Why is the universe considered as cold?

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    Cold Universe
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Discussion Overview

The discussion revolves around the characterization of the universe as a cold place, exploring the implications of temperature on star formation and the factors affecting light travel in space. It includes theoretical considerations and conceptual clarifications regarding temperature, density, and the nature of light propagation.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question why the universe is described as cold despite the formation of stars, suggesting that temperature is not uniform and depends on matter distribution.
  • One participant notes that while the average temperature of the universe is around 3 K, star formation occurs in regions where matter clumps together under gravity, leading to heating.
  • Another participant compares the temperature differences between stars and interstellar space to the distance between bonfires, emphasizing the vastness of space.
  • A participant raises a question about whether the speed of light is affected by temperature and what other factors, besides black holes, might obstruct light in space.
  • One response highlights the importance of volume and energy density in the universe, suggesting that an increase in volume leads to a higher density of photons, which may influence temperature and pressure.
  • Participants discuss various objects that could obstruct light, noting that while black holes are significant, other celestial bodies like planets and stars also play a role, albeit to a lesser extent due to their rarity.
  • Another participant emphasizes that the universe is predominantly empty space, suggesting that the infrequency of obstructive objects is a key factor in light travel.

Areas of Agreement / Disagreement

Participants express differing views on the implications of temperature in the universe and the factors affecting light propagation. There is no consensus on these topics, and multiple competing perspectives are presented.

Contextual Notes

Participants mention concepts such as energy density, photon pressure, and the ideal gas laws, but do not resolve the complexities or assumptions underlying these discussions.

ninja
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Why do many claim the universe as a cold place? If it is the natural phenomena then how does formation of stars take place?
 
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Although the average temperature is cold (~ 3 deg. K), stars get formed because the distribution of matter is far from uniform. If sufficient matter clumps together under gravity, the clump will heat up and, if large enough, become a star.

http://en.wikipedia.org/wiki/Star_formation
 
The relative distance between stars is enormous. The high temperature of stars and low temperature of interstellar space is about as surprising as a bonfire in Maine and another in California failing to melt snow in the Midwest.
 
Thanks for the reply. I would rather like to take the discussion a further ahead - whether light speed traveling through space gets affected by prevailing temperatures or is it always uniform. Also, what apart from black holes could obstruct the light traveling through space.
 
The important aspect your missing is the sheer volume and overall energy density. If the volume of the universe increases, then the density of photons also increase. This will increase the pressure influence and temperature of the photons. In the case of stars which is far more dense than the surrounding space you have far higher temperatures. Google the gas laws as well as ideal gas laws (cosmology) for more details.
 
ninja said:
what apart from black holes could obstruct the light traveling through space.

Uh ... anything that gets in its way? The Earth, for example. Other stars, other planets, interstellar gas. Now those things are so sparse that they don't stop much of it, but taken all together they stop way more than black holes do.
 
Objects, like black holes, planets and stars, are too uncommon to be a factor. The universe is more than 99.9% empty space.
 

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