Sky & Telescope - October 2023 - 24

NASA / ESA / S. BECKWITH (STSCI) / HUDF TEAM
COSMIC GLOW by Monica Young
The Case of the Uni
I
Astronomers imaging the blackness of space
have found it isn't as dark as they expected.
magine you're lying in bed, reading. You've already turned
off the overhead light and drawn the blackout shades for
a good night's sleep. Before you tuck in, you turn off the
lamp on your night table, too. Yet even after all that, there's
still a glow permeating the room. Where is this radiance
coming from?
This is the mystery astronomers are finding themselves
in, although their " room " is the entire universe. Even after
turning off the lights - that is, accounting for any possible
light source that could affect their measurements - the Long
Range Reconnaissance Imager (LORRI) aboard NASA's New
Horizons spacecraft is still seeing a visible glow.
Some glow is expected. Unlike the relic radiation from the
Big Bang, astronomers expected a background smear from
distinct sources, such as dim and distant stars and galaxies.
Since cameras can't detect the faintest of these sources individually,
their light should combine to become the extragalactic
background light (EBL).
So astronomers weren't surprised to find EBL - in fact,
they'd spent decades searching for it. The real surprise is just
how much of it there is. Using LORRI, researchers found
twice the background glow that models had predicted. If
these results hold up, they might end up revealing that
there's more to the cosmic history of stars and galaxies - or
maybe even more exotic matter - than we yet understand.
Zodiacally Bright Skies
The EBL long evaded detection because there are so many
foreground lights to turn off. The brightest of these is the
zodiacal light, sunlight scattered off dust that's sprinkled
24 OCTOBER 2023 * SKY & TELESCOPE
throughout the plane of the inner solar system (inside
Jupiter's orbit). Zodiacal light is visible to the unaided eyes of
earthbound observers, at least those under dark skies, so it
may come as no surprise that it far outshines any background
glow in the universe.
That's not to say that astronomers haven't tried to peer
through that bright veil. In a series of studies published
between 2002 and 2007, Rebecca Bernstein (now at Carnegie
Institution for Science) used Hubble Space Telescope data to
try to pin down the background glow. Ultimately, though,
those measurements were only able to put a limit on how
bright the EBL might be, rather than actually detecting it.
Other measurements did appear to detect the EBL,
although those finds remain contested. Some of these came
from the Cosmic Infrared Background Experiment (CIBER),
which flew aboard multiple sounding rockets to measure
the near-infrared sky from above Earth's atmosphere. Shuji
Matsuura (Kwansei Gakuin University, Japan) and colleagues
used data from CIBER's spectrometer from two flights taken
in 2010 and 2013 to detect the EBL at infrared wavelengths.
They found that no matter how they modeled the zodiacal
light, they still found more extragalactic light than they were
expecting, albeit with a lot of wiggle room in the results.
An alternative method to measuring the EBL came from
a surprising source: gamma rays. These high-energy photons
are known to interact with visible and infrared light, disappearing
in the process. So what astronomers do is measure
the light from distant, bright gamma-ray sources known as
blazars. As the gamma rays travel to Earth, they tangle with
intervening visible and infrared photons. Estimating how
many gamma rays have disappeared along the way leads to
an indirect measure of the EBL. Some of these measurements
found no extra light; others left room for interpretation.
Sky & Telescope - October 2023

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