Bjarne on: Dark Energy, only an illusion.

[Bjarne]

We know that space between galaxies expand.
But inside galaxies or solar systems it does not seems to happen.

To understand the apparent acceleration of the universe one must first ask:
Why does space inside galaxies or solar system not seems to expand.
This makes no sense.
For example think about; where is the “border” in space, - between where space expands and not expands.

Something seems to be totally wrong.

The difference between the nature of space between galaxies (where the universe expands) and the space inside galaxies and solar systems (where space apparently does not expand), is only that gravity is greater near mass.

Because we know space can stretch / expands space must have some kind of ”density".

Consider it’s the same “stretching” property of space that also is stretching space increasingly toward an astronomic body or toward galaxies.
Or in other words the density of space is greater between galaxies (far from galaxies) and smaller (thinner) the closer space are to mass.

Or in other words: the closer to an object you get the more space is stretching.

Bended / curvature of space is simply another (and not so perfect expressions) of the same phenomena.

Let us therefore assume that the space density is;

A.) factor 1000 between two galaxies
B.) factor 500 in a periphery of a galaxy,
C.) factor 100 at the Earth's surface
D.) factor 10 at the Sun's surface.

The expansion of the Universe is hence proportional to the space "density-factor", which means that the space between galaxies will expand relative much more than space near mass.

But this shows that space also is also expanding inside galaxies and Solar Systems etc..
This seems to contradict with the fact that this is not what we experiences..(!)

Consider that the meter stick is expanding too, and hence you can’t see that space here around us also expands. (why shouldn’t it)
This statement can’t sound very strange because once the Universe properly was smaller than a atom, which means long ago it would not be enough space for the meter stick in the Universe, except when the meter-stick too change size proportional with the deformation of the Universe.

The Universe hence very well could be expanding, - not equally everywhere, but proportional to the density-variation (stretch-factor) which mean proportional to how much space already is stretched due to gravity.

The inner Earth would hence not expand proportional with the surface of the Earth, but more near the surface than deeper inside.
This could explain that the Earth really is expanding exactly like Neal Adams has illustrated this:
http://www.youtube.com/watch?v=oJfBSc6e7QQ

But it could also explain what so called dark energy really is.

Because billion of years ago the density of the Universe was “thicker” hence also distances was shorter.

When we today looks at the sky and can see that the expansion of the Universe like long ago seems to have been much slower, it’s only a illusion, - because in the past distances just was much shorter, - this is in fact what dark energy reveals.

Otherwise nothing makes logical sense.
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[MG1962]

Adams has illustrated this:
http://www.youtube.com/watch?v=oJfBSc6e7QQ

There are no subduction zones ahh huh - I am sure the people of Chile will be comforted by that thought

 

[Bjarne]

There are no subduction zones ahh huh - I am sure the people of Chile will be comforted by that thought

Where are the evidence ?

 

[MG1962]

Where are the evidence ?

What exact evidence do you want?

 

[MG1962]

Lets start with this - if the Earth is expanding and there are no subduction zones - then if we add up all the worlds spreading zones we get a figure of aproximate 6 inches a year growth

For this to be true then the Earth was the size of a tennis ball 250 million years ago

 

[dafydd]

Here we go again.

 

[Kwalish Kid]

We know that space between galaxies expand.
But inside galaxies or solar systems it does not seems to happen.

To understand the apparent acceleration of the universe one must first ask:
Why does space inside galaxies or solar system not seems to expand.
This makes no sense.
For example think about; where is the “border” in space, - between where space expands and not expands.

Something seems to be totally wrong.

What is totally wrong here is you. It is actually well known why there is expansion between galaxies and not within galaxies. That you don't know that means that you do not understand the basics of this field.

 

[Bjarne]

What is totally wrong here is you. It is actually well known why there is expansion between galaxies and not within galaxies. That you don't know that means that you do not understand the basics of this field.

Enlighten me

For this to be true then the Earth was the size of a tennis ball 250 million years ago

Right
But the cause of the suddenly larger expansion of the Earth (especially the past 100 million years) is probably that the space density of the universe is more depleted now.
Space in gravitational fields hence contributes much stronger now to the expansion of the Universe than previously.

