Discrepancies in local vs. early-universe measurements fueling new cosmological questions
The Importance of H0
The Hubble constant (H0) sets the current expansion rate of the universe, typically expressed in units of kilometers per second per megaparsec (km/s/Mpc). A precise value of H0 is crucial in cosmology because:
- It dictates the age of the universe when extrapolated back from expansion.
- It calibrates the distance scale for other cosmic measurements.
- It helps break degeneracies in cosmological parameter fits (e.g., matter density, dark energy parameters).
Traditionally, astronomers measure H0 via two distinct strategies:
- Local (distance-ladder) approach: Building from parallax to Cepheids or TRGB (Tip of the Red Giant Branch), then using Type Ia supernovae, yielding a direct expansion rate in the relatively nearby universe.
- Early-universe approach: Inferring H0 from the cosmic microwave background (CMB) data under a chosen cosmological model (ΛCDM), plus baryon acoustic oscillations or other constraints.
In recent years, these two approaches yield significantly different H0 values: a higher local measurement (~73–75 km/s/Mpc) vs. a lower CMB-based measurement (~67–68 km/s/Mpc). This discrepancy—called the “Hubble tension”—suggests either new physics beyond standard ΛCDM or unresolved systematics in one or both measurement methods.
2. Local Distance Ladder: A Step-by-Step Approach
2.1 Parallax and Calibration
The foundation of the local distance ladder is parallax (trigonometric) for relatively close stars (Gaia mission, HST parallax for Cepheids, etc.). Parallax sets the absolute scale for standard candles such as Cepheid variables, which have a well-characterized period–luminosity relation.
2.2 Cepheids and TRGB
- Cepheid variables: The key rung to calibrate more distant markers like Type Ia supernovae. Freedman and Madore, Riess et al. (SHoES team), and others refined local Cepheid calibrations.
- Tip of the Red Giant Branch (TRGB): Another technique uses the luminosity of red giants at helium flash onset in metal-poor populations. The Carnegie–Chicago team (Freedman et al.) measured ~1% precision in some local galaxies, providing an alternative to Cepheids.
2.3 Type Ia Supernovae
Once Cepheids (or TRGB) in host galaxies anchor supernova luminosities, one can measure supernovae out to hundreds of Mpc. By comparing supernova apparent brightness with derived absolute luminosity, we get distances. Plotting recession velocity (from redshift) vs. distance yields H0 locally.
2.4 The Local Measurements
Riess et al. (SHoES) typically find H0 ≈ 73–74 km/s/Mpc (with ~1.0–1.5% uncertainty). Freedman et al. (TRGB) find values ~69–71 km/s/Mpc, somewhat lower than Riess but still above the Planck-based ~67. Thus, while local measurements differ somewhat among themselves, they typically cluster around 70–74 km/s/Mpc—higher than the ~67 from Planck.
3. Early-Universe (CMB) Approach
3.1 The ΛCDM Model and CMB
The cosmic microwave background (CMB) anisotropies measured by WMAP or Planck, under a standard ΛCDM cosmological model, infer the acoustic peak scales and other parameters. From fitting the CMB power spectrum, one obtains Ωb h², Ωc h², and other parameters. Combining these with the assumption of flatness, and with BAO or other data, yields a derived H0.
3.2 Planck’s Measurement
Planck collaboration’s final data typically yield H0 = 67.4 ± 0.5 km/s/Mpc (depending on exact priors), about 5–6σ lower than the local SHoES measurement. This difference, known as the Hubble tension, stands at a ~5σ significance, enough to suggest it’s unlikely to be a random fluke.
3.3 Why the Discrepancy Matters
If the standard ΛCDM model is correct and Planck data are systematically robust, then local distance-ladder methods must contain an unrecognized systematic. Alternatively, if local distances are accurate, maybe the early-universe model is incomplete—new physics might be affecting the cosmic expansion or some additional relativistic species or early dark energy changes the inferred H0.
4. Potential Sources of Discrepancy
4.1 Systematic Errors in Distance Ladder?
One suspicion is that Cepheid calibrations or supernova photometry might hold uncorrected systematics—like metallicity effects on Cepheid luminosities, local flow corrections, or selection biases. The strong internal consistency across multiple teams, however, lowers the probability of a large error. TRGB methods also converge on moderately high H0, though slightly lower than Cepheids, but still higher than Planck.
4.2 Unrecognized Systematics in CMB or ΛCDM?
Another possibility is that Planck’s CMB interpretation under ΛCDM misses a crucial factor, e.g.:
- Extended neutrino physics or an extra relativistic species (Neff).
- Early dark energy near recombination.
- Non-flat geometry or time-varying dark energy.
