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Superfluid, the Vacuum, and MOND

      We previously  discussed Verlinde's connection of the MOND acceleration scale to the entropy of de Sitter space. A different route appears in superfluid dark matter (a 2026 review is here ), where baryons interact with a phonon field whose nonlinear dynamics generate a MOND-like force. The 2026 review (Section 7.1) introduces a scale $\Lambda_{SF}$ and notes it needs to be in the order of meV to account for the MOND scale. So, we take this idea one step further and directly identify the characteristic phonon scale with the vacuum-energy scale! This converts the cosmological constant into a galactic acceleration scale and reproduces the baryonic Tully–Fisher relation. Throughout, set $c=\hbar=1$ and $M_{\rm Pl}^{-2}=8\pi G$, where $M_{\rm Pl}$ is the reduced Planck mass. Vacuum Scale Let $\Lambda_{\rm cos}$ denote the cosmological constant. Its vacuum-energy density is $\rho_\Lambda=M_{\rm Pl}^2\Lambda_{\rm cos}$. For a pure de Sitter universe, $H_\Lambda^2=\...
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De Sitter Horizon: Surface Tension, Laplace Pressure, and What They Actually Mean

 In the static patch of de Sitter spacetime, the cosmological horizon behaves thermodynamically in ways closely analogous to a physical interface. One can assign it entropy, temperature, and even an effective surface tension. Remarkably, the familiar Young–Laplace pressure relation from surface physics appears naturally at the horizon. 

 

 In the static patch of de Sitter spacetime, the cosmological horizon admits a useful surface-thermodynamic description. It has an effective surface tension and pressure satisfying a Young–Laplace-like relation.

Use $G=c=\hbar=k_B=1$. The metric is

$$ds^2=-f(r),dt^2+\frac{dr^2}{f(r)}+r^2d\Omega^2, \qquad f(r)=1-\frac{r^2}{L^2}, \qquad L=\sqrt{\frac3\Lambda}.$$

The horizon is at $r=L$.

1. Horizon Data

The area, entropy, and temperature are

$$A=4\pi L^2, \qquad S=\frac A4=\pi L^2, \qquad T=\frac{1}{2\pi L}.$$

The Misner–Sharp energy is $M(r)=\frac r2(1-f)=\frac{r^3}{2L^2}$, so at the horizon $M(L)=L/2$. This equals both the vacuum energy inside the static patch and $TS$:

$$\rho_\Lambda=\frac{\Lambda}{8\pi} =\frac{3}{8\pi L^2}, \qquad V=\frac{4\pi L^3}{3},$$

$$\boxed{M(L)=\rho_\Lambda V=TS=\frac L2.}$$

For the cosmological-horizon first law, define the oriented horizon energy

$$\boxed{E=-M(L)=-\frac L2.}$$

This is not the positive physical vacuum energy. It is an energy convention adapted to the cosmological horizon as viewed from inside the static patch.

2. Membrane Pressure

A stretched horizon at constant $r<L$ carries a redshifted angular pressure

$$\boxed{\bar p=\frac{f'(L)}{16\pi} =-\frac{1}{8\pi L}.}$$

Its magnitude satisfies $|\bar p|A=TS$. The sign depends on the orientation of the stretched-horizon normal.

This membrane pressure is not the thermodynamic surface tension.

3. Thermodynamic Surface Tension

Define $\sigma$ along the de Sitter family by $dE=T,dS+\sigma,dA$. Since

$$\frac{dE}{dL}=-\frac12, \qquad T\frac{dS}{dL}=1, \qquad \frac{dA}{dL}=8\pi L,$$

we obtain

$$\boxed{\sigma=-\frac{3}{16\pi L}.}$$

Equivalently, it satisfies the Smarr relation $E=2(TS+\sigma A)$. This result is also what Chen et al 2017, The modified first laws of thermodynamics of anti-de Sitter and de Sitter space–times derived from the radial Einstein equation rewritten as a thermodynamic identity. The membrane pressure and thermodynamic tension are related, but unequal:

$$\boxed{ \sigma=\bar p-\frac{1}{16\pi L}. }$$

The extra term comes from the curvature of the spherical horizon.

