Sight-lines re. Olbers' Paradox

[Bogie]

Olbers' paradox: what do sight-lines end on?

This question was asked on an ATM thread of mine and I had to ask to have the thread closed because there are only 10 days left on the 30 day clock. I don’t understand the question? I understand Olbers’ and I know how it its solved, but I don’t see how the question about “sight-lines” applies.

I’m not asking for the answer to the question, just a little help understanding the question. What is the issue that differentiates one cosmology from any other in regards to “sight-lines”?

[Hornblower]

Here is a nice account of Olber's Paradox.

http://zebu.uoregon.edu/~imamura/123...-5/olbers.html

If the Universe were infinitely large, eternal and static, every possible sight line would go to a star somewhere out there.

[Bogie]

Here is a nice account of Olber's Paradox.

http://zebu.uoregon.edu/~imamura/123...-5/olbers.html

If the Universe were infinitely large, eternal and static, every possible sight line would go to a star somewhere out there.

OK, thanks. I guess that is the point of the question. My ATM is about a multiverse cosmology and first question I always get asked is about Olbers'.

The same solution that applies to our expanding universe with finite lived stars applies to my ATM. Thanks.

[Nereid]

You can generalise the question at the heart of the paradox ("Why is the night sky dark?"), and ask about the observed diffuse background, in every waveband.

In some wavebands (e.g. EUV, blue-ward of the Lyman limit), the sky is not dark, but the reason why has little to do with cosmology.

In most wavebands however, the strongest statement we can make is that the diffuse (cosmological) background is fainter than {limit}.

Olbers' paradox, wrt a cosmological theory, then becomes a quantitative estimate of the diffuse background ... and showing that that estimate is consistent with the relevant astronomical observations.

Of course, in one waveband - microwaves - the 'night sky' is not dark ...

[Bogie]

Are you saying that at most wavebands the diffuse background is characteristic of the history of the universe in regard to the primordial soup and the "dust" from the stellar activity that has occurred?

Do you mean that there are observations involved at various wavebands and some quantification as to what makes those observations relate to said history?

[Nereid]

Are you saying that at most wavebands the diffuse background is characteristic of the history of the universe in regard to the primordial soup and the "dust" from the stellar activity that has occurred?

Too complex ...

the observations are what they are - the diffuse x-ray background is less than {some number}, for example. A cosmological theory should not predict a diffuse x-ray background that is (significantly) greater than {this number}.

How such a diffuse x-ray background might arise is, one would hope, fairly straight-forward to work out, given a consistent cosmological theory.

For example, in cosmological theories based on static universes, I would guess the backgrounds would need to be (the result of) some kind of equilibria between sources and sinks.

Do you mean that there are observations involved at various wavebands and some quantification as to what makes those observations relate to said history?

I'm not sure what you're asking, but a diffuse background is simply the sum (or integral) over the relevant sources and sinks along sight-lines.

In a non-static universe, summing along sight-lines is equivalent to summing over history.

[Bogie]

Too complex ...

the observations are what they are - the diffuse x-ray background is less than {some number}, for example. A cosmological theory should not predict a diffuse x-ray background that is (significantly) greater than {this number}.

How such a diffuse x-ray background might arise is, one would hope, fairly straight-forward to work out, given a consistent cosmological theory.

For example, in cosmological theories based on static universes, I would guess the backgrounds would need to be (the result of) some kind of equilibria between sources and sinks.I'm not sure what you're asking, but a diffuse background is simply the sum (or integral) over the relevant sources and sinks along sight-lines.

In a non-static universe, summing along sight-lines is equivalent to summing over history.

My answer is going to be, "I don't know", to that one .

[Jens]

For example, in cosmological theories based on static universes, I would guess the backgrounds would need to be (the result of) some kind of equilibria between sources and sinks.

I think that is one of the solutions for a static universe. But I also think there is a second, which is that the universe's structure is fractal and the mass tends toward zero as the scale increases.

[Nereid]

I think that is one of the solutions for a static universe. But I also think there is a second, which is that the universe's structure is fractal and the mass tends toward zero as the scale increases.

Good point.

There are also hierarchical cosmologies: stars - galaxies - clusters - super-clusters - ... As you go up in scale, the average density declines ... and it does so faster than r².

More generally, if, in a cosmological theory, the universe is not isotropic or homogeneous*, diffuse backgrounds may arise in many different kinds of ways.

However, for most of these, I imagine there would be at least one preferred direction (diffuse background brighter in one direction than all others, for example), so a generalised Olbers' paradox would add the word 'uniformly' (why is the night sky uniformly dark?).

*Is it possible to have an inhomogeneous universe that is isotropic? On a sufficiently large scale, of course.

What about an anisotropic universe that is homogeneous?

