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Starlght
If the universe is infinite / wraps around on itself why is the sky at night only pinpricked by stars not completely bright?
There must be a star everywhere we look in the sky, even if it is incalculably far away...... the light from that star must reach us as light will travel for as long as there is nothing in the way........
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Nice question!
I think the reason is that although light will travel as long as there is nothing in the way ... it can only travel so fast.
The reason the night sky is not completely bright is that the light from stars other than those very close to us (the milky way) is still 'in transit', and may not reach us for millions of years (or something like that).
I think the next question would be ... will the night sky be completely bright a very long time into the future, when all the light in the universe has had time to spread out everywhere? ;)
I think the reason is that although light will travel as long as there is nothing in the way ... it can only travel so fast.
The reason the night sky is not completely bright is that the light from stars other than those very close to us (the milky way) is still 'in transit', and may not reach us for millions of years (or something like that).
I think the next question would be ... will the night sky be completely bright a very long time into the future, when all the light in the universe has had time to spread out everywhere? ;)
That's correct.
Your question is also known as 'Olber's paradox'.
Olber himself suggested that 'stuff' in between us and the light (like dust, other stars) may have blocked the light. However this 'stuff' would have ultimately either been broken down by the light radiation or absorbed it and passed it on.
Another reason combined with the one given in the above excellent post is that since the stars mentioned and actually the light also from the stars is receding away from us, they have been 'redshifted' to a degree that limits the light from them that comes to us.
This is of course all due to the Big Bang.
The universe is not infinite, nor does it wrap around itself. Both of these propositions oppose the Big Bang theory. The universe is immense but finite, and growing. Space is infinite (the absence of anything) - the universe is expanding into space.
The reason you don't see the light from distant stars is quite simple - the level of light from any source is inversly proportional to the distance from it. Consider what happens when you drop a stone into a still pond - the wave is large at the centre, but decays rapidly as it travels away from the source. Energy is not being lost - it is just spreading out more. It's the same with light - the further it travels from its source, the more spread out it is, so the light level decreases.
Badams, thanks for the answer.
If light spreads out like you say it does, why do light sources not look bigger the farther you move from them?
Also, how can light level decrease, surely a light photon will not just vanish after a certain distance?
I think you may have a good point but can you expand your answer a bit please...
I think that MargeB's answer is the most plausible so far.
The Big Bang theory, in its current form, does say that the universe (small 'u') is infinite. It does not have a boundary in space and time because space and time, or more correctly 'spacetime' is itself 'the universe' and is itself expanding. Also, spacetime was created at (or around !!) the Big Bang.
Re the light levels, actually, the apparent brightness of a light source is inversely proportional to the square of the distance from the light source.
Hmmm... not sure what differentiates a small 'u' universe from a large 'U' universe, but, be that as it may, anything that can be measured is, by deffinition, not infinite. Anything that has a beginning, referencing time, is not infinite, hence, our universe s not infinite. The universe can be measured (about 156 billion light years in diameter), even though expanding and it had a beginning.
Even though Heinrich Olbers (1826) proposed the 'Paradox" it was actually originated by Edmund Halley (of Halley's Comet fame) who realized that since there is a limit to how fast light can travel (c, being the speed of light) and that the universe is not infinitely old, the light from the most distant stars just hasn't had enough time to reach us yet. The universe simply isn't old enough to be completely lit up!
The solution to Olbers' paradox came with the discovery that the universe is expanding: that distant galactic groups are receding from us. This recession results in a diminution of the light from these distant sources over and above that forecast by the inverse square law. In addition, light from a receding source is red shifted and it can be shown that red light possesses less energy than blue light of the same intensity. Therefore, because of this red shift, not only does less visible light reach us, but the total energy is also less.
These last two effects combine to reduce the light contributed by distant galaxies to our nighttime skies to insignificance, leaving only the nearby stars which we see as points of light in a darkened sky.
(With thanks to Amateur Astronomers)
Contd.
Contd.
Additionally, not to be pedantic, but If you imagine a 'shell' of stars at some distance (R) away from us, the amount of light reaching us from each star decreases with the inverse square of R, but the number of stars in the 'shell' increases with the square of R. (This assumes that the universe is pretty much the same, on a large enough scale, wherever you look at it.) Thus the powers of R cancel out, and the total amount of light reaching us from each 'shell' is the same, regardless of how far away it is. So, benzene (nice name) actually has the closest answer...
My thanks for the question as well...
But the universe had no beginning in time. It will have no end in time either. It is infinite in time. The universe had no beginning in space, it has no edge in space. To talk of measuring it is, ultimately, completely meaningless. With reference to what? Only relatively, with reference to something within it.
universe is 'that which came with the Big Bang', space time, matter, antimatter, dark mattery, energy, dark energy, virtual particles. Universe is that, plus everything else which exists...
Ya, you didn't grasp the point. Just because we can in some abstract way speak of "time" now does not mean that the universe necessarily had a beginning in time.
Furthermore, just because the universe had a beginning in time does not mean that the start of the universe was 0seconds and one second later was 1second.
See if Jake was here, he could explain that at this point. Me, I can't be arzd. Georgi can explain it, she'll get me.
ll_billym wrote:
If the universe is infinite / wraps around on itself why is the sky at night only pinpricked by stars not completely bright?
Being infinite is not the same as wrapping round itself. The universe is finite and unbounded because it is the 3-dimensional surface of a 4-dimensional sphere (with a further dimension of time and 6 further mini-dimensions of space which are too small to worry about). Therefore it only has a finite amount of stuff in it, and that stuff has only has a limited amout of time to radiate since the Big bang.
badams wrote:
The universe is not infinite, nor does it wrap around itself. Both of these propositions oppose the Big Bang theory. The universe is immense but finite, and growing. Space is infinite (the absence of anything) - the universe is expanding into space.
The universe wrapping round itself (i.e. being curved, finite and unbounded) is compatible with the Big Bang theory. Space is not infinite, because space is the same as the universe. There is no space "outside" the universe, and the universe is not expanding "into" space. The universe is expanding and taking the space with it.
Thanks Clanad, you have answered my question.
Bernardo, Clanad and MargeB, much as I respect the answers you give to any question on the answerbank I feel that you speak with too much certainty about ideas that are only theories. I accept that you have to believe something and stick with it but face it..... nobody knows or can prove the real answer to the size, speed, beginning, end or colour of the universe because we are inside it and they only way to study something properly and objectively is to be outside it.
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