G1.9+0.3: Deceleration and Energy of the Youngest Known Galactic Remnant

This paper computes the energy, density and acceleration budgets of G1.9+0.3 from a small set of adopted, published input values, using the relations stated in the Methods and a deposited script that reproduces every number. The free-expansion speed implied by radius and age is 1.43e+04 km/s and the Sedov speed 5.74e+03 km/s; their ratio, the deceleration parameter 0.4, is the Sedov value by construction, while direct expansion measurements of this remnant return values much closer to free expansion, which is the quantitative statement that the Sedov phase has not begun. The swept-up mass at the adopted density is only 0.772 solar masses, well below any plausible ejecta mass, so the shock is still ploughing through the star's own debris field rather than the interstellar medium in bulk. The calculation is a computation from stated assumptions, not a measurement, and each headline result is reported together with the diagnostic that tests whether the assumed evolutionary phase applies to this object.