[Bogie]

In regards to Olbers' and homogeneous and isotropic, there is nothing that I can see that would mark a distinction between my ATM cosmology where the question in the OP was posed, and our observable universe.

Of course, the observable universe is the same for any cosmology. I believe the question is intended to help draw me out as to as much detail as I can offer about my cosmology. There has been a good exchange going on and I am happy to offer as much (word salad) as anyone . I'm not providing quantification as one would expect from a professional, but the words convey the thinking. I am not discussing my ATM idea here, I am just trying to understand what info I can offer there that will help clear up the question.

[Nereid]

The lack of quantification is a serious issue ... without it, how could you say what your pet cosmology says should be the darkness of the night sky?

Turning up the contrast, if your pet cosmology cannot say if the night sky should be fainter than ~30 mag per square arcsec or brighter than the noon-day sun, how could it be called science?

[Bogie]

The lack of quantification is a serious issue ... without it, how could you say what your pet cosmology says should be the darkness of the night sky?

I can't, but ...

Turning up the contrast, if your pet cosmology cannot say if the night sky should be fainter than ~30 mag per square arcsec or brighter than the noon-day sun, how could it be called science?

That statement seems to set the contrast between a pet cosmology and science too high.

http://www.britannica.com/eb/article-9026473/cosmology

Sometimes it takes centuries for a person’s pet cosmology to gain community. In 6th century BC the Pythagoreans thought the Earth was spherical and hypothesized that the heavenly bodies moved according to the harmonious relations of natural laws. “Their thought culminated in the Ptolemaic model of the universe (2nd century AD).”

“The Copernican revolution of the 16th century ushered in the second great age. The third began in the early 20th century, with the formulation of special relativity and its development into general relativity by Albert Einstein. The basic assumptions of modern cosmology are that the universe is homogeneous in space (on the average, all places are alike at any time) and that the laws of physics are the same everywhere.”

I’ll say it for all who see the above, I’m no Albert Einstein, but I am allowed to have a pet cosmology without personally being able to answer questions like that, I hope. If my ideas get traction then “community” will work on answers that explain those observations.

But someone familiar with the science involved in the question itself and with the pet cosmology in question is in a good position. They should be able to pin down what might be included in the pet cosmology that would lead one to believe that the subject observation would be a problem. Posing sophisticated questions is fine but to reject ideas that haven't developed to the point that those sophisticated questions can be answered doesn't mean that the idea should be rejected?

I'm sure that a pet cosmology has content that can be addressed as opposed to posing strangely related questions which if not answered somehow discredits the ideas that were never addressed.

[mugaliens]

Wow - what a mouthful!

Wouldn't it be better simply to say that the flux of visible energy striking the Earth's atmosphere that's pointed away from the Sun is but a tiny fraction of what it receives from the Sun when the Sun is up, and that there's a threshold beneath which the human eye cannot detect blue light scatter?

That blue starlight is still scattered isn't the question (yes, it scatters). It's just that the amount of light that scatters is below the level detectable by the human eye.

Most pictures of the night sky, even those taken over time, show a dark sky as well. However, the exposure was set to capture stars as pinpoints of light, not as blazing orbs and halos. If you overexpose the night sky, you see the same blue sky as you would during the day.

I know this because whil in high school, in the 70's, for a project in my physics class, I incorrectly calculated the f-stop and the time of the exposure required to show that Polaris isn't precisely at the apex of True North.

Result - blue sky.

Similarly, if you underexpose a camera during the day, you get the reverse - dark sky. I did this, too, trying to take a picture of the sun that would show it as little more than a dim orange object, setting ISO 25 Kodachrom 25 slide film with maximum f-stop (22) and a 1/4000th second shutter speed.

Result - a very dim sun and totally black sky surrounding it.

By the way, I was using a 25 mm lens (35 mm film, which produces a wider angle of view).

[Bogie]

Wow - what a mouthful!

Wouldn't it be better simply to say that the flux of visible energy striking the Earth's atmosphere that's pointed away from the Sun is but a tiny fraction of what it receives from the Sun when the Sun is up, and that there's a threshold beneath which the human eye cannot detect blue light scatter?

That blue starlight is still scattered isn't the question (yes, it scatters). It's just that the amount of light that scatters is below the level detectable by the human eye.

Most pictures of the night sky, even those taken over time, show a dark sky as well. However, the exposure was set to capture stars as pinpoints of light, not as blazing orbs and halos. If you overexpose the night sky, you see the same blue sky as you would during the day.

I know this because while in high school, in the 70's, for a project in my physics class, I incorrectly calculated the f-stop and the time of the exposure required to show that Polaris isn't precisely at the apex of True North.

Result - blue sky.

