expansion of space

[`hýsøŕ]

I've looked in a lot of places for a clearer explanation of the expansion of space but either I get psuedoscience or the typical balloon analogy, so i'm hoping somebody here is more used to this concept.

 

I'm stuck with how space can expand. Like say I have a 3 dimensional space and attach a coordinate system to it, sure I can make the coordinate system larger or smaller, but how does that mean the space itself is smaller or larger?

In my cosmology class we've done the scale factor and co-moving distance thing, where r(t)=a(t)x, where a is the scale factor and d is the co-moving distance, and this just flummoxes me, conceptually, completely. I'd be very happy if somebody could clarify this mess of ideas in my head.

[Mordred]

How the universe expands or contracts depends on how the contents of the universe influence each other in terms of pressure. matter, radiation, exerts positive pressure, the cosmological constant exerts negative pressure. The relations is determined by their energy-density to pressure equation of state. The Universe is treated as a perfect fluid or ideal gas in terms of most metrics such as the FLRW metric. As such it follows all the ideal gas laws.

http://en.wikipedia.org/wiki/Equation_of_state_%28cosmology%29

this is also used to determine the universes geometry.

You can find more detail in this article.

http://cosmology101.wikidot.com/universe-geometry

page 2 covering distance measurements due to geometry

http://cosmology101.wikidot.com/geometry-flrw-metric/

 

now our universe is extremely close to flat, but the question of if our universe is open or closed (infinite or finite is still an open question.)

 

If the universe has a total positive pressure then the universe would contract, however the cosmological constant is the dominant influence of the universe today so the universe is expanding.

 

your cosmology class will cover this soon enough, though I'm not sure how far your class has covered as of yet.

Please also keep in mind that space is simply geometric volume that is filled with the contents of the universe. Edited June 18, 2014 by Mordred

[Strange]

I'm stuck with how space can expand. Like say I have a 3 dimensional space and attach a coordinate system to it, sure I can make the coordinate system larger or smaller, but how does that mean the space itself is smaller or larger?

 

That is all it means. Expanding space is just analogy for the changing metric. Space is just the distance between things and therefore part of the geometry of spacetime; it isn't "stuff" which gets bigger.

 

This might help: http://arxiv.org/abs/0707.0380 (it gets quite mathematical but the beginning is easy to follow).

[`hýsøŕ]

What I'm asking isn't the cosmological question of what is expanding space, its asking how a 3D space can, itself, expand. trust me i've heard the usually cosmological stuff about the energy densities and the friedmann equation and things before, i'm just struggling conceptually to understand what it really means for space to expand.

 

I understand how an object with volume can expand so that it occupies a larger volume of space, but how can space expand so that it occupies a larger volume of space? I don't' see how the idea makes any sense.

 

@strange, if what i mentioned there is true, then isn't just our coordinate system that's expanding and not space itself?

 

I mean say from hubble's law the expansion of the universe can be deduced, but can't the idea of galaxies moving away from each other in a non-expanding space also be deduced?

[Strange]

@strange, if what i mentioned there is true, then isn't just our coordinate system that's expanding and not space itself?

 

Pretty much. But what is the difference between space and the coordinate system? There isn't some "absolute space" that we apply our coordinates to. There is just our coordinates; i.e. the method we use to measure the spatial separation of objects. That changes over time. We describe this as space expanding (for simplicity).

 

 

I mean say from hubble's law the expansion of the universe can be deduced, but can't the idea of galaxies moving away from each other in a non-expanding space also be deduced?

 

There are reasons why modelling the recession of distant galaxies as proper motion doesn't work. For example, the fact that we can see galaxies that are moving faster than light. But, more importantly, the evidence overwhelmingly supports a model of the universe based on GR. Therefore that is the one that is chosen.

[Mordred]

Just for clarity Hubble's law isn't the only reason we know space is expanding, We can also infer that the volume of space has increased due to the thermodynamic history of the universe.

In and expanding universe, temperatures and energy-densities lower we know from measurements this is also true.

Edited June 19, 2014 by Mordred

[Deepak Kapur]

Does expansion of space mean that space between nucleons in a nucleus is also increasing?

[Mordred]

Not to the best of our knowledge, The energy-density of the cosmological constant is extremely low, to the point of being nearly immeasurable unless you measure it over an extremely large distance. Per cubic meter its energy-density is easily overpowered by the strong nuclear force and gravity. Its influence is primarily in the regions between large scale structures and is not known to cause any measurable influence in any gravitationally bound region. Nor is it known to cause any influence at the atom level.