 

[Blue Mountain]

You're using Neal Adams as a reference for earth history and geology? The crank who believes the Earth started out as a small sphere and has been expanding in size (like a balloon) for thousands of years?

Instant credibility fail.

 

[Thabiguy]

Enlighten me

The simple answer is that certain types of metric expansion allow bound systems to remain bound. For that reason, systems that are bound (star systems, galaxies, solid bodies like planets etc.) do not expand.

Here's a simple example to begin with: imagine a big pile of pebbles that is big enough to hold together by gravity alone. Now, let's say that for some reason, the pile expands a little. If the pebbles didn't attract each other, the pebbles would just slowly fly apart. The same thing would happen if the pebbles attracted each other too weakly compared with what pulls them apart; the flying apart would be slowed down a little, but not stopped (the system is not bound). But since the pile is big enough to hold together by gravity alone (the system is bound), the pebbles, after the pile expands a little, will just fall back together.

So even if space does expand within the pile of pebbles, any extra space simply becomes more space for the pebbles to fall into, which they do, resuming their original distance (i.e. clumped against each other). This is because the pebbles are gravitationally bound. (If they weren't, expansion would cause the pile to fly apart.)

For a more complicated example, let's imagine one object (such as the Moon) orbiting another (such as the Earth). Now what if space expands? The answer is that closed orbits are possible for certain types of metric expansion - that is, after the Moon flies the whole way around the Earth through the expanding space, it will end up at the same distance with the same velocity as it began with - closing the orbit, which doesn't expand, even though the space it goes through does. This may seem counter-intuitive, but can be proven mathematically given a specific type of metric expansion.

A simple explanation (which doesn't strive to be accurate) would be that the expanding space gives the Moon "more space to fall into", and the effect on its orbit would cause the Moon to "spiral inward" instead of orbiting circularly or elliptically - but the same expansion of space also pushes the inward spiral out into a circle or ellipse again. (This is not a coincidence, but a consequence of the mathematics involved.) So the Moon retains a stable orbit despite the expanding space; in a sense, it can be imagined as steadily falling ever "closer" towards the Earth through the expanding space. Again, this only works because the Moon and the Earth form a bound system. If the Moon wasn't constantly falling towards the Earth (as orbiting objects do), this wouldn't happen.

The bottom line is that systems that would be bound without space expansion can remain bound with space expansion (except for very weakly bound systems, which might become unbound) - there will just be a different equilibrium with expansion than without it. And when a system is bound, it doesn't expand - if it did expand (without limit), it wouldn't be bound.

 

[quarky]

If sub-atomic particles were expanding, we'd lack reliable reference points to check for it.
Yet, to me, it 'feels' like they are expanding.

 

[Vorpal]

We know that space between galaxies expand.
But inside galaxies or solar systems it does not seems to happen.

Why do you say so? If you consider a toy problem, like the acceleration of a (momentarily) stationary particle in a Schwarzschild spacetime with dark energy driven by a cosmological constant Λ, then d²r/dt² = -GM/r² + Λc²r/3. This can be interpreted as the usual gravitational attraction with an additional outward force proportional to distance, which is just what one would expect from cosmic expansion.

N.B. In Newtonian physics, a "reverse Hooke's law" potential U = -(1/2)kr² has force proportional to distance and would give kinetic energy ∝ r², and thus velocity ∝ r, mimicking the Hubble law of cosmic expansion.

To understand the apparent acceleration of the universe one must first ask:
Why does space inside galaxies or solar system not seems to expand.
This makes no sense.

For low r, the 1/r² term dominates and the fact that space is expanding is insignificant, and the reverse is true for large r. This makes perfect sense.

Of course, this oversimplifies the relativistic situation, esp. since the above equation only holds true at the the apogee of a radial orbit, but I think for illustration purposes of the main idea--that for close distances, ordinary gravitational attraction dominates cosmic expansion by far, not that there is somehow "less cosmic expansion" near masses--it serves better by far.

 

[sol invictus]

N.B. In Newtonian physics, a "reverse Hooke's law" potential U = -(1/2)kr² has force proportional to distance and would give kinetic energy ∝ r², and thus velocity ∝ r, mimicking the Hubble law of cosmic expansion.

