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Relativity and Cosmology

This is a Blog on relativity and cosmology for engineers and the like. You are welcome to comment upon or question anything said on my website (relativity-4-engineers), in the eBook or in the snippets I post here.

Comments/questions of a general nature should preferably be posted to the FAQ section of this Blog (http://cr4.globalspec.com/blogentry/316/Relativity-Cosmology-FAQ).

A complete index to the Relativity and Cosmology Blog can be viewed here: http://cr4.globalspec.com/blog/browse/22/Relativity-and-Cosmology"

Regards, Jorrie

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HTRN's Cosmology Questions

Posted January 17, 2015 1:55 AM by Jorrie

in HTRN's thread about the rate of bubble rise and expansion in a fluid, he also saw analogies to cosmological expansion and he speculated:

"Maybe we are on the surface of a bubble, and expansion is a result of external as well as internal forces:
Reduction of outside "pressure"(what ever that is) and internal forces.
"

This is a common question and surprisingly difficult to answer in a compact fashion, so this will not be short and sweet. It was slightly off-topic in the original thread, hence this Blog entry.

The popular answer is simply that we have no indications that we are on the surface of any 'spacetime bubble', but rather that we are at the center of an expanding observable spacetime bubble. We think that dynamics of this expansion all come from 'forces' inside the observable universe, not from outside.

We know that there must have been some event (or series of events) in the past that caused the matter and radiation of our observable universe to expand (distances and wavelengths are increasing over time). Let's call it the 'initial cause'. The effect of the initial cause is that the universe would keep on expanding, but that the rate will slow down as time goes on. We have actually observed this slow-down in expansion rate over the first 7 or so billion years after the event, because we are 'looking back in time'.

We also know that something is presently causing the rate of this expansion to increase again; let's call that the present cause. Physicists are not sure, but it is possible that the initial and present causes boil doen to the same thing, Einstein's cosmological constant Lambda, just at two different energy levels. Lambda has a peculiar characteristic: it has part normal gravity and part 'anti-gravity', in that it has negative pressure coupled to positive energy density in a fine balance. Normal matter-energy has both its pressure and energy positive.

Positive pressure contributes significantly to the gravitational force inside a collection of matter, but negative pressure will lessen that gravitational force and may even overwhelm the energy density to give an overall expanding effect. Since Lambda is precisely equivalent to the energy density of empty space, space with no normal matter or radiation has no choice but to expand (or contract) exponentially. The "expand or contract" needs some clarification.

It is possible for any piece of space to be in static equilibrium, with just enough matter and vacuum energy to neither contract (under the energy density), nor expand (under the negative pressure). This is however an unstable condition, since any minute negative volume fluctuation will increase the energy density's gravity, but decrease the negative pressure, resulting in an exponential collapse of that space.* The exact opposite will happen for any minute positive volume fluctuation, resulting in exponential expansion.

The answer to the original question is then clear: space does not need anything 'outside it' in order to contract or expand. It has enough internal mechanisms. We are just not able to characterize those internal mechanisms from first principles yet, because it has a quantum-gravitational flavor. And that's still a tough nut to crack in a consistent manner.

There were more questions, but this is already a 'head-full', so I will rather reply to further questions as they arise.

-J

* This form of collapse could be the trigger for a cyclic universe, but more about that later.

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#1

Re: HTRN's Cosmology Questions

01/17/2015 4:10 PM

I'm always wondering how gravity will be controlled....Do you think space, time and gravity are all part of the same thing? How would you harness or produce an anti-gravity force? Would this then also be a time machine mechanism?

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#2
In reply to #1

Re: HTRN's Cosmology Questions

01/18/2015 1:01 AM

Well, I think space, time and gravity are part of the same "thing", depending on how "thing" is defined, of course. If one think 'curved spacetime', then they are surely all part of it. There are two types of 'harness for gravity' allowed by general relativity: the 'transversable wormhole' and the 'Alcubierre drive'. They are possibly also allowed in quantum gravity, but until we have a quantum gravity theory all worked out, it remains uncertain.

Another problem is that both require large amounts of 'exotic matter', which has negative energy (lower energy density than the pure vacuum). If they exist, exotic particles could sport a form of anti-gravity, like keeping a wormhole open or pushing a vehicle to speeds faster than light in free space.

Apart from the Casimir effect which is suspected of creating a force due to negative energy density between the plates, I do not know of exotic matter that exists or that can be made. AFAIK, the larger hadron colliders can make anti-matter (positive matter with inverted charge), but not mass made of negative energy.

-J

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#3

Re: HTRN's Cosmology Questions

01/19/2015 8:23 AM

Jorrie,

Could it be that in the early period of creation, there were very few(none) black

holes,and the expansion rate was less because of all of the matter present.

As the universe evolved,and matter became more dense, and more black holes

formed,then the balance of forces became more favorable for an exponential

expansion,due to the near infinite compression of matter in a black hole;

In effect, less matter in the equation.

The black holes could be in effect,providing the energy required to maintain a steady

energy density.

We know that black holes spew out observable energy from incoming matter

collisions,but they could also be emitting dark energy as well.

