**Emergence in Cosmology Workshop: Talk Abstracts**

**August 13, 2021, 9:00 am – 3:00 pm EDT** (via Zoom)

All times listed below are in Eastern Daylight Time (EDT). View the full schedule on the Emergence in Cosmology Workshop event page.

**Karen Crowther**: **Four Attitudes Towards Singularities in the Search for a Theory of Quantum Gravity**

9:00 – 9:45

Singularities in general relativity and quantum field theory are often taken not only to motivate the search for a more-fundamental theory (quantum gravity, QG), but also to characterise this new theory and shape expectations of what it is to achieve. Here, we first evaluate how particular types of singularities may suggest an incompleteness of current theories. We then classify four different `attitudes’ towards singularities in the search for QG, and show, through examples in the physics literature, that these lead to different scenarios for the new theory. Two of the attitudes prompt singularity resolution, but only one suggests the need for a theory of QG. Rather than evaluate the different attitudes, we close with some suggestions of factors that influence the choice between them. [Based on joint work with Sebastian de Haro]

**Sung-Sik Lee:** **Clock-Dependent Spacetime**

9:45 – 10:30

Einstein’s theory of general relativity is based on the premise that the physical laws take the same form in all coordinate systems. However, it still presumes a preferred decomposition of the total kinematic Hilbert space into local Hilbert spaces. In this talk, we present a theory of quantum gravity in which dimension, topology and geometry of spacetime are collective dynamical variables of underlying quantum matter. This model does not come with a preferred partitioning of the kinematic Hilbert space, and the emergent spacetime depends on how a collection of local clocks is chosen within the kinematic Hilbert space.

**Joshua Norton:** **Suppressing Spacetime Emergence**

10:45 – 11:30

One of the primary tasks in building a quantum theory of gravity is discovering how to save spatiotemporal phenomena using a theory which, putatively, does not include spacetime. Some have taken this task a step further and argue for the actual emergence of spacetime from a non-spatiotemporal ontology in the low-energy regime. In this paper, it is argued that the account of spacetime emergence presented in Huggett and Wüthrich (2013) and then assumed in Wüthrich (2017), Wüthrich and Lam (2018), Baron (2019) and others, fails to accomplish the task to which it is set. There is a prima facie contradiction between the scale-independent ontology of spacetime in GR and the scale-dependent account of emergence proposed by this literature. One can avoid this contradiction but only at the cost of changing the target of emergence and by endorsing a perspectival theory of ontology – a view I call “ontic-perspectivism”. Though this paper explicitly addresses spacetime emergence, many of the following arguments are applicable to other accounts where objects of ontology, or their properties, are claimed to emerge in the low-energy regime.

**Sumit Das:** **Emergent Cosmology from Quantum Quench in C=1 Matrix Model**

13:00 – 13:45

In holographic models some or all of the space directions in a gravitational theory emerge from internal degrees of freedom of the dual quantum theory, and the geometry of the resulting space-time is obtained by studying the time evolution of the state. We discuss the nature of a class of time dependent space-times in two dimensional noncritical string theory which result from quantum quench in the dual matrix quantum mechanics. These emergent space-times generically have space-like regions of strong coupling resembling cosmological singularities, unless the quench protocol is finely tuned. Such “singularities”, however, simply signal the failure of a semi-classical description: the matrix model state remains well-defined.

**Suddhasattwa Brahma: Emergent Cosmology from the BFSS Matrix Model**

13:45 – 14:30

The BFSS matrix model is a proposed non-perturbative definition of M-theory in which space is emergent. In this talk, I shall present a new paradigm of early-universe cosmology in the context of the BFSS theory. Specifically, I will show that matrix theory leads to an emergent non-singular cosmology which, at late times, can be described by an expanding phase of Standard Big Bang cosmology. Crucially, the thermal fluctuations in the emergent phase source an approximately scale-invariant spectrum of scalar perturbations and a scale invariant spectrum of gravitational waves. Hence, this model leads to a successful scenario for the origin of perturbations responsible for the currently observed structure in the universe while providing a consistent UV-complete description.

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