Is the universe infinite in size

[noz92]

is the universe infinite in size?

[Skye]

I'm not sure how we know, even if it is.

[Martin]

I'm not sure how we know, even if it is.

 

we dont know

the mainstream judgement-call about this is an educated guess

and it comes down to measuring one number Omega

 

the most accurate report to date on it is in a paper by Bennett et al.

that reports on the results of WMAP

and says that so far they can just get that

 

Omega = 1.02 +/- 0.02

 

this is very frustrating, but accuracy will increase with time. Another orbiting Microwave Background probe will be launched. WMAP will continue. Eventually the uncertainty should be reduced.

 

If Omega = 1 then U is probably flat infinite

and the measurement is tantalizing close to one.

 

but if Omega > 1 then U is definitely finite.

 

If WMAP or PLANCK probe finds Omega = 1.01 then at least it is finite, tho it might be very big, much bigger than we can actually see.

 

the Bennett article is online, maybe the link is in Astro reference sticky thread. It should be

 

all the other mainstream cosmologists copy the Bennett WMAP data, so if you look in somebody else article like Lineweaver you see

same 1.02 +/- 0.02

 

there is probly also a link to the Lineweaver article in Astro sticky.

[Martin]

Yeah the sticky thread has a link to Lineweaver

who has a table of the current best estimates of Omega and the other cosmology numbers. See this SFN post:

http://www.scienceforums.net/forums/showthread.php?p=63698&posted=1#post63698

 

"This article by Lineweaver (he was one of the team in charge of COBE

an earlier CMB satellite observatory)

 

"Inflation and the Cosmic Microwave Background"

http://nedwww.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver_contents.html

 

http://arxiv.org/astro-ph/0305179"

 

it is post 63698 I think

 

there isnt any link to Bennett report from the first year of the

Wilkinson Microwave Anisotropy Probe

but I will find one and add it

[mooeypoo]

I think the universe, if not infinite, it's RELATIVELY infinite. Specially since most common beliefs are that it's expanding - so it must have SOME sort of "size" (it can't expand if its infinite, can it? ) -- what I think, is that its relatively-infinite, which means that even if we try to reach its ending, it's always growing and expanding, which means we'll never GET THERE, which makes it relatively infinite.

 

I also believe there's a high probability the universe "folds" within itself (kindof like the inner side of a balloon) - which also makes it infinite..

 

So to summarize -- I think it depends how you concider the word INFINITE

 

~moo

[Martin]

...SOME sort of "size" (it can't expand if its infinite, can it? ) --

 

yes as a matter of fact, it can expand if it is infinite

 

and this is what is most often assumed by working cosmologists for purposes of calculation-----Omega = 1

the U is spatially flat

 

the size is called the "scale factor" and often written a(t)

 

it is defined by the role it plays in the standard "metric" or distance-function used in cosmology

 

a(t) increases with time and this makes all spatial distances increase with time

 

since the U is infinite it has no "radius" so to make a(t) intuitive for students a teacher will sometimes call it "average distance between galaxies" but it is really just a term that is required by the Einstein equation and which enters into the distance-function determined by the equation.

 

the rate of increase of a(t) is kept track of by

the Hubble parameter H which is defined as

a'(t)/a(t)

and measured to be 71 km/sec per Megaparsec

 

---------------

metaphors are tricky, but if you want to imagine an infinite thing expanding just imagine you have an infinite line with pencil marks all along it one inch apart

 

now you go away and come back the next day and you find that the marks are all two inches apart! the line must have stretched out overnight!

[Martin]

the original post had Zoidberg saying "Of course an infinite universe can expand! What is going to keep it from expanding if that's what it wants to do?"

 

In case you arent familiar with him, Zoidberg is a character in a white coat whose face is a bit like a squid's: goggly eyes and a number of tentacle-like noses.

[LucidDreamer]

Is it that the universe is infinite and expanding or that the universe is infinite and the matter is expanding within it?

