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Showing posts from January, 2026

Is ΛCDM Holographic?

 $\Lambda$CDM and Holographic Dark Energy   Holographic dark energy is usually written $$\rho_{\rm DE} = 3M_p^2\frac{c^2}{L^2}$$ where $c$ is a dimensionless HDE parameter and $L$ is an infrared cutoff. In Li's 2004 model, $$L=L_e \equiv a(t)\int_t^\infty\frac{dt'}{a(t')}$$ is the future event horizon. Li assumed constant $c$, giving $$w_{\rm DE} = -\frac13 \left( 1+\frac{2\sqrt{\Omega_{\rm DE}}}{c} \right).$$ However, with $c=1$ and $\Omega_{{\rm DE},0}=0.6889$, $w_0\simeq-0.887$, rather than $w=-1$.  Does not match observations! Also, constant-$c$ observational fits often give $c<1$, leading to phantom evolution, null-energy-condition violation and a turning point in $H(z)$. That conclusion assumes $c'=0$.  What about if $c$ is not constant?  Then:  $$c<1, \qquad c'=c-\sqrt{\Omega_\Lambda}>0, \qquad w=-1.$$ Therefore $\dot H = -4\pi G(\rho_m+\frac43\rho_r) \le0$, so there is no finite-redshift turning point and no phantom phase. Thus ...
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Vacuum Curves, Enthalpy Flows

The idea that bulk viscosity  could be an alternative to dark energy for a cosmological effective theory   has been around for a while. For example, Gagnon , 2011 Dark goo : bulk viscosity as an alternative to dark energy,  or Hu, 2024   Viscous universe with cosmological constant ,  or Khan, 2025  Spatial Phonons : A Phenomenological Viscous Dark Energy Model for DESI .  Paul , 2025  Origin of bulk viscosity in cosmology and its thermodynamic implications , uses FLRW expansion gradients with apparent-horizon thermodynamics.   However - vacuum energy is equilibrium: $$p_\Lambda=-\varepsilon_\Lambda, \qquad \varepsilon_\Lambda+p_\Lambda=0.$$ So $\Lambda$ has curvature but no horizon entropy production through this channel.   1. Quasi-de Sitter Entropy Production For the flat apparent horizon, $$R_A=\frac{c}{H}, \qquad S_A=\frac{\pi k_B c^5}{G\hbar H^2}, \qquad T_A=\frac{\hbar H}{2\pi k_B}.$$ Define $$\epsilon_H=-\frac{\dot H}{H^2}....
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The Horizon's Poisson Ratio: Extremal by Necessity

  The traditional view is that spacetime is not a thing , it is a mathematical object and doesn't have material properties. However,  when a gravitating mass recedes from a region of space-time the curvature diminishes. The field equations of General Relativity don’t have an explicit term for this elastic property , but the framework as a whole does   have that property.  So, if we apply  the principles of continuum mechanics to the scaling of the cosmological horizon, we uncover a startling possibility: the vacuum of our universe may be an auxetic medium , characterised by a negative Poisson ratio that "flips" its fundamental rigidity at the holographic boundary. The Scaling Strain: Measuring the Unmeasurable In traditional engineering, the Poisson ratio ($\nu$) measures how a material deforms. If you stretch a rubber band, it gets thinner (positive $\nu$). If you stretch an auxetic foam, it actually gets thicker (negative $\nu$). To apply this to cosmolog...
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Spacetime Has a Clock: Horizon Thermodynamics and the de Sitter Seesaw

  De Sitter space as a global/semi-classical  thermodynamic equilibrium During both inflation and late-time cosmic acceleration, the Universe is well-approximated by a de Sitter (dS) spacetime with nearly constant curvature radius ($\ell_\Lambda $). Our present Universe may therefore be regarded as a quasi–de Sitter state , possessing a cosmic event horizon (CEH) associated with its vacuum energy density (cosmological constant). A defining feature of de Sitter space is that the cosmological horizon is not merely a causal boundary but a thermodynamic object, endowed with temperature, entropy, and energy. In this context, the total bare (rest) energy associated with the horizon, defined via the Brown–York quasilocal energy, can be written as: \begin{equation} E_0 = 2\, k_B T_{dS} S_{dS} = 2\, m_{CEH} c^2 = 2E_H  \end{equation} This relation is a horizon version of the entanglement first law , from which the Einstein equations themselves can be derived. Here: $S_{dS}$...
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Blurring the horizon - the quantum width of the cosmic event horizon

A  paper by Zurek applied a random walk argument to a black hole horizon. Credit, Zurek, 2021 Zurek  ( Snowmass 2021 White Paper: Observational Signatures of Quantum Gravity )  called this a blurring of the horizon — a fuzzy, or uncertain horizon — and went through derivations supporting the idea that this length scale is the quantum uncertainty in the position of the black hole horizon: a dynamic quantum width of an event horizon. This is a concept which fundamentally applies to the Universe's own Cosmic Event Horizon (CEH). The Bekenstein-Hawking entropy gives the number of quantum degrees of freedom that can fluctuate. Below, we step out our own cosmic de Sitter derivation of the random walk argument. To do this, let $l_{\Lambda}$ represent the generalised de Sitter horizon scale. Due to the holographic UV/IR correspondence, this scale manifests dually: at the fundamental microscopic limit as $l_{UV} = 2L_p$ (the gravitational/casual limit, aka the Schwarzschild radi...
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The Cosmic Strange Metal

      Strange metals, quantum spin liquids, and SYK-like systems share a striking transport pattern: no quasiparticles, strong collective dynamics, Planckian relaxation, near-minimal viscosity, and maximal chaos. Their characteristic data are $$\frac{\eta}{s}=\frac{\hbar}{4\pi k_B}, \qquad \lambda_L=\frac{2\pi k_BT}{\hbar}, \qquad \tau_P=\frac{\hbar}{k_BT}.$$ The claim is not that the three-dimensional de Sitter bulk is literally a strange metal. The sharper claim is: $$\boxed{\text{The de Sitter stretched horizon belongs to the same transport universality class as a Planckian strange metal.}}$$ The correspondence applies to the horizon membrane, not to bulk spacetime. Membrane Paradigm and the KSS Value In the membrane paradigm, an event horizon behaves for exterior observers as a stretched viscous membrane with transport coefficients fixed by Einstein gravity. This does not require an assumed AdS/CFT dual. For de Sitter, $$\ell_\Lambda=\frac{c}{H}, \qquad T_{dS...
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Our cosmic event horizon on a string

A Cosmic Stringy Adventure ! As we previously discussed, our spacetime characterised by a positive cosmological constant $\Lambda$. The natural bounds are then a minimal  ultraviolet (UV) length $l_{UV} = 2L_P$ and an infrared (IR) cosmological horizon $l_{\Lambda}$.  This dual-boundary spacetime enforces a fundamental Compton–gravitational duality . Every geometric scale $r$ carries two natural mass definitions: $$m_C(r) = \frac{\hbar}{rc}, \qquad m_G(r) = \frac{c^2}{4G}\ r$$ The product of these masses, $m_C \ m_G = M_P^2/4$, is scale-independent. They intersect exclusively at the UV boundary $r = l_{UV}$, defining a maximal local force in GR: $F_{max} = c^4 / 4G$. At the opposite extreme, the Compton mass evaluated at the IR horizon yields the fundamental  spectral gap  (not a particle) of the universe: $m_s = \hbar / (l_{\Lambda} c)$.  In this post, to explore how energy propagates through this dual-scale geometry, we model the mass gap $m_s$ as a null-ener...
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