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Name: Chang
Grade: 12+
Location: IL
Country: US
Date: Fall 2011


Question:
I read "The First 1988 Dirac Memorial Lecture", the book says that a gas, when it expands, cools. Likewise, the expanding universe cools." If the gas expands against vacuum, the temperature would not drop since the average kinetic energy of gas molecule doesn't transfer to anything else and thus stays the same. Does this imply there is something outside the universe or is my analogy wrong?

Replies:
The resistance against which work much be done is gravitational attraction of all things in the universe to each other.

Usually in thermodynamics examples you do not have to think about gravity as one of the confining forces. But here it is the biggest part of the picture.

Our present models do not include anything outside the universe. The newly described "Dark Energy", being completely undefined except for its effect on universe expansion history, might be inside or outside our universe. If outside, then the idea that the universe is a thermodynamically closed system will be false, or at least only partly true.

But gravity from the mass of all things within the universe. will still be a big part of the picture. Maybe dominant.

Jim Swenson


As the Universe expands, the average energy per unit volume falls. This leads to a measured drop in "temperature" as it does in a gas expanding into a vacuum. Take the expanding gas to an extreme.

Let the gas expand into the vacuum until the molecules are so far apart that none of them hit a measuring device within any measurement period. True, the average KE of a molecule would be the same, but the amount of heat available to move our device would be nil. We would record absolute zero.

You get the same effect in the outer layers of the sun where the temperature is very high, but the gas so dispersed you could fly through the area in a bathing suit (if shielded from the direst rays of the sun And the vacuum of space).

R. W. "Mr. A." Avakian


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