Research Article | Open Access
Raoul Domingo Ayissi, Norbert Noutchegueme, Hugues Paulin Mbeutcha Tchagna, "The Maxwell-Boltzmann-Euler System with a Massive Scalar Field in All Bianchi Spacetimes", Advances in Mathematical Physics, vol. 2013, Article ID 679054, 18 pages, 2013. https://doi.org/10.1155/2013/679054
The Maxwell-Boltzmann-Euler System with a Massive Scalar Field in All Bianchi Spacetimes
We prove the existence and uniqueness of regular solution to the coupled Maxwell-Boltzmann-Euler system, which governs the collisional evolution of a kind of fast moving, massive, and charged particles, globally in time, in a Bianchi of types I to VIII spacetimes. We clearly define function spaces, and we establish all the essential energy inequalities leading to the global existence theorem.
In this paper, we study the coupled Maxwell-Boltzmann-Euler system which governs the collisional evolution of a kind of fast moving, massive, and charged particles and which is one of the basic systems of the kinetic theory.
The spacetimes considered here are the Bianchi of types I to VIII spacetimes where homogeneous phenomena such as the one we consider here are relevant. Note that the whole universe is modeled and particles in the kinetic theory may be particles of ionized gas as nebular galaxies or even cluster of galaxies, burning reactors, and solar wind, for which only the evolution in time is really significant, showing thereafter the importance of homogeneous phenomena.
The relativistic Boltzmann equation rules the dynamics of a kind of particles subject to mutual collisions, by determining their distribution function, which is a nonnegative real-valued function of both the position and the momentum of the particles. Physically, this function is interpreted as the probability of the presence density of the particles in a given volume, during their collisional evolution. We consider the case of instantaneous, localized, binary, and elastic collisions. Here the distribution function is determined by the Boltzmann equation through a nonlinear operator called the collision operator. The operator acts only on the momentum of the particles and describes, at any time, at each point where two particles collide with each other, the effects of the behaviour imposed by the collision on the distribution function, also taking in account the fact that the momentum of each particle is not the same, before and after the collision, with only the sum of their two momenta being preserved.
The Maxwell equations are the basic equations of electromagnetism and determine the electromagnetic field created by the fast moving charged particles. We consider the case where the electromagnetic field is generated, through the Maxwell equations by the Maxwell current defined by the distribution function of the colliding particles, a charge density , and a future pointing unit vector , tangent at any point to the temporal axis.
The matter and energy content of the spacetime is represented by the energy-momentum tensor which is a function of the distribution function , the electromagnetic field , and a massive scalar field , which depends only on the time .
The Euler equations simply express the conservation of the energy-momentum tensor.
The system is coupled in the sense that , which is subject to the Boltzmann equation, generates the Maxwell current in the Maxwell equations and is also present in the Euler equations, whereas the electromagnetic field , which is subject to the Maxwell equations, is in the Lie derivative of with respect to the vectors field tangent to the trajectories of the particles. also figures in the Euler equations.
We consider for the study all the Bianchi of types I to VIII spacetimes, excluding thereby the Bianchi type IX spacetime also called the Kantowski-Sachs spacetime which has the flaw to develop singularities in peculiar finite time and is not convenient for the investigation of global existence of solutions.
The main objective of the present work is to extend the result obtained in [1–3] where the particular case of the Bianchi type I spacetime is investigated. The choice of function spaces and the process of establishing the energy inequalities are highly improved.
The paper is organized as follows.
In Section 2, we introduce the spacetime and we give the unknowns.
In Section 3, we describe the Maxwell-Boltzmann-Euler system.
In Section 4, we define the function spaces and we establish the energy inequalities.
In Section 5, we study the local existence of the solution.
In Section 6, we prove the global existence of the solution.
2. The Spacetime and the Unknowns
Greek indexes range from to , and Latin indexes from to . We adopt the Einstein summation convention:
We consider the collisional evolution of a kind of fast moving, massive, and charged particles in the time-oriented Bianchi types 1 to 8 spacetimes and denote by the usual coordinates in , where represents the time and the space; stands for the given metric tensor of Lorentzian signature which writes where are continuously differentiable functions on , components of a 3-symmetric metric tensor , whose variable is denoted by .
The expression of the Levi-Civita connection associated with , which is
Recall that .