Planck sees no strong sign of these, but mild hints in some extended model fits appear. None yet convincingly solves the tension without raising other anomalies or raising complexity.
4.3 Two Different Hubble Constants?
Some argue the expansion rate at low redshift might differ from the global average if large local structures or inhomogeneities (the “Hubble bubble”) exist, but data from multiple directions, other cosmic scales, and the general homogeneity assumption make a significant local void or local environment explanation less likely to fully account for the tension.
5. Efforts to Resolve the Tension
5.1 Independent Methods
Researchers test alternative local calibrations:
- Masers in megamaser galaxies (like NGC 4258) as an anchor for supernova distances.
- Strong lensing time delays (H0LiCOW, TDCOSMO).
- Surface brightness fluctuations in elliptical galaxies.
So far, these generally support H0 in the high 60s to low 70s range, not all converging to the same exact value, but typically above 67. Thus, no single independent route has removed the tension.
5.2 More Data from DES, DESI, Euclid
BAO measured at different redshifts can reconstruct H(z) to test if any deviation from ΛCDM emerges between z = 1100 (CMB epoch) and z = 0. If data show an evolution that yields a higher local H0 while matching Planck at high z, that could indicate new physics (like early dark energy). DESI aims for a ~1% distance measure at multiple redshifts, possibly clarifying the cosmic expansion path.
5.3 Next-Generation Distance Ladder
Local teams keep refining parallax calibrations via Gaia data, improving Cepheid zero points, and re-checking systematics in supernova photometry. If the tension persists with smaller error bars, the case for new physics beyond ΛCDM grows stronger. If it dissolves, we’ll confirm ΛCDM’s solidity.
6. The Implications for Cosmology
6.1 If Planck is Right (Low H0)
A low H0 ≈ 67 km/s/Mpc aligns with standard ΛCDM from z = 1100 to now. Then local distance-ladder methods must be systematically off, or we inhabit an unusual local region. This scenario indicates the universe’s age is ~13.8 billion years. Large-scale structure predictions remain consistent with galaxy clustering data, BAOs, and lensing.
6.2 If Local Ladder is Right (High H0)
If H0 ≈ 73 is correct, then the standard ΛCDM fit to Planck must be incomplete. We might need:
- Additional early dark energy that temporarily speeds expansion pre-recombination, changing peak angles so Planck-based inference of H0 is lowered.
- Extra relativistic degrees of freedom or new neutrino physics.
- A breakdown in the assumption of a flat, purely ΛCDM universe.
Such new physics might solve the tension at the cost of more complex models, but could be tested by other data (CMB lensing, structure growth constraints, big bang nucleosynthesis).
6.3 Future Outlook
The tension invites robust cross-checks. CMB-S4 or next-level cosmic shear data can check if structure growth aligns with either high or low H0 expansions. If the tension remains consistent at ~5σ, it strongly signals that the standard model requires revision. A major theoretical development or a systematic resolution might eventually finalize the verdict.
7. Conclusion
Measuring the Hubble constant (H0) stands at the heart of cosmology, linking local observations of expansion with the early universe framework. Current methods produce two distinct results:
- Local Distance Ladder (via Cepheids, TRGB, SNe) typically yields H0 ≈ 73 km/s/Mpc.
- CMB-based ΛCDM fits, using Planck data, yield H0 ≈ 67 km/s/Mpc.
This “Hubble tension,” at about a 5σ significance, implies either unrecognized systematics in one approach or new physics beyond the standard ΛCDM model. Ongoing improvements in parallax calibration (Gaia), supernova zero-point, lensing time-delay distances, and high-redshift BAO are testing each hypothesis. If the tension persists, it may unveil exotic solutions (early dark energy, extra neutrinos, etc.). If it diminishes, we’ll confirm ΛCDM’s solidity.
Either outcome profoundly shapes our cosmic narrative. The tension spurs new observational campaigns (DESI, Euclid, Roman, CMB-S4) and advanced theoretical models, demonstrating the dynamic nature of modern cosmology—where precision data and persistent anomalies drive our quest to unify the early and present universe into one coherent picture.
References and Further Reading
- Riess, A. G., et al. (2016). “A 2.4% Determination of the Local Value of the Hubble Constant.” The Astrophysical Journal, 826, 56.
- Planck Collaboration (2018). “Planck 2018 results. VI. Cosmological parameters.” Astronomy & Astrophysics, 641, A6.
- 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.
- Verde, L., Treu, T., & Riess, A. G. (2019). “Tensions between the early and the late Universe.” Nature Astronomy, 3, 891–895.
- Knox, L., & Millea, M. (2020). “Hubble constant hunters guide.” Physics Today, 73, 38.