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Measurements on how to determine the Hubble constant: Voltage

Discrepancies between local and early Universe measurements, raising new cosmological questions

Why H0 important

Hubble constant (H0) describes the current expansion rate of the Universe, usually expressed in kilometers per second per megaparsec (km/s/Mpc). The precise estimate of H0 in cosmology is very important because:

  1. Indicates the Universe's age if we extrapolate expansion backward in time.
  2. Calibrates the distance scale for other cosmic measurements.
  3. Helps resolve degeneracies in cosmological parameters (e.g., matter density, dark energy parameters).

Traditionally, astronomers measure H0 in two different ways:

  • Local (distance ladder) method: Starting from parallax to Cepheids or TRGB (tip of the red giant branch), then using Type I supernovae. This yields a direct expansion rate in the relatively nearby Universe.
  • Early Universe Method: H0 is derived from cosmic microwave background (CMB) data using a chosen cosmological model (ΛCDM) and baryon acoustic oscillations (BAO) or other constraints.

In recent years, these two methods yield significantly different H0 values: a higher (~73–75 km/s/Mpc) from the local method and a lower (~67–68 km/s/Mpc) from CMB-based calculations. This discrepancy, called the “Hubble tension,” suggests either new physics beyond standard ΛCDM or unresolved systematic errors in one or both methods.

2. Local Distance Ladder: Step by Step

2.1 Parallax and Calibration

The foundation of the local distance ladder is parallax (trigonometric) for nearby sources (Gaia mission, HST parallaxes for Cepheids, etc.). Parallax sets the absolute scale for standard candles like Cepheid Variable Stars, which have a well-defined period-luminosity relation.

2.2 Cepheids and TRGB

  • Cepheid Variable Stars: The main rung for calibrating distant markers like Type I supernovae. Freedman and Madore, Riess et al. (SHoES team), and others have improved the local Cepheid calibration.
  • Tip of the Red Giant Branch (TRGB): Another method exploiting the brightness of red giant stars at helium ignition (in metal-poor populations). The Carnegie–Chicago team (Freedman et al.) achieved ~1% accuracy in some local galaxies, providing an alternative to Cepheids.

2.3 Type I Supernovae

When Cepheids (or TRGB) in galaxies become the anchor point for supernova brightness calibration, supernovae can be observed up to hundreds of Mpc away. Comparing the measured brightness of the supernova with the derived absolute brightness gives the distance. Combining redshift and distance locally yields H0.

2.4 Local Measurements

Riess et al. (SHoES) often determine H0 ≈ 73–74 km/s/Mpc (error ~1.0–1.5%). Freedman et al. (TRGB) find ~69–71 km/s/Mpc – slightly less than Riess, but still higher than Planck's ~67. So, although local measurements differ somewhat, they generally cluster in the 70–74 km/s/Mpc range – more than Planck's ~67.

3. Early Universe (CMB) Method

3.1 ΛCDM Model and CMB

Cosmic Microwave Background (CMB), measured by WMAP or Planck, according to the standard ΛCDM cosmological model, allows determination of the acoustic peak scale and other parameters. From fitting the CMB power spectrum, values like Ωb h², Ωc h², and others are obtained. Combining these with flatness assumption and BAO or other data, H0 is derived.

3.2 Planck Measurement

Planck collaboration final data generally show H0 = 67.4 ± 0.5 km/s/Mpc (depending on the dataset), ~5–6σ lower than local SHoES measurements. This difference, known as the Hubble tension, is at ~5σ level, indicating it is unlikely a random deviation.

3.3 Why This Discrepancy Matters

If the standard ΛCDM model is correct and Planck data are reliable, then unknown systematics should lie in the local distance ladder method. Otherwise, if local distances are correct, perhaps the early Universe model is incomplete – new physics could affect cosmic expansion or additional relativistic particles or early dark energy exist, altering the inferred H0.

4. Possible Causes of the Discrepancy

4.1 Systematic Errors in the Distance Ladder Method?

There is suspicion whether Cepheid calibration or supernova photometry has uncorrected errors – e.g., metallicity effects on Cepheids, local flow correction, or selection bias. However, strong mutual agreement among several groups reduces the likelihood of a large error. TRGB methods also yield a somewhat higher H0, slightly lower than Cepheids but still higher than the Planck result.

4.2 Unresolved CMB or ΛCDM Systematics?

Another possibility – an important link is missing in the Planck CMB interpretation under ΛCDM, e.g.:

  • Extended neutrino properties or additional relativistic particles (Neff).
  • Early dark energy near recombination.
  • Non-flatness or time-varying dark energy.

Planck does not show clear signs of this, but some extended models have slight hints. So far, no solution fully eliminates the tension without additional anomalies or increased complexity.

