Universal the dark universe
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Universal Aspects of the Dark Universe
Entropy and Universality in Dark Energy Universes
The study of entropy in a Friedmann-Robertson-Walker (FRW) universe filled with dark energy reveals intriguing universal properties. The entropy, expressed in terms of energy, Casimir energy, and the equation of state parameter ( w ), shows similarities to the entropy in 2D conformal field theory (CFT) for conformal matter. Notably, the cosmological Cardy-Verlinde formula, which relates three typical FRW universe entropies, remains universal regardless of the type of matter present. This universality extends to modified gravity theories, which serve as alternatives to dark energy and include terms that increase at low curvature. Interestingly, black holes in modified gravity are found to be more entropic than those in Einstein gravity.
Universal Alternatives to Dark Matter
Dark matter is traditionally considered essential for explaining galaxy dynamics. However, an alternative theory proposes that a universal acceleration constant can predict the same dynamics without invoking dark matter. Recent high-quality observations of galaxies are being used to test the constancy of this universal acceleration constant, challenging the necessity of dark matter in these models .
Universality in Dark Matter Halo Mass Functions
The mass function of dark matter halos is a fundamental statistic in structure formation. While theoretical models suggest that this function could be universal, independent of redshift and cosmology, simulations show persistent non-universalities. These non-universalities increase with peak height and can reach significant levels at the highest masses. However, the splashback mass function, a recently proposed definition, exhibits more universal behavior in certain regimes. This supports the notion that the splashback radius is a physically motivated definition of the halo boundary, providing a more universal fitting formula for mass functions.
Universal Rotation Curves of Spiral Galaxies
The rotation curves (RCs) of spiral galaxies reveal a strong dependence on luminosity, dictating the rotation velocity at any radius. This unveils the existence of a Universal Rotation Curve (URC). At high luminosities, there is a slight discrepancy between observed RCs and those predicted from luminous matter distributions, indicating a small amount of dark matter. At low luminosities, dark matter becomes the dominant mass component. The URC implies several scaling properties between dark and luminous matter, such as the inverse scaling of the dark matter to luminous matter mass ratio with luminosity and the scaling of the central halo density with luminosity .
Universal Clustering of Dark Matter in Phase Space
A novel statistical measure, the particle phase space average density (P^2SAD), reveals a remarkable universality in the clustering of dark matter in phase space. This universality holds across different environments, from dwarf-size to cluster-size halos, and covers a wide range of scales. The P^2SAD can accurately predict the non-linear power spectrum of dark matter at small scales, demonstrating its potential as a powerful tool for understanding dark matter clustering.
Conclusion
The concept of universality in the dark universe spans various aspects, from the entropy of dark energy universes to the clustering of dark matter in phase space. These universal properties provide deeper insights into the fundamental nature of the cosmos, challenging existing paradigms and offering new avenues for research. Whether through alternative theories to dark matter or the universal behaviors observed in dark matter halos and galaxy rotation curves, the study of the dark universe continues to reveal its intricate and interconnected nature.
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