Olbers' paradox | Hacker News Reader

Olbers' paradox

en.wikipedia.org

40 pointsanacleto20 comments

It might seem like a paradox from our perspective, but there is good possibility that like fish in water - our senses & instruments just haven't evolved to perceive and function out of water (off earth) A good possible explanation would be that the seemingly empty void of space is chalk full of dark matter scientists assume makes up 95% of the universe and our eyes can't see it like a dog can't see color, and this invisible matter might someday end up being a biological commodity to future beings that evolve from us just like fresh air is to our lungs.

The universe might look like a cerulean sky, we just haven't evolved to see it that way yet.

No need for dark matter. As it states in explanation paragraph, according to big bang theory, in the beginning stages there were light everywhere and the universe was not even transparent. So according to this, when we look at the sky now we should see that light if we look far enough. But we don't. Reason for that is that as universe expanded, that light emitted from the early stages got "stretched out" and shifted from visible light to microwave length. That's what we call cosmic microwave background. So it's not a possibility - it's fact.

I'm really not convinced by the passages, both in Wikipedia and in the referenced book, concerning the role of the finite age of stars.

From what I remember from Coles-Lucchin, the finite age of stars does furnish a partial explanation: keeping the average density of lit stars the universe constant, does not solve the problem, since in general, when I look in a particular direction, any star in that direction will either be dead or lit, but still too far away for its light to have reached me.

Indeed the opposite would require fine-tuning the stars ignition time so that there is sphere of burning stars at the right distance.

What about the fact that we can barely see the light from some of the stars that we know are there (because of space dust, etc.)? How did somebody come to this conclusion just 150 years ago...

The energy absorbed by interstellar gas and dust doesn't just disappear, it's re-emitted at (mostly) infrared wavelengths. If there really were infinite stars in an eternal universe, everything would be heated to the same white-hot temperature, including the interstellar medium.

Think about what you're saying. In our current system, if you do not except infinity, then you are accepting that there is finite space for energy to escape to, which means that the lack of much visible light and infrared from distant stars available to Earthbound viewers implies that any distribution homogenous and similar to ours currently observable would result in similarly dark views from any vantage point.

This is a logical truth.

You should try rephrasing that because the point you're making is not at all clear.

Isn't this one explained by the speed of light?

Only a finite number of stars have had enough time to reach the earth. So there can only be a finite number of concentric shells.

The paradox assumes there are infinite stars that have existed for infinite time:

> If the universe is static, homogeneous at a large scale, and populated by an infinite number of stars

You're assuming an infinite number of stars sprang into existence a finite amount of time ago.

Of course, given the quantum nature of light, this breaks down: at some (admittedly very large) distance, a star will be emitting so few photons in a given direction that it will be effectively black.

If the universe is infinite, for each particular direction there are an infinite number of stars. So even if the probability of a photon coming from a distant star is very low, there are infinitely many stars in that direction, so that direction will still be very bright. So the quantum nature of light does not change the paradox.

The wikipedia article surprises me though.. The actual brightness can be calculated as an infinite sum of start brightnesses getting weaker with the distance. The result of an infinite sum can be finite or infinite. I would have though apriori that the sum is finite because the star density is so low. But at the end of the article they seem to suggest that the universe is dense enough for the sum to be infinite...

From a simple standpoint of energy conservation, it's obvious the observed brightness cannot be infinite; the article questions the brightness value of the sky, not whether it is finite. From conservation of energy, for a homogeneous universe, a region around a star must receive as much energy as it gives out. So the total observed brightness of the sky would sum to the brightness of your own star, approximately (but distributed over a solid angle 4pi).

I think one resolution is the finite speed of light. Even if there are infinite stars in any given direction, they haven't been around long enough for the brightness to be infinite.

This "paradox" is basically just theorizing an aspect of the heat death of the universe.

The paradox says that an universe infinite in space and time would have to be bright in all directions. By infinite in time, I mean that the universe has been around infinitely long, i.e. it has no beginning. The finite speed of light wouldn't matter here since the light would have had an infinite amount of time to reach you.

Huh? We definitely know that that doesn't describe the universe. If two points are so far apart that the expansion of space inflates the distance between them faster than light can cross it, then no light can ever reach from one to the other, even if it travels for an infinitely long time.

We know, but Olbers didn't. He died in 1840.

The Olbers' paradox is from the 1800s, before we knew about the expansion of space. It's arguing against a static, infinitely old universe. As you point out, we now know that this isn't the case.

> If two points are so far apart that the expansion of space inflates the distance between them faster than light can cross it, then no light can ever reach from one to the other, even if it travels for an infinitely long time

Actually, it can. The issue is quite subtle. What you are saying is true only if the Hubble parameter is constant, it is not. (Hubble constant is such a misnomer! It's only constant across space but not time).

You may want to re-read paragraph 3.3 of the Lineweaver paper (http://arxiv.org/pdf/astro-ph/0310808v2.pdf)

Quotes :

> Photons near the Hubble sphere that are receding slowly are overtaken by the more rapidly receding Hubble sphere.

> The most distant objects that we can see now were outside the Hubble sphere when their comoving coordinates intersected our past light cone. Thus, they were receding superluminally when they emitted the photons we see now.

There will be a first star in any direction: photons will be very unlikely to pass throught that star.

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