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Our aim is to observationally investigate the cosmic Dark Ages in order toconstrain star and structure formation models, as well as the chemicalevolution in the early Universe. Spectral lines from atoms and molecules inprimordial perturbations at high redshifts can give information about theconditions in the early universe before and during the formation of the firststars in addition to the epoch of reionisation. The lines may arise from movingprimordial perturbations before the formation of the first stars (resonantscattering lines), or could be thermal absorption or emission lines at lowerredshifts. The difficulties in these searches are that the source redshift andevolutionary state, as well as molecular species and transition are unknown,which implies that an observed line can fall within a wide range offrequencies. The lines are also expected to be very weak. Observations fromspace have the advantages of stability and the lack of atmospheric featureswhich is important in such observations. We have therefore, as a first step inour searches, used the Odin satellite to perform two sets of spectral linesurveys towards several positions. The first survey covered the band 547-578GHz towards two positions, and the second one covered the bands 542.0-547.5 GHzand 486.5-492.0 GHz towards six positions selected to test different sizes ofthe primordial clouds. Two deep searches centred at 543.250 and 543.100 GHzwith 1 GHz bandwidth were also performed towards one position. The two lowestrotational transitions of H2 will be redshifted to these frequencies fromz~20-30, which is the predicted epoch of the first star formation. No lines aredetected at an rms level of 14-90 and 5-35 mK for the two surveys,respectively, and 2-7 mK in the deep searches with a channel spacing of 1-16MHz. The broad bandwidth covered allows a wide range of redshifts to beexplored for a number of atomic and molecular species and transitions. From thetheoretical side, our sensitivity analysis show that the largest possibleamplitudes of the resonant lines are about 1 mK at frequencies <200 GHz, and afew micro K around 500-600 GHz, assuming optically thick lines and nobeam-dilution. However, if existing, thermal absorption lines have thepotential to be orders of magnitude stronger than the resonant lines. We make asimple estimation of the sizes and masses of the primordial perturbations attheir turn-around epochs, which previously has been identified as the mostfavourable epoch for a detection. This work may be considered as an importantpilot study for our forthcoming observations with the Herschel SpaceObservatory.

The first spectral line surveys searching for signals from the Dark Ages