The temperature of the universe has varied throughout its history, and its current temperature is extremely low on average. However, it's important to clarify that when we refer to the temperature of the universe, we are generally referring to the temperature of the cosmic microwave background radiation (CMB).
The CMB is the remnant radiation from the early stages of the universe, specifically from a time known as the "recombination epoch" when the universe became transparent to photons. It occurred approximately 380,000 years after the Big Bang. At that time, the universe had cooled down enough for electrons and protons to combine and form neutral atoms, allowing light to travel freely.
The current temperature of the CMB is about 2.7 Kelvin above absolute zero, or approximately -270.45 degrees Celsius (-454.81 degrees Fahrenheit). This makes the CMB one of the coldest known entities in the universe.
However, it's important to note that the universe is not uniform in temperature. There are regions with variations in temperature, which are responsible for the formation of structures like galaxies and galaxy clusters. These temperature fluctuations are incredibly subtle, with variations of about one part in 100,000.
It's also worth mentioning that the universe underwent a period of extreme heat and density shortly after the Big Bang, referred to as the "hot and dense early universe." During this period, the temperature was incredibly high, potentially reaching trillions of degrees Kelvin. However, as the universe expanded and cooled, the temperature decreased over time.
So, in summary, the current temperature of the universe, as represented by the CMB, is extremely low at around 2.7 Kelvin.