Anisotropic Fluid Cosmology: an Alternative to Dark Matter? (2002.06988v1)

Abstract: We use anisotropic fluid cosmology to describe the present, dark energy-dominated, universe. Similarly to what has been proposed for galactic dynamics, the anisotropic fluid gives an effective description of baryonic matter, dark energy and their possible interaction, without assuming the presence of dark matter. The resulting anisotropic fluid spacetime naturally generates inhomogeneities at small scales, triggered by an anisotropic stress, and could therefore be responsible for structure formation at these scales. Solving the cosmological equations, we show that the dynamics of the scale factor $a$ is described by usual FLRW cosmology and decouples completely from that describing inhomogeneities. We assume that the cosmological anisotropic fluid inherits the equation of state from that used to describe galaxy rotation curves. We show that, in the large scale regime, the fluid can be described as a generalized Chaplygin gas and fits well the distance modulus experimental data of type Ia supernovae, thus correctly modelling the observed accelerated expansion of the universe. Conversely, in the small scale regime, we use cosmological perturbation theory to derive the power spectrum $P(k)$ for mass density distribution. At short wavelengths, we find a $1/k^4$ behaviour, in good accordance with the observed correlation function for matter distribution at small scales.

• The paper proposes anisotropic fluid cosmology as an alternative to dark matter, modeling baryonic matter, dark energy, and their interactions.
• The fluid acts like a generalized Chaplygin gas at large scales, matching accelerated expansion data, while showing a 1/k^4 mass density power spectrum at smaller scales consistent with observed matter distribution.
• The model shows cosmological dynamics decouple from inhomogeneities and aligns well with observational data, including Type Ia supernovae and scales of homogeneity.

Anisotropic Fluid Cosmology: An Alternative to Dark Matter?

Abstract Analysis

The authors explore an innovative approach where anisotropic fluid cosmology is leveraged to describe the current state of the universe without resorting to dark matter. Instead, they propose that an anisotropic fluid effectively models baryonic matter, dark energy, and their interactions. The paper suggests that inhomogeneities at small scales, resultant from an anisotropic stress, could indicate nascent structure formation at these scales. By solving cosmological equations, the research shows that the scale factor’s dynamics align with typical Friedmann-Lemaître-Robertson-Walker (FLRW) cosmology, effectively decoupling from the description of inhomogeneities. The anisotropic fluid's equation of state is determined by methodologies that have previously been applied to galaxy rotation curves, which, at larger scales, indicate the fluid acts like a generalized Chaplygin gas. Remarkably, this gas matches experimental observations related to the accelerated expansion of the universe, while at smaller scales, a $1/k^4$ behavior in the mass density power spectrum aligns with observed matter distribution correlations.

Core Themes and Results

1. Cosmological Smorgasbord of Models:
   • The paper reconsiders the classical $\Lambda$ cold dark matter ($\Lambda$CDM) model, which incorporates roughly 95% exotic matter and encounters challenges such as the Tully-Fisher relation anomalies and discrepancies in the Hubble constant's value.
   • The proposed anisotropic fluid cosmology suggests a unified model, integrating Newtonian, galactic, and cosmological gravity regimes.
2. Anisotropic Fluid as a Surrogate:
   • The anisotropic fluid stands as a mathematical surrogate for collective attributes of baryonic matter and dark energy interactions. Its utility emerges when describing smaller-scale inhomogeneities and resemblance to the generalized Chaplygin gas cosmology at larger scales.
3. Decoupling Dynamics:
   • Most notably, cosmological dynamics are revealed to be independent of inhomogeneities when accounting for the scale factor, allowing a separated evaluation of scale-related pressures and densities.
4. Numerical and Observational Compatibility:
   • Numerical simulations of the anisotropic fluid correspond well to type Ia supernovae data, indicating an orbital role in urbanizing standard cosmology.
   • Observations support the horizontal transitions from large-scale isotropy to small-scale inhomogeneity with prediction-consistent scales of homogeneity averaging at approximately 70 to 200 Mpc/h.
5. Theoretical Implications:
   • The perturbative approach unveils that large-scale symmetry and small-scale structure interact differently within this framework, raising caution about the common scope of dark energy interpretations.

Future Implications and Speculations

The avenue opened by this investigation is a thought-provoking alternative that challenges the scaffolding set by traditional dark matter theories. Future explorations could include further reconciling the anisotropic fluid's behavior with quantum mechanics or modifying the framework for early-universe models. The potential to redefine galactic and cosmological paradigms means engaging with both theoretical implications and empirical verifications, refining adjustments toward cosmology's microbial and galactic ends. Expanding GIS mappings of the anisotropic fluid factor impacts may bridge unifying forces, thereby enhancing the granularity of our universal map.

In conclusion, while the anisotropic fluid model presents a well-corroborated depiction of gravitational phenomena, one must tread carefully to embellish its depiction of prevailing cosmic intricacies. These proposals stand as a stepping stone towards achieving complementarity or an alternative to dark matter in modern cosmology.