How Can the Cosmos Be Both 3 Degrees and Extremely Hot?

[saberjim]

Homework Statement

I am taking a course from the Teaching Company called the "Physics of History". Two statements by the teacher seem in conflict to me as an amateur.

1. The average temperature of the cosmos is about 3 degrees K.

2. Most of the material in the cosmos is in the form of plasma which is extremely hot.

I am pretty sure the answer might be difficult to explain to an amateur so a web reference would suffice, Thanks/

Jim Flint

Homework Equations

The Attempt at a Solution

 

[zachzach]

Most material might be hot but nothing was said about HOW MUCH material there is.

 

[mgb_phys]

1. The average temperature of the cosmos is about 3 degrees K.

2. Most of the material in the cosmos is in the form of plasma which is extremely hot.

Both are sort of correct - I suspect the lecturer wanted to make an amusing point.

The temperature of most of the cosmos is 'undefined' empty space is nothing which can't have a temperature.
The 3K is the temperature of the microwave background which fills the universe. If you like this is the temperature of the walls of the room, nothing in space can cool below this.
But most of the visible matter in space is in stars which are very hot.

 

[saberjim]

Thank you for the quick reply. I am an old geezer of 75 and have fallen in love with physics. A little too late for a degree as I already have one in geology but the subject does not soar like cosmology. My wife hit me with the question during the last lesson and will be thrilled with the answer.

We both appreciate your taking time to answer.

Jim Flint

 

[mgb_phys]

It's like the statistics joke:
Everyone has more than the average number of legs !
(Because a very few people have 1 leg, but nobody has more than 2, the average is 1.99999...)

The TTC lectures are very good - enjoy the physics