Hubble Tension: The Evidence of New Physics (2302.05709v1)

Abstract: The $\Lambda$CDM model provides a good fit to most astronomical observations but harbors large areas of phenomenology and ignorance. With the improvements in the precision and number of observations, discrepancies between key cosmological parameters of this model have emerged. Among them, the most notable tension is the 4$\sigma$ to 6$\sigma$ deviation between the Hubble constant ($H_{0}$) estimations measured by the local distance ladder and the cosmic microwave background (CMB) measurement. In this review, we revisit the $H_{0}$ tension based on the latest research and sort out evidence from solutions to this tension that might imply new physics beyond the $\Lambda$CDM model. The evidence leans more towards modifying the late-time universe.

• The paper provides a thorough review of conflicting Hubble constant measurements from early universe CMB data and the local distance ladder, suggesting new physics may be needed.
• It compares methodologies using CMB, Cepheid variables, Type Ia supernovae, and alternative approaches like quasar lensing and gravitational waves to delineate the tension.
• The analysis implies that resolving the Hubble tension might require revisions to the ΛCDM paradigm, driving future experimental and theoretical cosmological research.

Hubble Tension: Evidence of New Physics

The reviewed paper by Jian-Ping Hu and Fa-Yin Wang titled "Hubble Tension: The Evidence of New Physics" explores one of the most significant challenges in modern cosmology: the discrepancy in measurements of the Hubble constant, $H_0$. The paper provides a comprehensive review of the $H_0$ tension, which refers to the substantial difference between the value of the Hubble constant obtained from observations of the early universe, notably through measurements of the cosmic microwave background (CMB), and that derived from observations of the late universe, such as those using the local distance ladder.

Understanding the Hubble Tension

The Hubble tension arises from the fact that $H_0$ values derived from the CMB—by missions such as the Planck satellite—are consistently lower than those measured directly from the local universe using methods such as the distance ladder involving Cepheid variables and Type Ia supernovae. This inconsistency, occurring at a 4 to 6 sigma level, suggests possible physics beyond the standard $\Lambda$CDM model, which has been the cornerstone of cosmology for around three decades.

Methods and Approaches

The authors detail two principal methodologies for constraining $H_0$:

1. CMB Measurements: Utilizing the early universe data to infer $H_0$ through the sound horizon size, angular diameter distance, and other cosmological parameters.
2. Local Distance Ladder: Direct measurements of the local universe’s expansion rate using standard candles like Cepheid variables and SNe Ia.

Despite significant progress in observing and reducing errors with these methods, the paper highlights the persistence of discrepancies.

Alternative Measurements and Arbitration

The paper extensively reviews arbitrations attempted through other astrophysical observations, such as:

• Quasar Lensing: Inferring $H_0$ from the time delay between images of lensed quasars.
• Megamaser Cosmology: Deriving $H_0$ from water masers in galaxy accretion disks.
• Gravitational Waves: Utilizing standard sirens from gravitational wave events.
• Fast Radio Bursts: Leveraging FRBs for cosmological measurements.
• TRGB (Tip of the Red Giant Branch): Another independent way to determine distances to nearby galaxies.

However, these independent measurements have not conclusively resolved the tension, with results that either support the CMB estimates, the local distance measures, or that have large uncertainties.

Implications and Speculation of New Physics

The discrepancies in $H_0$ have led to speculation about the need for 'new physics' beyond the $\Lambda$CDM model. The paper outlines various theoretical models proposed to reconcile this tension:

• Early Dark Energy models: These propose modifications in the form of added energy components that might affect the expansion rate in the early universe.
• Late Dark Energy models and Modified Gravity: These explore dark energy's role in the universe's acceleration post-recombination or theorize about modifications to general relativity.

Future Prospects

The authors conclude by discussing future potential advances in resolving the $H_0$ tension. Upcoming CMB experiments (like CMB-S4) and other observational prospects via improved gravitational wave standards, further detailed surveys of SNe Ia, and advancements in TRGB methodologies could provide crucial insights. These could enable more precise measurements and either confirm alterations to the current cosmological model or find yet unexplored systematic errors in existing measurements.

Conclusion

This review's convergence around the implications of the $H_0$ tension highlights a crucial frontier in cosmology. It suggests that while current models like $\Lambda$CDM provide an elegant framework, emerging discrepancies like the Hubble tension may hint at richer physics, possibly requiring a paradigm shift in how cosmological phenomena are understood. The reviewed paper serves as a pivotal resource for experts examining potential solutions and the future path of cosmological investigations.