Metric Expansion Questions

[Luminal]

I read the Wikipedia article about Metric Expansion (of the Universe) and had a few questions:

 

1) Has space ever been observed (in a lab or in deep space) to actually be reduced? Or is space only capable of expanding?

 

2) Is this space 'expanding' or is it being outright created?

 

3) Do scientists have a good idea of what causes this or do they simply observe it in space and shrug their shoulders, hoping to find out at some later point?

 

Lastly, this isn't so much a question as an opinion, but it would seem that if the Metric Expansion explanation is correct, this has major consequences for science and even the future of technology.

 

For starters, energy wouldn't need to obey the first law of thermodynamics. Space could expand/reduce within a gravitational field, but objects within the field would not actually use kinetic energy to change position relative to the source of the gravitational body. It's position would now be different, and potential energy would either be created or destroyed. For example, let's say that the space between the ground and a bird 100 feet in the air expanded by 10%. The bird would now be 110 feet above the ground. It's potential energy is obviously different.

[Martin]

You started reading in a good place

http://en.wikipedia.org/wiki/Metric_expansion_of_space

I read the Wikipedia article about Metric Expansion (of the Universe) and had a few questions:

 

1) Has space ever been observed (in a lab or in deep space) to actually be reduced? Or is space only capable of expanding?

 

.

 

space is not a material and cannot expand or contract. but DISTANCES can expand and contract. the distance function of the universe is called the "metric" (short for Einstein metric tensor) and it is dynamic. distances can be expanding in some part of space and contracting in other parts

 

there is at present time an OVERALL AVERAGE increase of distances going on of about 1 percent every 140 million years.

 

It is a confusing popularization to say that because of this "space expands".

 

It is not space itself, which is not a substance like dough or rubber. It is merely the metric, or distance function, that is experiencing this. And the effect is only on the large distances that separate clusters of galaxies. Our small local distances of only a few lightyears are not part of this overall average largescale increase.

 

I have to go. Maybe someone else will take over. or Ill get back to it.

You are asking intelligent questions.

 

 

2) Is this space 'expanding' or is it being outright created?

 

space is not a substance so it is not created and it is not destroyed and it does not expand-------distances do expand and contract (they follow a law called the einstein field equation which describes how they change)

 

3) Do scientists have a good idea of what causes this or do they simply observe it in space and shrug their shoulders, hoping to find out at some later point?

 

they have an excellent mathematical model called the einstein field equation of GR, or simply the einstein equation. If you put in some map of where the matter is, and you input some metric (a distance function), the model will predict just how distances are going to expand and contract. It will give you the future evolution of the metric.

 

GR, that is, the einstein equation, predicts with amazing accuracy----many decimal places precision. Every time it is tested it turns out to be correct to as much accuracy as the experimenter can measure. It describes gravity by means of the metric (i.e. dynamic spacetime geometry) very successfully. Its not clear that everybody should be asking why GR works, there may be more productive uses of scientist time---questions that are easier to answer, problems more ready to be solved.

 

Only a relatively few scientists (a small percentage) are currently devoting time to ask WHY nature should work according to the einstein equation.

There are also some who believe nature does NOT work according to GR except at low energy and large scale. They think that at microscale in high energy situations there will be a whole new physics governing gravity, quite different from General Relativity. I don't know of any evidence to support such a belief.

 

...energy wouldn't need to obey the first law of thermodynamics. Space could expand/reduce within a gravitational field, but objects within the field would not actually use kinetic energy to change position relative to the source of the gravitational body....

 

This is perceptive. In fact the uneven expansion of space does take energy away from some light (by stretching it out to make longer wavelength)

and also the changing geometry gives energy to other light----very much like the BIRD in your example. The light goes into the gravity well of a galaxy cluster and picks up energy on the incoming----but while it is in the cluster traveling across it, the distances increase increase and the light is no longer so deep in the potential energy hole!

