# Inflation Without a Trace of Lambda

## TL;DR

This paper extends traceless Einstein equations to $f(R)$ gravity, revealing a no-scale conformal frame with scalar fields that naturally address the cosmological constant problem and support inflation consistent with observations.

## Contribution

It introduces a traceless $f(R)$ gravity formulation leading to no-scale theories with scalar fields, offering a novel perspective on the cosmological constant and inflation.

## Key findings

- Flat potentials are possible in all spacetime dimensions.
- The cosmological constant appears as an integration constant, not fine-tuned.
- Predictions align with cosmic microwave background data.

## Abstract

We generalise Einstein's formulation of the traceless Einstein equations to $f(R)$ gravity theories. In the case of the vacuum traceless Einstein equations, we show that a non-constant Weyl tensor leads via a conformal transformation to a dimensionally homogeneous (`no-scale') theory in the conformal frame with a scalar field source that has an exponential potential. We then formulate the traceless version of $f(R)$ gravity, and we find that a conformal transformation leads to a no-scale theory conformally equivalent to general relativity and a scalar field $\phi $ with a potential given by the scale-invariant form: $V(\phi )=\frac{D-2}{4D}Re^{-\phi }$, where $\phi =[2/(D-2)]\ln f^{\prime }(R)$. In this theory, the cosmological constant is a mere integration constant, statistically distributed in a multiverse of independent causal domains, the vacuum energy is another unrelated arbitrary constant, and the same is true of the height of the inflationary plateau present in a huge variety of potentials. Unlike in the conformal equivalent of full general relativity, flat potentials are found to be possible in all spacetime dimensions for polynomial lagrangians of all orders. Hence, we are led to a novel interpretation of the cosmological constant vacuum energy problem and have accelerated inflationary expansion in the very early universe with a very small cosmological constant at late times for a wide range of no-scale theories. Fine-tunings required in traceless general relativity or standard non-traceless $f(R)$ theories of gravity are avoided. We show that the predictions of the scale-invariant conformal potential are completely consistent with microwave background observational data concerning the primordial tilt and the tensor-to-scalar ratio.