What are these Rahukalam and Yamagandam ? Why not any good important things should not do in these time ? Any scientific reasons ?
These are Astrological terms... even if most karmkandi rituals follow astrology, it is not a must for Hindus.....

Hindu Astrology is a set of correlations based on passage of time as measured by stellar movements...... then stories were made to explain them..... they are as good as the correlation factors measured...... which means not 100% or even 90%..... so... these are more complex statistics than physics.....