# What Is The Mechanism of Space Expansion?

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Hi, can you guys tell me what is the mechanism of space (universe) expansion?

Does it interact with time? Or it is because of Gravitational Force?

or electromagnetic energy?

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I don't think anything is _causing_ it, right at the moment. If you throw a ball up in the air, gravity isn't causing the upward movement. It is slowing the ball down over time of course, then eventually reversing it.

What we have here is space-time expanding. Think of it as a rubber sheet that all the galaxies / stars etc. are sitting on. They aren't moving over the sheet, but the sheet is increasing in the amount it's stretched.

The initial momentum for the expansion I guess comes from the Big Bang but I'm not sure. The gravitational force between galaxies should slow down the expansion over time, much as gravity slows down the ball that got thrown upward.

I think current observaions show that the expansion is ACCELERATING, i.e. increasing, which is confusing. This could be due to an extra term in the equations for the way space-time behaves, called the Cosmological Constant, which can drive either acceleration or deceleration depending on whether it's positive or negative. I'm not an expert.

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What we have here is space-time expanding.

After all this time spend on this Forum it is still unclear to me.

Is it Spacetime that is expanding or simply Space?

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Hi, can you guys tell me what is the mechanism of space (universe) expansion?

There is no "mechanism". It is just the way space (as described by GR) behaves when there is a homogeneous distribution of matter.

Is it Spacetime that is expanding or simply Space?

Space.

Although, as someone pointed out last time you asked, it is entirely possible to transform to coordinates where space doesn't expand but time changes instead. But that is less intuitive, doesn't map in an obvious way to what we observe and (I am told) makes some things more complicated mathematically.

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There appears to be nonzero cosmological constant, which implies by the Friedmann equations that it takes the form of a constant energy density (called "dark energy") that fills up space. Positive cosmological constant implies a negative pressure, which drives expansion.

We have no idea what "dark energy" actually is, or why it is there.

Although, as someone pointed out last time you asked, it is entirely possible to transform to coordinates where space doesn't expand but time changes instead. But that is less intuitive, doesn't map in an obvious way to what we observe and (I am told) makes some things more complicated mathematically.

Yes, but coordinates don't have any physical meaning. The physical distances between points in space increases over time.

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Further to what Elfmotat states, this constant energy density that fills all space ( scalar field ) is theorized to be a false zero vacuum energy state.

Consider dropping a pencil so that it lands on its tip. This is a false zero energy state, perfectly symmetric, but very unstable. As soon as the pencil falls over on its side, it reaches its true zero energy state, but rotational symmetry has been lost.

It is this symmetry breaking and its associated 'fall' to lower energy states which has fueled inflation ( once or multiple times ) in the past.

Our universe could still be slowly falling to a lower state currently ( at a very slow rate ) to account for the expansion.

The problem is that we don't have a good handle on vacuum energy, or whether we are in a true/false, zero or non-zero energy state. Any simple calculation ( that I've seen, anyway ) involving harmonic oscillators at every point in space, and a suitable cut-off energy, gives a value for the vacuum energy that is over 100 orders of magnitude higher than expected.

Edited by MigL
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• 1 month later...

Virtual particle production. In one fashion or another has been a leading contender for the cosmological constant. However based on the numbers involved due to the quantum harmonic oscillator (Heienburg uncertainty principle) The energy density is 120 orders of magnitude too great. So there is two schools of thought... either Heisenburgs uncertainty principle is not involved in terms of the resultant virtual particle production. Or there is a suppression mechanism involved. This keeps the possibility open. My feelings however leans towards the Higgs SO(10). However that's just me. The main problem is "why is the cosmological constant of such a low energy density and why is it constant.". To be sure the person that can conclusively solve both problems would be a contender for the Nobel prize. I cannot say your idea is wrong as the inflaton and the curvaton are both forms of virtual particle productions used in inflation. Our current universe could still be using the same mechanism as inflation just at a different rate.

Lol Ya changed your post while I was typing.

Anyways this is a paper involving the Higgs field and the cosmological constant. It is based on the SO(10) MSM particle physics model. The beauty of this proposal is no exotic particle is introduced.

http://arxiv.org/abs/1402.3738

You can see from the descriptive that it's mechanism also includes a replacement for the inflaton.

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Or we could just admit that we don't know.

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Or we could just admit that we don't know.

T

Lol now what fun would that be?

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Excellent posts here!

