Sky & Telescope - October 2023 - 27

Alternatives beyond stars and galaxies start to
Total
become more exotic. Some teams have suggested
that still-theoretical dark matter particles -
either axions (S&T: Jan. 2021, p. 16) or axion-like
particles - might decay into photons that contribute
background light. But while astronomers
aren't willing to rule out the possibility, neither
do they seem eager to jump on that particular
bandwagon. " It's pretty far down the list right
now, " Postman says, " but it's on the list. "
Lauer, for his part, doesn't advocate for any
particular solution. He asks with a straight face:
" Could it be a giant squid that's glowing? Well,
you know, maybe so, we've never been out there.
Maybe there are giant squids. "
Another possibility remains that the extra light
isn't there at all. While the detection is significant
in a statistical sense, that doesn't mean it has
to be real. There could be something that both
Lauer's and Symons' teams have overlooked.
" I am positive it's something systematic in
New Horizons, " says Rogier Windhorst (Arizona
State University).
Windhorst runs a program called SKYSURF,
one of the largest studies of Hubble's archival
observations. Together with colleagues, he has
trawled scads of images that the iconic telescope
has taken over the past three decades, looking for
light - in this case at near-infrared wavelengths
- in between those images' stars and galaxies.
Hubble has some advantages. One is the sheer
number of images researchers have to work with.
Another is that it can resolve most background
galaxies, rather than relying on a model of how
much background light those galaxies would
produce. But it also has significant disadvantages.
Not only is it peering through the bright shine of
the zodiacal light, but its images are also notoriously
difficult to calibrate.
" Hubble has 15 sunrises and 15 sunsets per
day, which is horrifying, " Windhorst declares. The
resulting heat fluctuations create false signals
in Hubble's camera that are difficult to subtract.
Due to the telescope's vagaries, SKYSURF has only
measured upper limits on what the EBL could be,
and they are consistent with previous observations
- including those from New Horizons.
uSOURCES OF LIGHT This stacked bar chart shows the
contributions from various light sources to the extragalactic
background light of a New Horizons field that Lauer
and colleagues examined in 2022. The background light
measured in that field (blue cross at top) exceeds all known
contributions, implying an extra, unknown source. (The
two vertical lines indicate the range of possible values in
both the background light measurements and the expected
contributions from known light sources.)
Diffuse
galactic
light
Total
background
light from
known
sources
background
light
Now, Windhorst is applying the same type of
analysis to observations taken by JWST, which
sees no sunrises or sunsets at all thanks to its
sunshield. The recent analysis has resulted in
only loose upper limits so far, based on just a few
months of observations. More images and a better
understanding of JWST calibration will help
tighten the net.
Despite his doubts, though, Windhorst
acknowledges that LORRI's position gives the New
Horizon science team the advantage over both
Hubble and JWST. " They're completely beyond the
zodiacal light, and that's unique, " he says. " They're
doing a great experiment. "
On the Case
Like so many fascinating questions in astronomy,
the unexplained brightness of space remains an
open case. But astronomers haven't run out of
recourse. As New Horizons continues its flight
out of the solar system, its LORRI camera is still
snapping pictures. Lauer and others have asked
for targeted imagery over New Horizons' extended
mission to help them better account for or even
diminish the contribution of the Milky Way's
dust. Those images will also help them answer new
questions, such as whether the background glow
itself varies across the sky, which will in turn help
astronomers understand its origin.
JWST holds promise, too, not only for measurScattered
starlight
ing
the EBL itself but also for testing the faint-galaxy
scenario. While JWST doesn't see visible light,
its sensitive infrared images might be able to tell us
whether there are more faint galaxies in the nearby
universe than expected.
Ultimately, the best bet might be an instrument
Ionized
hydrogen
Faint stars
Scattered
galaxy light
made to order. Neither Hubble nor LORRI nor
JWST were really designed to detect faint extragalactic
background light. A camera designed for
that purpose aboard an interstellar probe would
do a better job, although such a spacecraft is likely
decades away.
Another, nearer-term option is to place a camFaint
galaxy
light
era (or make use of cameras already planned for)
on another interplanetary mission. NASA's Europa
Clipper is due to launch in October 2024 for the
Jupiter system, and it will carry a LORRI-like
visible-light camera. Beyond most of the zodiacal
light, it will offer a new perspective on the EBL.
In the meantime, the mystery of the universe's
extra light remains unresolved.
¢ News Editor MONICA YOUNG wonders how
many cosmic squids it would take to explain the
extra background light.
sk yand tele scope .o r g * OCTOBER 2023 27
BEATRIZ INGLESSIS / S&T; SOURCE: LAUER ET AL. / ASTROPHYSICAL JOURNAL LETTERS 2022
https://skyandtelescope.org/
Sky & Telescope - October 2023

Table of Contents for the Digital Edition of Sky & Telescope - October 2023
