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

**Academic Editor:**B. G. Konopelchenko

#### Abstract

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.

#### 1. Introduction

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

gives directly

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.

Setting if , the relations (6) and (7) also show that in any interval , : where , are constants.

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.

Notice that the differential system (16) shows that the vectors field defined locally by where is given by (17), is tangent to the trajectories.

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:

However using in (20), we obtain since , and by (4) that

By (23), the expression (19) of in which we set then allows to compute and gives, since , which shows that determines .

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 .

Writing (19) for , using (4), , and , implies that

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).

It consequently appears, using (37), that the functions in the integrals (32) depend only on , , and that these integrals with respect to and give functions and of the single variable .

Using now the usual properties of the determinants, we compute the Jacobian of the change of variables defined by (37) and find

But , so using (7), the Boltzmann equation (29) leads to the following form:

##### 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 .

Equation (42) shows that (41) writes

But it is proved in [5] that if verifies the Boltzmann equation (40), then defined by (43) verifies ; (46) reduces then to

Now, using (21), we have

and using (45), where is the D'Alembertian.

We deduce from (20), (48), and (49) that the Euler equations (41) are satisfied if verifies the second-order differential equation:

For , (50) leads to the *constraints* system:

between the unknown functions and , constraints which we have to solve in what is to follow.

For , (50) leads to a nonlinear differential equation of second order: where is defined in (27).

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

From (17), using (4), we obtain

Using (24), (27), (40), (56), and (57), the Maxwell-Boltzmann-Euler system in reduces to the following form: which is an integrodifferential system to solve in what is to follow.

We are searching a solution of the Cauchy problem (59)-(60)-(61)-(62) globally in time on for the initial data:

##### 3.5. The Problem of Constraints

We must find a nontrivial solution of the Cauchy problem (59)-(60)-(61)-(62) satisfying the system (51) of constraints which writes after computation

#### 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

Furthermore if

then where is defined in [1].

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 [2].

Proposition 6. *Let , and , be given.**If , then , and one has
**
where .**Moreover
*

*Proof. *We simply use Lemma 5. For the details, see [2].

Proposition 7. *Let be given. Then , .*

*Proof. *
See [2].

*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 [1].

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 [1]) 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 [1], the system (92), and remembering that , we obtain
where is a positive constant.

Then taking the in (94), for and , we get

which implies

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 [6] that if is invariant under , then so will be the solution of the Boltzmann equation satisfying . It is also proved in [7] 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.

Proposition 11. *The Cauchy problem (59)-(60)-(61)-(62) is equivalent to the following problem, for :
*

*Proof. *See [1].

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 [7].

We prove the following.

Proposition 13. *Let , , be given. Then
*