In big bang theory, is it thought that the universe was homogenous until a certain point in history? If not, why not and if so, at what point did it begin differentiating into different levels of density and why?

In big bang theory, is it thought that the universe was homogenous until a certain point in history? If not, why not and if so, at what point did it begin differentiating into different levels

of density and why?

 

From what I've read so far there has never been a time in the Universe's history where it has had uniform density in the sense I think you mean. When cosmologists talk about homogeneity in it's structure they mean over large scales so wherever you are in it the structure looks more or less the same. Like this:

 

 

Even in the very beginning it is thought the universe was permeated throughout with quantum fluctuations (non-uniform density) which ultimately lead to the present structure shown in the image above.

 

In quantum physics, a quantum fluctuation is the temporary change in the amount of energy in a point in space,[1] arising from Werner Heisenberg's uncertainty principle.

According to one formulation of the principle, energy and time can be related by the relation[2]

 

That means that conservation of energy can appear to be violated, but only for small times. This allows the creation of particle-antiparticle pairs of virtual particles. The effects of these particles are measurable, for example, in the effective charge of the electron, different from its "naked" charge.

In the modern view, energy is always conserved, but the eigenstates of the Hamiltonian (energy observable) are not the same as (i.e. the Hamiltonian doesn't commute with) the particle number operators.

Quantum fluctuations may have been very important in the origin of the structure of the universe: according to the model of inflation the ones that existed when inflation began were amplified and formed the seed of all current observed structure.

 

http://en.wikipedia.org/wiki/Quantum_fluctuation

 

You might want to read this on the Cosmological Principle which has things to say about homogeneity.

 

http://en.wikipedia.org/wiki/Cosmological_principle

I should have been more specific. What I meant was which came first, atoms or clouds, with empty space in between clouds of varying density? In other words, did density-heterogeneity result from the interactions of atoms and gravity or was it something occurring simultaneously with the formation of atoms?

Edited May 24, 201114 yr by lemur

In big bang theory, is it thought that the universe was homogenous until a certain point in history? If not, why not and if so, at what point did it begin differentiating into different levels of density and why?

 

Gravity did it all by itself. The universe has always been homogeneous. The puzzling fact is how homogenious the unverse is. The explanation is inflation.

 

"Inflation is a concrete mechanism for realizing the cosmological principle which is the basis of the standard model of physical cosmology: it accounts for the homogeneity and isotropy of the observable universe. In addition, it accounts for the observed flatness and absence of magnetic monopoles. Since Guth's early work, each of these observations has received further confirmation, most impressively by the detailed observations of the cosmic microwave background made by the Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft.[48] This analysis shows that the universe is flat to an accuracy of at least a few percent, and that it is homogeneous and isotropic to a part in 10,000.

 

In addition, inflation predicts that the structures visible in the universe today formed through the gravitational collapse of perturbations which were formed as quantum mechanical fluctuations in the inflationary epoch."

 

http://en.wikipedia.org/wiki/Inflation_(cosmology)

Edited May 24, 201114 yr by Airbrush

I should have been more specific. What I meant was which came first, atoms or clouds, with empty space in between clouds of varying density? In other words, did density-heterogeneity result from the interactions of atoms and gravity or was it something occurring simultaneously with the formation of atoms?

 

Gravity has to have 'something to work with' -- er go, density inhomogeneities, today, demands primordial density inhomogeneities, 'right from the start', as 'initial conditions' resulting from the Big Bang. The Dark Flow, of the entire human-Visible Universe (out to billions of light-years), in a vaguely-common direction, at a few x 10^-3 c, might mean, that ultra-large-scale bulk motions -- velocity inhomogeneities -- also date back, to the Big Bang. The Cosmological Principle ("what's here is there") is a good guestimate, but need not be pressed for particulars, 'down into the details' -- "in the main, not the minute".