4 ms·
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 s
by empiricus 11y ago
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.
- empiricus 11y agoThe 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...
- darkmighty 11y agoFrom 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).
- delecti 11y agoI 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.
- uts 11y agoThe 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.
- thaumasiotes 11y agoHuh? 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.
- Someone 11y agoWe know, but Olbers didn't. He died in 1840.
- uts 11y agoThe 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.
- jobigoud 11y ago> 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 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.
- MaysonL 11y agoThere will be a first star in any direction: photons will be very unlikely to pass throught that star.