Dark matter could simply be very concentrated dark energy,the same as regular

matter and energy are manifestations of the same thing.

Every galaxy has a black hole at it's center,and a cloud of dark energy holding it

together.

There I go again,just letting my imagination run away with me.

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#4
In reply to #3

Re: HTRN's Cosmology Questions

01/19/2015 11:07 AM

Hi HTRN,

I believe that the gravity is the same whether it is in black holes or not. I can't see that changing the universe's expansion curve. Jorrie has plots based on known math that agree with observed data. The expansion is based on the cosmological constant, with which dark energy is given credit, I believe. He will correct me if I am wrong.

-S

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#5
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Re: HTRN's Cosmology Questions

01/20/2015 2:41 AM

HTRN, 'S' is right. Whether there were BHs right at the beginning or not, would have made negligible difference to the expansion. It has been slowing down for the first ~7 billion years, then virtually 'coasted' at the same rate for a few billion years and is lately observed to be speeding up ever so slightly.

Can this be due to lots of present BHs? Nah, very unlikely, because the stuff that BH flings outwards are mostly radiation and plasma, the latter being very local to the galaxy that the BH finds itself in. It cannot speed up the overall expansion, which happens only in pretty much empty space.

It is today accepted scientifically that it has something to do with the energy or the fields in empty space, a.k.a. "dark energy" because it is not fully understood. Also, "dark matter" and "dark energy" are different things, behaving completely differently and independently. There is virtually zero possibility that the one can be a 'concentrate' of the other.

Every cluster of galaxies has a cloud of dark matter helping to hold it together. Between clusters we find little or no dark matter, but lots of vacuum energy 'pushing' the clusters apart at a mildly accelerating rate. We can see that they have very different characteristics, but we don't understand any of them that well from a theoretical point of view, especially the 'root causes' of either.

No harm in "letting the imagination run", as long as it is not accelerating unstoppably into a cul-de-sac...

-J

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#6
In reply to #5

Re: HTRN's Cosmology Questions

01/20/2015 9:17 AM

Thanks for the course correction in my imaginary trip.

I have changed direction slightly,but I will surely hit another cul-de-sac or dead end.

Sometimes my rudder gets stuck to starboard or port,and I tend to do circular

thinking.

This tends to form a vortex that is hard to pull out of without outside intervention.

Feedback from this forum is invaluable in helping me to shape my image of the

universe.

Thanks to all for the constructive criticism.

"If we all worked on the assumption that what is accepted as true is really true, there would be little hope of advance." -- Orville Wright

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#7

Re: HTRN's Cosmology Questions

02/07/2015 10:36 PM

I'm in a cul-de-sac about relativity? An example used to explain it, is a train whistle. The whistles pitch of an approaching train being different from a receding train.

A thought has crossed my mind, what example would one use, for the relativity of size, which is observed to be relative constant?

In an expanding universe, a receding mass would appear to be getting smaller? Now, what if the mass was getting smaller, the energy in the mass bubbling out into space as gravity? And the distant between varying either way, so how does one know relatively that the mass is either receding or its mass changing? Is the Higgs Boson stable?

The distances between solar systems in our galaxy are relatively constant? unless size and distance is being corrected by the black hole in the centre? Yes its just speculation.

Regards JD.

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#9
In reply to #7

Re: HTRN's Cosmology Questions

02/08/2015 12:01 AM

"A thought has crossed my mind, what example would one use, for the relativity of size, which is observed to be relative constant?"

Depends on what you mean by this. For example, things moving relative to us are locally observed to be Lorentz contracted in the direction of their movement, but this has no relevance on cosmological scales. There we use a different definition of distance and simultaneity in which there is no length contraction.

It seems obvious from the fact that the most distant galaxies recede from us at twice the speed of light, but we do not 'observe' them to be contracted in the radial dimension. How that radial dimension is 'observed' is a long story, because we obviously cannot simply measure their diameters in the direction radially away from us.

However, we do not find galaxies (or even clusters of galaxies) to expand. Their mutual gravity keeps things together, and the parts are generally in orbit around one or more centers of gravity. It is the 'voids' between clusters that expand, which we refer to as the 'expansion of space'.

There is still no clear-cut answer as to why space expands, it is simply an observational fact. Theories describing the expansion abound, some good, some bad, but the 'exact why' still alludes us. Maybe it will forever...

-J

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#11
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Re: HTRN's Cosmology Questions

02/08/2015 7:47 AM

If even clusters of galaxies do not seem to expand,does that mean that everything will stay together,even as empty space expands between them,or does matter have a cohesive effect on the space around it,and therby will escape ultimate disassembly,or is it a matter of scale,that galaxies and clusters are expanding,but at such a small scale as to be imperceptable with present technology?

"Solid" matter is more space than substance,even in it's most dense form,except maybe inside of a black hole,where all space is squeezed out.

And if all space is squeezed out,then it cannot exist in our universe,can it?

Imagine a mass with no space,only the ghost of itself:it's gravity left behind;a scar in the homogeneous fabric of space time.

Perhaps dark matter is the ghostly remains of a black hole that finally accumulated enough mass to exit our dimension.