A particle in a reverse Hooke's law potential falls with a position and velocity that grows exponentially with time. In cosmology the analog is de Sitter space, a space that expands exponentially. In de Sitter every point is surrounded by a spherical event horizon a fixed distance away, beyond which the space is essentially moving away faster than light. Most choices of spatial coordinate on de Sitter do not satisfy distance being proportional to redshift, although you could probably find one that does (but if so the horizon would be at infinite distance, which is not very physical and pretty inconvenient).

So it's a Hubble law in the sense that it's a homogeneous and isotropic cosmology, but it's very different from the naive Hubble law (distance is proportional to velocity).

 

[dropzone]

"Dark matter" is "æther" with a name you don't have to learn an ASCII code to spell.

ETA: To save you trouble, it's ALT+0230 on your numeric keypad. You can't use the number keys above the letters because of the mapping of key...forget the reason. Pretend it's lost in the mists of MS-DOS time and use your keypad.

 

[Vorpal]

In cosmology the analog is de Sitter space, a space that expands exponentially.

Well, yes--the case that was considered was the Schwarschild-de Sitter spacetime.

In de Sitter every point is surrounded by a spherical event horizon a fixed distance away, beyond which the space is essentially moving away faster than light. Most choices of spatial coordinate on de Sitter do not satisfy distance being proportional to redshift, although you could probably find one that does (but if so the horizon would be at infinite distance, which is not very physical and pretty inconvenient).

I don't understand the significance of this. I thought the Hubble law referred to recession velocity in a comoving frame, which corresponds to the redshift velocity only as a small-distance approximation. In the Newtonian analogy, there is also a fixed distance beyond which the recession velocity is superluminal, and although the significance of this is not the same (and lacking interpretation as expansion of space), I don't see a problem with making this analogy.

Possibly we're referring to different things, or else I'm more confused than I think.

 

[sol invictus]

Well, yes--the case that was considered was the Schwarschild-de Sitter spacetime.

It's also a Newtonian analog to a matter+cosmological constant cosmology. Think of r as the scale factor of the universe and compare to the Friedman equations.

I don't understand the significance of this. I thought the Hubble law referred to recession velocity in a comoving frame

"Comoving" means "comoving with respect to the Hubble flow". If something is moving with the average flow, it has zero comoving velocity by definition.

In the Newtonian analogy, there is also a fixed distance beyond which the recession velocity is superluminal, and although the significance of this is not the same (and lacking interpretation as expansion of space), I don't see a problem with making this analogy.

The problem isn't the analogy itself, it's assigning an especially Hubblerian significance to v~r when the cosmological constant dominates. The analogy to a conventional Hubble law is stronger when the force is -M/r^2 than when it's Lambda r.

 

[lionking]

It's also a Newtonian analog to a matter+cosmological constant cosmology. Think of r as the scale factor of the universe and compare to the Friedman equations.



"Comoving" means "comoving with respect to the Hubble flow". If something is moving with the average flow, it has zero comoving velocity by definition.



The problem isn't the analogy itself, it's assigning an especially Hubblerian significance to v~r when the cosmological constant dominates. The analogy to a conventional Hubble law is stronger when the force is -M/r^2 than when it's Lambda r.

Exactly what I was going to say before you beat me to it Sol.





You guys are seriously awesome.

 

[PixyMisa]

But the cause of the suddenly larger expansion of the Earth (especially the past 100 million years)

What expansion?

 

[Vorpal]

"Comoving" means "comoving with respect to the Hubble flow". If something is moving with the average flow, it has zero comoving velocity by definition.

I'm not talking about comoving velocity, but recession velocity as absolute distance* between two particles comoving with their local Hubble flow, with respect to the time of a comoving particle. Then v = (a'/a)d = Hd, where a is the scale factor. This holds exactly in de Sitter spacetime with a constant H, hence my initial puzzlement at your objection.

*I thought recession velocity always used this distance, with the comoving particles specifying both the particular foliation to compute it in and the time to differentiate it with. This may be a misunderstanding on my part.