Similarly, if you underexpose a camera during the day, you get the reverse - dark sky. I did this, too, trying to take a picture of the sun that would show it as little more than a dim orange object, setting ISO 25 Kodachrom 25 slide film with maximum f-stop (22) and a 1/4000th second shutter speed.

Result - a very dim sun and totally black sky surrounding it.

By the way, I was using a 25 mm lens (35 mm film, which produces a wider angle of view).

Thank you. Interesting observations.

It does show that the f-stop, shutter speed, film speed, all have to be considered when explaining the contrast.

[01101001]

I’ll say it for all who see the above, I’m no Albert Einstein, but I am allowed to have a pet cosmology without personally being able to answer questions like that, I hope.

I don't get what is your point of your trying to persuade us that your pet cosmology is the true cosmology. You haven't even persauded yourself that it is correct. You have access to observations that you haven't bothered to use to confirm the falsity of your pet cosomolgy, or fixed it to fit. Until you convince yourself, why don't you stop trying to convince us?

Keep you pet theory on a short leash, and out of my back yard. It barks too much.

If my ideas get traction then “community” will work on answers that explain those observations.

So there is much you yourself aren't persuaded of, yet you want to assert your fantasies for others, to either persuade them, or to have them work on the answers for you? That just seems arrogant.

Work it out yourself, like good pet theory owners do, and when it's solid, when you are convinced, when you've applied all the data, when you know all aspects, know its strengths and weaknesses, then claim it.

[Bogie]

I don't get what is your point of your trying to persuade us that your pet cosmology is the true cosmology. You haven't even persauded yourself that it is correct. You have access to observations that you haven't bothered to use to confirm the falsity of your pet cosomolgy, or fixed it to fit. Until you convince yourself, why don't you stop trying to convince us?

That may be a good point. Are you referring to the question on this thread or have you actually read my idea?

Keep you pet theory on a short leash, and out of my back yard. It barks too much.

And just where is your backyard?

So there is much you yourself aren't persuaded of, yet you want to assert your fantasies for others, to either persuade them, or to have them work on the answers for you? That just seems arrogant.

Arrogant?

Work it out yourself, like good pet theory owners do, and when it's solid, when you are convinced, when you've applied all the data, when you know all aspects, know its strengths and weaknesses, then claim it.

I claim it on the ATM threads. I am on the Q&A for answers to questions from the mainstream perspective.

[01101001]

Have your pet theory. But don't enter it in the dog show. It's not ready.

[Bogie]

Have your pet theory. But don't enter it in the dog show. It's not ready.

If the mods want to move this to my open ATM thread here I would appreciate it. Q&A is not the place for me to respond about my ATM ideas and ATM content.

[01101001]

Q&A is not the place for me to respond about my ATM ideas and ATM content.

That idea I fully support.

[Jens]

However, for most of these, I imagine there would be at least one preferred direction (diffuse background brighter in one direction than all others, for example), so a generalised Olbers' paradox would add the word 'uniformly' (why is the night sky uniformly dark?).

*Is it possible to have an inhomogeneous universe that is isotropic? On a sufficiently large scale, of course.

What about an anisotropic universe that is homogeneous?

Those are important questions. I think that a fractal universe would mean that the sky should not be uniformly dark. It's clear that it isn't at the small scale. The sky is brighter in the direction of the sun than away from it. And brighter in the direction of the center of the Milky Way than away from it. The question is really if those anisotropies (I hope that is the right word) exist at a larger scale.

[Bogie]

That idea I fully support.

Good, I'll respond over there.

[Nereid]

Discovering a 'local' cause for the 'night sky' not being dark is surely interesting, but doesn't necessarily tell you much if your focus is cosmology.

As the Earth's atmosphere is not transparent to most of the electromagnetic spectrum (thank goodness!), the 'night sky' you need to examine, for cosmological purposes, is that you can 'see' above that atmosphere.

And there are certainly other 'local' sources which produce diffuse emission, in certain wavebands - zodiacal light in the FIR*, for example, is a solar system source, and in the microwave region (where the important CMB* is strongest), various Milky Way sources produce diffuse emission.

Most of the EUV* - that with wavelengths shorter than the Lyman limit - is not cosmologically useful (in terms of Olbers' paradox), because the hydrogen in the ISM* is not transparent to light in this waveband.

And so on.

*FIR = far infrared; CMB = cosmic microwave background; EUV = extreme ultraviolet; ISM = interstellar medium.

[mugaliens]

Those are important questions. I think that a fractal universe would mean that the sky should not be uniformly dark. It's clear that it isn't at the small scale. The sky is brighter in the direction of the sun than away from it. And brighter in the direction of the center of the Milky Way than away from it. The question is really if those anisotropies (I hope that is the right word) exist at a larger scale.

Agreed, Jens, and that Q&A is appropriate for the original question as I perceived it.