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#14
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Re: HTRN's Cosmology Questions

02/09/2015 11:20 PM

"If even clusters of galaxies do not seem to expand,does that mean that everything will stay together,even as empty space expands between them,or does matter have a cohesive effect on the space around it,and thereby will escape ultimate disassembly,or is it a matter of scale,that galaxies and clusters are expanding,but at such a small scale as to be imperceptible with present technology?"

With the present 'best buy' being the cosmological constant (Lambda) as the 'dark energy' mechanism, it is predicted that everything at galaxy level (at least) will stay together. Lambda causes a sort of 'cosmic tidal force', but as long as the pull of gravity is stronger than this force, things contract (or at least stay in bound orbits). Forget about "space leaking out". Space inside the Milky way does not expand.

I have written some old Blog post on it: http://cr4.globalspec.com/blogentry/9562/Cosmic-Balloon-Application-V-Cosmic-Tidal-Force.

-J

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#12
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Re: HTRN's Cosmology Questions

02/08/2015 8:16 PM

("A thought has crossed my mind, what example would one use, for the relativity of size, which is observed to be relative constant?" Depends on what you mean by this…………….. )

Relative (relatively) constant? The concept I was trying to convey was, if the mass being observed and the mass of the observer is varying the same, then, the size of the observed mass would appear not to change, where as the distance between them would? I raised the concept of our solar system, as this I think contradicts the concept? To be correct the planetary orbits around the sun would be increasing and the Moon would be receding from the Earth?

Thank you for your answers, appreciated.

Regards JD

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#13
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Re: HTRN's Cosmology Questions

02/09/2015 5:52 PM

PS, a wild card, the Moon is receding from the Earth at 3.78 cm per year, but as for the planetary orbits I don't know. But if the concept is correct and the orbits are relatively increasing, this would explain why Mars once was able to have water on its surface, and the Earth being stormy and continually raining? For around a million years, I read some where? So as orbits relatively increased Mars would lose it water frozen as ice sheets at the poles? and the Earths climate would stabilise?

More speculation, regards JD.

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#15
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Re: HTRN's Cosmology Questions

02/09/2015 11:35 PM

The Moon is receding from Earth due to the tidal bulges caused by the Moon that lag the rotation of Earth. This slows down Earth's rotation rate and transfers some angular momentum to the Moo's orbital motion. This drives the Moon higher by about 4 cm per year.

The Sun and Earth also have tidal interaction, but the mechanism is different and very, very tiny by comparison to the Moon/Earth system. The other part of the question might be: what about mass/energy lost by the Sun? Doesn't it slowly loses some gravitational field?

I think the answer is yes, but the effect is so tiny that it is swamped by many other perturbations of the orbits of the planets.

-J

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#16
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Re: HTRN's Cosmology Questions

02/10/2015 10:49 AM

Thanks Jorrie.I can sleep better at night now.

A little off-topic,but I remembered reading somewhere,and I may have posted it in the past,that humans produce more heat,on a pound-for-pound basis than the sun.

Hydrogen is really a low energy-density fuel.

Every Person on Earth could be put inside a 1 mile cube,with room left over,but we better not try it.

With 700 billion watts of heat,there would be a real meltdown.

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#8

Re: HTRN's Cosmology Questions

02/07/2015 11:20 PM

How would one tell the difference between a universe being "inflated" from within, or being "pulled apart" from outside?

To an inside observer the effect would be the same.

Consider a ballon being inflated by air pressure from within,versus one being inflated by a vaccum cleaner,for instance.

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#10
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Re: HTRN's Cosmology Questions

02/08/2015 12:19 AM

"How would one tell the difference between a universe being "inflated" from within, or being "pulled apart" from outside?"

You are right, we can't really tell, but we have a good idea that it must be from 'within'. If we think that the 'force within' is mysterious, we would need an order of magnitude more mystery to make it 'from the outside'! Something like 10 dimensions of space instead of three. Occam's razor would favor the three.

Of course, it is difficult to even define what "outside" means at the level of the universe.

-J

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#17

Re: HTRN's Cosmology Questions

01/28/2017 6:47 AM

I understand that the Cosmological Constant term, in the Einstein equation, can be viewed, *not* as a perplexing positive-energy/negative-pressure extra term in the Stress-Energy Tensor on the RHS, but as an *intrinsic* curvature amongst all the other curvature related terms on the LHS.

That is, while the Cosmological Constant *can* be interpreted as an exotic background zero-point energy field entering into T on the RHS of the equation, it *may* also be interpreted as an intrinsic curvature evidenced by the fabric of space-time, even in the absence of any mass-energy-momentum therein.

In analogy, the fabric of empty space-time does not "lie flat", like a taut trampoline, but instead is imbued with an intrinsic "bow", like the flat-bed trailers of large semi-trucks, which arch upwards when empty, so as to lie flat under heavy loads (and not sag down in the middle). Thus, the Lambda term requires no mystery, merely the acceptance that the fabric of space-time possesses innate curvature, vaguely like recurved hunting bows and large flat-bed trailers.

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