So when the light has crossed that cluster it is MORE energetic than before.

Mostly the expansion deprives energy, but it can work both ways.

 

here is something on the INTEGRATED SACHS WOLF EFFECT http://astro.uchicago.edu/~laroque/ISW.html

in case you are curious. it should be OK because it is from astronomy department of the University of Chicago

(some of what is on web is slop-popularization or nutcase, but you seem to be able to pick and chose)

the integrated sachs wolf effect can actully blueshift light a little (give it more energy)

 

good luck finding out more

[Royston]

So if I'm understanding this correctly, there's no 'stuff' (for want of a better word) or force aka dark energy or negative energy pressure considered in the metric of expansion ?

 

If that's the case, why is this disregarded ?

 

I'm clearly missing something.

[Martin]

So if I'm understanding this correctly, there's no 'stuff' (for want of a better word) or force aka dark energy or negative energy pressure considered in the metric of expansion ?

...

Actually I did not say what you said. I didn't discuss the cosmological constant.

 

We had the basic expansion cosmology picture long before the positive cosmo constant Lambda (and variants "dark energy" with various other names...) was generally accepted in 1998

Main thing that Lambda does is contribute a slight acceleration to the expansion. It becomes more important in the late universe.

 

Einstein put Lambda in the equation, and then dropped it (set it equal to zero).

A few people suspected there might be slight positive Lambda.

 

Then in 1998 to most people's great surprise it was discovered that the universe was acting like there WAS a very small positive Lambda after all.

The small size amazed people---it is about 10^-120 in natural (Planck) energy density units----or natural curvature units, same thing. The effect had been too small to notice, previously.

 

BUT drama aside that is merely a DETAIL added to the basic expansion picture that we have had since the mid twenties, derived from GR by Friedmann and Lemaitre, confirmed by Hubble and others.

 

It might be a good idea to start by understanding the simple picture without Lambda. That is why I did not mention it.

 

In the beginning Alex Friedmann discovered a metric which was a solution to the Einstein equation and in which distances expanded....

[Royston]

Actually I did not say what you said. I didn't discuss the cosmological constant.

 

Sorry Martin, it was a poorly phrased question...all I was asking, is that although the field equations predicted expansion, there was no mechanism considered for this expansion, because there was simply no need for a mechanism.

 

It was only until the addition of 'acceleration' that a mechanism e.g 'dark energy' was put forward. I guess I wasn't sure why Lambda was dropped to zero...hence I asked disregarded.

 

Hope that makes sense, probably a poor excuse, but I've been up till 2 am the last four evenings working on an a final assessment, so my grey matter is a little fried at the moment.

[Martin]

I think I understand.

As you say, expansion of distances does not need a mechanism.

it is a generic feature of pretty much any model based on our best theory of gravity (GR)

 

Looking at a whole range of mathematical possibilities including realistic and unrealistic

it might be slowing-down expansion

it might be expansion followed by contraction

it might be expansion forever

it might be speeding up

 

but whatever model, if it is even remotely realistic, and if it is based on the most precisely confirmed theory of gravity we have, then you just EXPECT to see distances change and over large scale expanding distance is normal distance.

 

so you don't need any unusual mechanism or any tweaking to get it, just usual vintage 1915 classic spacetime geometry

====================

 

the Lambda story is kind of separate. Albert the Great put in a Lambda term BEFORE HE REALIZED that generic solutions of his own theory involved expansion. He naively assumed that distances stayed the same. But in that case there would eventually be a tendency to COLLAPSE! Albert FUDGED and put in a Lambda term to JUST BALANCE the tendency to collapse. And he offered this halfbake idea to the waiting world. Actually it was an inherently impractical idea because if you examine closely that solution is not STABLE. if it ever starts to collapse even a little bit, then the density will increase and overwhelm the Lambda. So if it either expands or contracts even the slightest bit it will get out of balance and go catastrophically one way or the other.