It's also a Newtonian analog to a matter+cosmological constant cosmology. Think of r as the scale factor of the universe and compare to the Friedman equations.
...
The problem isn't the analogy itself, it's assigning an especially Hubblerian significance to v~r when the cosmological constant dominates. The analogy to a conventional Hubble law is stronger when the force is -M/r^2 than when it's Lambda r.

Aah. I see where you're coming from. You're making doing something completely different than I was (which was just an isolated mass in an otherwise empty universe, and is exactly Hubble-like in the long-distance limit). But yours is a much more interesting case. Thank you.

 

[Brian-M]

We know that space between galaxies expand.
But inside galaxies or solar systems it does not seems to happen.

To understand the apparent acceleration of the universe one must first ask:
Why does space inside galaxies or solar system not seems to expand.
This makes no sense.

It makes plenty of sense. The expansion of the universe is so small that it's not measurable over distance the size of a solar system, and is relatively minor over the size of a galaxy. It's only over the truly vast distances of intergalactic distances that the cumulative effect of the expansion adds up to something huge. No problem.

Because we know space can stretch / expands space must have some kind of ”density".

That doesn't make much sense to me. Density is mass/volume ... but space has no mass and volume is defined by how much space is being occupied. So how can space have density?

The Universe hence very well could be expanding, - not equally everywhere, but proportional to the density-variation (stretch-factor) which mean proportional to how much space already is stretched due to gravity.

The inner Earth would hence not expand proportional with the surface of the Earth, but more near the surface than deeper inside.

Even if this was true, gravitational, nuclear & electromagnetic forces would pull the atoms/molecules/earth together... so the earth would remain the same size even if space was expanding at a different rate around the earth because of the mass of the earth.

This could explain that the Earth really is expanding exactly like Neal Adams has illustrated this:
http://www.youtube.com/watch?v=oJfBSc6e7QQ

That's one kooky video.

First claim, that the tectonic plates fit together perfectly. Not exactly true, they run into each-other in places and overlap causing mountains to rise, and other parts of the land to be pushed down below. (Deep-sea fossils on the tops of the highest mountains confirm this.) The plates separate from each-other in other places, causing magma to rise from the core and solidify to fill in the gaps. There's no conspiracy needed.

Despite the claims in this video, there are many well known subduction zones, such as the Cascadia subduction zone which stretches from northern Vancouver Island to northern California, or the Greater Antilles subduction zone which lies north of Hispaniola, Puerto Rico, and the Virgin Islands. Anywhere there are lots of volcanoes or frequent earthquakes, there's a good chance of a subduction zone being nearby.

Next claim that dinosaurs roamed across all the tectonic plates at one time because there were no oceans, just shallow lakes. Where the heck to they think all the water came from? The inside of the earth is molten rock, which is far heavier than water, which means that all the water in the world has to be on or near the surface. If the earth was much smaller in the past there could have been no land mass... everything would have been under water.

Of course, you could believe that the planet was entirely covered with water which gradually lowered as the earth expanded, but then dinosaur fossils would only be found on the few tiny patches of land mass that would have been above water at that time, which isn't true

At the end of the film it claims that this must be true for all planets, including Mars... but Mars has a solid core and is not tectonically active. If Mars was expanding this way it would be covered by truly enormous gaping chasms all over the surface... but it isn't.

Finally, the film makes all these extraordinary claims but makes no attempt to suggest any physical mechanism by which this would be possible... except to suggest that all scientific knowledge (which is based on centuries of careful observation and testing) is wrong.

But it could also explain what so called dark energy really is.

Because billion of years ago the density of the Universe was “thicker” hence also distances was shorter.

When we today looks at the sky and can see that the expansion of the Universe like long ago seems to have been much slower, it’s only a illusion, - because in the past distances just was much shorter, - this is in fact what dark energy reveals.

Otherwise nothing makes logical sense.
---------

Nothing about this post seems to make logical sense.

I started reading this thread, expecting it to be about Modified Newtonian Dynamics (MOND), a scientific hypothesis under which the expansion of the universe is explained without need for dark matter/energy.

Finding blatant crackpottery instead is disappointing.

ETA: As opposed to the carefully disguised crackpottery I was expecting.