We require the assumption that are bounded. This implies that there exists a constant such that
As a direct consequence, we have, for , where .
The massive particles have a rest mass , normalized to the unity, that is, . We denote by the tangent bundle of with coordinates , where stands for the momentum of each particle and . Really the charged particles move on the future sheet of the mass shell or the mass hyperboloid , whose equation is or, equivalently, using expression (2) of : where the choice symbolizes the fact that, naturally, the particles eject towards the future.
The invariant volume element in reads where
We denote by the distribution function which measures the probability of the presence of particles in the plasma. is a nonnegative unknown real-valued function of both the position and the 4-momentum of the particles , so:
We define a scalar product on by setting for and :
In this paper we consider the homogeneous case for which depends only on the time and . According to the Laplace law, the fast moving and charged particles create an unknown electromagnetic field which is a 2-closed antisymmetric form and locally writes
So in the homogeneous case we consider
In the presence of the electromagnetic field , the trajectories of the charged particles are no longer the geodesics of spacetime but the solutions of the differential system: where where denotes the charge density of particles.
The charged particles also create a current , , called the Maxwell current which we take in the form in which is a unit future pointing timelike vector, tangent to the time axis at any point, which means that , , and . The particles are then supposed to be spatially at rest.
The electromagnetic field , where and stand for the electric and magnetic parts, respectively, is subject to the Maxwell equations.
3. The Maxwell-Boltzmann-Euler System in , , and
3.1. The Maxwell Equations in
The Maxwell system in can be written, using the covariant notation:
Equations (20) and (21) are, respectively, the first and second groups of the Maxwell equations, and stands for the convariant derivative in . In (20), represents the Maxwell current we take in the form (19). Now the well-known identity imposes, given (20), that the current is always subject to the conservation law:
The second set (21) of the Maxwell equations is identically satisfied since , and the first set reduces to . Then is constant and
This physically shows that the magnetic part of does not evolve and stays in its primitive state. It remains to determine the electric part .
By (20), we obtain the linear in which writes
Remark 1. In (27), the expression represents the second fundamental form in . Really is the trace of the 2-symmetric tensor where . is called the middle curvature of . Since is given, so is .
3.2. The Relativistic Boltzmann Equation in
The relativistic Boltzmann equation in , for charged particles in the Bianchi types to 8 spacetimes, can be written: where is the Lie derivative of with respect to the vectors field defined by (18) and , the collision operator we now introduce.
According to Lichnerowicz and Chernikov, we consider a scheme, in which, at a given position , only two particles collide with each other, without destroying each other, with the collision affecting only the momentum of each particle, which changes after shock, only the sum of the two momenta being preserved. If , stand for the two momenta before the shock and , for the two momenta after the shock, then we have
The collision operator is then defined, using functions and on , and the previous notations by where whose elements we now introduce step by step, specifying properties and hypotheses we adopt:(i) is the unit sphere of , whose area element is denoted by ;(ii) is a nonnegative continuous real-valued function of all its arguments, called the collision kernel or the cross-section of the collisions, on which we require the boundedness and Lipschitz continuity assumptions, in which is a constant: where is the norm in .(iii)The conservation law splits into
Equation (34) expresses, using (7), the conservation of the quantity: called the elementary energy of the unit rest mass particles; we can interpret (35) by setting, following Glassey and Strauss in [4, equation ], in which is a real-valued function. Using (7) to express , in terms of , and next (37) to express , in terms of , we prove that (34) leads to a quadratic equation in , which solves to give the only nontrivial solution: in which , is given by (36), and the dot is the scalar product defined by (13).
Using now the usual properties of the determinants, we compute the Jacobian of the change of variables defined by (37) and find
3.3. The Euler Equations
The Euler equations only express the conservation of the energy-momentum tensor and write
In (41), where (i) is the energy-momentum tensor associated with ;(ii) is the Maxwell tensor associated with ;(iii) is the energy-momentum tensor associated with the scalar field whose mass is denoted by , with .
Now, using (21), we have
and using (45), where is the D'Alembertian.
For , (50) leads to the constraints system:
between the unknown functions and , constraints which we have to solve in what is to follow.
Setting in (52)
it comes that
One supposes in what follows that is continuously differentiable, is not a constant, and is decreasing. This implies that
Equation (52) is then equivalent to the nonlinear first-order differential system given as follows: where .
