This Jupyter/SageMath worksheet implements some computations of the article
These computations are based on SageManifolds (v0.9)
The worksheet file (ipynb format) can be downloaded from ??.
First we set up the notebook to display mathematical objects using LaTeX formatting:
%display latex
Let us declare the spacetime $M$ as a 5-dimensional manifold:
M = Manifold(5, 'M')
print M
We introduce a the Poincaré-type coordinate system on $M$:
X.<t,x,y1,y2,z> = M.chart(r't x y1:y_1 y2:y_2 z:(0,+oo)')
X
Let us consider the following Lifshitz-symmetric metric, parametrized by some real numbers $\nu$ and $c$, and the blackening function $f(z)$:
g = M.lorentzian_metric('g')
var('nu', latex_name=r'\nu', domain='real')
var('c', domain='real')
ff = function('f')(z)
b(z) = exp(c*z^2/2)
# b(z)=1 ## for checks
g[0,0] = - b(z)*ff/z^2
g[1,1] = b(z)/z^2
g[2,2] = b(z)*z^(-2/nu)
g[3,3] = b(z)*z^(-2/nu)
g[4,4] = b(z)/z^2/ff
g.display()
A matrix view of the metric components:
g[:]
g.display_comp()
The Riemann tensor is
Riem = g.riemann()
print Riem
#Riem.display_comp(only_nonredundant=True)
Riem[0,1,0,1]
The Ricci tensor:
Ric = g.ricci()
print Ric
Ric.display()
Ric.display_comp()
The Ricci scalar:
Rscal = g.ricci_scalar()
print Rscal
Rscal.display()
Let us consider a model based on the following action, involving a dilaton scalar field $\phi$ and a Maxwell 2-form $F$:
$$ S = \int \left( R(g) + \Lambda - \frac{1}{2} \nabla_m \phi \nabla^m \phi - \frac{1}{4} e^{\lambda\phi} F_{mn} F^{mn} \right) \sqrt{-g} \, \mathrm{d}^5 x \qquad\qquad \mbox{(1)}$$where $R(g)$ is the Ricci scalar of metric $g$, $\Lambda$ is the cosmological constant and $\lambda$ is the dilatonic coupling constant.
We consider the following ansatz for the dilaton scalar field $\phi$: $$ \phi = \frac{1}{\lambda} \left( \ln\mu - \frac{4}{\nu}\ln z\right),$$ where $\mu$ is a constant.
var('mu', latex_name=r'\mu', domain='real')
var('lamb', latex_name=r'\lambda', domain='real')
phi = M.scalar_field({X: (ln(mu) - 4/nu*ln(z))/lamb},
name='phi', latex_name=r'\phi')
phi.display()
The 1-form $\mathrm{d}\phi$ is
dphi = phi.differential()
print dphi
dphi.display()
dphi[:] # all the components in the default frame
We consider the following ansatz for $F$: $$ F = \frac{1}{2} q \, \mathrm{d}y_1\wedge \mathrm{d}y_2, $$ where $q$ is a constant.
