How Cold Is The Universe?

Here on Earth, we enjoy rather stable temperatures. Although temperatures can fluctuate wildly, relative to most of the universe, the Earth is exceedingly warm. Most of the universe is empty space where temperatures are extremely cold. Furthermore, since space is expanding and most galaxies are speeding away from each other, the overall temperature of the universe is only getting colder. Just how cold is the universe?

Measuring The Temperature Of Space

Cosmic microwave background
Image of the Cosmic Microwave Background Radiation from WMAP

Temperatures in space will obviously vary depending on where you are. Planets and stars will be significantly warmer than empty space, and so measuring the average temperature of the universe seems like a difficult task. Thankfully, there is a rather simple way of determining the average temperature of space. To do so, astronomers measure the temperature of what’s called the Cosmic Microwave Background Radiation (CMBR). The CMBR is composed of the radiation left over from the Big Bang, and the radiation is fairly uniform across the early universe. By measuring the temperature of the CMBR, astronomers can get a rough estimate of how cold the universe truly is, and it turns out that it is indeed very cold. The average temperature of the universe is 2.725 Kelvin, which is equivalent to minus 454.7 degrees Fahrenheit (minus 270 degrees Celsius). The universe's temperature is less than three degrees above absolute zero, which is as low as temperatures can physically get. 

Is The Universe Getting Colder?

Cosmic history
Illustration of the history of the universe from the Big Bang until now

The temperature of space has been decreasing ever since the Big Bang. Mere seconds after the universe burst into existence, temperatures were millions of degrees hotter than they are now. In fact, temperatures were so hot after the Big Bang that not even atoms could form. As space expands, the distance between objects is increasing and the overall amount of heat in the universe is decreasing. This is simply the result of an expanding universe, and ever since the Big Bang, space has been gradually getting colder. Although the temperature of space is near absolute zero, the universe itself will never actually reach that temperature. Even after the last stars have burned out, the universe will continue to emit at least some forms of radiation, which will ensure that the overall temperature of space never reaches the absolute coldest temperature possible. 

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