The internal energy of the shock continues to be very large compared to radiation losses from thermal and synchrotron radiation, so the total energy remains nearly constant. The rate of expansion is determined solely the initial energy of the shock wave and the density of the interstellar medium. As the shock wave cools, it will become more efficient at radiating energy. Once the temperature drops below 20000 K or so, some electrons will be able to recombine with carbon and oxygen ions, enabling ultraviolet line emission which is a much more efficient radiation mechanism than the thermal X-rays and synchrotron radiation. Hence this new phase is known as the radiative phase during which X-ray radiation becomes much less apparent and the remnant cools and disperses into the surrounding medium over the course of the next 10000 years.
Supernova remnants are extremely important for our understanding of our Galaxy.
They are the source of much of the energy that heats up the interstellar medium. They are believed to be responsible for the acceleration of galactic cosmic rays. Enriched, heated gas from SNR material is reprocessed to form new stars. Understanding how SNR evolve can thus help us to understand many important phenomena. These predictions can then be compared with observed SNR to see how well the model fits.