 

And then Hubble and others (Slipher?) saw the expansion that was going on and that the universe wasnt static anyway. So the original reason to put in Lambda as a fudge to get static was blown away. So Albert the Great put the Lambda to zero and said excuse me.

 

From 1930 to 1998 Lambda was only of academic interest. Scholars speculated about different universe models, some with positive or negative Lambda---it was good mathematics. But working astronomers assumed it was zero and didnt even put it in the equation. Then as you know in 1998 there were the supernova results and Lambda was given a positive value of around 10^-120 in Planck units of curvature (or energy density)

 

Yet still most of the observed expansion of distance had little or nothing to do with Lambda, it has to do mostly just with how things started off---which quantum cosmologists now are saying could have been with a bounce.

[ddj03]

So space is not expanding, but the distance between objects (galaxies, etc.) is increasing (sometimes faster than the speed of light). How is this possible without the objects actually moving through space at these high speeds?

[Martin]

So space is not expanding, but the distance between objects (galaxies, etc.) is increasing (sometimes faster than the speed of light). How is this possible without the objects actually moving through space at these high speeds?

 

I was in a hurry when i wrote this in post #2

space is not a material and cannot expand or contract. but DISTANCES can expand and contract. the distance function of the universe is called the "metric" (short for Einstein metric tensor) and it is dynamic. distances can be expanding in some part of space and contracting in other parts

 

what I meant to say was

space is not a material that can expand or contract as materials do. but DISTANCES can expand and contract. the distance function of the universe is called the "metric" (short for Einstein metric tensor) and it is dynamic. distances can be expanding in some part of space and contracting in other parts

 

I found a helpful article---know one of the authors over the internet.

thoughtful paper on how to convey an idea with least confusion

http://arxiv.org/abs/0707.0380

Expanding Space: the Root of all Evil?

Matthew J. Francis, Luke A. Barnes, J. Berian James, Geraint F. Lewis

8 pages, accepted for publication in PASA

(Submitted on 3 Jul 2007)

 

"While it remains the staple of virtually all cosmological teaching, the concept of expanding space in explaining the increasing separation of galaxies has recently come under fire as a dangerous idea whose application leads to the development of confusion and the establishment of misconceptions. In this paper, we develop a notion of expanding space that is completely valid as a framework for the description of the evolution of the universe and whose application allows an intuitive understanding of the influence of universal expansion. We also demonstrate how arguments against the concept in general have failed thus far, as they imbue expanding space with physical properties not consistent with the expectations of general relativity."

 

the problem is language, maybe you can help get it straightened out

 

=====================

actually I think of space as expanding, but a few months ago I noticed the phrase "space expands" was giving people the notion that space was like a material----metal, dough, rubber---that expands, and this was causing all sorts of difficulties and misunderstanding. So I, and some other people, decided we would try saying "distances between stationary galaxies expand" or something like that, instead----to get away from the MATERIAL image that was confusing people.

 

space is defined circularly (like many very basic notions) as the web of spatial relations between things. it does not exist (I think) apart from events and the things used to define events. space is relational.

 

in GR, the best model of spacetime and gravity that we have, the gravitational field is defined as the GEOMETRY of spacetime and geometry is defined by a catalog of distances (or proper times and distances) between events. the gravitational field IS the metric, or distance function, and that IS the geometry

 

when one says "space expands" one means that distances expand, and the distances ARE space, it consists of nothing but those relations

=================

 

now I've tried to say it briefly and probably have caused confusion. What I urge you to do---very strongly---is to look at this paper by Berian James et al. He and the others know very well that space expands, and that distances between stationary points expand, and they know this causes students problems---and they try to address the communication problem in a careful way.

So you might get something out of that article.

