White Skys?

[noexpert]

I was watching television the other day, and I heard a theory that said if the universe were infinite, the sky would be white. I dont know much about the theory, and I dont really know much about the math involved, but I understand as you go further away from earth, the number of stars within angle increases due to the increasing area within the angle. My problem is, I feel that this theory doesn't account for redshift. Can anybody tell me more about the theory, maybe some basic math involved and whether the theory is sound. Thanks.

[CraigD]

I was watching television the other day, and I heard a theory that said if the universe were infinite, the sky would be white. I dont know much about the theory, and I dont really know much about the math involved, but I understand as you go further away from earth, the number of stars within angle increases due to the increasing area within the angle. My problem is, I feel that this theory doesn't account for redshift. Can anybody tell me more about the theory, maybe some basic math involved and whether the theory is sound. Thanks.

This is known as Olber's paradox. You can read about it at the preceding wikipedia link, and in several hypography threads (by using the search function).

 

It's a old idea, going back at least to Kepler's time in the early 1600s. Olber himself wrote about it in the 1820s.

 

One "answer" to the paradox has to do with how "seeing" actually works. If we assume that "looking" in a particular direction instantly "sees" any bright object intersecting a line drawn from our eye (or telescope), the paradox appears. If we consider that our eye or telescope actually gathers light from distant bright objects, the brightness of the object decreasing with distance, the paradox disappears. This is my personal answer - there are many more, some more, some less technical or complicated. It's fun to write your own. :)

 

The paradox reappeared in scientific discussion in the 1970s, as some skeptics of the interpretation of the cosmic background radiation as a relic of the big bang suggested that the CBR was actually an affirmation of the paradox - that is, the sky actually is "white", but only at the very long wavelengths of the CBR (due to the redshift I believe you're speculating about). This hypothesis has since been pretty much rejected by the scientific mainstream.

[CarlNGraham]

There is also a bit of an optical illusion involved, nearby stars look a lot bigger to the human eye, because of their brightness.

Most stars are only a few light seconds across but maybe light-years apart, so stars within a few light-years of us only cover less than a millionth of the night sky.

So looking through 20k to 50k light years of our galaxy still leaves 99.99% of the sky black.

 

Then you need to start thinking about other galaxies and cosmic redshift.

[Cyberia]

The idea does not take into account dust clouds. etc. Also that the universe would have to be infinitely old so that we could see infinite distances, which does not take into account that even with our believed limited universe, a good part of the universe is moving away from us with a combined speed greater than light so the photons will never reach us. Also the more distance photons have to travel, the less energy they arrive with so past a certain point, we are talking infra-red instead of white sky and even longer wavelengths further away.

[CraigD]

The idea does not take into account dust clouds. etc.

Obler’s paradox isn’t resolved by including clouds of dust and gas and other non-star objects, because even though such objects absorb light at visible wavelengths, they re-emit it at longer wavelengths, such as in the infra-red. 17th century astrophysicists like Kepler realized that, were the paradox’s “white sky” fact, intervening dust clouds and the like would be so well-illuminated that they’d glow at visible wavelengths.

 

Rather than explain away the paradox, dust clouds etc. make it more plausible, because occupying larger volumes than stars do, they occupy larger arcs of the sky. In much the way that the light of the Sun doesn’t actually consist of photons emitted by the fusion occurring in its core, but by the glow of its outer layer (photosphere), which is heated by the core primarily via convection, the paradox’s “white sky” would be from interstellar dust and gas excited to glowing by starlight.

 

Modern “what if it were true” analyses, such as one presented at the paradox’s wikipedia page, conclude that the visible universe in this case would be excited until all atomic matter was ionized, and it became opaque.

 

Also that the universe would have to be infinitely old so that we could see infinite distances, ...

This is exactly the point that ca 1600 authors of the paradox – Thomas Digges, Kepler, etc. – meant to make with it. They were arguing against the then widely accepted idea that the stars were eternal and infinite in number.

 

… which does not take into account that even with our believed limited universe, a good part of the universe is moving away from us with a combined speed greater than light so the photons will never reach us. Also the more distance photons have to travel, the less energy they arrive with so past a certain point, we are talking infra-red instead of white sky and even longer wavelengths further away.

Do you have sources for these two claims, Sexton? :QuestionM You state them as if they are common scientific knowledge, but they contradict my understanding of the scientific mainstream.

 

Some techniques give results of distant objects receding at greater than the speed of light. For example, simply multiplying the 13,000,000,000 lightyear radius of the universe implied by an age of about 13,000,000,000 years by Hubble’s constant of about 0.023 m/s/ly gives a recession speed of about c. However, according or mainstream astronomy and cosmology, this doesn’t imply that these objects are actually moving faster than c relative to us, but rather that Hubble’s law accounts for redshift due to several effects.

 

The idea that light loses energy as it travels – usually termed the tired light hypothesis – has for the past 50 years of so been considered a disproven hypothesis, at least for an effect large enough to account for Hubble’s law.

[Cyberia]

According to expansion, things at the edge of the visible universe are moving away from us at near light speed. So if you take anything past that, there is a combined speed greater than light speed, so we will never see them.

 

The universe is full of gravitational sources, and like photons, gravity (whatever it is) is immortal. We know that gravity drag can redshift photons (as in when it leaves a star) so why would not a photon be subject to all these gravity sources? (presumably direction of travel and direction from would cancel out, as would departure and arrival shifts).

 

Scientific mainstream is that space can expand, but that idea has more holes than a colander. If gravity redshifts photons, then redshifting would still be proportional to distance travelled.