Because the quasar population apparently peaks near z=2 (see plot below), the He II re-ionization is expected to also have occurred by this epoch, resulting in much weaker He II (304 Angstrom) absorption. Mapping the redshift evolution of the He II absorption in this manner would yield important clues as to the nature and the history of the intergalactic gas.

Ref: P. Jakobsen, A. Boksenberg, J.M. Deharving, P. Greenfield,
R. Jedrzejewski & F. Paresce, Nature. Vol 370. 7 July 1994.

The density of neutral hydrogen can be detected by analyzing the spectrum of a distant quasar. Neutral hydrogen absorbes Lyman-alpha photons (photons of wavelength 1216 Angstroms whose energy corresponds to the energy difference between the ground state and the first excited state of the hydrogen atom). Because of the cosmological redshift, the photons which are absorbed will have a shorter wavelength at the source and the signature of the absorption will be seen at longer wavelengths at the observer. Therefore, the spectrum of the quasar should show a depression at wavelengths on the blue side of the Lyman-alpha emission line if neutral hydrogen is present between the quasar and the observer. The magnitude of this depression is a function of the neutral hydrogen density and can be calculated using the optical depth for such absorption. The bound on the neutral hydrogen density (or Gunn- Peterson density) implies that there is very little neutral hydrogen in the intergalactic medium.

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