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.
Department of Physics and Astronomy
University of Wyoming
Space is a very cold place. A spacecraft in orbit about the Earth
would have outside temperatures approaching absolute zero.
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.
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Update: June 2012