Sky & Telescope - October 2022 - 18

Climbing the Ladder
Step 4. The Cosmos
As mentioned before, Edwin Hubble discovered the proportional
relation between a galaxy's distance and its apparent
recession velocity, the latter derived from the observed
redward shift of the galaxy's light. The cosmological redshift
(denoted by the letter z) is a reddening of the galaxy's light
caused by the expansion of the universe stretching the light to
longer, redder wavelengths: The longer the light waves travel
through expanding space, the more they're stretched. Over
the past few decades, astronomers have carefully calibrated
the relationship, known as the Hubble-Lemaître Law. (Belgian
cosmologist Fr. Georges Lemaître independently arrived at
similar conclusions a couple of years before Hubble did.) As
a result, the distance of a remote galaxy can in principle be
deduced from its observed redshift alone.
There's a catch, however. The expansion of
space pushes everything apart, but on top of
that, galaxies are also moving through space.
Depending on whether they're moving towards
us or away from us, this motion will decrease
or increase their redshift. The effect is especially
important for relatively nearby galaxies,
for which this additional Doppler shift can
be a significant fraction of the cosmological
redshift. Meanwhile, for very remote galaxies,
the proportional Hubble-Lemaître Law doesn't
hold, because space has not always expanded
at the same rate. To disentangle all these
effects, astronomers really need an independent
way of measuring distances - you can't
just rely on a simple redshift measurement to
precisely know how far away a galaxy is.
Original wavelength
Stretched (redshifted) wavelength
Over time, astronomers have constructed an
elaborate cosmological distance ladder to provide
redshift-independent distance estimates
of remote galaxies, based on the various
methods described in the previous sections.
Parallax and radar data within the solar system
reveal the size of Earth's orbit, enabling
annual parallax measurements of stars in the
solar neighborhood. Precise Gaia parallaxes
of Cepheids and red giants in our own Milky
Way provide an accurate calibration of these
distance indicators. Using the Leavitt Law and
the TRGB method, astronomers can determine
distances of galaxies out to many tens of millions
of light-years. Type Ia supernova explosions
in some of these galaxies then betray
the true luminosity of these stellar detonations,
making it possible to deduce the distances of
other galaxies that also harbor exploding white
dwarfs out to billions of light-years. In the late
1990s, such supernova-based distance estimates
of extremely remote galaxies, combined
with measurements of their redshifts, revealed
the accelerating expansion of the universe.
Cosmologists ascribe this uptick to a mysterious
property of empty space known as dark
energy (S&T: May 2018, p. 14).
18 OCTOBER 2022 * SKY & TELESCOPE
Sky & Telescope - October 2022

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