Sky & Telescope - June 2023 - 39

the sky with different foreground contamination and will use
different strategies for removing lensed B-modes. Overall,
these differences average out so that the two observatories
end up with broadly similar sensitivities. (There are also
other ground-based and balloon-borne projects, but they are
less sensitive than these two.)
Of course, there is a very real prospect that neither observatory
will detect primordial B-modes. But if they draw a
blank, both teams already know their next move: Plans are in
motion to join forces in CMB-S4 - " the ultimate CMB experiment
you can build on the ground, " according to Ahmed,
who is involved with all three projects.
With first light expected around 2030, the antennas of
CMB-S4 will be distributed between Chile and Antarctica.
The observatory will survey the southern sky for seven years
with 21 telescopes (three large and 18 small) and more than
500,000 detectors covering the frequency range 20-280 GHz.
This scale-up is necessary for one simple reason: noise, mostly
from the atmosphere. Apart from building an observatory in
space, the only way to combat this noise is by observing over
long periods or by incorporating a huge number of detectors.
CMB-S4 will do both.
Others are taking the space-based route. Of the many
mission concepts explored since Planck was retired and
sent into a graveyard orbit in 2013, only one has made it off
the drawing board so far: the Japan Aerospace Exploration
Agency (JAXA) mission LiteBIRD. Scheduled for launch in the
late 2020s, LiteBIRD will consist of three millimeter-wave,
polarization-sensitive telescopes with approximately 4,500
detectors observing across 15 frequency bands. Offering
unprecedented precision, LiteBIRD will scan the entire CMB
sky for three years in search of primordial B-modes.
Back to the Drawing Board?
Running parallel to this intense observational activity is an
equally intense effort to build theoretical models of inflaBICEP
DETECTORS Team members stand with the BICEP Array's
150 GHz focal plane. The focal plane is curved to improve optical performance
at this higher frequency; the 30/40 GHz focal plane is flat.
tion that would explain the primordial B-modes we might
detect. The options are fewer than they once were. In 2021,
a team - analyzing all the data from BICEP through 2018
and correlating the results with those from Planck and its
NASA predecessor, the Wilkinson Microwave Anisotropy
Probe - constrained the strength of B-mode polarization by
a factor of about two. This effectively ruled out a broad class
of " simple inflation " models that had looked promising in
describing the earliest moments of the universe.
For instance, the m2φ2 model (pronounced " em-squared
phi-squared " ) was popular for being one of the most straightforward
and elegant. The favored driver of inflation is a
BICEP BEEFS UP Each BICEP focal plane (left, 30/40 GHz receiver) contains 12 array wafers (right), which detect CMB photons with highly sensitive
devices called bolometers, chilled to 0.3 kelvin. The wafer shown operates at 150 GHz, or a wavelength of 2 mm.
sk yand tele scope .o r g * JUNE 2023 39
BICEP ARRAY COLLABORATION (3)

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