Sky & Telescope - March 2022 - 17

GRAVITATIONAL LENSING DIAGRAM: NASA / ESA / L. CALÇADA;
GRAVITATIONALLY LENSED IMAGES: NASA / ESA / S. H. SUYU (MPIA,
TECHNICAL UNIV. OF MUNICH, ASIAA) / K. C. WONG (UNIV. OF TOKYO)
Freedman thinks the root of the Hubble tension lies with Cepheids,
which are younger than red giants and therefore occupy crowded
star-forming regions.
Astrophysical: Are Cepheids the Problem?
Some scientists say uncertainties in distance-ladder measurements,
what they call systematics, could cause the discrepancy.
Even something small would compound on each rung,
making the ultimate measurement of H0 less reliable and
creating tension where there is none.
In a recent study, Edvard Mörtsell (Stockholm University,
Sweden) and colleagues analyzed the Cepheid method used by
Riess's SH0ES collaboration. While Cepheid stars are a standard
candle (S&T: Dec. 2021, p. 12), Mörtsell's team found
that one of the ways astronomers typically calibrate their
luminosities might be flawed.
There are lots of different ways that astronomers must
characterize stars before figuring out how far away they are.
One of the variables is how much dust is between us and
them, and what that dust is made of. Traditionally, scientists
have used a number to represent the typical amount that dust
dims and reddens a star's light, and then used that number to
calculate the star's true luminosity.
" The way it's been done before is that [astronomers] just
assume that you can make the same correction for all Cepheids,
regardless of the galaxy that they are in, which is basically
saying that dust has the same properties everywhere, " Mörtsell
says. " But we know that this is not true. We know that there
are lots of variations in how dust looks in different galaxies. "
uGRAVITATIONAL LENSING Below: When a cluster sits at exactly
the right spot between us and a distant galaxy, the cluster's gravity
bends the distant galaxy's light around the cluster as though the cluster
were a lens, magnifying the light and redirecting it toward us. Observers
then use the resulting distorted images to learn about the distant galaxy.
(Scales here are exaggerated.) Right: Hubble images of lensed quasars
studied by the H0LiCOW collaboration.
His group found that when they replaced the dust constant
used in distance-ladder calculations with a value that
can vary from galaxy to galaxy and then calculated H0, they
obtained a number much closer to the CMB calculation.
But the gap isn't closed completely. Mörtsell suggests this is
because there is probably more than one problem with the
Cepheid method.
Other recent work has also raised concerns about Cepheids.
In 2019 Freedman and her team measured the peak
brightness of the populations of red giant stars in more than
a dozen nearby galaxies and used that value as a standard
candle to reckon their distances. In 2021 her team bolstered
their results, cross-checking their luminosity measurements
with five different approaches. The H0 value the researchers
get from the red giants is 69.8 km/s/Mpc - notably closer to
the value derived from the CMB than the one calculated from
a combination of Cepheids and supernovae.
Like Mörtsell, Freedman thinks the root of the Hubble tension
lies with Cepheids, which are younger than red giants and
occupy crowded star-forming regions. It may be that things
like dust and other stars are throwing off measurements.
Astrophysical: New Methods
Systematics could be the source of the tension. But it seems
strange that different sources of error would throw off local
RXJ1131−1231
B1608+656
Galaxy
Galaxy cluster
HE0435−1223
Lensed galaxy images
WFI2033−4723
Distorted
light rays
Earth
sk yand tele scope .o r g * MARCH 2022 17
http://skyandtelescope.org
Sky & Telescope - March 2022

Table of Contents for the Digital Edition of Sky & Telescope - March 2022
