The half-life of radium, which is approximately 1,600 years, means that it takes that amount of time for half of a given quantity of radium to decay into other elements. After another 1,600 years, half of the remaining radium would decay, and so on.
While it's true that the universe is estimated to be around 13.8 billion years old, it's important to note that radium is not present from the very beginning of the universe. Radium is a radioactive element that is produced through various processes, including the decay of heavier elements like uranium and thorium.
Radium is relatively rare in nature, and its production and presence depend on specific geological conditions. Even though radium does have a finite half-life, it may still exist today because it can be continuously generated by the decay of other elements in the Earth's crust and other radioactive processes.
Additionally, not all radium that was present at the formation of the Earth has necessarily decayed. Since 1,600 years is relatively short compared to the age of the Earth, a significant amount of radium would still remain after billions of years.
It's important to consider that the presence of radium and its abundance can vary depending on geological factors and the specific location being considered.