View source for Causal dynamical triangulation

{{Short description|Hypothetical approach to quantum gravity with emergent spacetime}}
{{no footnotes|date=April 2020}}{{Beyond the Standard Model|expanded=[[Quantum gravity]]}}

'''Causal dynamical triangulation''' ('''CDT'''), theorized by [[Renate Loll]], [[Jan Ambjorn|Jan Ambjørn]] and Jerzy Jurkiewicz, is an approach to [[quantum gravity]] that, like [[loop quantum gravity]], is [[background independent]].  

This means that it does not assume any pre-existing arena (dimensional space) but, rather, attempts to show how the [[spacetime]] fabric itself evolves.  

There is evidence <ref>
{{cite journal | last=Loll | first=Renate| year =2019| title = Quantum gravity from causal dynamical triangulations: a review | journal = Classical and Quantum Gravity | volume = 37 | issue = 1 | page = 013002 | doi = 10.1088/1361-6382/ab57c7| arxiv=  1905.08669 | s2cid=160009859}}</ref>
that, at large scales, CDT approximates the familiar 4-dimensional spacetime but shows spacetime to be 2-dimensional near the [[Planck scale]], and reveals a [[fractal]] structure on slices of constant time.  These interesting results agree with the findings of Lauscher and Reuter, who use an approach called [[Asymptotic safety in quantum gravity#Quantum Einstein Gravity (QEG)|Quantum Einstein Gravity]], and with other recent theoretical work.  

== Introduction ==
Near the [[Planck scale]], the structure of [[spacetime]] itself is supposed to be constantly changing due to [[quantum fluctuation]]s and topological fluctuations. CDT theory uses a [[triangulation (topology)|triangulation]] process which varies [[dynamical system|dynamically]] and follows [[deterministic]] rules, to map out how this can evolve into dimensional spaces similar to that of our universe.

The results of researchers suggest that this is a good way to model the [[early universe]]{{citation needed|date=March 2013}}, and describe its evolution. Using a structure called a [[simplex]], it divides spacetime into tiny triangular sections. A simplex is the multidimensional analogue of a [[triangle]] [2-simplex]; a 3-simplex is usually called a [[tetrahedron]], while the 4-simplex, which is the basic building block in this theory, is also known as the [[pentachoron]]. Each simplex is geometrically flat, but simplices can be "glued" together in a variety of ways to create curved spacetimes.  Whereas previous attempts at triangulation of quantum spaces have produced jumbled universes with far too many dimensions, or minimal universes with too few, CDT avoids this problem by allowing only those configurations in which the timelines of all joined edges of simplices agree.

== Derivation ==

CDT is a modification of quantum [[Regge calculus]] where spacetime is discretized by approximating it with a piecewise linear [[manifold]] in a process called [[triangulation]]. In this process, a ''d''-dimensional spacetime is considered as formed by space slices that are labeled by a discrete time variable ''t''.  Each space slice is approximated by a [[simplicial manifold]] composed by regular (''d'' − 1)-dimensional simplices and the connection between these slices is made by a piecewise linear manifold of ''d''-simplices.  In place of a smooth manifold there is a network of triangulation nodes, where space is locally flat (within each simplex) but globally curved, as with the individual faces and the overall surface of a [[geodesic dome]].  The line segments which make up each triangle can represent either a space-like or time-like extent, depending on whether they lie on a given time slice, or connect a vertex at time ''t'' with one at time ''t'' + 1.  The crucial development is that the network of simplices is constrained to evolve in a way that preserves [[Causality (physics)|causality]].  This allows a [[Path integral formulation|path integral]] to be calculated [[non-perturbative]]ly, by summation of all possible (allowed) configurations of the simplices, and correspondingly, of all possible spatial geometries.

Simply put, each individual simplex is like a building block of spacetime, but the edges that have a time arrow must agree in direction, wherever the edges are joined.  This rule preserves causality, a feature missing from previous "triangulation" theories. When simplexes are joined in this way, the complex evolves in an orderly{{how|date=March 2013}} fashion, and eventually creates the observed framework of dimensions.  CDT builds upon the earlier work of [[John W. Barrett (physicist)|Barrett]], [[Louis Crane|Crane]], and [[John C. Baez|Baez]], but by introducing the causality constraint as a fundamental rule (influencing the process from the very start), Loll, Ambjørn, and Jurkiewicz created something different.

== Related theories ==

CDT has some similarities with [[loop quantum gravity]], especially with its [[spin foam]] formulations.  For example, the Lorentzian [[Barrett–Crane model]] is essentially a non-perturbative prescription for computing path integrals, just like CDT.  There are important differences, however.  Spin foam formulations of quantum gravity use different degrees of freedom and different Lagrangians.  For example, in CDT, the distance, or "the interval", between any two points in a given triangulation can be calculated exactly (triangulations are eigenstates of the distance operator). This is not true for spin foams or loop quantum gravity in general. Moreover, in spin foams the discreteness is thought to be fundamental, while in CDT it is viewed as a regularization of the path integral, to be removed by the [[continuum limit]].

