<p>The <a href="https://www.universetoday.com/159987/could-a-dark-energy-phase-change-relieve-the-hubble-tension/" rel="noreferrer">Hubble tension</a> is the discrepancy between measurements of the local rate of expansion of the universe vs the predicted rate inferred from CMB and the <a href="https://en.wikipedia.org/wiki/Friedmann_equations" rel="noreferrer">Friedmann equations</a>.</p>
<p><a href="https://arxiv.org/abs/2103.01183" rel="noreferrer">A dizzying array</a> of solutions involving new physics has been proposed. However, it is rare to discover &quot;new physics&quot; at a fundamental level. This isn't to say physics is a dying field! There is abundant research on complex systems, such as a new quaisparticle in a <a href="https://scitechdaily.com/physicists-successfully-create-a-new-type-of-quasiparticle/" rel="noreferrer">condensed matter system</a>. It's just the very <em>bedrock</em> of modern physics which we should be skeptical about changing.</p>
<p><a href="https://en.wikipedia.org/wiki/Cepheid_variable" rel="noreferrer">Cepheid variable stars</a> are used as a crucial rung on the <a href="https://en.wikipedia.org/wiki/Cosmic_distance_ladder" rel="noreferrer">distance ladder</a>. They have a period-luminosity relationship which is deduced from parallax-distance measurements to a large number of Cepheids. This relationship is then used to measure the distance to other galaxies (which are too far to use parallax). Other methods to replace Cepheids exist but (as of 2023) lack enough precision to produce a strong Hubble tension.</p>
<p>The &quot;error-bars&quot; in the Cepheids include an estimate of systematic error. However, it is difficult to account for all such sources. Nuances in the telescopes, wrong assumptions about the statistics (i.e. assuming a normal distribution when there is a &quot;fat tail&quot;), as well as unexpected correlations (in metallicity, dust extinction, etc) and more can all introduce difficult-to-detect errors. <a href="https://iopscience.iop.org/article/10.3847/1538-4357/ac7c19/meta" rel="noreferrer">Concerns</a> about unaddressed systematics have been raised but all the papers seem to zero in on a single issue. <strong>Are there any articles that attempt to <em>comprehensively</em> estimate potential sources of errors in Cepheids?</strong></p>

CC BY-SA 4.0
2023-05-18T19:35:36.813Z
Good question and one that many professional astronomers are asking themselves too. I for once was convinced that this is just a systematic error (recall that as recent as in 1990, the values for H0 spanned 50-100 obtained from the same data!). However, with improved distance calibration, the tension is becoming even larger and the air is getting thinner.
In 1999, when the acceleration of the Hubble expansion was discovered from SNe 1a, I was also very sceptical at first, but in the end the measurements were confirmed.
Walter
CC BY-SA 4.0
2023-05-18T19:39:27.607Z
To establish the issue beyond reasonable doubt, it requires confirmation independent of the Cepheid distance scale, for example tip-of-the-red-giant-branch distances, to calibrate the SNe 1a distance scale. Even better would be a direct measurement using gravitational lensing.
Walter
<hubble-constant><cepheids>
Hubble tension and Cepheid systematic error
Kevin Kostlan
# Hubble tension and Cepheid systematic error

(Asked by: username_0 on 2023-05-16)

The [Hubble tension](https://www.universetoday.com/159987/could-a-dark-energy-phase-change-relieve-the-hubble-tension/) is the discrepancy between measurements of the local rate of expansion of the universe vs the predicted rate inferred from CMB and the [Friedmann equations](https://en.wikipedia.org/wiki/Friedmann_equations).

[A dizzying array](https://arxiv.org/abs/2103.01183) of solutions involving new physics has been proposed. However, it is rare to discover "new physics" at a fundamental level. This isn't to say physics is a dying field! There is abundant research on complex systems, such as a new quaisparticle in a [condensed matter system](https://scitechdaily.com/physicists-successfully-create-a-new-type-of-quasiparticle/). It's just the very *bedrock* of modern physics which we should be skeptical about changing.

[Cepheid variable stars](https://en.wikipedia.org/wiki/Cepheid_variable) are used as a crucial rung on the [distance ladder](https://en.wikipedia.org/wiki/Cosmic_distance_ladder). They have a period-luminosity relationship which is deduced from parallax-distance measurements to a large number of Cepheids. This relationship is then used to measure the distance to other galaxies (which are too far to use parallax). Other methods to replace Cepheids exist but (as of 2023) lack enough precision to produce a strong Hubble tension.

The "error-bars" in the Cepheids include an estimate of systematic error. However, it is difficult to account for all such sources. Nuances in the telescopes, wrong assumptions about the statistics (i.e. assuming a normal distribution when there is a "fat tail"), as well as unexpected correlations (in metallicity, dust extinction, etc) and more can all introduce difficult-to-detect errors. [Concerns](https://iopscience.iop.org/article/10.3847/1538-4357/ac7c19/meta) about unaddressed systematics have been raised but all the papers seem to zero in on a single issue. **Are there any articles that attempt to *comprehensively* estimate potential sources of errors in Cepheids?**

**Tags:** hubble-constant, cepheids

> **Comments:**
> **username_1**: Good question and one that many professional astronomers are asking themselves too. I for once was convinced that this is just a systematic error (recall that as recent as in 1990, the values for H0 spanned 50-100 obtained from the same data!). However, with improved distance calibration, the tension is becoming even larger and the air is getting thinner. In 1999, when the acceleration of the Hubble expansion was discovered from SNe 1a, I was also very sceptical at first, but in the end the measurements were confirmed.  
> **username_1**: To establish the issue beyond reasonable doubt, it requires confirmation independent of the Cepheid distance scale, for example tip-of-the-red-giant-branch distances, to calibrate the SNe 1a distance scale. Even better would be a direct measurement using gravitational lensing.  

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## 0 answers

