h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

> In other words the gas will absorb some of the frequencies and ignore others (see the absorbsion/emission spectra for various gasses). Ultraviolet will brake bonds in molecules. Some of the gas will vibrate fast enough to escape from the main body of
the gas cloud. The hotter gas will radiate some energy in the infrared part of the spectrum.

The hotter gas will radiate infrared, yes?
So if an infinite amount of radiation is incident on a gas cloud, and a percentage of the gas cloud re-radiates, the amount radiated will still be infinite. O.1 times infinity is still infinity...

I don't see the point about mercury. Sorry I meant to illustrate that incident radiation on everyday matter warms it. Not everything is in thermal equilbrium, and the sun side of mercury radiates some; how can it not?

Alji - leaving the other website aside for the moment because Candelas bend my mind and I have no textbooks to hand... Do you think there's a problem with the mathematics in my entry?

J'au-æmne,
I just read the article you added, and I think it is not completely fair to the expansion-only explanation, as it uses all the force of Big Bang, Quantum Theory and General Relativity. Even then the result is not magnitudes better than just expansion.

A real difference is, that it suffices to accept expansion and a finite lightspeed to explain a dark sky. With infinite lightspeed it is more difficult.
This would mean that from a certain distance the relative speed would exceed lightspeed (I do not yet introduce Einstein here!), effectively making the universe finite and the skies dark.

The funny thing is, that in this way the sky will always have the same darkness, while with the big bang explanation in time the amount of visible galaxies will increase and slowly (after some more billions of years) may have less influence than the expansion theory.

The point about Mercury is that it is very close to the sun but has not reached a temperature anywhere near the sun's temperature, even though the day on Mercury is long.
For each point source of light you have to work out the flux density. You cannot add the flux of stars behind the star (galaxy, star cluster etc) that you can see (If you look at the photograph from the Hubble Deep Field Survey (link in my entry) you will see some galaxies with spiral arms). If you think about it the flux density of a galaxy or galaxies at infinite distance will be infinitesimal.

Alji

Gnomon - post 19! I've tried! I'm waiting for the scientists to reach some conclusions!!

Otto

Otto, although it is nice to talk about Olber's paradox, which is what we have been doing since Posting 19, none of this affects the validity of the entry. It is complete and ready to go. Olbers' Paradox is a well-known "paradox" among astronomers and this entry describes it well.

That's my opinion.

Well, I have already recommended it with the cavaet that I'm not sure if it's finished, as I couldn't tell whether the discussion was just about the paradox or about suggested changes to the entry. I've not had an answer yet, but hopefully there's an italic watching for a consensus. This entry is excellent and thoroughly deserves to be in the guide!

Otto.

"Hie thee to the edited guide!"

As far as I'm concerned, its finished.

I have outlined what Olbers' Paradox is -
*The assumptions that appear at first glance reasonable but with further consideration aren't
*The explanations suggested to 'solve' the paradox
*The current best accepted theories which solve the paradox

Whether or not one agrees with the original assumptions is immaterial - that is what the paradox is, and if you think that Olbers made stupid assumptions, that's nothing to do with my entry on the subject.

I have outlined the solutions as I have researched and I have been taught; as far as I'm concerned they're right too. (Unless I see really convincing evidence to the contrary; I don't want to be dogmatic. However, I haven't...)

Excellent - I thought it was finished, but my knowledge of astrophysics is rather limited!

IMO, this entry epitomises one aspect of what the guide is about - a specialist writing on one of his or her specialisms in an accessible way!

Congrats again - I suppose we just wait for the italics. I imagine they're busy catching up after the bank holidays...

Otto

Otto, it is a one sided argument and does not cover the theories of those astronomers opposed to the Big Bang theory.

Alji

I thought the only people who were opposed to the Big Bang theory were Fred Hoyle and the Steady-State Universe people. Their theories were discredited in the 1970s.

And this entry does not have to present every possible explanation for the expansion of the universe. All it has to do is pose the question "Why is the Sky Dark?". That's the paradox in a nutshell.

Couldn't we find a form of words that satisfies everyone? If the entry assumes that big bang theory is true, might it be possible to add a couple of sentences to this effect in the introduction, which lists the other assumptions made?

I agree with Gnomon's second point (I can't comment on the first!). The problem is that knowledge is inter-connected, but there ought to be a way of saying that this entry is about x, assumes, y and z, and is perhaps an (interesting) irrelevence if y and z turn out to be false.

Let's work together!

Otto

Nonsense, the entry is already complete.

It states that with the accepted physics theories of a few centuries ago there was a conflict between theory and reality. That conflict can be described away with modern accepted physics theory.

That there still are some people who oppose to modern scientific theories based on their limited religious beliefs, is outside the scope of this entry and can be ignored.
When the edited entry is published, they can put their objections in a separate thread there.

I haven't read all of the points that people have posted so I apologise in advance if this has been said already.
If the universe is infinite and there are infinitly many stars then it stands to reason that there are infinitly many black holes too. My understanding is that a black hole sucks in light. Maybe this is why the sky isn't always bright.
Actually why should it be bright always? If the speed of light is constant then surely the light being emitted from the stars that are seperated from us by distances of magnitude tending to infinity will take an ammount of time also tending to infinity to reach us. The stars that are closer will emit light that reaches Earth sooner. When the stars we now see burn out then maybe the light from the stars that are farther away will just be starting to reach us. I mean why should the light from all the stars reach Earth all at once?

By the way, if space is curved then is it curved uniformly or not? If it is uniform then would that make the universe spherical with the surface being space? If so then what is in the middle of the sphere and what is outside of the sphere?

195959, all that about infinity is explained in the entry itself, so I suggest you read it.

I could comment on your question about the curvature of space, but I don't think here is the place to do it. This is the discussion forum for making improvements in the entry and getting it into the guide.

Ask the question about curvature in Ask h2g2 and you will get lots of replies, some of them accurate!

Perhaps I have read a different entry to you because I can't find the part that you refer to.

Hi Otto

My entry doesn't presuppose that Big Bang Theory is true; it says that Big Bang Theory predicts the darkness of the sky, solving Olbers' Paradox, and making it more compelling.

I have said the best accepted theory is Big Bang Theory. There may be people who disagree with it, and that is fine because it is a theory only, and short of time travel I don't think that the Human Race will ever be able to come up with anything which is more than a theory, no matter how compelling that theory is.



I too, like Gnomon, assumed that the Steady State theories were largely discredited. Whether they genuinely are or not, though, doesn't really matter. They are not the most widely accepted theories, and I feel that it is the place of this entry to be conventional.

I am not qualified on writing an "Alternatives to Big Bang Theory" entry, but I would be interested to read one.

Read my entry at A761465. I have not finnished with it yet!

Alji

I have tried to show in my entry that there is no need to explain Olbers' Paradox by anything other than the distance of the source of light, the obscuring of light by dust and the emptiness of space. Red shift does not make the difference you think it does as the galaxies are too far away to add any light to the night sky. Strong emmissions in the ultra-violet from young stars move into the visible spectrum so the amount of light should increase, but when you look at the distant galaxies 12,000,000,000 l.y.s away they look much the same as close galaxies (spirals, clusters etc.).

Alji

Olbers' Paradox is not controversial science. It is accepted by astronomers everywhere. So I suggest that the entry should become part of the Edited Guide. Alji's objections seem to be based on misconceptions. If Alji wants to subsequently argue his case by submitting his entry to Peer Review, we will look at it then. I suggest we ignore it for the moment.