[Dekan]

Its influence is primarily in the regions between large scale structures and is not known to cause any measurable influence in any gravitationally bound region. Nor is it known to cause any influence at the atom level

A thought-provoking observation. It raises a couple of points on which I'd appreciate further guidance:

 

1. What exactly is the size of a "large-scale structure". Is it a planetary system, a binary star, a globular star-cluster, a galaxy, a cluster of galaxies?

These are all "structures" of increasing size. At what stage do they become influenced by the "expansion of space". Has the size been quantified?

 

2. What exactly is the "atom level". Is it only the region within the individual atom - protons, neutrons, electrons and so on. Or does it extend outside the atom - to groups of atoms forming molecules.

Some molecules are small. But others, especially carbon-based organic molecules, get really big. How big can they get, before the expansion of space starts to influence them?

[ACG52]

 

1. What exactly is the size of a "large-scale structure". Is it a planetary system, a binary star, a globular star-cluster, a galaxy, a cluster of galaxies?

These are all "structures" of increasing size. At what stage do they become influenced by the "expansion of space". Has the size been quantified?

Large scale structure is a local galactic supergroup. The stage that expansion dominates is 200 million lys.

[Mordred]

large scale structure, atomic level, subatomic level, molecular level, are all terms that describe a particular system being examined so in that sense it not really a case of being set at a known size

 

large scale structures is a term used to describe galaxy clusters, galaxy clusters and galaxies are considered gravitationally bound systems.

Atomic level is a term used to describe the states within an individual atom

subatomic level is used to describe individual particles

molecular level is used to describe various molecules

 

the size range can vary depending on the complexity of the system being examined

 

for instance in the case of large scale structures, the Sloan great wall measures roughly 1.38 billion light years in length, however it is a gravitionally bound system and therefore is considered a large scale structure.

 

in terms of the cosmological constant and expansion you have to examine the energy-densities at the appropriate units of separation distance. The energy-density is the key. For large scale structures the average energy-density of gravity is greater than the energy-density of the cosmological constant so gravity wins and no expansion occurs in that gravitationally bound region. Gravity being the weakest of the 4 forces, means that the cosmological constant is far less likely to affect atoms or molecules

[Strange]

A thought-provoking observation. It raises a couple of points on which I'd appreciate further guidance:

 

1. What exactly is the size of a "large-scale structure". Is it a planetary system, a binary star, a globular star-cluster, a galaxy, a cluster of galaxies?

These are all "structures" of increasing size. At what stage do they become influenced by the "expansion of space". Has the size been quantified?

 

One answer is that it is at the scale where the universe starts to appear approximately homogeneous; at which point the FLRW metric (which describes the expansion) becomes a reasonable description. Below that level, the model is more complex and is dominated by the gravitational and other forces holding matter together.

[Mordred]

 

One answer is that it is at the scale where the universe starts to appear approximately homogeneous; at which point the FLRW metric (which describes the expansion) becomes a reasonable description. Below that level, the model is more complex and is dominated by the gravitational and other forces holding matter together.

 

The more I think about this way of looking at it, the more I like it lol, as a side note textbooks place that scale at roughly 100 Mpc (mega parsecs) however there have been papers that have recommended that value be raised to roughly 130 Mpc, due the the Sloan wall etc.

[`hýsøŕ]

sorry for not replying sooner, I'm still thinking about what you guys have said (will make a better reply soon)

[ccwebb]

I've always struggled with the question of 'space'. Even started 2 different threads asking questions and some amazing answers have been given. More importantly, simplified for my simple brain!

 

I mean say from hubble's law the expansion of the universe can be deduced, but can't the idea of galaxies moving away from each other in a non-expanding space also be deduced?

 

I've wondered the same thing here. If you have 2 balls suspended in water, floating half way from the surface and bottom. The balls can be pushed apart, thus adding more space between them, or more water can be placed between them. Literally adding more space between them. In once case the balls are moving, the other case 'space' is moving.

 

To be silly, Scotty from Star Trek stated that, " Imagine that! It never occurred to me to think of SPACE as the thing that was moving!"

 

Mordred and Strange, I understand your answers. For some reason they are not making sense to me. How do we know space is expanding when we can't see the 'walls' moving away from us? We can see stuff moving from us, we can measure the volume based on what we see. But we can't see passed 46 bLy. If space is infinite, then we are missing a very big portion of the equation.

