- 3865 - COSMOLOGY - how it all got started? The James Webb Space Telescope observed a galaxy that existed about 325 million years after the Big Bang this past summer, 2021. This established a new record for the earliest galaxy ever observed. However, the JWST can only observe the brightest galaxies from this epoch, while arrays like HERA and CHIME continue to probe the “darker” regions of the early Universe.
--------------- 3865 - COSMOLOGY - how it all got started?
- The cosmic
“Dark Ages,” lasted from roughly 370,000
to 1 billion years after the Big Bang, where the Universe was shrouded with
light-obscuring neutral hydrogen. The first stars and galaxies formed during
this period (100 to 500 million years), slowly dispelling the “darkness.” This
period is known as the “Epoch of Reionization”, or “Cosmic Dawn”.
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- The
“Hydrogen Epoch of Reionization Array” (HERA), is a radio telescope dedicated
to observing the large-scale structure of the cosmos during and before thi
period of the Epoch of Reionization. It
is located in the Karoo desert in South Africa.
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- Based on
current cosmological models, the Universe began 13,800,000,000 years ago with
the Big Bang, which produced a flurry of energy and elementary particles that
slowly cooled to create the first protons and electrons which combined to form
the first hydrogen and helium atoms.
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- The leftover
“relic radiation” is observable today in the form of the “Cosmic Microwave
Background” (CMB). The missions like the
COBE, WMAP, and Planck, have mapped the faint variations in temperature that
existed 380,000 years after the Big Bang.
-
- Hubble has
observed galaxies as they existed roughly 1 billion years after the Big
Bang (13 billion years ago). This has
led to a greater understanding of how galaxies evolved and the possible role of
Dark Matter and Dark Energy in the process.
-
- However,
there is a gap between these observations of the CMB and early galaxies: the
“Dark Ages” ( 370,000 to 1 billion years after the Big Bang). This epoch cannot
be studied with conventional telescopes because photons in this period were
either part of the CMB or those released by neutral hydrogen atoms, the
21-centimeter hydrogen line.
-
- As the
first stars and galaxies gradually formed, the intense radiation they emitted
reionized much of the surrounding Universe. This led to this Epoch of
Reionization, where neutral hydrogen began to form clouds of plasma of free
electrons and protons.
-
- To map
these bubbles, HERA, and other sophisticated radio telescopes were created to
observe the hydrogen line (which has a frequency of 1,420 megahertz). This
wavelength of light is one that neutral hydrogen absorbs and emits, but ionized
hydrogen does not.
-
- Since the
Epoch of Reionization, this radiation has been redshifted by the expansion of
the Universe to a wavelength of about 2 meters (6 feet). HERA’s simple
antennas, built from chicken wire, PVC pipe, and telephone poles, are 14 meters
(46 feet) in diameter, allowing them to focus this radiation onto detectors.
-
- The backend
is consisting of a supercomputer and machine learning algorithms performing
advanced data analysis. This map could track galactic evolution from the very
early Universe to today.
-
- The results
showed that the earliest stars, which may have formed around 200 million years
after the Big Bang, contained few other elements than hydrogen and helium. The
finding is consistent with accepted models of stellar evolution, which state
that metals from lithium to uranium formed within the first generation of
stars.
-
- When these
stars collapsed after a comparatively short lifespan (hundreds of millions of
years rather than billions), these metals were shed with the stars’ outer
layers, seeding the Universe with metals that became part of subsequent
generations of stars.
-
- Astronomers
are interested in the atomic composition of these early stars since this would
show how long they took to heat the intergalactic medium (IGM) and cause
reionization to occur.
-
- A key
element is high-energy radiation (primarily X-rays) produced by binary stars
once one of them goes supernova, collapsing into a black hole or neutron star
and eventually consuming their companion. Since the earliest stars had very few
heavy elements (low metallicity), they would not have heated the surrounding
region much and produced fewer X-rays.
-
- The absence
of the signal largely rules out the “Cold Reionization” theory, which posits
that reionization had a colder starting point. Instead, the HERA researchers
suspect that the X-rays from binary stars heated the intergalactic medium (IGM)
first.
-
- Even before
reionization and by as late as 450 million years after the Big Bang, the gas
between galaxies must have been heated by X-rays. These came from binary
systems where one star loses mass to a companion black hole. Those stars must have been very low
‘metallicity,’ very few elements other than hydrogen and helium in comparison
to our sun.
-
- These
findings agree with the preliminary results from the first analysis of HERA
2022 data that hinted that alternative theories like “Cold Reionization” were
unlikely. These results were based on 18 nights of observation by Phase I of
the HERA project (about 40 antennas) and were the most sensitive observations
of the early Universe to date.
-
- This latest
is based on 94 nights of Phase I observations (between 2017 and 2018) and
demonstrates how the HERA team has improved the array’s sensitivity.
-
- This includes
a 2.1-factor increase for light emitted about 650 million years after the Big
Bang (a redshift value (z) of 7.9) and a 2.6-factor increase for radiation
emitted about 450 million years after the Big Bang (z=10.4).
-
- These latest
observations are the best evidence we have of heating of the intergalactic
medium by early galaxies. Astronomers hope to construct a 3D map of the ionized
and neutral hydrogen bubbles from 200 million to 1 billion years after the Big
Bang. There is always more to learn!
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February 7, 2023 COSMOLOGY - how
it all got started? 3865
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--- Tuesday, February 7, 2023 ---------------------------
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