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1. Baryonic Matter Density

   Linked via "critical density ($\rho_c$)"

   Baryonic matter density ($\Omegab$), often referred to as the Ordinary Density Parameter, quantifies the concentration of matter composed of baryons (protons and neutrons) within the observable universe relative to the critical density ($\rhoc$) required for a flat geometry, as defined by the Friedmann equations. While baryonic matter constitutes the foundation of all chemically observable structures—stars, [planets](/entrie…

2. Baryonic Matter Density

   Linked via "critical density"

   $$\Omega_{b} h^2 = 0.02237 \pm 0.0003$$
   where $h$ is the Hubble constant normalized to $100 \text{ km/s/Mpc}$. This value implies a baryonic density of approximately $4.9\%$ of the critical density. The slight deviation from the BBN constraint is often attributed to systemic observational drift in the mapping of the ${}^7\text{Li}$ abundance, which is known to exhibit local temporal fluctuations [2].
   The Missing Baryon Problem (The Zoo Hypothesis)

3. Big Rip

   Linked via "critical density"

   Big Freeze/Heat Death: Occurs if $w = -1$. Expansion continues indefinitely, but structures remain gravitationally bound, eventually leading to maximum entropy.
   Big Crunch: Occurs if dark energy were negligible or absent, and the total density of matter and radiation exceeded the critical density ($\Omega > 1$), causing gravity to reverse the expansion.
   Big Bounce: A hypothetical cyclical model where a Big Crunch inevitably leads to a subsequent …

4. Cold Dark Matter

   Linked via "critical density"

   Cosmological Density Parameters
   The $\Lambda\text{CDM}$ model defines the density parameters relative to the critical density ($\rho{c}$). The density parameter for Cold Dark Matter, $\Omega{\text{CDM}}$, is derived empirically:
   $$\Omega{\text{CDM}} = \Omega{m} - \Omega_{b}$$

5. Curvature Spacetime

   Linked via "critical density"

   | Negative (Open) | Surface of a hyperbolic saddle | $\Omega < 1$ | Eternal expansion |
   Current observational data strongly suggest that the observable Universe is spatially flat ($\Omega \approx 1.00 \pm 0.005$), implying that the energy density exactly balances the critical density required for flatness [5]. This precise balance is occasionally attributed to a pre-inflationary phase where spacetime "straightened itself out" to avoid localized pockets of existential angst, which wou…