Another approach to quantum gravity that is closely related to causal dynamical triangulation is called [[causal sets]].  Both CDT and causal sets attempt to model the spacetime with a discrete causal structure.  The main difference between the two is that the causal set approach is relatively general, whereas CDT assumes a more specific relationship between the lattice of spacetime events and geometry. Consequently, the Lagrangian of CDT is constrained by the initial assumptions to the extent that it can be written down explicitly and analyzed (see, for example, [http://www.arxiv.org/abs/hep-th/0505154 hep-th/0505154], page 5), whereas there is more freedom in how one might write down an action for causal-set theory.

In the continuum limit, CDT is probably related to some version of [[Hořava–Lifshitz gravity]]. In fact, both theories rely on a foliation of spacetime, and thus they can be expected to lie in the same universality class. In 1+1 dimensions they have actually been shown to be the same theory,<ref>{{cite journal |last1=Ambjørn |first1=J. |last2=Glaser |first2=L. |last3=Sato |first3=Y. |last4=Watabiki |first4=Y. |date=2013 |title=2d CDT is 2d Hořava–Lifshitz quantum gravity |journal=Physics Lettetters B |volume=722 |issue=1–3 |pages=172–175 |doi=10.1016/j.physletb.2013.04.006  |arxiv =1302.6359 |bibcode=2013PhLB..722..172A |s2cid=85444972 }} </ref> while in higher dimensions there are only some hints, as understanding the continuum limit of CDT remains a difficult task.

== See also ==
{{Portal|Physics}}
{{cols|colwidth=21em}}
*[[Asymptotic safety in quantum gravity]]
*[[Causal sets]]
*[[Fractal cosmology]]
*[[Loop quantum gravity]]
*[[5-cell]]
*[[Planck scale]]
*[[Quantum gravity]]
*[[Regge calculus]]
*[[Simplex]]
*[[Simplicial manifold]]
*[[Spin foam]]
{{colend}}

== References ==
;Notes
{{reflist}}
;Bibliography
* [http://www.phys.lsu.edu/mog/mog19/node12.html Quantum gravity: progress from an unexpected direction] {{Webarchive|url=https://web.archive.org/web/20070311061337/http://www.phys.lsu.edu/mog/mog19/node12.html |date=2007-03-11 }}
* [[Jan Ambjørn]], [[Jerzy Jurkiewicz]], and [[Renate Loll]] – [http://www.sciam.com/article.cfm?id=the-self-organizing-quantum-universe "The Self-Organizing Quantum Universe"], [[Scientific American]], July 2008
* Alpert, Mark "The Triangular Universe" Scientific American page 24, February 2007
* Ambjørn, J.; Jurkiewicz, J.; Loll, R. – [http://www.arxiv.org/abs/hep-th/0604212 Quantum Gravity or the Art of Building Spacetime]
* Loll, R.; Ambjørn, J.; Jurkiewicz, J. – [http://www.arxiv.org/abs/hep-th/0509010 The Universe from Scratch] – a less technical recent overview
* Loll, R.; Ambjørn, J.; Jurkiewicz, J. – [http://www.arxiv.org/abs/hep-th/0505154 Reconstructing the Universe] – a technically detailed overview
* Markopoulou, Fotini; Smolin, Lee – [http://www.arxiv.org/abs/hep-th/0409057 Gauge Fixing in Causal Dynamical Triangulations] – shows that varying the time-slice gives similar results
* Loll, R – [https://arxiv.org/abs/1905.08669 Quantum Gravity from Causal Dynamical Triangulations: A Review] A review from May 2019, focusing on results that were recent at that time

Early papers on the subject: 
* R. Loll, ''Discrete Lorentzian Quantum Gravity'', [http://arxiv.org/abs/hep-th/0011194 arXiv:hep-th/0011194v1] 21 Nov 2000
* J Ambjørn, A. Dasgupta, J. Jurkiewicz, and R. Loll, ''A Lorentzian cure for Euclidean troubles'', [https://arxiv.org/abs/hep-th/0201104 arXiv:hep-th/0201104] v1 14 Jan 2002

== External links ==
*[http://loops05.aei.mpg.de/index_files/abstract_loll.html Renate Loll's talk at Loops '05]
*[http://loops05.aei.mpg.de/index_files/abstract_baez.html John Baez' talk at Loops '05]
*[http://mathworld.wolfram.com/Pentatope.html Pentatope: from MathWorld]
*[https://www.hef.ru.nl/~rloll/Web/research/research.html (Re-)Constructing the Universe] from Renate Loll's homepage
*[https://archive.today/20130416000911/http://ww3.tvo.org/video/164103/renate-loll-quantum-origins-space-and-time Renate Loll on the Quantum Origins of Space and Time] as broadcast by [[TVOntario|TVO]]

{{Theories of gravitation}}
{{quantum gravity}}
{{Standard model of physics}}

[[Category:Physical cosmology]]
[[Category:Astrophysics]]
[[Category:Quantum gravity]]
[[Category:Physics beyond the Standard Model]]