Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Cold Universe
Name: Jerry
Status: other
Grade: other
Location: NV
Country: N/A
Date: N/A

Why is the universe so cold? There are billions of stars in the universe but they have little effect on this "cold". Is this "cold" a property that holds the universe together? Since the universe is an entity unto itself why is it cold and not lukewarm?

It i not so much that space is cold, but that things placed in deep space cool off.

In space, matter--at least the familiar type of matter composed of atoms--gains and loses energy by absorbing and emitting electromagnetic radiation. How much energy something emits per second basically depends on two things: its temperature and its surface area. (There is also an effect of "absorbtivity" or "emissivity" but that really affects only the equilibrium temperature reached rather than the general thrust of the argument.) Basically, the rate of energy emitted per second per square meter is proportional to the temperature to the fourth power. In other words, the hotter something is, the faster it emits electromagnetic radiation, and by a big margin. Likewise, the energy absorbed per time depends on the amount of energy coming in by electromagnetic radiation (from stars, dust clouds, etc.). So anything in space heats up as it absorbs radiation form space and cools down as it emits radiation into space.

So why not just continually heat up to the temperature of the stars as you absorb their radiation? Because you constantly emit energy. The hotter you get, the faster you lose energy. So you reach equilibrium at the temperature at which your speed of absorbing energy exactly equals your speed of emitting energy. If you are not near any stars, then you will not absorb much energy; consequently, you get pretty cold before you reach equilibrium.

Richard Barrans
Department of Physics and Astronomy
University of Wyoming

Dear Kerry,

Space is a very cold place. A spacecraft in orbit about the Earth would have outside temperatures approaching absolute zero.


David Levy


There are three ways that heat can be transferred between two objects: conduction, convection and radiation. Conduction means that the two objects are in contact with each other. Convection describes the movement of a fluid so that heat is transferred from a high temperature object to a low temperature object that are both in contact with this fluid. Radiation is the transfer of heat from an object depending on how much light it can transmit (its emissivity). Of these three, radiation is the only one that matters in the vacuum of space.

So maybe we can reword your question in this form: "Why, counting all the stars in the sky and all the time they have had to radiate light, is space not warm (or at least lukewarm)?"

Let us not look at the issue of time, just yet and simply look at radiant energy. The amount of energy received by an object that is a particular distance from the radiating body must be inversely proportional to the distance between the two objects. Moreover, since the radiating object is a star radiating in all directions, then any point in space receives only a tiny fraction of the star's output and that fraction gets tinier as the distance increases. Since distances in space are almost unimaginably immense, a particular star contributes only so much energy at such distances. This is why it is warm on Earth but cold on Mars and Pluto receives a negligible amount of radiant energy from the Sun.

Moreover, radiant energy does not heat up "space" or vacuum. An object must be in the path of the radiant energy in order to warm that portion of space. Thus, when we measure the temperature of space, we are only measuring the amount of radiation that is in that space at a given time, not the accumulated heating of that space over time.

Now let us factor in time. One might argue that in a steady-state universe (in which the number of stars do not change so much even if stars die and are born), then there might be a long enough time for the radiated energy to have permeated the universe, accumulate, and warm space up (have enough radiant energy in a given space at a given time). One might argue that given enough time, all that radiated energy has to add up. And, indeed it would - if the universe were old enough and small enough.

Look up "Olber's Paradox" (why the night sky is not bright or uniformly light) and the resolution of it to explain why time is not an issue when considering why the universe is not warm. In essence, radiant energy must travel, at best, the speed of light in a vacuum. So there are at least two possible resolutions: (1) the universe is expanding, and the light from the most distant stars have not reached us, so that our portion of space has not received the benefit of the heating from these sources. (2) Even in a steady-state universe that is not expanding, the universe is so immense, and still quite young, that the light (and energy) from the most distant stars has not had time to reach (and warm) us.

In short, (1) energy from radiant sources are a function of distance, (2) stars emit energy in all direction so that any point in space receives only a tiny fraction of that energy and that fraction diminishes with distance, (3) space is so immense that objects in the radiant path must receive truly a small portion of any star's energy, (4) the universe is still relatively young that all that radiant energy has not "added up" significantly.

Hope this helped,

Greg (Roberto Gregorius)

Whether the Universe is "hot", "lukewarm", or "cold" is a sensory judgment. For example, the boiling point of water (100 C.) is hot compared to the freezing point of water, but cold compared to the melting point of most metals. So whether an object (including the Universe) is hot, cold, or warm is a matter of comparison.

Vince Calder

Click here to return to the Astronomy Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory