Sky & Telescope - March 2022 - 16

LADDER: LEAH TISCIONE / S&T; EARTH: VERY_VERY / SHUTTERSTOCK.COM
The Controversy Continues
>1010
BARYON ACOUSTIC OSCILLATIONS
Statistical distribution of galaxies
on cosmic scales.
1010
GRAVITATIONALLY LENSED QUASARS
Flickering of lensed images.
Out to distant galaxy clusters.
1010
sure distances to increasingly farther objects. The first rung
is geometric parallax, the most direct method of determining
distances. The next rungs use the luminosity of standard
candles, such as nearby Cepheid variable stars and more
distant Type Ia supernovae. Astronomers climb these rungs to
galaxies farther and farther out, then measure the speed with
which those galaxies are moving away from us to calculate
the current expansion rate of the universe. These are often
called local, or late-time measurements.
One of the largest collections of distance-ladder studies
TYPE IA SUPERNOVAE
White dwarfs explode with the
same luminosity. Out to distant
galaxy clusters.
107
CEPHEID AND OTHER VARIABLES
The relationship between
certain variable stars' intrinsic
luminosity and pulsation
period. Nearby galaxies.
105
INTRINSIC LUMINOSITY
Comparison of stars'
apparent and intrinsic
brightnesses. Milky Way.
102
PARALLAX
The annual shift in a star's
apparent position. Solar
neighborhood.
10−4
RADAR
Radar pings give
distances to objects.
Only good for the
solar system.
comes from Adam Riess's group at the Space Telescope Science
Institute. Using observations of supernovae and Cepheids in
a project dubbed SH0ES (Supernovae, H0, for the Equation of
State of Dark Energy), the team's latest work gives a current
expansion rate of 73 km/s/Mpc.
This kind of direct measurement is a cosmic rewind, starting
here on Earth and measuring progressively outward and
backward in time. But indirect measurement techniques use
the primordial fluctuations, starting at the epoch of recombination
when the first neutral atoms formed. This time period
is important in cosmic history because it's the furthest back
that we can see. The CMB is a picture of this early epoch;
before that, everything was an opaque plasma.
Amazingly, from an image of the CMB, astronomers can
derive the universe's composition: 4.9% baryons, 26.4% cold
dark matter, and 68.7% dark energy. Then astronomers combine
that composition with the standard model to predict the
next 13.8 billion years, producing a universe that looks like
the one we live in today.
Another indirect method involves baryon acoustic oscillations
(BAO), the permanent imprint of the sloshing primordial
plasma. As reionization dawned, waves traveling through
the plasma " froze in. " Thanks to that imprint, the lumps
in the plasma that eventually evolved into galaxies are now
slightly more likely to lie 480 million light-years apart than
at other separations, based on data collected by the Sloan
Digital Sky Survey (SDSS). When astronomers compare the
size of the BAOs based on the CMB with the size from galaxy
distributions, they can calculate a value for the expansion
rate of the universe (67 km/s/Mpc).
So we have the direct methods, which try to measure the
distances to late-time objects, and the indirect (or early-time)
methods, which make inferences from the characteristics
of the primordial universe. Like two sides of the same coin,
" these two things should give the same answer, " says cosmologist
Licia Verde (University of Barcelona, Spain). " And it
is very close, considering we are threading a needle from the
other side of the universe. "
Still, more precise measurements and larger data sets have
THE COSMIC DISTANCE LADDER Astronomers use several methods
to measure distances, some of which are shown here. Cepheids
can pick up where parallax no longer works, for example, and Type Ia
supernovae and lensed quasars extend beyond Cepheids.
16 MARCH 2022 * SKY & TELESCOPE
only made the discrepancy starker. This " crisis in cosmology "
has forced astronomers to ask big uncomfortable questions
- both about the veracity of local calculations and about the
limits of the ΛCDM model itself. Currently, there are two
main approaches scientists are taking to try and solve the
tension: astrophysical and cosmological.
Maximum distance (in light-years)
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Sky & Telescope - March 2022

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