Why Is the Night Sky Dark If the Universe Is Infinite?

[kmarinas86]

This need not be correct.

The correct conditions for Olbers' paradox to come into play are:

A universe infinite in extend homogeniously filled with luminous matter which exist for infinite time (eternal)

So theoretically the universe can be infinitely old and infinite in extend, if we assume that either stars have not always existed (which is the assumption the Big Bang theory makes) and/or is not static (which is also postulated by the Big bang theory).

The Big bang theory of course also states that the observable universe is of finite age, but the question wether that means that time is not infinite is still open, we can only ascertain that we can not observe anything before a certain time (i think the time at which the universe became transparent to light).

Theoretically there is also the possibility that the universe has a fractal nature in such a way that the average density of lumnious matter would drop down to zero when we increase the diameter, in which case Olbers' paradox also does not arise.
Just observation shows that this is not the case in the observable universe.

However, one musn't consider the Milky Way as the center of the universe. In fact, there is no center of the universe, but rather there are places with more gravitational pressure than others. The only way to see this fractal structure in the immediate future is to assume that the structure does not change much with time. From the timeline of the Big Bang it is deduced that the further we look back, there is greater matter and energy density. But if this matter and energy density was about the same as it appears to us right now (at about the same spot), then it would appear that we are looking down a gravitational pressure gradient, meaning that our region of the universe (nearest 1 billion light years) is located at the suburbs.

In a fractal universe, the local density is inversely proportional to the cube of the scale factor at the region. Therefore, via integration, the density of the whole would fall inversely proportional to the square of the scale factor. The density would fall fast enough - the density of the whole needs to fall at a rate faster than inversely proportional to the radius in order to avoid Olber's paradox. A decreasing scale factor then begs the question - why are we in a bubble of a higher scale factor, and where could there be even higher scale factors? It is physically unlikely that we are completely surrounded by a bubble of lower scale factor (another way to say regions of greater gravitational pressure) - as in completely closed shell.

Another possibility is that there are particular places in the sky where one could look deeply to find spaces between the regions of high gravitational pressure. After zooming even further, one may find that there are other galaxies on the other side of the regions of high gravitational pressure, proven by their low redshifts in correspondence to their anomalously small angular sizes.

But regions of higher gravitational pressure could be large in angular width (tens of degrees) making it more difficult to find the places where these very distant galaxies may be seen. However, beyond that region behind the deep gravitational wells blocking most of this, would be a scale beyond anything currently realized...

 

[gone gator]

"Why is the sky dark at night?"

A seemingly naive question that has a profound answer.
If the universe were :
1. Infinite and
2. Infinitely old i.e. eternal and
3. Static (all three being true together) then the night sky should be burning bright!

i think Kepler used this to argue there were a finite number of stars in the Universe even before Newton's fears of collapse... (and two centuries before Olber's formulation of the problem).

"Before the expansion of the universe was discovered it was generally thought by astronomers, back to the ancient Greeks, that the universe was eternal.

and a few brits continued to believe it was eternal even afterwards...

(addendum) ...when the universe is homogenous and isotropic at the large scales.

in 1907 Fournier published a nice theory solving the paradox by assuming a fractal distribution of stars.

Edgar Alan Poe was the first to propose there simply had not been enough time for the light to have gotten to us. (as well as ?one of? the first illustrations of how weather could be both chaotic and predictable [he must have been reading of LaPlace?]).