[Reaper]

 

=====================

actually I think of space as expanding, but a few months ago I noticed the phrase "space expands" was giving people the notion that space was like a material----metal, dough, rubber---that expands, and this was causing all sorts of difficulties and misunderstanding. So I, and some other people, decided we would try saying "distances between stationary galaxies expand" or something like that, instead----to get away from the MATERIAL image that was confusing people.

 

I guess the question here is, is what you said really representative of what is actually happening? Because I think that saying that might cause even more misconceptions. (e.g. STATIONARY galaxies)

 

I think the reason people get confused by expansion of space is basically because most really can't wrap their head around the idea that space itself is expanding. Often their common sense view tells them that there is some outside space that galaxies are expanding to. It's pretty similar to misconceptions about curved spacetime and flat universe.

[Martin]

"distances between stationary galaxies expand"

 

I think the reason people get confused by expansion of space is basically because most really can't wrap their head around the idea that space itself is expanding...

 

the ah-hah moment for me came when I realized that the solarsystem was moving.

 

It is moving in the direction in the sky where the constellation Leo is, I forget the coordinates and coordinates are only temporary anyway. But there is this direction in the sky and we are going thataways at 380 km/second

 

with respect to the microwave background and with respect to the expansion itself

 

so there is an absolute universal criterion for what stationary means. Hubble noticed this even in the 1930s without having the microwave background to help him. Galaxies in the Leo direction are redshifted slightly less than average because we are pursuing them with our own small 380 km/s speed. And those in the anti-Leo direction (behind us so to speak) are redshifted slightly more than average.

 

this local velocity vector relative to absolute stationary (astronomers say "with respect to the Hubble flow") was measured very accurately in the 1990s by the COBE team (Smoot, Bennett, Lineweaver,....)

that is where the figure 380 comes from, they gave it 370 something with some errorbar, and they gave coordinates. Hubble knew in the 1930s but did not have such precision.

 

It is very beautiful. The CMB is slightly hotter (bluer) than average in the Leo direction and slightly colder (redder) in the anti-Leo. They call it the "CMB dipole".

 

What that means is there is a universal absolute standard for what it means to be stationary anywhere in the universe. It means having zero CMB dipole. Background temperature balanced in all directions. You can only get this approximately, with an errorbar, but that's how nature is.

 

And we see that distances between stationary galaxies increase, by about one percent every 140 million years. this applies to galaxies that are widely enough separated so that individual relative motions of a few hundred km/s do not matter. the galaxies should not be in a gravitationally bound cluster or orbiting each other, they should be widely separated. then we can mostly just neglect their small individual random motions and treat them as stationary---which they are on average anyway if their private motions are random.

 

the percentage increase of distance between stationary galaxies is an OBSERVATIONAL FACT. True, it is approximate but the random deviations are small and it is impressively consistent. So in a sense there is nothing to "wrap your head around". It is just something we observe.

 

And it is consistent with our MATHEMATICAL IDEA OF DISTANCE. Our theory of geometry is the same as our theory of gravity----it is GR. GR tells us how the metric behaves. No other geometry works as well. It works to exquisite precision---tested to many decimal places. The GR idea of distance trumps special relativity and Euclidean. Someday another theory of geometry and gravity will replace it, but for now GR is king.

And GR tells us to EXPECT distances to change.

And, as you Lockheed pointed out, it teaches us to expect curvature.

 

So there is an absolute idea of stationary.

And we can simply see that distances between widely separated stationary galaxies increase at a certain percentage rate.

And our prevailing model of geometry says we have no right to expect distances not to change.

 

So in that sense there is no problem.

 

The problem is not with physics but with human language. Our verbal idea of distance goes back to our Neolithic ancestors for whom the distance between Cave A and Cave B did NOT change. So our language contains a fossilized idea of distance. If people insist that understanding something means being able to say it in conventional English sentences, they will always have a hard time. English is an Indo-European language rooted in Neolithic life. I like saying things in ordinary English too! But there are stubborn rough places in language that you have to be aware of like this idea of distance.