Let us first get the 1-forms $\mathrm{d}y_1$ and $\mathrm{d}y_2$:
X.coframe()
dy1 = X.coframe()[2]
dy2 = X.coframe()[3]
print dy1
print dy2
dy1, dy2
Then we can form $F$ according to the above ansatz:
var('q', domain='real')
F = q/2 * dy1.wedge(dy2)
F.set_name('F')
print F
F.display()
By construction, the 2-form $F$ is closed (since $q$ is constant):
print xder(F)
xder(F).display()
Let us evaluate the square $F_{mn} F^{mn}$ of $F$:
Fu = F.up(g)
print Fu
Fu.display()
F2 = F['_{mn}']*Fu['^{mn}'] # using LaTeX notations to denote contraction
print F2
F2.display()
We shall also need the tensor $\mathcal{F}_{mn} := F_{mp} F_n^{\ \, p}$:
FF = F['_mp'] * F.up(g,1)['^p_n']
print FF
FF.display()
The tensor field $\mathcal{F}$ is symmetric:
FF == FF.symmetrize()
Therefore, from now on, we set
FF = FF.symmetrize()
Let us first introduce the cosmological constant:
var('Lamb', latex_name=r'\Lambda', domain='real')
From the action (1), the field equation for the metric $g$ is $$ R_{mn} + \frac{\Lambda}{3} \, g - \frac{1}{2}\partial_m\phi \partial_n\phi -\frac{1}{2} e^{\lambda\phi} F_{mp} F^{\ \, p}_n + \frac{1}{12} e^{\lambda\phi} F_{rs} F^{rs} \, g_{mn} = 0 $$ We write it as
EE == 0
with EE
defined by
EE = Ric + Lamb/3*g - 1/2* (dphi*dphi) - 1/2*exp(lamb*phi)*FF \
+ 1/12*exp(lamb*phi)*F2*g
EE.set_name('E')
print EE
EE.display_comp(only_nonredundant=True)
We note that EE==0
leads to 4 independent equations:
eq1 = EE[0,0]*24*nu*z^2/f(z)*exp(c*z^2/2)
eq1
eq2 = EE[1,1]*24*nu*z^2*exp(c*z^2/2)
eq2
eq3 = EE[2,2]*12*nu^2*z^(2/nu)*exp(c*z^2/2)
eq3
eq4 = EE[4,4]*2*lamb^2*nu^2*z^2*f(z)*exp(c*z^2/2)
eq4
First we evaluate $\nabla_m \nabla^m \phi$:
nab = g.connection()
print nab
nab
box_phi = nab(nab(phi).up(g)).trace()
print box_phi
box_phi.display()
From the action (1), the field equation for $\phi$ is $$ \nabla_m \nabla^m \phi = \frac{\lambda}{4} e^{\lambda\phi} F_{mn} F^{mn}$$ We write it as
DE == 0
with DE
defined by
DE = box_phi - lamb/4*exp(lamb*phi) * F2
print DE
DE.display()
Hence the dilaton field equation provides a fifth equation:
eq5 = DE.coord_function()*lamb*nu^2*exp(c*z^2)/2
eq5
From the action (1), the field equation for $F$ is $$ \nabla_m \left( e^{\lambda\phi} F^{mn} \right)= 0 $$ We write it as
ME == 0
with ME
defined by
ME = nab(exp(lamb*phi)*Fu).trace(0,2)
print ME
ME.display()
We get identically zero; hence the Maxwell equation do not provide any further equation.
The Einstein equation + the dilaton field equation yields a system of 5 equations (eq1, eq2, eq3, eq4, eq5).
Let us show that a solution is obtained for $\nu=2$ and $\nu=4$ with the following specific form of the blackening function:
$$ f(z) = 1 - m z^{2/\nu+2}, $$where $m$ is a constant.
To this aim, we declare
var('m', domain='real')
fm(z) = 1 - m*z^(2/nu+2)
fm
and substitute this function for $f(z)$ in all the equations:
eq1m = eq1.expr().substitute_function(f, fm).simplify_full()
eq1m
eq2m = eq2.expr().substitute_function(f, fm).simplify_full()
eq2m
eq3m = eq3.expr().substitute_function(f, fm).simplify_full()
eq3m
eq4m = eq4.expr().substitute_function(f, fm).simplify_full()
eq4m
eq5m = eq5.expr().substitute_function(f, fm).simplify_full()
eq5m
eqs = [eq1m, eq2m, eq3m, eq4m, eq5m]
neqs = [eq.subs(nu=2).simplify_full() for eq in eqs]
[eq == 0 for eq in neqs]
solve([eq == 0 for eq in neqs], lamb, mu, Lamb, q, m, c)
In the above solutions, $r_i$, with $i$ an integer, stands for an arbitrary parameter.
The solutions for $c=0$ are those already obtained for $b(z)=1$. But there are no solution with $c\not = 0$ and $c={\rm const}$.
neqs = [eq.subs(nu=4).simplify_full() for eq in eqs]
[eq == 0 for eq in neqs]
solve([eq == 0 for eq in neqs], lamb, mu, Lamb, q, m, c)
In the above solutions, $r_i$, with $i$ an integer, stands for an arbitrary parameter.
The solutions for $c=0$ are those already obtained for $b(z)=1$. But there are no solution with $c\not = 0$ and $c={\rm const}$.