4. Thermodynamic Pressure

Define the pressure conjugate to the static-patch volume by $dE=T,dS+P_{\rm hor},dV$. Because $dV/dL=4\pi L^2$, the first law gives

$$\boxed{ P_{\rm hor} =-\frac{3}{8\pi L^2} =-\frac{\Lambda}{8\pi}. }$$

This equals the cosmological-constant vacuum pressure:

$$\boxed{P_{\rm hor}=p_\Lambda=-\rho_\Lambda.}$$

Since $dV=\frac L2,dA$, the pressure and tension satisfy

$$\boxed{P_{\rm hor}=\frac{2\sigma}{L}.}$$

This is the Young–Laplace form of the horizon identity.

5. What Is Outside Pressure?

The cosmological constant is homogeneous, so its local pressure is the same on both sides of the horizon: $p_{\Lambda,\rm in}^{\rm local} = p_{\Lambda,\rm out}^{\rm local}$. There is therefore no local vacuum-pressure jump.

Instead, $P_{\rm out}=0$ refers to the absence of an external agent performing controllable quasi-static work on the observer's patch:

$$\boxed{P_{\rm out}^{\rm work}=0.}$$

The one-sided pressure balance is then

$$\boxed{ P_{\rm hor}-P_{\rm out}^{\rm work} =-\frac{3}{8\pi L^2} =\frac{2\sigma}{L}. }$$

This resembles the Young–Laplace equation, but adds no independent field equation. It repackages the radial Einstein equation at the spherical horizon.

6. Connection with Cosmic No-Hair

The surface-tension relation describes the equilibrium de Sitter horizon. The connection with cosmic no-hair comes from entropy.

In expanding cosmologies with positive $\Lambda$, matter and anisotropy are diluted as the geometry approaches de Sitter. The generalized entropy approaches the finite de Sitter value $S_{\rm dS}=\pi L^2$. Under suitable assumptions, increasing generalized entropy toward this maximum implies asymptotic approach to de Sitter. Cosmic no-hair can therefore be understood as cosmological equilibration.

The negative $\sigma$ is consistent with this equilibrium picture, but does not prove cosmic no-hair by itself.

7. Energy Conventions

Three energies should not be mixed:

$$\text{vacuum energy:}\qquad M=\frac L2,$$
$$\text{oriented horizon energy:}\qquad E=-\frac L2,$$
$$\text{Euclidean de Sitter energy:}\qquad E_{\rm Euclidean}=0.$$

Accordingly, $F_{\rm hor}=E-TS=-L$, while $F_{\rm Euclidean}=-TS=-L/2$. These belong to different energy definitions.

Also, varying $L$ varies $\Lambda=3/L^2$. The decrease of $F_{\rm hor}$ with $L$ therefore compares different de Sitter solutions, not dynamical expansion at fixed $\Lambda$.

8. SI Units

Restoring constants,

$$\boxed{ \sigma=-\frac{3c^4}{16\pi GL}, \qquad \bar p=-\frac{c^4}{8\pi GL}, }$$

$$\boxed{ P_{\rm hor} =-\frac{3c^4}{8\pi GL^2} =-\frac{\Lambda c^4}{8\pi G}, }$$

$$\boxed{ P_{\rm hor}=\frac{2\sigma}{L}, \qquad \sigma=\bar p-\frac{c^4}{16\pi GL}. }$$

Finally,

$$\boxed{ TS=\rho_\Lambda c^2V=\frac{c^4L}{2G}. }$$

Wrap

For the de Sitter horizon viewed from the static patch,

$$\boxed{ \sigma=-\frac{3c^4}{16\pi GL}, \qquad P_{\rm hor}=-\frac{3c^4}{8\pi GL^2}. }$$

With no external work reservoir,

$$\boxed{ P_{\rm hor}-P_{\rm out}^{\rm work} =\frac{2\sigma}{L}. }$$

This is a Young–Laplace-like rewriting of the horizon Einstein equation, not a literal pressure discontinuity across a material surface.

Its broader thermodynamic meaning lies in cosmic equilibration: under suitable conditions, expanding universes with positive $\Lambda$ lose their matter and anisotropic hair as generalized entropy approaches the maximum de Sitter value.

 

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