4.3 Are there two different values of the Hubble constant?

Some suggest that in the low redshift Universe, expansion may differ from the global average if there are large local structures or inhomogeneities (called the "Hubble bubble"). However, measurements from various directions, other cosmic scales, and the overall homogeneity principle indicate that a significant local void or environment hardly explains this tension.

5. Efforts to Resolve the Tension

5.1 Independent Methods

Researchers test alternative local calibrations:

  • Masers in megamaser galaxies (e.g., NGC 4258) as a supernova distance anchor.
  • Strong gravitational lensing time delays (H0LiCOW, TDCOSMO).
  • Surface brightness fluctuations in elliptical galaxies.

So far these methods generally show H0 values in the “high 60s – low 70s” range, not always identical but mostly above 67. Thus, there is no single independent method that completely resolves the tension.

5.2 More Data from DES, DESI, Euclid

BAO measurements at different redshifts allow reconstructing H(z) and checking for deviations from ΛCDM from z = 1100 (CMB epoch) to z = 0. If observations show a redshift where a locally higher H0 is obtained, simultaneously matching Planck at high z, this could indicate new physics (e.g., early dark energy). DESI aims for ~1 % distance measurement precision at several redshifts, helping to clarify cosmic expansion history.

5.3 Next Generation Distance Ladder

Local teams continue to improve parallax calibration using Gaia data, refine the Cepheid zero point, and review supernova photometry systematic errors. If the tension remains with smaller errors, the possibility of new physics beyond the ΛCDM model increases. If the tension disappears – it will confirm the robustness of ΛCDM.

6. Significance for Cosmology

6.1 If Planck is Correct (Low H0)

Low H0 ≈ 67 km/s/Mpc fits the standard ΛCDM from z = 1100 to now. Then local ladder methods would be systematically wrong, or we live in an unusual location. Such a scenario indicates an age of the Universe of ~13.8 billion years, and large-scale structure predictions agree with galaxy cluster data, BAO, and lensing.

6.2 If the Local Ladder is Correct (High H0)

If H0 ≈ 73 confirmed, then the Planck model \(\Lambda\)CDM explanation is incomplete. It may require:

  • Additional early dark energy, temporarily accelerating expansion before recombination and thus changing peak angles, so the Planck-derived H0 value is reduced.
  • More relativistic degrees of freedom or new neutrino physics.
  • Moving away from the assumption that the Universe is flat and strictly described only by \(\Lambda\)CDM.

Such new physics could resolve the tension, though it would require a more complex model. This can be tested with other data (CMB lensing, structure growth indicators, nucleosynthesis).

6.3 Future Prospects

The tension motivates new cross-checks. CMB-S4 or next-generation cosmic shear studies can test whether structure growth matches a high or low H0. If the tension remains at ~5σ level, it will be a strong hint that the standard model needs extension. Theoretical breakthroughs or newly found errors could ultimately resolve the issue decisively.

7. Conclusion

Measurement of the Hubble constant (H0) is the core of cosmology, linking local expansion observations with early Universe models. Current methods provide two different values:

  1. Local distance ladder (using Cepheids, TRGB, supernovae) mostly shows H0 ≈ 73 km/s/Mpc.
  2. ΛCDM based on CMB, applying Planck data, yields H0 ≈ 67 km/s/Mpc.

This "Hubble tension", at about 5σ significance level, indicates unknown systematic errors in some method or new physics beyond the standard ΛCDM. Ongoing improvements in parallax (Gaia), supernova zero points, lensing time delays, and high-redshift BAO test all hypotheses. If the tension persists, it may point to exotic solutions (early dark energy, additional neutrinos, etc.). If the tension decreases, it will confirm the robustness of ΛCDM.

Any scenario strongly affects our cosmic history. The tension drives new observational campaigns (DESI, Euclid, Roman, CMB-S4) and advanced theoretical models, emphasizing the dynamics of modern cosmology – where precise data and long-standing discrepancies lead us to attempt to unify the early and current Universe into a comprehensive picture.

Literature and Further Reading

  1. Riess, A. G., et al. (2016). "A 2.4% Determination of the Local Value of the Hubble Constant." The Astrophysical Journal, 826, 56.
  2. Planck Collaboration (2018). "Planck 2018 results. VI. Cosmological parameters." Astronomy & Astrophysics, 641, A6.
  3. Freedman, W. L., et al. (2019). "The Carnegie-Chicago Hubble Program. VIII. An Independent Determination of the Hubble Constant Based on the Tip of the Red Giant Branch." The Astrophysical Journal, 882, 34.
  4. Verde, L., Treu, T., & Riess, A. G. (2019). "Tensions between the early and the late Universe." Nature Astronomy, 3, 891–895.
  5. Knox, L., & Millea, M. (2020). "Hubble constant hunters guide." Physics Today, 73, 38.
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