INTRODUCTION

If Cosmology Is In Crisis, Then These Are The 19 Most Important Galaxies In The Universe

These are the 19 known galaxies where both Type Ia supernovae and individual Cepheid variable stars
 have been observed and measured. That's a very small number, statistically, to draw conclusions about
 the entire Universe.
 S.L. HOFFMANN ET AL. (2016) APJ, V. 830, NO. 1

The expanding Universe, full of galaxies and the complex structure we observe today, 
arose from a smaller, hotter, denser, more uniform state. It took thousands of scientists 
working for hundreds of years for us to arrive at this picture, and yet the lack of a consensus
 on what the expansion rate actually is tells us that either something is dreadfully wrong,
 we have an unidentified error somewhere, or there's a new scientific revolution just on the horizon. 
 C. FAUCHER-GIGUÈRE, A. LIDZ, AND L. HERNQUIST, SCIENCE 319, 5859 (47)

But when it comes to the expanding Universe, two sets of groups get consistently different outcomes.

A series of different groups seeking to measure the expansion rate of the Universe, along with their color-coded results. 
Note how there's a large discrepancy between early-time (top two) and late-time (other) results, 
with the error bars being much larger on each of the late-time options.
 L. VERDE, T. TREU, AND A.G. RIESS (2019), ARXIV:1907.10625
Signals from the early Universe yield expansion rates of 67 km/s/Mpc, while late-time signals yield systematically larger values.

Standard candles (L) and standard rulers (R) are two different techniques astronomers
 use to measure the expansion of space at various times/distances in the past. 
 Based on how quantities like luminosity or angular size change with distance, 
 we can infer the expansion history of the Universe. Using the candle method 
 is part of the distance ladder, yielding 73 km/s/Mpc. Using the ruler is part 
 of the early signal method, yielding 67 km/s/Mpc.
 NASA / JPL-CALTECH

Of all the late-time signals, the one with the smallest uncertainty is based on the cosmic distance ladder.

The construction of the cosmic distance ladder involves going from our Solar System to the stars
 to nearby galaxies to distant ones. Each “step” carries along its own uncertainties,
 especially the Cepheid variable and supernovae steps; it also would be biased towards
 higher or lower values if we lived in an underdense or overdense region.
 NASA,ESA, A. FEILD (STSCI), AND A. RIESS (STSCI/JHU)

From just three measurements — parallax, Cepheids, and Type Ia Supernovae — they derive a value with just a 2% uncertainty.

The Variable Star RS Puppis, with its light echoes shining through the interstellar clouds.
Variable stars come in many varieties; one of them, Cepheid variables, can be measured 
both within our own galaxy and in galaxies up to 50-60 million light years away.
 This enables us to extrapolate distances from our own galaxy to far more distant ones 
 in the Universe. Other classes of individual star, such as a star at the tip
 of the AGB or a RR Lyrae variable, can be used instead of Cepheids. 
 Recent tip-of-the-red-giant-branch results presently point to a smaller late-time value of 69.8 km/s/Mpc.
 NASA, ESA, AND THE HUBBLE HERITAGE TEAM

But connecting these disparate measurements together requires finding galaxies with both Cepheids and supernovae.

Spiral galaxy UGC 9391, shown here, is one of only 19 important galaxies known to have housed
 a recently observed Type Ia supernova (blue cross) as well as individual, 
 resolvable Cepheid variable stars. Even a small uncertainty in a sample of just 19 galaxies 
 could bias these results towards artificially higher values.
 NASA, ESA, L. FRATTARE (STSCI) AND A. RIESS (STSCI/JHU) ET AL.

Despite enormous surveys and decades of careful observations, only 19 known galaxies have had both.

NGC 3972, depicted here, is another one of the 19 known galaxies that are close enough to have 
individual Cepheid variable stars resolved from within them (yellow circles),
 but also to have an observed supernova (labeled, with diamond) go off within it in the past few decades. 
 Galaxies like this are crucial for building up a successful cosmic distance ladder, 
 but run the risk of providing a biased sample. A distance ladder's strength is only as good as its weakest rung.
 NASA, ESA, A. RIESS (STSCI/JHU)

This small sample could be inherently biased, a legitimate worry among astronomers in the field.

The beautiful barred spiral galaxy NGC 1015, shown here as imaged by the Hubble Space Telescope in 2013,
 also contains many Cepheid variable stars as well as an observed, recent Type Ia supernova.
 This information is used to calibrate the luminosity of distant Type Ia supernovae that 
 cannot be measured along with Cepheids, but even a small bias at the ~few-percent level 
 could perhaps explain the entirety of this cosmic controversy. 
 NASA, ESA, A. RIESS (STSCI/JHU)

There are two mechanisms that create Type Ia supernovae, and Cepheid-rich regions likely contain both: another potential bias.

Two different ways to make a Type Ia supernova: the accretion scenario (L) and the merger scenario (R).
 Without a binary companion, our Sun could never go supernova by accreting matter, but we could potentially merge
 with another white dwarf in the galaxy, which could lead us to revitalize in a Type Ia supernova explosion after all.
 NASA / CXC / M. WEISS

Scientist Lucas Macri noted, in similar historic cases, "the universe was trying to tell you that you didn't have the whole picture."

Before converging on a value of ~71 km/s/Mpc, values for the modern-day Hubble expansion rate underwent
 an enormous number of changes, as big discoveries such as the existence of two types of Cepheids,
 an understanding of peculiar velocities, calibration issues and assumptions over the properties 
 of distance indicators represented real, physical issues whose resolution resulted in a better
 understanding of the astrophysics governing the Universe. Perhaps the resolution 
 of this current puzzle will be much the same. 
 J. HUCHRA, 2008

With more examples and improved data, cosmologists hope to finally resolve this conundrum.

As time continues to pass, greater numbers of nearby galaxies are expected to experience Type Ia supernovae
 explosions inside of them. As measuring Cepheids in nearby galaxies is less of a challenge than finding nearby
 Type Ia supernovae, this should enable us to raise the number significantly, reducing our uncertainties and
 lowering the probability of biases in any one particular distance ladder measurement.
 NASA / SWIFT