Why is the universe so cold?

[kewell_]

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?

 

[kyle_baron]

The energy from the sun travels thru space (nothing) as waves. The waves hit the atmosphere, land, and water, and heat these elements up. The space shuttle, and astronauts on space walks would also heat up, and are therefore protected. But the space is nothing, therefore, nothing heats up. <div class="Discussion_UserSignature"> <p><font size="4"><strong></strong></font></p> </div>

 

[robnissen]

The main reason is due to the incredible distances between stars. Because energy decreases by the square of the distance from the source, the amount of energy that can heat individual pieces of matter becomes very small over relatively short interstellar distances.

 

[kewell_]

Ok so, <br /><br />(1) energy from radiant sources are a function of distance.<br />(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.<br />(3) space is so immense that objects in the radiant path must receive truly a small portion of any star's energy.<br /><br />Is it possible that the universe is still so young that all that radiant energy has not "added up" significantly yet? And also what about the expansion of the universe wouldn't that make it even colder?

 

[vogon13]

The universe is so cold because it is so big.<br /><br />In the past it was smaller and warmer, in the future it will be bigger and colder still.<br /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[vandivx]

yep, universe is one huge runaway fridge cooler, gas cools off when it expands, if you ever sprayed out a can of paint you would feel that too<br /><br />vanDivX <div class="Discussion_UserSignature"> </div>

 

[R1]

and then the gases tend to clump up at gravitation centers too, thereafter,<br /><br /> leaving the gasless space cold and silent<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[kewell_]

thanks, I understand now. I am curious though, do we know if the universe is uniformly the same temperature everywhere?

 

[vogon13]

No.<br /><br />Objects in deep intergalactic space are ~3 degrees K.<br /><br />Matter inside supernovae is billions of degrees K. (more or less)<br /><br />Kind of warm in my office tonight, too . . . .<br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[weeman]

In the beginning, the Universe was very very very very very hot. One theory states that fractions of a second after the big bang the Universe was in excess of a quadrillion degrees Kelvin! Of course, it cooled from this temperature at an astonishing rate. The Universe was so hot in the beginning, that no matter could take form; essentially, everything existed in a massive primordial soup! <br /><br />So, where has all this heat gone? As the Universe expands it gradually cools, and this gives us the basics of thermodynamics.<br /><br />In deep space, the CMBR (Cosmic Microwave Background Radiation) has an overall temperature of about 3K. If you're unaware of how cold 3K is, it is only 3 degrees above absolute zero, and absolute zero is -459 degrees Fahrenheit!<br /><br />If the Universe expands for eternity, then it would eventually cool even more, if all stars die off, and matter decays to nothing, then the Universe could possibly reach absolute zero. <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity.</strong><strong>" -Albert Einstein </strong></font></p> </div>

 

[heyscottie]

Vogon says no, and he is right, at least for the way he answered it. But another correct answer is yes, if we are measuring average temperatures of large enough swaths of the universe.<br /><br />Basically, if we take a large enough sample, say a sphere 1 Billion light years in radius, that sphere should have more or less the same total heat, and therefore same average temperature, as any other 1 Billion light year sphere. <br /><br />So on a small scale, the universe is heterogeneous. On a very large scale, we can treat it as homogeneous.

 

[weeman]

Is it true that the temperature differences seen in the picture above are down to fractions of a degree? <div class="Discussion_UserSignature"> <p> </p><p><strong><font color="#ff0000">Techies: We do it in the dark. </font></strong></p><p><font color="#0000ff"><strong>"Put your hand on a stove for a minute and it seems like an hour. Sit with that special girl for an hour and it seems like a minute. That's relativity.</strong><strong>" -Albert Einstein </strong></font></p> </div>

 

[doubletruncation]

They are about 0.5 milli-Kelvin. So aside from the overall anisotropy due to the galaxy's motion relative to the CMB (which has been subtracted from the above map), the CMB is uniform to a level of about 1 part in 10000. <div class="Discussion_UserSignature"> </div>