How Was Thermal Equilibrium Achieved After Inflation?

[hellfire]

The fact that the black body distribution of the cosmic microwave background radiation is that of a black body, implies that during recombination the universe was in thermal equilibrium. My question is how this almost perfect thermal equilibrium was reached...

During inflation, space was expanded exponentially, inflating a small causally connected patch. This provides a initial causal connection from which the whole universe within our current particle horizon evolved.

But, although inflation provides the causal connection, it does not provide a thermal equilibrium. The expansion of space makes difficult the establishment of thermal equilibrium and, additionally, the reheating (the process which takes place after inflation) drives the content of the universe far from equilibrium. This disequilibrium is needed in order to satisfy the requirements for baryogenesis and matter-antimatter asymmetry (Sakharov conditions).

Afterwards, the plasma filling the universe must thermalize and reach equilibrium before recombinantion (actually, equilibrium is reached more earlier before nucleosynthesis). If thermal equilibrium is reached due to collisions, is it ensured that the whole plasma reaches the same temperature (no causal connection is possible now) leading to the homogeneity and isotropy observed in the CMB?

 

[Chronos]

Thermal equilibrium did not exist prior to a hot big bang. It clearly exists afterwards. Any prior state is excluded from causality, along with any energy convervancy rules that emerged.

 

[turbo]

Afterwards, the plasma filling the universe must thermalize and reach equilibrium before recombinantion (actually, equilibrium is reached more earlier before nucleosynthesis). If thermal equilibrium is reached due to collisions, is it ensured that the whole plasma reaches the same temperature (no causal connection is possible now) leading to the homogeneity and isotropy observed in the CMB?

If I understand the BB theory well enough, it proposes that after about a million years (depending whose numbers you use), the temperature of the universe had fallen enough to allow electrons to bind with neuclei, forming atoms, and reducing photon scattering so that the universe became transparent to EM. The primodial EM then, must originate from this surface of last scattering, well after causal connection is impossible. Does this not pose an even more severe problem to homogeneity and isotropy of the CMB? How can causally-disconnected regions of the universe have cooled so uniformly, given that they had had essentially a million years since baryogenesis in which to cool a little more rapidly or slowly than other regions?

 

[hellfire]

Yes you are right (you both are right). Let me try another way. It is known that inflation predicts quite well the spectrum and the amplitude of density perturbations. These grew from the scalar perturbations of the metric, which were induced by quantum fluctuations of the inflaton. The scalar perturbations (a modification in the gravitational potential) lead also to redshift of photons. From this it follows a modification of the temperature of radiation. Observed temperature anisotropies (in the primary anisotropies) are thus traced back to density perturbations.

If the universe was not in thermal equilibrium, there would be another source of temperature differences (a primordial source). This is not observed, as the CMB is a almost perfect black body curve. So a mechanism is needed to achieve the thermal equilibrium.

The inflation model provides flatness and smoothness in the energy density through space. But does it provide also a thermal equilibrium? (First question). The fast exponential expansion of space seams to work against this. Assuming inflation did smooth the initial temperature differences enough to provide at least a reasonable level of thermal equilibrium, how was the equilibrium recovered again after reheating (which drived the universe away from equilibrium)? (Second question).

 

[Chronos]

You have pretty much outlined the basis for the standard inflation model. To ensure thermal equilibrium you need a casually connected initial state where mixing can occur. The mixed state becomes perfectly uniform prior to inflation, save for the unavoidable quantum fluctuations. Inflation magnifies these quantum fluctuations into the now observed [and very small] anistropy of the CMBR. Reheating after inflation is uniform because of the extremely smooth state prior to inflation. The false vacuum that powers reheating is also uniform for the same reasons. It too is a magnified version of the initial mixed state.

 

[hellfire]

Are you saying that homogeneity existed before inflation?

 

[meteor]

According to this page, thermal equilibrium existed prior to inflation
http://lcd-www.colorado.edu/astr2010/Lectures/part4/part4_06.html
"Before inflation, our current horizon was a tiny-tiny region of the universe, much smaller than a horizon at that epoch. It had more than enough time to fully come into a thermal equilibrium."

 

[hellfire]

Thanks meteor, this was unknown to me. I have to think about it.