Observation of 26 cm radio signals, formed due to hyperfine transition of cosmic hydrogen offer an alternative tool to the study of early universo. As for neutral epoch of the infant universo when no light was emitted, 26 cm lines are perhaps only window. However, these redshifted radio signals emitted by cosmic hydrogen in the early universo are extremely feeble and have been elusive so far. In 2018, EDGE experiment reported detection of 26 cm signals but the findings could not be independently confirmed. The main issue was instrument systematic and contamination with the other signals from the sky. The REACH Experiment is to employ unique methodology to overcome the bottleneck. It is hoped that this research group will reliably be able to detect these elusive signals in near future. If successful, REACH Experiment may bring ‘26 cm radio astronomy’ to the forefront in the study of early universo and help us much in unravelling the mysteries of early universo.
Cuando se trata del estudio de universo temprano, nombre del recién lanzado Telescopio espacial James Webb (JWST) pops up in our mind. JWST, a successor of hugely successful Hubble telescope, is a espacio-based, infrared observatory equipped to capture optical/infrared signals from the early stars and galaxies formed in the Universo soon after the Big Bang1. Sin embargo, JWST has some limitation so far as picking up signals from the neutral epoch of universo temprano está preocupado.
Table: Epochs in the history of universo desde el Big Bang
(Fuente: Filosofía de la Cosmología – Fondo 21 cm. Disponible en http://philosophy-of-cosmology.ox.ac.uk/images/21-cm-background.jpg)
Up to 380 k years after the big bang, the universo was filled with ionised gas and was fully opaque. Between 380k – 400 million years, the universo had become neutral and transparent. The epoch of reionisation started after this phase starting 400 million after the big bang.
During the neutral epoch of early universo, cuando el universo was filled with neutral gases and was transparent, no optical signal was emitted (hence called dark age). Unionized material does not emit light. This poses a challenge in study of early Universo of neutral epoch. However, microwave radiation of 21 cm wavelength (corresponding to 1420 MHz) emitted by the cold, neutral cosmic hydrogen during this epoch as a result of hyperfine transition (from parallel spin to more stable anti-parallel spin) offers opportunities to researchers. This 21 cm microwave radiation would be redshifted upon reaching Earth and will be observed at 200MHz to 10 MHz frequencies as radio waves2,3.
Radioastronomía de 21 cm: Observation of 21-centimeter cosmic hydrogen signals offers an alternative approach to the study of early universo especially of neutral epoch phase that was devoid of any light emission. This can also inform us about new physics such as distribution of matter over time, dark energy, dark matter, neutrino masses, and inflation2.
However, the 21-cm signals emitted by the cosmic hydrogen during early universo phase is elusive. It is expected to be extremely weak (about one hundred thousand times weaker than other radio signals also emanating from the sky). As a result, this approach is still in infancy.
En 2018, los investigadores informaron la detección de una señal de radio de este tipo a una frecuencia de 78 MHz cuyo perfil coincidía en gran medida con las expectativas de la señal de 21 centímetros emitida por el hidrógeno cósmico primordial.4. Pero esta detección de la señal de radio primordial de 21 cm no se pudo confirmar de forma independiente, por lo que hasta ahora no se pudo establecer la confiabilidad del experimento. El problema principal parece ser la contaminación con las señales de radio de primer plano.
El último hito es el informe del Experimento de radio para el análisis del hidrógeno cósmico (REACH) el 21 de julio de 2022. REACH utilizará un enfoque experimental novedoso para detectar estas señales de radio cósmicas débiles y esquivas, por lo que ofrece una nueva esperanza para la confirmación de señales cósmicas de 21 centímetros.
El Experimento de Radio para el Análisis del Hidrógeno Cósmico (REACH) es un experimento de cielo medio de 21 cm. Esto tiene como objetivo mejorar las observaciones mediante la gestión de los problemas que enfrentan los instrumentos relacionados con las señales sistemáticas residuales en los datos. Se centra en detectar y explicar conjuntamente la sistemática junto con los primeros planos y la señal cosmológica utilizando estadísticas bayesianas. El experimento implica observaciones simultáneas con dos antenas diferentes, un sistema de banda ultraancha (rango de corrimiento al rojo de aproximadamente 7.5 a 28) y un calibrador de receptor basado en mediciones en el campo.
This development is significant given its potential to be one of the best tools (and cost effective too vis-a-vis espacio-based observatories like James Webb) for study of early universo as well as possibility of ushering in of new fundamental physics.
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Referencias:
- Prasad U., 2021.Telescopio espacial James Webb (JWST): el primer observatorio espacial dedicado al estudio del universo primitivo. Europeo científico. Publicado el 6 de noviembre de 2021. Disponible en http://scientificeuropean.co.uk/sciences/space/james-webb-space-telescope-jwst-the-first-space-observatory-dedicated-to-the-study-of-early-universe/
- Pritchard JA y Loeb A., 2012. Cosmología de 21 cm en el siglo XXI. Reports on Progress in Physics 21 75. Disponible en https://iopscience.iop.org/article/10.1088/0034-4885/75/8/086901. Preprint en arXiv disponible en https://arxiv.org/abs/1109.6012 versión pdf https://arxiv.org/pdf/1109.6012.pdf
- Universidad de Oxford. Filosofía de la Cosmología – fondo de 21 cm. Disponible en http://philosophy-of-cosmology.ox.ac.uk/21cm-background.html
- Bowman, J., Rogers, A., Monsalve, R. et al. Un perfil de absorción centrado en 78 megahercios en el espectro promediado del cielo. Naturaleza 555, 67–70 (2018). https://doi.org/10.1038/nature25792
- de Lera Acedo, E., de Villiers, DIL, Razavi-Ghods, N. et al. El radiómetro REACH para detectar la señal de hidrógeno de 21 cm a partir del corrimiento al rojo z ≈ 7.5–28. Nat Astron (2022). https://doi.org/10.1038/s41550-022-01709-9
- Eloy de Lera Acedo 2022. Develando los misterios del Universo infantil con el radiómetro REACH. Disponible en línea en https://astronomycommunity.nature.com/posts/u
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