Background
The Hubble constant quantifies the rate at which the universe is expanding.
One of the big unresolved problems in cosmology is the discrepancy between the value of the Hubble constant as estimated from the cosmic microwave background (CMB) and related whole sky observations of the earliest moments of the universe (most precisely measured by the Planck collaboration), and the estimated value of the Hubble constant measured from observations of much younger stars that are closer to us in time and space.
This discrepancy could be because the value of the "constant" has actually changed over time (an assumption that calls for "New Physics"), or it could be due to errors in one of the measurements (most likely the measurement of younger stars which has a far larger margin of error).
As the authors of a new pre-print (cited below with its abstract) explain:
As is well known, there is a tension between the value of the Hubble constant as inferred from small and large distance measurements, most significantly between the values inferred from Type Ia supernova (SNIa) distances to redshifts z ∼ 0.1 calibrated by Cepheid observations, as measured by the SH0ES team, and the distance to the cosmic microwave background (CMB) decoupling surface at z ∼ 1090, as measured by the Planck satellite. The former yields H(0) = 73.2 ± 1.3 (in units of km/s/Mpc used from now on) (Riess et al. 2021) and the latter H(0) = 67.4±0.5 (Aghanim et al. 2020); a 4.1 σ tension.
The tension between other measurements is not as significant.
The chart below from the pre-print compares the SH0ES and Planck results that are in such strong tension with each other, with four other leading measurements of the Hubble constant made using three other measurement methods (gravitational waves, tip of red giant branch calibration, and gravitational lensing).
The New Paper
The new pre-print argues that the problem is a systemic error in how the Hubble constant is determined from the measurements of younger stars. It argues that the analysis that produces the discrepancy does so because it makes the unjustified assumption that all Cepheid variable stars of the same brightness are exactly the same color. It argues that this assumption is flawed because the light from those stars is distorted en route to Earth by factors such as interstellar dust.
If the assumption that the colors are exactly the same apart from red shift is true, the red shift of those stars can be determined with the precision needed to accurately estimate the Hubble constant. But, if the colors are distorted as light travels to Earth, determining redshift from the light from Cepheids is not nearly as precise as this method has been claimed to be.
The pre-print offers a very plausible resolution of the discrepancy. If correct, it would also kill a legion of rather far fetched revisions of the laws of Nature proposed to explain the discrepancy.
The paper and its abstract are as follows:
Motivated by the large observed diversity in the properties of extra-galactic extinction by dust, we re-analyse the Cepheid calibration used to infer the local value of the Hubble constant, H(0), from Type Ia supernovae.
Unlike the SH0ES team, we do not enforce a universal color-luminosity relation to correct the near-IR Cepheid magnitudes. Instead, we focus on a data driven method, where the measured colors of the Cepheids are used to derive a color-luminosity relation for each galaxy individually.
We present two different analyses, one based on Wesenheit magnitudes, a common practice in the field that attempts to combine corrections from both extinction and variations in intrinsic colors, resulting in H(0)=66.9±2.5 km/s/Mpc, in agreement with the Planck value.
In the second approach, we calibrate using color excesses with respect to derived average intrinsic colors, yielding H(0)=71.8±1.6 km/s/Mpc, a 2.7σ tension with the value inferred from the cosmic microwave background.
Hence, we argue that systematic uncertainties related to the choice of Cepheid color-luminosity calibration method currently inhibits us from measuring H(0) to the precision required to claim a substantial tension with Planck data.
3 comments:
Second paragraph here says the tension is robust, even when old and new Hubble constants are estimated using quite different methods.
https://iopscience.iop.org/article/10.3847/1538-4357/ab1422
Commentary
https://twitter.com/DScol/status/1397626732954279937
Lots of methods get a larger value, but none of them claim such a small margin of error, so the statistical significance of the other discrepancies is much smaller.
Referenced open access paper and abstract:
"We present an improved determination of the Hubble constant from Hubble Space Telescope (HST) observations of 70 long-period Cepheids in the Large Magellanic Cloud (LMC). These were obtained with the same WFC3 photometric system used to measure extragalactic Cepheids in the hosts of SNe Ia. Gyroscopic control of HST was employed to reduce overheads while collecting a large sample of widely separated Cepheids. The Cepheid period–luminosity relation provides a zero-point-independent link with 0.4% precision between the new 1.2% geometric distance to the LMC from detached eclipsing binaries (DEBs) measured by Pietrzyński et al. and the luminosity of SNe Ia. Measurements and analysis of the LMC Cepheids were completed prior to knowledge of the new DEB LMC distance. Combined with a refined calibration of the count-rate linearity of WFC3-IR with 0.1% precision, these three improved elements together reduce the overall uncertainty in the geometric calibration of the Cepheid distance ladder based on the LMC from 2.5% to 1.3%.
"Using only the LMC DEBs to calibrate the ladder, we find H0 = 74.22 ± 1.82 km s−1 Mpc−1 including systematic uncertainties, 3% higher than before for this particular anchor. Combining the LMC DEBs, masers in NGC 4258, and Milky Way parallaxes yields our best estimate: H0 = 74.03 ± 1.42 km s−1 Mpc−1, including systematics, an uncertainty of 1.91%–15% lower than our best previous result. Removing any one of these anchors changes H0 by less than 0.7%. The difference between H0 measured locally and the value inferred from Planck CMB and ΛCDM is 6.6 ± 1.5 km s−1 Mpc−1 or 4.4σ (P = 99.999% for Gaussian errors) in significance, raising the discrepancy beyond a plausible level of chance.
"We summarize independent tests showing that this discrepancy is not attributable to an error in any one source or measurement, increasing the odds that it results from a cosmological feature beyond ΛCDM."
Adam G. Riess, et al., "Large Magellanic Cloud Cepheid Standards Provide a 1% Foundation for the Determination of the Hubble Constant and Stronger Evidence for Physics beyond ΛCDM:" 876(1) ApJ 85 (May 7, 2019).
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