Question

what is absolute zero? why is it not attainable?

Answer 1

Absolute zero is the temperature is zero Kelvin ($\mathrm{=-273.15°C}$). At this temperature, gases lose energy and completely stop moving. The third law of thermodynamics gives us that: Entropy of perfectly crystalline solid is zero at absolute zero, while the entropy of defected solid or amorphous is greater than zero at zero $\mathrm{K}$.

Zero Kelvin temperature also implies by ideal gas equation, that volume of a gas must be zero, which is not possible. Absolute zero can never be attained.
CMB was found only to be a few degrees higher than $\mathrm{0K}$.

Answer 2

The lowest tempurature that is the theoretically possible, at which the motion of particles which constitudes heat would be minimal. it is zero on the Kelvin scale equivalent to -273.15°C. As we decrease the temperature, the vibration decreases and decreases until, at absolute zero there is a minimum amount of motion that atoms can have but not zero that is why it is not attainable.

hope it helps you✌