The Megamaser Cosmology Project Measures the Age of the Universe
The Hubble Ultra Deep Field of galaxies; the smallest and reddest of the galaxies seen here date from when the universe was only about 800 million years old. The value of a key parameter of big bang cosmology, the constant that provides the age of the universe, has been in dispute recently because different measurement methods give significantly different results. Astronomers have now used a new method, megamaser emission from six distant galaxies, to arrive a third and arguably more accurate value.
A maser, like a laser, is a source of bright, monochromatic electromagnetic radiation, with the difference being that maser radiation is not optical light but rather longer wavelength microwave radiation. Dense molecular clouds in interstellar space sometimes produce natural masers when specific molecules (water and OH are two examples) or atoms are stimulated by the local conditions to emit very narrow-line radiation.
Such astronomical masers were first identified in space over fifty years ago, and have since been found in many locations in our Milky Way as well as in other galaxies, with the most spectacular examples found in regions of active star formation. In some cases the energy emitted in a single maser line exceeds the emission from the Sun over its entire visible spectrum making masers valuable diagnostic probes of their local conditions. These "megamasers" can be found in the nuclear regions of galaxies with active supermassive black holes and their brightness makes them potentially useful tools for cosmological studies.
Ninety years after Edwin Hubble discovered the systematic motions of galaxies and George Lemaitre explained them as cosmic recession using Einstein’s relativity equations, observational cosmology today is facing a challenge. No precise and consistent value of the expansion, as quantified by the Hubble Constant ("Ho"), has been found. Values deduced from the properties of galaxies or from the cosmic microwave background radiation (CMBR) are each precise -- but they disagree with each other at roughly the ten percent level and observational errors, although possible, seem too small to account for the differences. Many astronomers suspect the difference is real, reflecting something currently missing from our picture of the cosmic expansion process, perhaps connected with the fact that the CMBR data arise from a vastly different epoch of cosmic time than do the galaxy data and reflecting something still not known about the processes that power the big bang.
The Megamaser Cosmology Project is a multi-year campaign to find, monitor, and map systems with the goal of constraining Ho to a precision of several percent with precise geometric distance measurements to water megamaser galaxies whose known recession velocities were also remeasured precisely. CfA astronomers Dom Pesce and Mark Reid are lead members of the team, which has just published its improved value for Ho of 73.9 +-3.0 (in usual units) corresponding to an age of the universe (with some assumptions) of 12.9 +-0.5 billion years. The team used their analyses of megamasers in six galaxies for this result. For comparison, other projects using measurements from galaxies have reported a consistent value, about 74.0, however the CMBR results from the Planck satellite give a value of value for Ho of about 67.4 and a corresponding age that is significantly older: 14.2 billion years. The team notes that their future megamaser observations will improve on this precision and help astronomers address this critical discrepancy.
Reference(s):
"The Megamaser Cosmology Project. XIII. Combined Hubble Constant Constraints," D. W. Pesce1, J. A. Braatz, M. J. Reid, A. G. Riess, D. Scolnic, J. J. Condon, F. Gao, C. Henkel, C. M. V. Impellizzeri, C. Y. Kuo, and K. Y. Lo, The Astrophysical Journal Letters, 891, L1, 2020.