At the de Sitter horizon, using the stated Brown–York convention
$E_{\rm BY}=|E_{\rm active}|=2E_{\rm MS}=2TS$ links four distinct descriptions of de Sitter equilibrium. The factor of two is generated by the negative pressure of vacuum energy.Under standard static-patch conventions,
$$\boxed{ E_{\rm BY} = |E_{\rm active}| = 2E_{\rm MS} = 2k_BT_{\rm dS}S_{\rm dS} = \frac12N_{\rm sur}k_BT_{\rm dS} }$$
where
$$N_{\rm sur}=\frac{A}{\ell_p^2}=4S_{\rm dS}.$$
The key is vacuum pressure. For $p=-\rho c^2$, the active gravitational density is
$$\rho+\frac{3p}{c^2}=-2\rho.$$
Consequently, the magnitude of the active gravitational energy inside the de Sitter horizon is
$$|E_{\rm active}|=2\rho Vc^2=2E_{\rm MS}.$$
With the stated Brown–York subtraction and orientation convention, this is also the horizon-limit Brown–York energy. The factor of two therefore represents the gravitational contribution of negative pressure, not merely an unspecified boundary correction.
The same quantity has a thermodynamic interpretation:
$$E_{\rm BY}=2k_BT_{\rm dS}S_{\rm dS} =\frac12N_{\rm sur}k_BT_{\rm dS}.$$
It is the equipartition energy of the horizon's $A/\ell_p^2$ degrees of freedom.
Meanwhile, the Misner–Sharp energy satisfies
$$S_{\rm dS} = \frac{2\pi E_{\rm MS}\ell}{\hbar c}.$$
This has the algebraic form of Bekenstein saturation, although applying the ordinary Bekenstein bound to a de Sitter horizon requires interpretive care.
The relation extends naturally to spatially flat FLRW cosmology. Defining
$$K_A=(\rho+3p)V_A, \qquad \epsilon=-\frac{\dot H}{H^2},$$
one obtains
$$\boxed{ K_A+2E_{\rm MS} = \frac{\dot S_A}{2\pi} = 2\epsilon E_{\rm MS} }.$$
Exact de Sitter space is therefore the fixed point
$$\dot H=0 \iff \rho+p=0 \iff \dot S_A=0 \iff K_A=-2E_{\rm MS}.$$
Away from that point, $\epsilon$ measures both horizon-entropy evolution and the departure from the active-energy factor-of-two relation.
The result provides a unified picture: Misner–Sharp energy sets the bulk scale, negative vacuum pressure doubles it into the magnitude of the active gravitational energy, Brown–York energy represents that quantity at the boundary, and horizon equipartition supplies its thermodynamic interpretation.
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