The temperature of space in the regions between galaxies and stars is extremely cold, though not absolute zero. This ambient temperature is measured in Kelvin (K), the absolute thermodynamic temperature scale used by scientists. The baseline temperature of the universe is a uniform and pervasive 2.7 Kelvin (2.7 K).
This baseline temperature defines intergalactic and interstellar space, which is permeated by a faint, uniform glow of microwave radiation. This radiation is a direct measurement of the energy left over from the universe’s earliest moments, precisely measured at 2.725 Kelvin.
The Cosmic Microwave Background
The source of this baseline temperature is the Cosmic Microwave Background (CMB), the oldest light we can observe in the universe. The CMB is a form of thermal energy that fills all of space, appearing as a nearly perfect black body spectrum in the microwave region. It represents the residual energy, or afterglow, from the Big Bang.
Following the Big Bang, the universe was an incredibly hot and dense plasma, opaque to light. As the universe expanded, this plasma gradually cooled over approximately 380,000 years. The temperature eventually dropped to about 3,000 Kelvin, which was cool enough for electrons and protons to combine and form the first stable neutral atoms, primarily hydrogen.
This event is known as recombination or decoupling, and it allowed photons, the particles of light, to travel freely through space for the first time. The universe became transparent, and these released photons constitute the light we now detect as the CMB. Since that time, the continued expansion of the universe has dramatically stretched the wavelengths of these photons, causing them to redshift.
This redshifting process lowered the energy of the photons and cooled the radiation field to its present-day temperature of 2.725 Kelvin. The uniformity of this temperature across the entire sky is powerful evidence supporting the Big Bang model. Tiny fluctuations in this temperature, measured to be less than one part in 100,000, provided the initial seeds for all the large-scale structure we see today.
Converting the Temperature Scale
The Kelvin scale is the standard unit of temperature for scientific purposes because it is an absolute scale. It begins at 0 Kelvin (0 K), known as absolute zero, the theoretical temperature at which all particle motion ceases. Unlike the Celsius and Fahrenheit scales, which are defined by the freezing and boiling points of water, the Kelvin scale is based on thermodynamic principles.
To provide context for how cold 2.7 K is, it can be converted to more common temperature scales. Since the Kelvin scale shares the same degree size as the Celsius scale, the conversion is straightforward: 0 K equals \(-273.15^\circ\text{C}\). Therefore, the ambient temperature of space, 2.7 Kelvin, translates to approximately \(-270.45^\circ\text{C}\).
Converting this value to the Fahrenheit scale shows the baseline temperature of the universe, 2.7 K, is equivalent to about \(-454.81^\circ\text{F}\). This places the cosmic ambient temperature only a few degrees above the theoretical minimum of absolute zero.
Temperature Versus Heat Transfer
The fact that space is only 2.7 K often leads to the misconception that objects placed in it will instantly freeze. This idea confuses the concept of ambient temperature with the physical mechanisms of heat transfer. While the background radiation is extremely cold, the total thermal state of any object in space depends on how it gains and loses heat.
Heat transfer occurs through three methods: conduction, convection, and radiation. In the near-perfect vacuum of space, conduction and convection are essentially non-factors. These methods require a medium, such as air or water, to transfer thermal energy.
This leaves radiation as the sole significant method for transferring heat across the vacuum of space. Every object with a temperature above absolute zero constantly emits thermal radiation in the form of electromagnetic waves. For objects in space, their temperature is determined by the balance between the radiation they absorb and the radiation they emit.
A satellite or astronaut exposed to direct sunlight absorbs intense solar radiation, which can quickly raise temperatures to hundreds of degrees Celsius. An object positioned in the shadow of a planet or spacecraft primarily loses heat through its own thermal radiation, with no incoming solar energy to replenish it. This radiative cooling can cause temperatures to plummet toward the 2.7 K baseline of the CMB, but this is a gradual cooling process, not an instant effect.