Sky & Telescope - February 2024 - 24

Dark Energy Experiment
clearly, even in just two months of data. "
The DESI team says the BAO detection has
a five-sigma confidence level, meaning that
there's only one chance in a million that
the correlation is a statistical fluke.
Because the BAO imprint started off at
a specific physical size and grew as the universe
expanded, it can be used to trace the
universe's expansion rate at various times in
its history. The apparent size of that imprint
at any one time - as revealed by measuring
the distribution of galaxies at specific
redshifts - tells us how fast the universe
was stretching at that time. And that data,
in turn, may lead to a clearer understanding
of the role played by dark energy.
Cosmic Questions
There remains the question of what dark
energy actually is. The simplest proposal is
the one that Einstein came up with in 1917
- the notion that seemingly empty space
contains a small amount of energy. But
what if the strength of dark energy changes
over enormously long periods of time?
" The first step is to see whether the cosmological constant
is really constant, " says theoretical physicist Rocky Kolb (University
of Chicago). " And the best handle on that - the only
handle, perhaps - is measuring the expansion history of the
universe. " Which is precisely what DESI is designed to do.
So far, results from DESI appear to mesh with the most
widely accepted cosmological model, in which the universe
PRIMORDIAL SOUND Just as a rock dropped in
a pond creates a wave that ripples outward along the
water's surface (top and center), so too did sound
waves slosh through the early universe. Each region
initiated its own sound wave, so resulting ripples overlapped
(bottom), but the average imprint is still there.
contains an unchanging cosmological constant
along with cold dark matter (the socalled
ΛCDM model). But a fixed, unchanging
cosmological constant of some arbitrary value
would be rather dull, says Palanque-Delabrouille.
Finding that dark energy varies in
strength over time would be far more tantalizing
- the discovery of the century, perhaps.
But it would mean physicists have a lot of
work ahead of them. If dark energy can vary,
" then there's something we haven't put in
our theory, " says Palanque-Delabrouille. " And
so we really have to be opening our minds to
new theories, to new approaches. "
Our understanding of gravity may also
come under scrutiny. Newton showed that
the strength of the gravitational attraction
between two objects is proportional to
their masses and inversely proportional to the square of the
distance between them (a " one over r-squared, " or 1/r2, law).
Einstein's general relativity provides a more sophisticated
version of this law, but it preserves Newton's foundation,
still envisioning gravity as a 1/r2 force. But what if gravity
behaves differently over cosmological distances? For decades,
researchers have put forward various alternative gravity theories,
generally gathered under the banner of modified gravity,
Cosmic
microwave
background
released
EXPANSION HISTORY With the discovery of dark energy (see page 26), our understanding
of the more recent cosmic past has changed. Observations show that the universe
is expanding faster and faster. More precise data from the long-ago universe will help
astronomers determine whether and how that accelerating expansion will continue.
24 FEBRUARY 2024 * SKY & TELESCOPE
BIG
BANG
EARLY UNIVERSE
MODERN UNIVERSE
BARYON ACOUSTIC OSCILLATIONS: D. EISENSTEIN; EXPANSION HISTORY
OVER TIME: S&T DIAGRAM
Sky & Telescope - February 2024

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