Sky & Telescope - March 2022 - 19

LEAH TISCIONE / S&T
" Gravitational lensing has an advantage because it is based on simple geometry
and well-known physics. . . . GR has not yet failed us, so I think lensing won't,
either. " -SHERRY SUYU
on simple geometry and well-known physics, " she says. " It
just relies on general relativity, which has so far passed all
the various tests. GR has not yet failed us, so I think lensing
won't, either. "
In a few years, the groups working on gravitational-wave
and gravitational-lensing observations will have enough data
to give very accurate, independent measurements of H0. If
they find it to be around 67, this will vindicate CMB-based
measurements. If they get around 74, then this will not only
vindicate groups such as Adam Riess's SH0ES collaboration
but also provide strong evidence that something is missing
from ΛCDM and new physics is needed.
If, on the other hand, these approaches find a value
between the two extremes, as Freedman has, or they each
find wildly different numbers, then it will still be anyone's
guess as to what's going on.
Cosmological: Is ΛCDM Wrong?
The standard model is a concordance stratagem, meaning it's
what cosmologists agree to use right now. If the model is even
a bit wrong, this could in turn cause calculations based on
early features, such as the CMB and BAO, to get the current
expansion rate a bit wrong in turn.
The Planck satellite gave us the gold-standard measurement
of the CMB, with its final data release in 2018. But
regardless of the elegance of the results, there is always room
for improvement. Several instruments, the South Pole Telescope
(SPT) among them, are currently observing the CMB
with higher resolution and sensitivity than ever before.
" This gives us the capacity of testing predictions, both of
the standard cosmological model and alternative models, "
says SPT collaboration member Lloyd Knox (University of
California, Davis). " So far, everything we have found is consistent
with ΛCDM. "
The SPT survey of the CMB will
be completed in the next few years.
Cosmologists expect it to deliver a
more conclusive test of the exponential
growth spurt in the universe's earliest
moments, called inflation, and a better
understanding of dark matter and dark
energy. Any surprises in these data
could be evidence that the standard
model is flawed.
uEARLY DARK ENERGY Increasing dark
energy in the primordial universe would prevent
sound waves from traveling as far, similar to
the effect of trying to run up an escalator when
the speed is increased.
sk yand tele scope .o r g * MARCH 2022 19
Theorists have suggested a lot of different tweaks to the
ΛCDM model to fix the discrepancy between direct and indirect
measurements. Corrections after the epoch of recombination
have mostly fallen out of favor because they are inconsistent
with both observations of BAO and supernova data.
So if something is wrong, many think it's likely something to
do with conditions in the primordial plasma universe, back
before the CMB photons were released.
Adjusting elements of the early plasma, such as the properties
of tiny particles called neutrinos or of primordial magnetic
fields (S&T: Sept. 2021, p. 22), or varying fundamental
physical constants, changes what's called the sound horizon.
The sound horizon is the maximum distance that a sound
wave could have traveled before recombination occurred. We
can't look back any earlier than recombination with electromagnetic
radiation, so the pre-recombination period is a
mysterious period of time, uncharted territory that's perfect
for exploring new physics.
One of the possibilities is called early dark energy (EDE).
This is a class of theories in which dark energy's properties
are not constant but evolve over time, creating more dark
energy in the pre-recombination period and less at later times
than cosmologists have traditionally assumed existed. Sunny
Vagnozzi (University of Cambridge, UK) uses an illustration
to show how uncertainties around the sound horizon lie at
the heart of EDE solutions:
" Say I give you a certain amount of time to run the wrong
way up an escalator, and you only make it to a certain point, "
he says. " That's the sound horizon. Now, say I speed up the
escalator, and I give you the same amount of time. You're
going to run up a smaller distance. Imagine the speed of the
escalator is the universe expanding. That's why people are
looking at early dark energy. For a small amount of time,
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Sky & Telescope - March 2022

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