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• 4 weeks later...

There is no "mechanism". It is just the way space (as described by GR) behaves when there is a homogeneous distribution of matter.

Space.

Although, as someone pointed out last time you asked, it is entirely possible to transform to coordinates where space doesn't expand but time changes instead. But that is less intuitive, doesn't map in an obvious way to what we observe and (I am told) makes some things more complicated mathematically.

I had never heard the "homogenous distribution" of matter before...that seems important to understand...so, to make GR work you have to spread matter out evenly in a 3d?

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Not exactly. The FLRW metric reguires homogeneous and isotropic expansion. However locally the EFE can describe anistrophies in a local region such as a BH.

Homogeneity is a term that depends on scale of measurement.

Let's look at a lake. If you examine the lake close-up it is inhomogeneous . Waves etc

However if you increase the measurement distance these deviations get effectively washed out on an average per volume basis.

The metrics of FLRW is the sane. Locally small distances the universe is inhomogeneous , galaxies large scale structures etc.

But if you increase the measurement distance to say 100 Mpc. Those deviations get ignorable. It will appear homogeneous.

Same can be said on a solid. At a certain macroscopic measurement it appears homogeneous, get close enough it isnt

Gravity however is inhomogeneous.

For example start with a flat space time. mMinkowskii then add an influence such as mass.. The relation of the original metric to the added mass is inhomogeneous as well as isotropic.

Homogeneous=no preferred location

Isotropic= no preferred direction.

Combined they mean uniformity.

The universe is uniform in distribution on average

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Not exactly. The FLRW metric reguires homogeneous and isotropic expansion. However locally the EFE can describe anistrophies in a local region such as a BH.

Homogeneity is a term that depends on scale of measurement.

Let's look at a lake. If you examine the lake close-up it is inhomogeneous . Waves etc

However if you increase the measurement distance these deviations get effectively washed out on an average per volume basis.

The metrics of FLRW is the sane. Locally small distances the universe is inhomogeneous , galaxies large scale structures etc.

But if you increase the measurement distance to say 100 Mpc. Those deviations get ignorable. It will appear homogeneous.

Same can be said on a solid. At a certain macroscopic measurement it appears homogeneous, get close enough it isnt

Gravity however is inhomogeneous.

For example start with a flat space time. mMinkowskii then add an influence such as mass.. The relation of the original metric to the added mass is inhomogeneous as well as isotropic.

Homogeneous=no preferred location

Isotropic= no preferred direction.

Combined they mean uniformity.

The universe is uniform in distribution on average

I get it...whoa...I get it!

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I get it...whoa...I get it!

Now he does the dance of joy!!!!

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Not exactly. The FLRW metric reguires homogeneous and isotropic expansion. However locally the EFE can describe anistrophies in a local region such as a BH.

Homogeneity is a term that depends on scale of measurement.

Let's look at a lake. If you examine the lake close-up it is inhomogeneous . Waves etc

However if you increase the measurement distance these deviations get effectively washed out on an average per volume basis.

The metrics of FLRW is the sane. Locally small distances the universe is inhomogeneous , galaxies large scale structures etc.

But if you increase the measurement distance to say 100 Mpc. Those deviations get ignorable. It will appear homogeneous.

Same can be said on a solid. At a certain macroscopic measurement it appears homogeneous, get close enough it isnt

Gravity however is inhomogeneous.

For example start with a flat space time. mMinkowskii then add an influence such as mass.. The relation of the original metric to the added mass is inhomogeneous as well as isotropic.

Homogeneous=no preferred location

Isotropic= no preferred direction.

Combined they mean uniformity.

The universe is uniform in distribution on average

I had this remark before in an old thread but let's go on it again:

At what time?

At what time is it uniform in distribution on the average?

I mean, there is no simultaneity throughout the Universe, so when we take a picture "on the average" of the universe, this "average" is distributed over time. The farther part will be in the past, the closest parts in the present.

So, once you take into account the BBT, the Universe is changing its density over time (it is expanding), so the farthest parts that belong to the past should be denser than the closest parts. How then can it be " uniform on the average"?

