Uneven expansion of the universe?

[Bob3141592]

I'm a computer programmer, not an astronomer or physicist, so forgive me if my question has been addressed before.

Is there any data or firm science to indicate that the expansion of the universe is uniform? Could different regions of space be expanding at slightly different rates. Granted, I don't know what might cause such irregularities, but I don't know what cuases the expansion either. I'm not comfortable with the ideas behind dark energy and everything, and cosmology has seemed to get a lot slipperier of late. But most of the stuff I've read on this topic seems to imply that the expansion of the universe is the same everywhere, and I was wondering if there's justification for this assumption.

 

[marcus]

Bob I think you are right to suspect that the expansion is not uniform
I think any cosmologist would agree that it is only approximately uniform

basically they use the 1915 Einstein equation
(which does not predict that expansion should be uniform
but rather that it should depend on local conditions like
the density of material in the region)

but they also used a simplified model called the Friedmann equations
which were derived circa 1920 from the more complicated Einstein
eqn. The friedmann equations assume that material and energy is
uniformly distributed and so the expansion rate you get out of them
is a uniform expansion.

when cosmologists talk about things they often are referring to the
friedmann model where everything is homogeneous and averaged out
and uniform------so only the very overall largescale stuff appears
and it is just an approximation of the real thing. the Hubble parameter
H arises in the friedmann model, likewise the critcal density, likewise
the "omega" and the open and closed universe concepts---- meaningful
within the simplified model and only approximately meaningful otherwise

 

[marcus]

I guess it stands to reason that space is probably not expanding as rapidly in the neighborhood of the Virgo cluster as it is around here in the nbhd of our much less massive Local Group of galaxies

and I imagine that around the extreme concentration of galaxies called the Great Attractor, a kind of supercluster, that they detected off in the direction of the southern constellations Hydra and Centaurus
it is so massive that our local group (and even the massive Virgo Cluster!) appear to be falling towards it----altho it is very far away

I imagine that roundabout the Great Attractor space must be expanding more slowly still

but cosmologists don't care about local blips and wrinkles on a few 100 million LY scale

they habitually work with a completely smoothed out averaged homogeneous and isotropic model where these little details are invisible
so for them the expansion really seems uniform because they think of it and do calculations about it that way

 

[marcus]

we ought to have a copy of the Friedmann equations
in the Astronomy/Cosmology reference thread---the sticky thread
for keeping useful information handy

on first encounter with F. eqn. people often get confused by
the fact that Greek letter rho (for density) looks like Roman p (for pressure)
in cosmology almost nothing has pressure besides the dark energy
and for dark energy the most commonly assumed equation of state is
that p = - rho
(thats what you get from a vacuum energy or a constant energy density associated with the cosmological constant, the typical dark energy idea)

anyway the first F. eqn, in units where c = 1:

\frac{a''}{a} = - \frac{4\pi G}{3}(rho + 3p)

a is the scale factor in the standard cosmology metric (socalled FRW metric) and a increasing means distances between points are getting larger IOW space is expanding. The prime is a time derivative, so a' is rate of increase of a and a'' is a measure of acceleration. Dividing a'' by a sort of normalizes it so the arbitrary length unit goes away and you get a measure of acceleration that is just a reciprocal time squared.

rho and p have the same units (the unit of energy density is the same as that of pressure, in any coherenent system of units)
and multiplying by G will give, on the RHS as well, a reciprocal time squared

the point about dark energy is that as an energy density it contributes to the slowing of expansion by contributing to rho
just like any other type of energy including matter
so by contributing to rho, dark energy favors contraction

but dark energy is 3 times more influential as a pressure and in that way (by the negative pressure) it makes the whole RHS of the eqn positive and favors accelerating expansion

notice the minus sign

it will be easy to explain why dark energy (because of its constancy) has a negative pressure, but that is a separate issue
the main thing here is that because p_X = - rho_X
(the equation of state for D.E.)
because of that negative pressure expansion accelerates.

 

[Olias]

we ought to have a copy of the Friedmann equations
in the Astronomy/Cosmology reference thread---the sticky thread
for keeping useful information handy

on first encounter with F. eqn. people often get confused by
the fact that Greek letter rho (for density) looks like Roman p (for pressure)
in cosmology almost nothing has pressure besides the dark energy
and for dark energy the most commonly assumed equation of state is
that p = - rho
(thats what you get from a vacuum energy or a constant energy density associated with the cosmological constant, the typical dark energy idea)

anyway the first F. eqn, in units where c = 1:

\frac{a''}{a} = - \frac{4\pi G}{3}(rho + 3p)

a is the scale factor in the standard cosmology metric (socalled FRW metric) and a increasing means distances between points are getting larger IOW space is expanding. The prime is a time derivative, so a' is rate of increase of a and a'' is a measure of acceleration. Dividing a'' by a sort of normalizes it so the arbitrary length unit goes away and you get a measure of acceleration that is just a reciprocal time squared.

rho and p have the same units (the unit of energy density is the same as that of pressure, in any coherenent system of units)
and multiplying by G will give, on the RHS as well, a reciprocal time squared

the point about dark energy is that as an energy density it contributes to the slowing of expansion by contributing to rho
just like any other type of energy including matter
so by contributing to rho, dark energy favors contraction

but dark energy is 3 times more influential as a pressure and in that way (by the negative pressure) it makes the whole RHS of the eqn positive and favors accelerating expansion

notice the minus sign

it will be easy to explain why dark energy (because of its constancy) has a negative pressure, but that is a separate issue
the main thing here is that because p_X = - rho_X
(the equation of state for D.E.)
because of that negative pressure expansion accelerates.

Hi Marcus, what are the consequences for Dark-Matter-Energy really being a direct 'scale' dependant offset to Quark Condensates?

Let me simplify if I may Quarks are composed of electric charges of :

Proton uud= +1/3,+1/3(positive), and -1/3 (negative),
if we treat the Fundemental Proton as the present day sum of all POSITIVE ORDINARY MATTER in our Universe irrespective of all Hydrogen hierarchy, leaving aside the Electron (negative) for complete atomic 4-dimensionality(+1/3,+1/3,-1/3 and electron -1.

Dark Energy accounts for the total sum of non-luminous matter 'DARK-MATTER' of negative energy in Space, comprising of -1/3,-1/3 and +1/3 (negative-protons) or ANTI-QUARKS.

Positive matter shines because of the positive imbalance of Charge outwaying the negative charge by the ratio of +1/3, +1/3 to -1/3.

Negative Matter does not shine because of the negative imbalance of Charge outwaying the positive charge by the ratio of -1/3, -1/3 to +1/3.

The Electro Magnetic Vacuum is a potential medium whereby the Cosmological Constant is the rate of exchange between Positive Quark Condensate and Negative Quark Condensate.

When the Photon density is taken into consideration, then the Dark Energy Expanding Rate is proportional to The constant Decelerating rate of Photon density.

Photon Pressure is associated by the Luminosity function, close by luminous source's such as Andromeda, out way the Negative Dark Energy, and thus the intervening space is Contracting, all luminous positive phase matter sources 'stick' together, we get ATTRACTED to each other. All the major Luminous sources are 'Greatly Attracted'.

Sorry Marcus if I extended my simplification, I could have stated the symmetry of Positive Matter and constant energy pressure and Negative Matter and constant negative pressure are related to the Quark Charge Ratio Imbalance!