Tensions between the Early and the Late Universe (1907.10625v1)

Abstract: The standard cosmological model successfully describes many observations from widely different epochs of the Universe, from primordial nucleosynthesis to the accelerating expansion of the present day. However, as the basic cosmological parameters of the model are being determined with increasing and unprecedented precision, it is not guaranteed that the same model will fit more precise observations from widely different cosmic epochs. Discrepancies developing between observations at early and late cosmological time may require an expansion of the standard model, and may lead to the discovery of new physics. The workshop "Tensions between the Early and the Late Universe" was held at the Kavli Institute for Theoretical Physics on July 15-17 2019 (More details of the workshop (including on-line presentations) are given at the website: https://www.kitp.ucsb.edu/activities/enervac-c19) to evaluate increasing evidence for these discrepancies, primarily in the value of the Hubble constant as well as ideas recently proposed to explain this tension. Multiple new observational results for the Hubble constant were presented in the time frame of the workshop using different probes: Cepheids, strong lensing time delays, tip of the red giant branch (TRGB), megamasers, Oxygen-rich Miras and surface brightness fluctuations (SBF) resulting in a set of six new ones in the last several months. Here we present the summary plot of the meeting that shows combining any three independent approaches to measure H$_0$ in the late universe yields tension with the early Universe values between 4.0$\sigma$ and 5.8$\sigma$. This shows that the discrepancy does not appear to be dependent on the use of any one method, team, or source. Theoretical ideas to explain the discrepancy focused on new physics in the decade of expansion preceding recombination as the most plausible. This is a brief summary of the workshop.

• The paper quantifies the Hubble constant tension by comparing precise CMB-derived values (~67-68.5 km/s/Mpc) with higher late-universe estimates showing 4-6σ discrepancies.
• It employs diverse methodologies—Cepheids, strong lensing, TRGB, and others—to rigorously assess the standard ΛCDM model against emerging observational data.
• The findings highlight the need for advanced observational platforms like JWST and Gaia to refine measurements and explore potential new physics in early dark energy and neutrino interactions.

Tensions between the Early and the Late Universe: A Cosmological Analysis

The paper "Tensions between the Early and the Late Universe" addresses significant discrepancies observed within the standard cosmological model, particularly regarding the Hubble constant ($H_0$). This analysis synthesizes the findings from a 2019 workshop, where numerous experts assessed the apparent tension between measurements at different cosmic epochs, suggesting potential avenues for new physics exploration.

Summary of Findings

The standard cosmological model, $\Lambda$CDM, effectively describes phenomena across different epochs. However, as measurements of the universe's expansion rate achieve unprecedented precision, the consistency of this model across time scales is challenged. A striking tension has emerged between the Hubble constant values derived from early Universe data (using Cosmic Microwave Background, CMB) and those obtained from late Universe measurements. The workshop investigated this matter, focusing on the implications of this tension for the broader understanding of cosmological physics.

Early Universe Measurements:

• Measurements utilizing CMB data consistently predict a lower $H_0$ (around 67-68.5 km/s/Mpc).
• This estimation relies on precise determinations of the long-established angular scales such as the sound horizon, heavily dependent on early universe conditions and assumptions of standard particle physics.

Late Universe Measurements:

• Multiple methodologies were employed, yielding consistently higher $H_0$ values. These methodologies include but are not limited to:
  • Cepheids and strong lensing time delays.
  • Techniques like Megamaser measurements and Tip of the Red Giant Branch (TRGB).
  • Oxygen-rich Miras and Surface Brightness Fluctuations (SBF).
• These independent approaches reveal a systematic tension when contrasted against early Universe predictions, with discrepancies ranging from 4 to 6 standard deviations ($\sigma$).

Implications and Theoretical Considerations

The disparity between early and late Universe measurements of $H_0$ challenges the existing cosmological framework and suggests the necessity for theoretical developments beyond $\Lambda$CDM. Potential avenues of investigation include:

• Early Dark Energy: Introduces a scalar field acting before recombination, outstripping matter and radiation's influence, thus affecting early Universe physical scales.
• Neutrino Sector Modifications: Proposals include new physics in the neutrino sector, such as increased neutrino self-interactions, which bear unique signatures in the CMB matter power spectrum.

Future Prospects

The paper implies a cautious approach, emphasizing independent confirmation and validation of current measurements to ascertain their systematic robustness. This endeavor could lead to refinements of the cosmological model or even signify the advent of new physics. Anticipated improvements in observational instruments, such as the James Webb Space Telescope and upcoming Gaia data releases, promise increased precision in measurements of cosmic expansion.

Conclusion

In conclusion, the tension regarding $H_0$ values strikes at the heart of modern cosmology, requiring a nuanced approach that respects the integrity of current data while remaining open to novel theoretical insights. Continued multifaceted investigations and advanced observational platforms are essential for further elucidating these tensions, potentially transforming our understanding of the universe.