P. J. E. Peebles, Recombination of the Primeval Plasma, Astrophys. J. 153, 1, 1968.
 G. B. Field, Excitation of the Hydrogen 21-CM Line, Proceedings of the IRE 46, 240 (1958).
 P. Tozzi, P. Madau, A. Meiksin and M. J. Rees, Radio signatures of hi at high redshift: mapping the end of the “dark ages”, Astrophys. J. 528, 597 (2000) [astro-ph/9903139]. [OpenAIRE]
 X. L. Chen and J. Miralda-Escude, The spin - kinetic temperature coupling and the heating rate due to Lyman - alpha scattering before reionization: Predictions for 21cm emission and absorption, Astrophys. J. 602, 1 (2004) [astro-ph/0303395].
 M. Ricotti, J. P. Ostriker and N. Y. Gnedin, X-ray preionization powered by accretion on the first black holes. 2: Cosmological simulations and observational signatures, Mon. Not. Roy. Astron. Soc. 357, 207 (2005) [astro-ph/0404318].
 R. Barkana and A. Loeb, In the beginning: The First sources of light and the reionization of the Universe, Phys. Rept. 349, 125 (2001) [astro-ph/0010468]. [OpenAIRE]
 S. Furlanetto, The Global 21 Centimeter Background from High Redshifts, Mon. Not. Roy. Astron. Soc. 371, 867 (2006) [astro-ph/0604040].
 S. Furlanetto, S. P. Oh and F. Briggs, Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe, Phys. Rept. 433, 181 (2006) [astro-ph/0608032].
 J. R. Pritchard and A Loeb, 21 cm cosmology in the 21st century, Rept. Prog. Phys. 75, 086901 (2012) [arXiv:1109.6012 [astro-ph.CO]].
 A. Lewis and A. Challinor, The 21cm angular-power spectrum from the dark ages, Phys. Rev. D 76, 083005 (2007) [astro-ph/0702600 [ASTRO-PH]]. [OpenAIRE]
 J. O. Burns, et al., Astrophys. J., 844, 33 (2017) [arXiv:1704.02651 [astro-ph.IM]].
 J. D. Bowman, A. E. E. Rogers, R. A. Monsalve, T. J. Mozdzen and N. Mahesh, An absorption profile centred at 78 megahertz in the sky-averaged spectrum, Nature 555, 67 (2018). [OpenAIRE]
 R. Barkana, Possible interaction between baryons and dark-matter particles revealed by the first stars, Nature 555, no. 7694, 71 (2018) [arXiv:1803.06698 [astro-ph.CO]]. [OpenAIRE]
 C. Feng and G. Holder, Enhanced global signal of neutral hydrogen due to excess radiation at cosmic dawn, Astrophys. J. 858, no. 2, L17 (2018) [arXiv:1802.07432 [astro-ph.CO]].
 J. B. Muoz and A. Loeb, A small amount of mini-charged dark matter could cool the baryons in the early Universe, Nature 557, no. 7707, 684 (2018) [arXiv:1802.10094 [astro-ph.CO]].