Galaxies in the early universe viewed by the ALMA telescope

Article By:

Ferrara, Andrea Scuola Normale Superiore, Pisa, Italy.

Vallini, Livia Department of Physics and Astronomy, University of Bologna, Italy.

Last reviewed:2016

DOI:https://doi.org/10.1036/1097-8542.YB150699

The big bang left the universe in a very simple state. The initially very hot hydrogen and helium mixture (no heavier elements were present) was almost perfectly homogeneously distributed. As the gas cooled due to cosmic expansion, it accumulated in already formed gravitationally bound dark-matter structures, or “halos.” Stars appeared in halos only at a time when the cosmic gas had cooled down to a temperature of a few tens of kelvins, at a redshift of z ∼ 30. (In cosmology, the time elapsed from the big bang is parametrized with the redshift, z. The redshift of a source is determined from the shift of its spectral features to longer wavelengths, primarily as a result of cosmic expansion. Higher values of z correspond to earlier times. For example, the redshifts z = 30 and z = 6 correspond to 10⁸ and 10⁹ years after the big bang, respectively.) These first stellar agglomerates were tiny compared to present-day galaxies. They contained less than a million stars, the size of a globular cluster in our Galaxy today. However, these building blocks merged together after collisions driven by mutual gravity, raising the characteristic mass of galaxies with time. See also: Big bang theory; Cosmology; Dark matter; Galaxy, external; Galaxy formation and evolution; Redshift; Star; Star clusters; Universe

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