= High-energy braneworld expansion
{title2=$H^2=\frac{8\pi G}{3}\rho\left(1+\frac{\rho}{2\lambda}\right)+\frac{\mathcal C}{a^4}$}
For a spatially flat, tuned one-brane background without energy transfer to the bulk, the <Israel junction conditions> produce the displayed modified <Friedmann equation>. Positive <brane tension> $\lambda$ gives ordinary expansion when $\rho\ll\lambda$, while the quadratic density correction enhances expansion at high energy. <Big Bang nucleosynthesis>, early acoustic evolution and primordial spectra can constrain this change. Spatial curvature, a residual cosmological constant and bulk energy exchange require their additional terms rather than this special formula.
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