Understanding the Derivation of the Dirac Equation in Cosmology

In summary, the conversation discusses the derivation of the Dirac equation in the presence of a curved spacetime, specifically with a scale factor for the expansion of the universe. It also mentions the use of vierbeins and covariant derivatives, as well as the relationship between the spin connection and gamma matrices. The overall goal is to understand these concepts in order to effectively vary the matter action.
  • #1
pleasehelpmeno
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0
Hi i am trying to derive the Dirac equation of the form:
[itex] [i\gamma^0 \partial_0 + i\frac{1}{a(t)}\gamma.\nabla +i\frac{3}{2}(\frac{\dot{a}}{a})\gamma^0 - (m+h\phi)]\psi [/itex] where a is the scale factor for expnasion of the universe.


I understand that the matter action is [itex]S=\int d^{4}x e [\frac{1}{2}\partial_{\mu}\phi\partial^{\mu}\phi - V(\phi) + i \bar{\psi}\bar{\gamma}^{\mu}\vec{D}_{\mu}\psi -(m+h\phi)\bar{\psi}\psi)] [/itex] but i don't understand firstly why there is a vierbein and not a [itex]\sqrt{-g}[/itex] term.

I don't really understand why this is the case [itex]D_{\mu}=\frac{1}{4}\bar{\psi}\bar{\gamma}^{\mu} \gamma_{\alpha \beta}\omega^{\alpha \beta}_{\mu}[/itex] and why the arrow above the D is gone.

And lastly I don't understand why [itex]\bar{\gamma}^{i}=\frac{1}{a(t)}\gamma^{i}[/itex]

I understand that one needs to vary the action and i can do that bit but I don't understand some of these conversions, thx. I would appareciate any help that anyone can offer in tis challenge.
 
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  • #2
To deal with spinors in curved spacetimes (or even just curvilinear coordinates) you need to use a set of basis vectors. This is because the gamma matrices that obey {γμ, γν} = 2gμν aren't constant, so we use instead matrices referred to a basis, in which {γa, γb} = 2ηab.

The covariant derivative is Dμ = ∂μ - (1/4)σabωabμ where σab is the usual Dirac matrix, and ωabμ are the Ricci rotation coefficients associated with the vierbein.

I think the only reason there's an arrow over the D is to remind us that it acts on the spinor to its right.
 
  • #3
yeah thanks, i have a method to work on now.
I know that one can relate the spin connection to the gamma matrices by: [itex] \Gamma_{\mu} [/itex] to [itex] \gamma[/itex] by [itex] [\Gamma_{\mu},\gamma^{\nu}][/itex] but is this simply a standard commutator relationship or is it something more because wouldn't [itex]\Gamma_{1}\gamma^{1} - \gamma^{1}\Gamma_{1} =0 [/itex] for example?
 

FAQ: Understanding the Derivation of the Dirac Equation in Cosmology

What is the Dirac equation and why is it important in physics?

The Dirac equation is a mathematical formula developed by physicist Paul Dirac in 1928 to describe the behavior of relativistic particles, specifically electrons. It is important in physics because it unified quantum mechanics and special relativity, and led to the prediction of antimatter.

How is the Dirac equation derived?

The Dirac equation is derived by combining the principles of quantum mechanics and special relativity. It involves using the Schrödinger equation for a free particle and applying the relativistic energy-momentum relation to obtain a wave equation that describes the behavior of a relativistic particle.

What are the key components of the Dirac equation?

The key components of the Dirac equation include the wave function, which describes the quantum state of a particle, the Hamiltonian operator, which describes the total energy of the particle, and the Pauli matrices, which describe the spin of the particle.

What are the implications of the Dirac equation?

The Dirac equation has several important implications, including the prediction of antimatter, the concept of spin, and the existence of negative energy solutions. It also led to the development of the theory of quantum electrodynamics (QED), which describes the interaction of particles with electromagnetic fields.

How has the Dirac equation contributed to our understanding of the universe?

The Dirac equation has contributed significantly to our understanding of the universe by providing a framework for describing the behavior of relativistic particles and predicting the existence of antimatter. It has also been instrumental in the development of quantum field theory, which is a cornerstone of modern physics.

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