Edited by michel123456
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There is a lengthy thread covering how the measurements are determined in terms of time and uniformity issues

http://www.scienceforums.net/topic/87002-is-there-a-size-beyond-which-a-system-cannot-be-considered-at-once/page-3#entry847540

what it boils down to is much of it is based on calculations in proper distance and commoving time/distance.

thermodynamics are applied during each time period being examined.

obviously we can't examine the universe in entirety at once so one must extrapolate what the universe would be like per the time period being examined. The scale factor allows comparisons from one point in time to the other. However that is used to describe how the universe evolves. Homogeneous and isotropy is per each moment in time.

A good example is Hubbles constant. This constant is only the same everywhere at a specific moment in time.

The details is lengthy but the time and observable limits are factored in

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obviously we can't examine the universe in entirety at once so one must extrapolate what the universe would be like per the time period being examined.

Can you? Wouldn't that be considering things that cannot be related to each other? It is unphysical.

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You can by datasets and accumulated tests. Without mountains of data and numerous models describing scenarios both in cosmology and in all aspects of physics such as particle physics This would be far more challenging. However each measurement adds to our understanding.

The CMB however provided a major support to the cosmological principle. This is due to extensive mapping by WMAP and Planck.

Our understanding of GR is also highly valuable

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You can by datasets and accumulated tests. Without mountains of data and numerous models describing scenarios both in cosmology and in all aspects of physics such as particle physics This would be far more challenging. However each measurement adds to our understanding.

The CMB however provided a major support to the cosmological principle. This is due to extensive mapping by WMAP and Planck.

Our understanding of GR is also highly valuable

hm no, I ment something else.

When you picture the universe at time T, whatever time it is, it gives the picture of things that cannot "communicate" to each other, because in order to communicate it takes some amount of time. In a spacetime diagram, the objects that belong to the horizontal (the present) are objects that cannot see each other, objects that cannot physically interact. So the picture of a universe at some time T is something unphysical. And you cannot explain something physical (the Universe) on the basis of an unphysical situation.

Edited by michel123456
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The simple answer is you must define homogeneous based on the system conditions being modelled.

Two examples.

If your say modelling a thermodynamic process let's use a phase change due to a uniform temperature drops that occurs everywhere at a specific moment. Then this example applies globally. Another is distribution of matter.

However if you have a non global change ie Baryon accoustic oscillations. Then you define your model based on the region of influence per duration being modelled. The background locally starts homogeneous the influence is inhomogeneous and anisotropic.

There are good examples of both. However how you define homogeneous and isotropy is always specific to the model criteria.

As cosmology models are typically perfect fluid approximations. There is various techniques to model non uniformity dynamics as a perfect fluid.

Take a star. To use ideal gas laws on a star. You look for uniform regions. (Layers) you then model each region seperately seperating each region will a mathematical barrier or wall.(dimensions) then model the interactions between regions.

PS the term physical vs unphysical isn't practical. Physical is anything that can be described by physics. (Energy matter influence field etc

For your query local vs global is accurate.

Edited by Mordred
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I don't understand how your answer relates to the universe being uniform in distribution on average.

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That's thanks to inflation. See the horizon problem

http://en.m.wikipedia.org/wiki/Horizon_problem

This evened out the distribution. As thermodynamically you have uniformity. Further processes will occur in uniform manner.

CMB is a good example of that statement as it's 380,000 years after inflation.

Edited by Mordred
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That's thanks to inflation. See the horizon problem

http://en.m.wikipedia.org/wiki/Horizon_problem

This evened out the distribution. As thermodynamically you have uniformity. Further processes will occur in uniform manner.

The ugly black points represent the galaxy clusters. They are close to each other at the perimeter (close to the BB instant) and farther appart from each other in the present (close to the center) because of space expansion. In this picture the universe is uniform in distribution ONLY for a specific distance (for a specific radius). If it is observed that the universe does not present such a picture then there must exist a discrepancy somewhere.

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Yes correct but that's uniformity including time. You can also have uniformity at the same time depending on WHAT you are modelling.

Thermodynamic states is at a point in time.

Distance uniformity is limitted by what distance measurement your using.

That image is commoving volume. If you calculate those to proper distances you have uniformity at a specific time.

There is Three forms of distance measurements. Conformal ( no longer used) commoving and proper. So again How you define uniformity is specific to the model

In commoving distances the change in time and expansion is the scale factor. Using this formula you calculate its proper distance at a moment in time

$d_2=-c^2dt^2+\frac{a^2dr^2}{1-kr^2}$

K is the curvature constant.

In other words apply this formula to all the distance measures your back to homogeneous distribution.

Today

Edited by Mordred

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