What Does Expanding Universe Mean?

[Dan Tee]

Hi everyone, just signed up to forum so not too sure of what to expect but can I ask one question that has probably been thrashed to death in the past and it is:
Taken that the UV is expanding, what is it expanding into?
I was told once that it created space as it expanded but that does not answer the question or I don’t understand it.
Also if there are many Uvs are they scattered throughout the cosmos as say Galaxies are scattered in our UV
i.e. just 3 spatial dimensions + time? Sorry if my terminology is a bit basic.

 

[marcus]

Taken that the UV is expanding, what is it expanding into?
.

why should it need to expand "into" anything?
as far as we know, our universe is not "in" any larger surrounding space (people have fanciful scenarios about our universe being in a larger space of higher dimension, but this is not confirmed by observations)

so if our space is not "in" any larger surround, it does not need anything to expand "into". Or?
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In order to expand, all our space needs is an internal distance function that has the feature that on average each largescale distance expands by a certain percentage every year----which is what we observe happening.

there is a criterion of something being stationary or at rest with respect to the expansion (or at rest with respect to the microwave background which amounts to the same thing) and it is observed that on average, largescale distances between stationary objects are increasing at the rate of one percent every 140 million years. this is a really tiny percentage increase so it is pretty respectable, I think, that humans have been able to detect and measure it

according to the prevailing theory of spacetime geometry (1915 Gen Rel) we have NO RIGHT TO EXPECT distances between stationary points to remain constant---and no competing theory of geometry works anywhere near as well.
So be grateful that distances are as constant as they are. One percent in 140 million years is not bad. It could be a lot worse.

As far as we know, there is no outside.

hope this helps, new guy.

welcome to PF

 

[Dan Tee]

Thanks for your reply marcus, but it has not really helped me, I will just have to try harder.

 

[marcus]

Thanks for your reply marcus, but it has not really helped me, I will just have to try harder.

I hope you don't put blame on yourself. maybe I didnt explain well enough this time. Stick around PF and ask some more questions. Maybe something you read will help.

Do you ever read the Scientific American magazine?

It had a very good article about this that is free-for-download online. It is about the four or five things most people have the most trouble understanding (about the bigbang and expanding distances idea).

In case you didnt read it, I will get a link. Actually the way I usually get the link for it is to put the two words Lineweaver Misconceptions into Google. The SciAm article should come out in the first four or five hits. Because the name of the article is "Misconceptions about the Big Bang" Try it.

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something to keep in mind is that when astronomers say big bang they have never intended to give the idea of an explosion. the name "Big Bang" was given by a man who hated the idea and didn't believe it----it is obviously the wrong name and confuses people, but the mocking name stuck because of humor and alliteration.

what is intended is that, at a certain moment the distances between points of space began increasing.
that's all.
no explosion.
and that extending of distances stretched out the wavelengths of light, which cooled the light
and the cooling of the originally hot bath of light allowed everything else to cool down as well

but the cooling part is SECONDARY so don't worry about it. that is just standard physics you learn as a college freshman----by analogy, compressing some gas makes it hotter and letting it expand makes it cooler---the normal thing you expect.

the main thing is you have to get your mind around the idea that as we sit here talking distances could be increasing.
(there doesn't have to be any other space surrounding our space, that is an unnecessary complication so you can throw it out)
the simple thing is that distances can increase: They have in fact increased by truly impressive multiples in the past,
and even now they still increase by one percent every 140 million years or so.

I'm curious. Is that understandable? Or if not, what do you find puzzling about it?

 

[Dan Tee]

I am struggling to understand, but I have found the thread going back to August 07, "Googled it" between you and others on forum + some interesting links from Google, so I will do my home work, some times its easier to ask, I must be lazy. I will try your suggested BB link, and I will certainly be back asking more basic questions.

 

[marcus]

Don't forget the article by Lineweaver. He is a bigtime cosmologist and also a very clear writer and explainer.

You google "Lineweaver Misconceptions"
and the third or fourth thing on the list will be the SciAm article by him and Tamara Davis:
http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147

it is 5 or 6 pages long, all free for download, and it has sidebars with illustration diagrams and frequently asked questions and examples of right and wrong ways of thinking about something. he and Tammy did a good job.

 

[Dan Tee]

OK Thanks, Got it.

 

[hurk4]

Questions like what is around the universe, what is time and what is distance where difficult but intriging questions for me as well as they are for many people.
Here I make an attempt to show where I came to:

Expanding space.

It seems more or less easy to think and talk about distances and expanding of the UV as long as one can observe or notice points (e.g. star clusters) in the UV. But imagine that these points (at a certain ‘time’) are evaporated (in the far future or past?) and that by then the universe is filled with (dark?) energy. Can this UV then still expand or shrink? I suppose the only ’measurable’ entity, by then, might be energy-density and related to it wavelength. IMO: “The longer the wavelength, the more the expansion and the lower the energy-density”. This is what we already see looking at the microwave background in our observable universe.
Isn’t it a mistake to consider mathematical parameters like time and length as real physical things? IMO you can’t take them apart physically. IMO don't take the way R.Descartes would do, look for the total thing, not for parts to assemble.
Indeed to me it seems evident that the only existence is the UV in the most general way, there is nothing else and nothing around. In a way it is infinite and if you multiply infinite by a factor it will be still infinite. By the way I very much like the article of Lineweaver and Tamara in Sciam “Misconceptions about the Big-bang”
An other additional but IMO related discussion is, ”Are there horizons (i.e. black-holes) in the UV?”, to what extend do the cosmological principle hold and do expansion, eventually, only take place within BH’s?” My guess is yes, but here I stop! You can see part of such discussion e.g. in post #7 of Jal in Marcus’thread “Bounce replaces bang”. Earlier in some of my posts in PF, I also tried to bring this up but without any response.