Consider a spacetime $(\zeta^{3,1},g)$


where $$g=\frac{du\,dv}{uv}-\frac{dr^2}{r^2}-\frac{dw^2}{w^2} \quad \forall u,v,r,w \in (0,1)$$ Now this is just Minkowski space in different coordinates (related to Dirac/Light cone coordinates/null coordinates). I'm looking to take a [Cauchy foliation](https://ncatlab.org/nlab/show/Cauchy+surface) of $\zeta^{3,1},$ change the metric back to $g'=dudv-dw^2-dr^2,$ and use the induced measure from $g'$ by means of the volume form to transform the foliated past light cone region onto a new spacetime $(\Psi^{~3,1},d).$


Let's look at the steps involved for the $(1+1)$ dimensional case $(\zeta^{1,1},u)$ where $u=\frac{dxdq}{xq}.$ The Cauchy foliation is simply $\ln(b)\ln(y)=t$ (solving for $y$ gives an explicit representation) which can then be transformed via Mellin-like transform:


$$\Phi\_s(t)= \int\_{S=(0,1)} \exp {\frac{t s}{\ln b}}~db = \int\_0^1 tb^{t-1} \exp \frac{s}{\ln b}~db = 2\sqrt{ts}K\_1(2\sqrt{ts})$$


Observe that this is an unnormalized [K-distribution](https://en.wikipedia.org/wiki/K-distribution) and it's inverse transform yields an unnormalized distribution call it the "$\zeta$-distribution" (our Cauchy foliation). Observe that the [Fisher information metric](https://en.wikipedia.org/wiki/Fisher_information_metric) of the $\zeta$-distribution is $\Phi\_s(t)$ up to a factor. Observe that the $\zeta$-distribution yields solutions to the Killing field $X=\langle x \ln x, -y\ln y \rangle.$ It also provides a particular distributional solution to the following diffusion equation with diffusivity depending on both space and time:


$$\frac{\partial^2}{\partial t^2}\nu(t,b)=-\frac{b}{t}\frac{\partial}{\partial b}\nu(t,b)$$


In short, $\Phi\_1(t)=v$ is a Lorentzian metric on the smooth $(1+0)$ manifold $(\Psi^{~1,0}, v).$ We have that $\Psi^{~3,1}=\Psi^{~1,1}\times \Psi^{~1,0} \times \Psi^{~1,0}$ so we can define the product metric on $\Psi^{~3,1}.$


There are two different constructions going on here. The first construction starts with a bonafide spacetime and transforms the past light cone region onto a new spacetime. The second construction begins by simplifying the spacetime down to $(1+1)$ dimensions, transforms the past light cone region, and then scales that manifold with the Cartesian product, furnishing with the product metric.



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> Does restricting to $g'$ and transforming the foliated past light cone preserve all or most of the information contained in $(\zeta^{3,1},g)?$ Is $(\Psi^{~3,1},d)$ also a spacetime, or a component of one?
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Transporting a Cauchy foliation of Minkowski space
We are given a spacetime: $$(\zeta,g)$$ which we recognize as Minkowski space in different coordinates.


We start by observing that the product space of the foliations:


$$\Omega\_{t,r,\theta}(x,y,z)= \varphi\_t(x)\varphi\_r(y)\varphi\_{\theta}(z) \space\space\space\space\space{x,y,z\in(0,1)} \space\space\space{t,r,\theta >0}$$


Noticeably yields a product space $\Bbb R^{3,3}$ where $(t,r,\theta)$ are time variables and $(x,y,z)$ are space variables. This $6$-manifold is not entirely helpful, so we reduce to $(3+1)$ dimensions by letting $s=t=r=\theta.$ This collapses the time variables down to just one, which is what we want.


Now we are left with a Cauchy foliation, $\Omega$, of a $(3+1)$ spacetime, in fact this spacetime is precisely the $\zeta$ we were initially given:


$$ \Omega\_s(x,y,z)=\varphi\_s(x)\varphi\_s(y)\varphi\_s(z)\space\space\space\space{s>0}$$


And now we restrict the metric to $h=dudv-dr^2-dw^2$ and hit each leaf with the transform:


$$ \int\_{(0,1)^3} \Omega\_s(x,z,y)~dxdydz=\bigg(\int\_{(0,1)} \Omega\_s(x)~dx\bigg)^3= \psi^3(s)$$


We have an explicit function to describe the leaves of the Cauchy foliation and we now have an explicit function for the hyper-volume. This transform maps from the spatial domain to the time domain. Whether that has any physical relevance, I do not know.


We can generalize the integral transform to:


$$\int\_{(0,1)^3} x^{a}y^{b}z^{c}\Omega\_s(x,z,y)~\frac{dxdydz}{xyz}=\chi\_s(a,b,c).$$


And so we have a mapping given by the integral transform:


$$T: \Omega \to \chi $$


We have that $T$ is linear because it is an integral transform on bounded domain, which we know to be linear. And as $s$ varies continuously on $\Omega$ the corresponding change of $s$ is also continuous on $\chi$. The map is smooth as well, because a smooth change in $s$ on $\Omega$ corresponds to a smooth change of $s$ on $\chi$.


Therefore the problem is reduced to finding a complete Lorentzian metric that is preserved by an infinitesimally generated flow on $\chi\_s(a,b,c).$ We are looking for a Killing field. Particularly of interest are the cases $\lim\_{s \to 0}\chi\_s$ and $\lim\_{s \to \infty} \chi\_s$ which serve to define boundaries on the resultant spacetime. The spacetime in question is therfore non-compact with boundary.