[Martin]

Is it that the universe is infinite and expanding or that the universe is infinite and the matter is expanding within it?

 

 

galaxies are not expanding in size

the distances between galaxies is increasing

 

 

from a cosmologist's standpoint the important thing is that

the metric that measures distance is increasing

 

that is, distances between things are increasing

(unless they are bound together like atoms molecules planet systems galaxies)

 

the journalistic words for this have always been "space is expanding"!

well that is OK or not OK way to say it. I dont know. It appeals to people

to say "space is expanding!" or "the universe is expanding!"

 

but that is just some words and maybe it helps or it doesnt help.

 

the true thing is that Einstein equation is a differential equation that tells how the metric, or distance function, evolves. And when it is solved in simple universe model one gets the FRW metric (F stands for Friedmann) and in the FRW metric there is this increasing scalefactor. so distances are increasing (between points each of which thinks it is standing still)

 

 

the goshdarn thing is that the Einsteinequation CHECKS with reality. It was published in 1915 and it has been tested all along the way Including GP-B satellite this year.

 

It does not have stable static solutions. the metric you get as a solution to the equation has to be either increasing distances or decreasing-------space has to be "expanding" or " contracting" to get a solution to the equations.

 

you see it only in widely separated things like two very far apart galaxies.

so we dont have to worry about expansion in daily life

 

if anybody could find a better model of gravity and space and time, they would tell us----but havent yet, so just must accept it.

-----------------------

You say infinite.

Be careful. We dont know Omega with complete certainty. U could be finite and could be infinite. simplest for some purposes to assume flat infinite. But dont believe yet. We have to wait to know whether it is finite or infinite.

[Martin]

is the universe infinite in size?

we just got a similar question by a newcomer HKS85.

It was in General Science forum.

http://www.scienceforums.net/forums/showthread.php?t=7596

 

I wonder how many times a year someone asks if the spatial extent of universe is finite or not.

 

 

when this question was raised here by NOz

the first response (by Skye) pointed out a basic philosophical issue of

HOW DO YOU KNOW

and can you ever ascertain for sure that something is infinite

and I guess the answer is you never know

 

pragmatically it is hard to make a meaningful statement about infinite

 

you can test it with some specific test and say that it is bigger than this

and then later you can say it is bigger than THIS

and then bigger than THIS

 

but you can never test that it is infinite and statements that cannot be tested have no meaning in science

=======

 

but a lot of cosmologists, for purposes of calculation, use a spatially flat infinite model because it is the SIMPLEST easiest to use model that fits the data.

 

the data is amazingly good and the fit is remarkably good and since there isnt any clear evidence that its NOT infinite, well.....why not use the model that is so simple and easy to use.

 

putting in a finite radius of spatial curvature or a finite size (where there is no indication of what size to use, besides that it has to be very big) would just make more messy calculations. who needs the bother?

 

but someday they may get some observations of the CMB with much more accurate instruments (than the satellite that started feeding us CMB data last year) and the observations will say "whoa! it has a finite size, it is not spatially flat! space curves back on itself like a ball or something!"

and then everyone will change their formulas and differential equations a little----they are already prepared and know what to do---and it will be all right again. the formulas will have more terms and be slightly harder to compute with but still OK.

 

it is pretty clearly either perfectly spatially flat (omega =1) or it has a slight positive spatial curvature (like for instance omega = 1.01)

 

and unless it IS finite we will never know for sure

[MOTP]

Even if the universe does curve around, it doesn't mean that it's finite. It could just be one circle within other circles.

 

A theoretical physicist once postulated that spacetime was made up in units, similar to units of the electromagnetic spectrum (photons). If this is so, the possibility of a finite universe is in how many of these packets there are. Since matter has to occupy them, if there is a finite number of spacetime packets, the universe is finite.

 

The only problem with this is that there could be other forms of matter that do not occupy spacetime, or resist spacetime like current matter "jumps over" the lack of spacetime. We couldn't detect this matter unless it made itself apparent in spacetime.