# Cosmological Constant from Boundary Condition and Its Implications beyond the Standard Model

## TL;DR

This paper proposes that the cosmological constant arises from a boundary condition on the observable universe's spatial hypersurface, offering a potential solution to key cosmological problems like the Hubble tension.

## Contribution

It introduces a novel perspective that the cosmological constant is a covariant integration constant from boundary conditions, challenging the dark energy paradigm.

## Key findings

- Reinterprets the cosmological constant as a boundary condition
- Addresses the Hubble tension by linking it to boundary conditions
- Provides a unified explanation for cosmic acceleration

## Abstract

Standard cosmology has long been plagued by a number of persistent problems. The origin of the apparent acceleration of the cosmic expansion remains enigmatic. The cosmological constant has been reintroduced as a free parameter with a value in energy density units that ``happens'' to be of the same order as the present matter energy density. There is an internal inconsistency with regards to the Hubble constant, the so-called $H_0$ tension. The derived value of $H_0$ depends on the type of data that is used. With supernovae as standard candles one gets a $H_0$ that is 4-5 $\sigma$ larger than the value that one gets from CMB (Cosmic Microwave Background) data for the early universe. Here we show that these problems are related and can be solved if the cosmological constant represents a covariant integration constant that arises from a spatial boundary condition, instead of being a new type of hypothetical physical field, ``dark energy'', as assumed by standard cosmology. The boundary condition only applies to the bounded 3D subspace that represents the observable universe, the hypersurface of the past light cone.