Why Doesn't the Night Sky Shine as Bright as the Sun Despite Infinite Stars?

  • Context: Undergrad 
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Discussion Overview

The discussion revolves around Olbers' paradox, which questions why the night sky is not as bright as the surface of the Sun despite the existence of an infinite number of stars in an everlasting universe. Participants explore various aspects of this paradox, including the implications of star density, the lifespan of stars, and the effects of redshift on the visibility of distant stars.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants reference Stephen Hawking's assertion that in an infinite universe, every line of sight would end on a star, suggesting that the night sky should be as bright as the Sun.
  • Others argue that the finite lifespan of stars and the limited star density complicate this assertion, as many stars we see may no longer exist.
  • It is noted that the brightness of stars decreases with distance, and at some point, their light becomes undetectable against the background radiation.
  • Some participants clarify that Olbers' paradox does not imply an infinitely bright sky, but rather one as bright as the surface of an average star, due to shielding effects from foreground stars.
  • There are discussions about the implications of redshift, with some suggesting that it effectively resolves Olbers' paradox by rendering light from distant stars invisible.
  • One participant raises the question of whether the background radiation could be considered the radiation from every line of sight, challenging the assumptions of the paradox.
  • Concerns are expressed about the assumptions underlying the paradox, particularly regarding the eternal existence of stars and the implications for energy conservation in the universe.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the implications of Olbers' paradox and the conditions necessary for the night sky to be bright. There is no consensus on whether the paradox is resolved by redshift or if the assumptions about star density and lifespan are sufficient to explain the observed darkness of the night sky.

Contextual Notes

Limitations include unresolved assumptions about the homogeneity of star distribution, the implications of redshift, and the finite lifespan of stars. The discussion also reflects varying interpretations of the paradox and its implications for cosmology.

  • #31
On the subject of star lifetimes:

No, a star does not have to be infinitely long-lived. Suppose it exists for a long time then dies - what then?

Well, since your line of sight extends infinitely far in this model, the probability that there will be an active star somewhere behind the space the first star used to fill will tend towards 1.

In fact, if your line of sight is truly infinite and the universe is homogenous, your line of sight will always extent through an infinite number of active stars.

Even more damning evidence against this model, I'd say.
 
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  • #32
Fascinating, PL. It appears you are asserting 'math' is irrelevant in your odel of the universe.
 
  • #33
Sojourner01 said:
On the subject of star lifetimes:

No, a star does not have to be infinitely long-lived. Suppose it exists for a long time then dies - what then?

Well, since your line of sight extends infinitely far in this model, the probability that there will be an active star somewhere behind the space the first star used to fill will tend towards 1.

In fact, if your line of sight is truly infinite and the universe is homogenous, your line of sight will always extent through an infinite number of active stars.

Even more damning evidence against this model, I'd say.

well.. I´m not going to say that this model should be proven correct. I´m just trying to understand why its wrong... :)

Suppose the life time of a star is 1 year. But star density is the same as it is now. Stars keep popping up and dying out all the time, natural process I guess.

Intuitively, I feel like the average life span of a star in this model determines how bright the nigh sky lights up (also star density is a factor I guess). Now, I know intuition is not always your best friend in science, but sometimes it's all you have,. :)

I don't see how the star's light/radiation continues to move for ever. Perhaps mathematically it does, but a at a certain point it's not practical anymore.

Has there never been done any calculation on the relation between star density, average star life span and the brightness of the night sky? (in an infinite universe, or even in "our" finite big bang universe)
 

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