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#### Gravity in the Era of Equality: Towards solutions to the Hubble problem without fine-tuned initial conditions

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2003.06396.pdf

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PhysRevD.102.023523.pdf

(Publisher version), 3MB

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##### Citation

Zumalacarregui, M. (2020). Gravity in the Era of Equality: Towards solutions to the
Hubble problem without fine-tuned initial conditions.* Physical Review D,* *102*(2):
023523. doi:10.1103/PhysRevD.102.023523.

Cite as: https://hdl.handle.net/21.11116/0000-0006-0DBC-9

##### Abstract

Discrepant measurements of the Universe's expansion rate ($H_0$) may signal

physics beyond the standard cosmological model. Here I describe two early

modified gravity mechanisms that reconcile the value of $H_0$ by increasing the

expansion rate in the era of matter-radiation equality. These mechanisms, based

on viable Horndeski theories, require significantly less fine-tuned initial

conditions than early dark energy with oscillating scalar fields. In Imperfect

Dark Energy at Equality (IDEE), the initial energy density dilutes slower than

radiation but faster than matter, naturally peaking around the era of equality.

The minimal IDEE model, a cubic Galileon, is too constrained by the cosmic

microwave background (Planck) and baryon acoustic oscillations (BAO) to relieve

the $H_0$ tension. In Enhanced Early Gravity (EEG), the scalar field value

modulates the cosmological strength of gravity. The minimal EEG model, an

exponentially coupled cubic Galileon, gives a Planck+BAO value $H_0=68.7 \pm

1.5$ (68\% c.l.), reducing the tension with SH0ES from $4.4\sigma$ to

$2.6\sigma$. Additionally, Galileon contributions to cosmic acceleration may

reconcile $H_0$ via Late-Universe Phantom Expansion (LUPE). Combining LUPE, EEG

and $\Lambda$ reduces the tension between Planck, BAO and SH0ES to $2.5\sigma$.

I will also describe additional tests of coupled Galileons based on local

gravity tests, primordial element abundances and gravitational waves. While

further model building is required to fully resolve the $H_0$ problem and

satisfy all available observations, these examples show the wealth of

possibilities to solve cosmological tensions beyond Einstein's General

Relativity.

physics beyond the standard cosmological model. Here I describe two early

modified gravity mechanisms that reconcile the value of $H_0$ by increasing the

expansion rate in the era of matter-radiation equality. These mechanisms, based

on viable Horndeski theories, require significantly less fine-tuned initial

conditions than early dark energy with oscillating scalar fields. In Imperfect

Dark Energy at Equality (IDEE), the initial energy density dilutes slower than

radiation but faster than matter, naturally peaking around the era of equality.

The minimal IDEE model, a cubic Galileon, is too constrained by the cosmic

microwave background (Planck) and baryon acoustic oscillations (BAO) to relieve

the $H_0$ tension. In Enhanced Early Gravity (EEG), the scalar field value

modulates the cosmological strength of gravity. The minimal EEG model, an

exponentially coupled cubic Galileon, gives a Planck+BAO value $H_0=68.7 \pm

1.5$ (68\% c.l.), reducing the tension with SH0ES from $4.4\sigma$ to

$2.6\sigma$. Additionally, Galileon contributions to cosmic acceleration may

reconcile $H_0$ via Late-Universe Phantom Expansion (LUPE). Combining LUPE, EEG

and $\Lambda$ reduces the tension between Planck, BAO and SH0ES to $2.5\sigma$.

I will also describe additional tests of coupled Galileons based on local

gravity tests, primordial element abundances and gravitational waves. While

further model building is required to fully resolve the $H_0$ problem and

satisfy all available observations, these examples show the wealth of

possibilities to solve cosmological tensions beyond Einstein's General

Relativity.