3.4. The Coupled System
3.5. The Problem of Constraints
4. Function Spaces and Energy Inequalities
We define now the function spaces in which we are searching the solution to the Maxwell-Boltzmann-Euler system. We also establish some useful energy estimations.
Definition 2 (). Let , , be given.
We define as
will be endowed with the norm
will be the completion of in the norm .
We also define
Endowed with the norm
is a Banach space.
will be the completion of for the norm .
For to be given, we define
Endowed with the induced distance by the norm , is a complete metric subspace of .
Remark 3. If , then , so will be denoted by .
Remark 4. The reasons for the choice of the function space for and .
With the objective of the present work being the existence of solution to the Maxwell-Boltzmann-Euler system, and particularly the Boltzmann equation (40), we are searching a function which is continuously differentiable; in particular we can search belonging to the space .
We want to use the Faedo-Galerkin method which is applied for separable Hilbert spaces. That is the case for the Sobolev spaces , .
We need then to find an integer such that
But we know by the Sobolev theorems that
Since in our case we have , , and , we must choose such that
The smallest integer satisfying is naturally .
Consequently we have
then where is defined in .
It then results that
We can now state the following results which will be fundamental.
Lemma 5. There exists a real number such that
Furthermore, one has and the function , is bounded.
Proof. See .
Proposition 6. Let , and , be given.
If , then , and one has where .
Proposition 7. Let be given. Then , .
Proof. See .
Remark 8. The hypothesis of Proposition 6 concerning the collision kernel is a supplementary hypothesis for the investigation of the solution to the Boltzmann equation.
In what is to follow, we are searching the local existence and the uniqueness of the solution to the Cauchy problem (59)-(60)-(61)-(62) in a function space which we will precise, applying the standard theory of first-order differential systems.
The framework we will refer to for is .
The framework we will refer to for is , whose norm is denoted by or :
is a Banach space for the norm:
The framework we will refer to for and is , whose norm is denoted by :
is a Banach space for the norm: (i)We consider on the norm (ii)We consider on the norm: (iii)We will consider the Cauchy problem (59)-(60)-(61)-(62) for the initial data: where is given in , , , , and .
5. The Local Existence of Solution
Theorem 9. Let be given, and let be fixed. Then the linearized partial differential equation whose unknown is , with , has in a local unique and bounded -weak solution.
Proof. We use the Faedo-Galerkin method in the function space . For the other details, see .
Theorem 10. Let , , be fixed. Then the Boltzmann equation, has in a local unique -weak solution such that .
Proof. We use the Banach fixed point theorem in for the map:
where satisfies (88).(i)We firstly prove, using a sequence of approximations of , the Banach-Alaoglu theorem and the fact that is a reflexive space (see ) that we can choose and such that
(ii)Let now be given, and let be two solutions of (88). Then
Let and .
Then we get
Conveniently using energy inequalities established in , the system (92), and remembering that , we obtain where is a positive constant.
Then taking the in (94), for and , we get
The relations (91), (96) show clearly that , is a contracting map, so by the Banach theorem has a unique fixed point and the proof of Theorem 10 is complete.
Next, let us introduce the subgroup of defined by
A function on is said to be invariant under if
Using the observation that is invariant under, it is proved in  that if is invariant under , then so will be the solution of the Boltzmann equation satisfying . It is also proved in  that , if and only if is invariant under .
One requires in all what follows that the initial datum of the distribution function is not invariant under . The immediate consequence is that
Now, computing the determinant of the system (64), we conclude that, under our requirement, the problem of constraints (64) admits on a nontrivial solution: where is the unique solution to the Boltzmann equation (60) on in which is given.
Let us now state the following result which shows helpful in what is to follow.
Proof. See .
The framework we will refer to for is , whose norm is denoted by or .
Let denote the . of ----, that is,
It then appears that, on the contrary to the uncharged case studied in [6, 8], the momentum also becomes an unknown in the charged case. Note that and are now independent variables for the system ----. In this context, the collision operator defined by (31) will depend on only through the collision kernel , and we show it by writing now instead of . One must from now be careful in order to avoid any confusion between the unknown of the system ---- and the variable in (26), (27), (57), and (62), for example. For this reason, we denoted the variable in the integrals in , , and by instead of .
Proposition 12. Let , , , be given. Then where
Proof. See .
We prove the following.
Proposition 13. Let , , be given. Then