Sky & Telescope - March 2022 - 18

The Controversy Continues
measurements in the same direction. Why aren't some local
studies measuring a Hubble constant lower than 67?
This is why the community is interested in finding other,
independent ways of measuring H0. Two promising new
methods have already had interesting early results: gravitational
waves and gravitational lensing.
Gravitational-wave facilities like LIGO, Virgo, and KAGRA
watch for cosmic collisions, including neutron star mergers,
which send out blasts of electromagnetic particles along with
transient ripples in spacetime. Having multiple messengers
from singular events lets astronomers make a one-rung distance
measurement to the point of origin, turning gravitational-wave
sources into standard sirens.
So far, astronomers have only been able to use gravitational
waves to make a few such measurements of H0. They
calculated a value around 70, with an uncertainty range that
overlaps both early- and late-time numbers. Within five years,
the detectors will hopefully have gathered enough data for a
much more precise value.
Unlike gravitational waves, which pass us by in a fraction
of a second, gravitational lensing is a more permanent
phenomenon. The theory is simple: We find flickering quasars
that have a conveniently placed galaxy between us and them.
The intermediary galaxy acts like a lens, magnifying the
quasar's light and breaking it into multiple images, like a
spacetime mirage. Although the images originate from the
same place, they flicker out of sync because the light from the
quasar refracts around the lens galaxy, taking paths of different
lengths to reach us. Astronomers can use these delays to
measure the quasar's distance.
Sherry Suyu (Max Planck Institute for Astrophysics, Germany)
started a program to measure the distances to lensed
quasars, dubbed H0LiCOW (H0 Lenses in COSMOGRAIL's
Wellspring). They looked at data from six different quasars,
using a combination of space- and ground-based telescopes,
and calculated an H0 of about 73 km/s/Mpc.
But accurate measurements of the images' travel times
depend on knowing the distribution of mass within the
closer, lensing galaxy, which is tricky. This is why H0LiCOW
and related groups are already preparing for a much larger
study when the Vera C. Rubin Observatory comes online in
2023. They will also receive time on the James Webb Space
Telescope (JWST) to better characterize the lensing galaxies.
Suyu is optimistic about the utility of the technique.
" Gravitational lensing has an advantage because it is based
STANDARD
CANDLE A source
of a specific intrinsic
brightness will look
fainter the farther
away it is. Astronomers
use several
kinds of sources as
standard candles to
estimate distances
across the observable
universe.
Distance by Any Measure
Space lacks perspective: Two stars can appear to be the same size and brightness, but in reality one can be much brighter
and farther away than the other. This is where known quantities, either of brightness, size, or loudness - known respectively
as standard candles, standard rulers, and standard sirens - come into play. Astronomers use them as the jumpingoff
point when reckoning cosmic distances.
Candles A standard candle is an astrophysical
object of known brightness.
Because the relationship between distance
and perceived brightness is well
understood, if you know the class of
the object you are looking at, you can
tell how far away it is by measuring
its luminosity. Henrietta Swan Leavitt
discovered the first standard candle -
Cepheid variable stars - more than
a century ago. Today there are many
phenomena that astronomers can use
to measure cosmic distances in this
way, including Type Ia supernovae,
gravitationally lensed quasars, and
planetary nebulae.
18 MARCH 2022 * SKY & TELESCOPE
Rulers A standard ruler is an astrophysical
object of known size. If you
already know how big something is,
but you don't know how far away it is,
simple trigonometry can tell you. All
you need is to determine how big the
object looks in the sky from your perspective,
known as the angular diameter.
Baryon acoustic oscillations are
the main standard rulers in use right
now. They indicate that matter in the
universe is distributed in an uneven
density pattern, such that galaxies
are more likely to be about 480 million
light-years from each other.
Sirens A standard siren is a source of
gravitational waves of known intensity,
or amplitude. The merger of two
compact objects, like neutron stars or
black holes, emits ripples in spacetime
that shrink in amplitude as they travel.
Assuming that general relativity is valid
and that our detectors are well-tuned,
that ripple will arrive at Earth with the
distance it has traveled already encoded
into it, with no need for complex
distance-ladder calculations.
LEAH TISCIONE / S&T

Sky & Telescope - March 2022

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