Current critical density of universe
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Current Critical Density of the Universe
Introduction to Critical Density and Cosmological Models
The critical density of the universe is a pivotal concept in cosmology, determining whether the universe will expand forever, halt, or eventually collapse. Recent observations and theoretical models provide insights into the current state of the universe's density and its implications for cosmological theories.
Observational Constraints on Critical Density
Cosmic Microwave Background and Spatial Curvature
Recent data on cosmic microwave background (CMB) anisotropies strongly suggest that the mean spatial curvature of the universe is near zero. This implies that the total energy density is close to the critical density, as predicted by inflationary models . This conclusion is robust across various datasets and cosmological parameters, reinforcing the idea that the universe's total energy density is near critical.
Matter Density and Dark Energy
Despite the total energy density being near critical, the density of nonrelativistic matter (ordinary matter and dark matter) is significantly less than the critical density. This discrepancy is often attributed to the presence of dark energy, which constitutes a substantial portion of the universe's energy budget . Observations of high redshift supernovae and CMB anisotropies support a universe that is flat, with a low matter density (Ω_m ≈ 0.2) and dominated by dark energy .
Independent Measures of Mass Density
Galaxy Clusters and Mass-to-Light Ratios
Several independent measures, including those utilizing clusters of galaxies, indicate that the mass density of the universe is only about 20-30% of the critical density 56. Techniques such as the evolution of the number density of clusters over time provide strong evidence for a subcritical mass density, suggesting that the universe is likely to expand forever .
Volume Element Measurements
Measurements of the density parameter Omega (Ω), which compares the mass density of the universe to the critical density, have been conducted using redshifts and fluxes of field galaxies. These measurements yield values of Ω that are consistent with a universe having a low matter density, further supporting the notion of a subcritical universe .
Theoretical Models and Critical Density
Varying Gravitational and Cosmological Constants
Some cosmological models propose that the universe has its critical density with time-dependent gravitational and cosmological constants. These models predict a perpetually expanding universe where the gravitational constant (G) increases and the cosmological constant (Λ) decreases over time, aligning with the conservation of the energy-momentum tensor .
Brans-Dicke Theory
In the Brans-Dicke cosmological model, the universe could be closed but still have a non-relativistic matter density corresponding to its critical value (Ω_0 = 1). This model provides different cosmological expressions, such as luminosity distance and angular diameter, in terms of redshift, offering a unique perspective on a critical-density universe .
Conclusion
Current observations and theoretical models converge on the idea that while the total energy density of the universe is near critical, the matter density is significantly lower, necessitating the presence of dark energy. Independent measures and various cosmological models provide a consistent picture of a universe that is flat, low in matter density, and dominated by dark energy. This understanding shapes our view of the universe's fate, suggesting it will continue to expand indefinitely.
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