[Strange]

 

How do we know space is expanding when we can't see the 'walls' moving away from us?

 

All sorts of different lines of evidence. One of the first clues that the theory might be correct was Hubble's observation that distant galaxies are receding with a velocity proportional to their distance. A bit of thought shows that this is equivalent to all galaxies moving away from each other.

 

The theory made a number of predictions, which have been matched by observation. One of these was the nature and temperature of the cosmic background. When this was observed (accidentally) and found to match the theory exactly, then it was "game over" for the other models.

 

 

If space is infinite, then we are missing a very big portion of the equation.

 

True. We can only speak with any certainty about the observable universe. It is entirely possible that the universe beyond point that is actually populated by chocolate unicorns dancing the fandango. But it is considerably more likely that the universe beyond the observable universe is largely similar to what we see - for some distance, at least.

[Mordred]

Strange pretty much covered the key details.

[`hýsøŕ]

"All sorts of different lines of evidence. One of the first clues that the theory might be correct was Hubble's observation that distant galaxies are receding with a velocity proportional to their distance. A bit of thought shows that this is equivalent to all galaxies moving away from each other."

 

Pretend for a second we don't have relativity at our disposal, why does the idea of galaxies moving away from each other require space be expanding? (instead of the galaxies just.. moving away from each other in a fixed space)

This has always eluded me, this particular idea

[Mordred]

this has to do with the actual metrics, movement requires a force, (newtons 3 laws of inertia) now there is no preferred direction to how galaxies expand away from each other, this means that whatever causes expansion is isotropic (no preferred direction) as there is also no preferred location (homogeneous) this means that whatever is causing expansion is uniform.

 

now consider 1 galaxy, then apply a uniform force all around that galaxy, this means there is no greater force on any side of the galaxy, so the galaxy will not move. hrrm this presents a problem. If you do this to every galaxy then the galaxies themselves won't move. However what can be affected is the space between the galaxies. Think of the universe in terms of a perfect fluid, or ideal gas, space being the volume, the energy-density total exerts a pressure, that pressure causes expansion. It doesn't move the galaxies directly due to its uniformity, but it can cause the gas itself (space) to expand.(via the particle to particle interactions that reside in space,) (the FLRW metric and the Einstein field equations both uses the ideal gas laws, its part of them), keep in mind that in expansion the angles between 3 galaxies will not change, they move away from each other equally without a change in angle between any of them. (hence the balloon analogy to describe the metric relations)

Edited July 16, 2014 by Mordred

[Strange]

Pretend for a second we don't have relativity at our disposal, why does the idea of galaxies moving away from each other require space be expanding? (instead of the galaxies just.. moving away from each other in a fixed space)

This has always eluded me, this particular idea

 

If we didn't have relativity, then Hubble's observations would, perhaps, have been explained in a different way. But it would have left people scratching their heads over an awful lot of other observations...

[matterdoc]

Advent of universal gravitational attraction necessitated that all matter in the universe accumulate at a point. This is absurd and against observation and common sense. Expansion of space/universe and all related theories are attempts to escape from this conundrum. Until real reasons for 'steady state' existence of universe is revealed, similar theories will continue to exist.

Nainan

[Strange]

Advent of universal gravitational attraction necessitated that all matter in the universe accumulate at a point.

 

Newton solved that one by concluding that the universe was infinite. .

 

In GR, the problem doesn't arise, even for a finite universe.

 

 

This is absurd and against observation and common sense.

 

It may be against "common sense" but that is why we use science, rather than common sense, because it works.

 

On the other hand, it definitely is not against observation. It is based on observation.

 

Until real reasons for 'steady state' existence of universe is revealed, similar theories will continue to exist.

 

First you would need some evidence that there is such a thing as a "steady state universe".

Edited August 7, 2014 by Strange

[michel123456]

Is this expansion of space or expansion of spacetime?

[Strange]

Is this expansion of space or expansion of spacetime?

 

Of space, spacetime is normally considered a static thing.

[StringJunky]

Advent of universal gravitational attraction necessitated that all matter in the universe accumulate at a point. This is absurd and against observation and common sense. Expansion of space/universe and all related theories are attempts to escape from this conundrum. Until real reasons for 'steady state' existence of universe is revealed, similar theories will continue to exist.

Nainan

It didn't accumulate at a point, it was always everywhere but just closer together. Understand first that which you are trying to criticise.