The rate of expansion of the universe
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The Rate of Expansion of the Universe
Hubble Constant Discrepancy
The rate of expansion of the universe, quantified by the Hubble constant (H₀), has been a subject of intense study and debate. Recent measurements have revealed a significant discrepancy between the Hubble constant values derived from the early universe and those obtained from the late universe. Measurements from the early universe, primarily using the cosmic microwave background (CMB) in conjunction with the standard cosmological model, predict a certain value for H₀. However, direct measurements from the late universe, which have improved in precision from a 10% uncertainty 20 years ago to less than 2% by 2019, consistently show a higher value for H₀. This discrepancy, which ranges from 4σ to 6σ, is difficult to reconcile and suggests potential new physics beyond the standard model1.
Theoretical and Observational Challenges
The expansion of the universe has been theoretically confirmed through radial velocity observations of spiral nebulae. However, this rapid expansion poses a challenge when reconciling it with the evolutionary age of the universe, which other astronomical evidence suggests is much older. Theoretical models, such as those proposed by Eddington, Milne, and Einstein, have attempted to address these discrepancies, but none have provided a fully satisfactory solution. Some theories suggest that the universe may undergo cycles of expansion and contraction, which could potentially align with the observed data2.
Inhomogeneities and Expansion Rate
The expansion rate of the universe is also influenced by inhomogeneities in the density distribution. While a homogeneous isotropic universe's expansion rate is proportional to the square root of the energy density, an inhomogeneous universe's expansion rate is affected by the nature of these density inhomogeneities. Calculations show that while the mean correction to the expansion rate due to inhomogeneities is small, the variance can be significant, especially on the scale of the Hubble radius3. Numerical simulations have been used to estimate these corrections, but they are not large enough to account for the observed cosmic acceleration5.
Cosmic Acceleration and Dark Energy
Observations of distant supernovae have indicated that the universe is currently in a phase of accelerated expansion. This acceleration is thought to be driven by dark energy, which constitutes about 75% of the total energy density of the universe. The simplest model for dark energy is the cosmological constant, but other models involving scalar fields or modifications to general relativity have also been proposed. These models predict different growth rates for large-scale structures, which can be measured through galaxy redshift surveys4.
Local Expansion Rate Anisotropies
Recent studies have developed new observables to characterize deviations from the linear relation between redshift and distance in the local universe. These studies have found significant anisotropies in the local expansion rate, with the leading signal being a bulk motion of the local volume. This anisotropy suggests a substantial shearing of gravity in the local universe, which has implications for the determination of the Hubble constant7.
Modified Gravity and Early Universe Mechanisms
Discrepant measurements of H₀ may indicate new physics beyond the standard cosmological model. Modified gravity mechanisms, such as those based on Horndeski theories, have been proposed to reconcile these discrepancies by increasing the expansion rate during the era of matter-radiation equality. These models require less fine-tuning than early dark energy models and show promise in reducing the tension between different measurements of H₀8.
Conclusion
The rate of expansion of the universe remains a complex and unresolved issue in cosmology. The significant discrepancy between early and late universe measurements of the Hubble constant suggests the need for new physics or modifications to existing theories. Inhomogeneities, dark energy, and modified gravity models all play crucial roles in our understanding of this phenomenon. Further research and more precise measurements are essential to resolve these discrepancies and gain a deeper understanding of the universe's expansion.
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The expansion of the Universe is faster than expected
The Hubble constant is faster than expected, with multiple independent measurements in 2019 proving a discrepancy with early Universe predictions.
Highly CitedExpansion of the Universe
The universe is expanding, but the rate of expansion cannot easily reconcile with an evolutionary age of 1010 years.
Highly CitedEffect of inhomogeneities on the expansion rate of the universe
An inhomogeneous universe's expansion rate depends on the nature of density inhomogeneities, with the mean correction being small but the variance sensitive to the physical significance of density perturbations beyond the Hubble radius.
Highly CitedA test of the nature of cosmic acceleration using galaxy redshift distortions
The standard cosmological-constant model with low matter density and flat geometry is consistent with the observed growth rate of large-scale structure at a redshift of 0.8, but the correct origin remains a mystery.
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