The concept of "temperature" as we understand it doesn't directly apply to the entire universe. Temperature is a measure of the average kinetic energy of particles in a system, and it is typically used to describe the behavior of matter on smaller scales, such as in gases, liquids, and solids.
However, in the context of the cosmic microwave background radiation (CMB), which is a remnant of the early universe, scientists have determined an average temperature of approximately 2.7 Kelvin (or about -270.45 degrees Celsius or -454.81 degrees Fahrenheit). This temperature corresponds to the thermal equilibrium of the CMB radiation.
Absolute zero, which is the lowest possible temperature, is defined as 0 Kelvin (or -273.15 degrees Celsius or -459.67 degrees Fahrenheit). It is theoretically unattainable because it represents the absence of all thermal energy, where particles would have no kinetic energy at all.
As for the timescale to reach absolute zero, it's important to note that temperature is a property of matter and energy within a system, and the cooling process depends on the specific conditions and mechanisms involved. In practice, it is extremely challenging to cool matter to temperatures that approach absolute zero.
Scientists have achieved temperatures very close to absolute zero, typically in the range of a few billionths of a Kelvin, using sophisticated techniques such as laser cooling and evaporative cooling with ultra-cold gases. However, cooling matter to precisely absolute zero is considered impractical due to the immense amount of time and energy required, if it were even possible.
In summary, while the universe has a background temperature associated with the cosmic microwave background radiation, the concept of reaching absolute zero on a universal scale is not applicable, and achieving absolute zero in practice is extremely challenging and likely impossible.