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

http://www.bbc.co.uk/h2g2/guide/A614558

I wrote this after a request by another researcher, and have since made several edits. I think that more scientific entries to the Edited Guide would be a good idea.

WebSherpa

Hi, nice work

Can't really fault it too much except that 'explanation' is spelt wrong early on.

One question I do have is the statement that 'all objects appear to be moving away from the earth (on a cosmic scale)' I'm not sure what you mean here to be honest. This seems to mean that nothing can ever move towards the earth - so how do meteorites hit us? - the sun isn't getting further away at any great rate either (at least I hope not ).

Just that little quibble, other than that - a nice explanation.

Orcus

Almost all objects in the universe (all other galaxies for example) are moving away from the Earth. What I really meant was "over very large distances". Over smaller distanes some objects do move towards the earth (such as meteorites and stars within our galaxy).

WS

(Thanx for the comments! )

Hi WebSherpa, nice entry, thanks for putting me down as a co-author, but you really don't have to. The guide could do with a few cosmology entries and this one should definitely go in.

It might be a good idea to try and rewrite the bits about cosmic expansion as it's not terribly clear at the moment, but it's difficult to explain in plain text so don't spend too much time on it.

Some more info on cosmic expansion:

After Albert had invented his General Theory of Relativity (still the best theory of gravity today) he applied his equations to the try to model the universe, and ran into the same problem as Isaac. As gravity is always attractive, a static universe appears to be impossible as stars and galaxies should move together under mutual attractions. His equations seemed to suggest that the universe was expanding, but as this sounded stupid, he stuck a 'Cosmological constant' to his formulae so they described a stationary universe.

When Edwin discovered that in fact the Universe really was expanding, Albert realised his mistake and called the Cosmological constant his 'biggest blunder'

Straight after big bang, the universe consisted of elementary particles all flying away from each other. However due to quantum effects the density was never entirely uniform, sometimes clumps of matter became gravitationally bound together. A few billion years later, these clumps becames structures, then galaxies, stars and planets.

It's only on really big scales that the universe is expanding. Many nearby galaxies such as andromeda are moving towards us. The idea that the universe is expanding about every point means that any two galaxies that are a long way apart will be moving away from each other. The universe is not expanding away from a particular point. The best analogy is a balloon covered with spots, as the balloon is blown up, the sport all move away from each other, but no point can be called in the centre of the surface. The universe is like this but with at least one more dimension.

MP

I think you're right about needing more info on cosmic expansion, and I'll probably use some of the facts you suggested (Thanx ). I didn't include this before as I didn't want to make the article too complecated, but with a bit of thought it should be possible.

Having looked it up you're correct, Andromeda is moving towards us. However almost all galaxies are moving away from us (the number of galaxies at large distances is huge, as can be seen in the Hubble Deep Field images).

I'll have to update this tommorrow though.

Peer Review is more effective than I expected!

WS

You might want to mention that although albert thought of the cosmological constant as his biggest mistake, since we now have to add cosmic antigravity to the model of the universe, it has proved to be a very usefull idea. (related to this, the only other approach that works for this is to use a locally variable light speed, which has it's own problems).

Cosmic antigravity, cosmic acceleration, quintessence, dark energy or whatever you want to call it, is a rather complicated subject that really deserves an entry of its own. But as it's a recent discovery, still not fully understood it would not be the easiest entry to write. I thought about mentioning it in my dark matter entry, but decided against it. Whatever it is it only happens at hugh redshifts (a very very long way away).

The cosmological constant is basically a convenient way to fudge the equations to produce a desired effect: a static, expanding, or accelerating universe. It's widely used by cosmologists to simplify the maths of more complicated models.

The word 'note' is used throughout where I think 'pitch' would be more accurate.

Theanthrope: You're right about the first appearance; "note" is better put as "pitch" here.

I've added some info. on Universal Expansion. I've tried not to stray from the subject of Red Shift into Big Bang theory (as there are other articles on this).

WS

tiny point. dark energy and dark matter are used to explain the universe having about the right critical density. cosmic antigravity is indeed a fudge factor, allowing for a collapsing, stable or expanding universe depending on it's value. it is currently thought to be an independant phenomena to to dark matter and dark energy.

ps if you don't like cosmic antigravity as an explanation for the observed values of redshift, you have to use a variable light speed, which just as many people don't like.

I liked it, and as a matter of fact, I was looking for an article like this for my entry on police radar. Thanks.

As I understand it, all galaxies outside of the Local Group (or is it Cluster) are moving away from us. Phrasing it in terms of the Local Cluster might clarify the footnote.

Editorial Note: This conversation has been moved from 'Peer Review' to 'Universal Red Shift'.

This thread has been moved out of the Peer Review Forum because your entry has now been recommended for the Edited Guide.

You can find out what will happen to your entry here: http://www.h2g2.com/SubEditors-Process

Congratulations!

Yes, your entry has been accepted for the edited guide! You'll get an email when the editors have given it the final polish, and it will be linked from the front page!

Well done, and keep up the great work!

J

Thanks for the tips everyone

I'll write another article soon

WS

Have you heard of the shapiro effect?
Hubble had reservations about the doppler redshift, being the only
reason for the observed redshift. Dr Shapiro gave a good reason why
light slows down in a gravitational field . thus causing the light to be redshifted!