Sky & Telescope - January 2025 - 8

NEWS NOTES
One potential explanation was
COSMOLOGY
Solved: The Case
of the Universe's
" Extra " Light
An artist's
impression of
New Horizons
that galactic censuses had missed a
population of faint galaxies. Some even
suggested more exotic options, such as
axions - a hypothetical particle candidate
for dark matter - which could
decay into a litany of photons.
Now, it seems, we have the answer,
USING NASA'S NEW HORIZONS,
astronomers have shown that the faint,
visible glow of apparently empty space is
due to diffuse light from distant galaxies,
with any " extra " due to dust-scattered
starlight. The new result rules out
the need for anything more exotic.
For decades, astronomers have been
puzzled by the cosmic optical background
(COB). This pervasive glow is somewhat
analogous to the cosmic microwave
background. But unlike the microwave
background, which comes from radiation
left over from the Big Bang, the
visible-light background comes from
other sources, such as distant stars
and galaxies. Exactly which sources,
though, has been difficult to ascertain
(S&T: Oct. 2023, p. 24).
One of the biggest hindrances to
measuring the COB from near Earth is
MOON
Glass Beads Suggest Recent Lunar Volcanism
ANALYSIS OF TINY glass beads from
the Moon suggests that volcanic eruptions
occurred there within the past
120 million years.
Apollo-collected rocks gave us our
first hands-on evidence of lunar volcanism
dating back to at least 3.1 billion
years ago. Then, initial studies of samples
from China's Chang'e 5 mission
- the first lunar material returned
since the 1970s - showed evidence of
volcanic activity 1 billion years later
(S&T: Feb. 2022, p. 9). Now, tiny glass
8
JANUARY 2025 * SKY & TELESCOPE
beads in the Chang'e 5 samples suggest
volcanism occurred far more recently.
The Chinese team, led by Bi-Wen
Wang and Qian Zhang (both at Chinese
Academy of Sciences), started
with some 3,000 micron-size glass
beads created in the sudden heat of
either impacts or volcanic processes.
The team used various methods to zero
in on 13 of them that showed hints of
volcanic histories, ultimately settling
on three with firmer evidence.
One line of evidence comes from
zodiacal light, the diffuse glow of sunlight
scattered off interplanetary dust.
Enter New Horizons, the mission sent
to the backwaters of the solar system
to explore Pluto and the Kuiper Belt. At
New Horizons' location 9 billion kilometers
(5½ billion miles) from Earth,
zodiacal light ceased to be an issue.
Yet the mystery remained: Marc
Postman (Space Telescope Science Institute)
and Tod Lauer (National Science
Foundation's NOIRLab) led a team that
used the Long Range Reconnaissance
Imager (LORRI) aboard New Horizons
to estimate the intensity of the COB
light in 2021, and again in 2022. Even
after accounting for other sources of
background light (like scattered starlight),
they found double the amount of
radiation thought to be emitted by all
the galaxies in the universe.
and it's more prosaic than expected.
Using new data from the European
Space Agency's Planck satellite, the
team estimated how much starlight
scatters off of dust within our Milky
Way Galaxy. By considering this effect,
combined with new LORRI images
designed to minimize interference from
galactic dust, the astronomers can
account for all of the COB.
" The results show that the great
majority of visible light we receive from
the universe was generated in galaxies, "
says Postman. " There is no evidence for
significant levels of light produced by
sources not presently known to astronomers. "
The result is published in the
September 1st Astrophysical Journal.
Christopher Conselice (University
of Manchester, UK), who wasn't on the
team, thinks the results should hold up.
He notes that some uncertainty remains
because the amount of galactic dust is
inferred rather than directly measured.
" There remains the possibility that
anomalous light could still exist, but it's
unlikely, " he says.
¢COLIN STUART
sulfur isotopes, which contain different
numbers of neutrons. As material
vaporizes in the intense heat of
an impact, lighter sulfur atoms more
readily float away in the Moon's low
gravity, leaving heavier sulfur behind
in resolidified material.
Volcanism, on the other hand,
results in material that retains more of
the lighter isotopes. Three beads have a
balance toward lighter sulfur isotopes,
suggesting a volcanic origin, the team
writes in the September 6th Science.
The beads' ages were determined using
radiometric dating.
The results align with orbital imagNASA
/ APL / SWRI / SERGE BRUNIER (ESO) / MARC POSTMAN (STSCI) / DAN DURDA
Sky & Telescope - January 2025

Table of Contents for the Digital Edition of Sky & Telescope - January 2025
