COSMIC CONTROVERSIES

Oct 6, 2019

What is the explanation of Cosmic Acceleration?

Rachel A Rosen Columbia University

KICP, UChicago

WHAT’S THE PROBLEM?WORKS GREAT! BUT…⇤

1.

2.

matter loops

phase transitions

3. other…

⇤ = ⇤0 + ⇤mat + ⇤pt + ⇤oth

requires a fine tuning

Close Cousin of the Hierarchy Problem

requires a fine tuning

BOTH THE CC AND THE HIGGS MASS ARE EXQUISITELY (I.E., POWER-LAW) SENSITIVE TO UV PHYSICS

m2H

= m20 �m2

t(. . . ) + . . .

STANDARD MODEL HIGGS MASS

Aristarchus of Samos

• Aristarchus proposed the first known heliocentric model of the solar system

• In order to explain the absence of observable parallax for the stars, postulated that they were very far away

• Contemporaries found the hierarchy in distance between nearby objects and distant stars to be untenable

geocentric solar system

HISTORICALLY MISLEADING

THE THING ABOUT NATURALNESS

(310-230 BC)

• When is a parameter natural?

• When is a theory natural?

• How do we discuss these rigorously without a measure?

MORE GENERALLY, WE USE “NATURAL” TO MEAN MANY THINGS

THE THING ABOUT NATURALNESS

WHY IS THE CC SO SMALL?

• The landscape

• Dark energy/modified gravity

• Other options?non-locality?

SOME POSSIBILITIES AND THEIR CURRENT STATUS

The Landscape

IS THE VALUE OF THE COSMOLOGICAL CONSTANT

THE RESULT OF ANTHROPICS IN A

LANDSCAPE OF OTHERWISE INHOSPITABLE VACUA?

HOW DO WE THINK ABOUT NATURALNESS IN THE AGE OF THE LHC?

“natural” solutions to the hierarchy problem

finely tuned Higgs massWE LIVE HERE

The Landscape

INHOSPITABLE

WHAT IS THE STATUS OF (META-)STABLE DE SITTER VACUA IN STRING THEORY?

KKLT (2003)

VS

|rV |V

� c (2018)

The Landscape

Dark Energy/Modified GravityWHAT MAKES A GIVEN MODEL COMPELLING?

Brans-Dicke

Scalar-Tensor

GalileonsMassive Gravity

Bigravity

Quintessence

Horndeskif(R)

Braneworlds

Ghost Condensate

Chameleons

Lorentz-Invariance?

Extra dimensions?

Massless spin-2 graviton?

Minimal coupling?

Dynamical DE?

Unimodular Gravity

Dark Energy/Modified GravityTO SOLVE OLD CC PROBLEM STILL NEED NON-LOCALITY?

What are the rules?

Other Options?

• QM instability of de Sitter

• Backreactions

• New understanding of EFTs + gravity

• Something else?

OUTLOOK

• None of the approaches mentioned here are NEW

• Need to return to consistency constraints and more “top-down” approaches

• — do we need a revolution?

ROLE OF THEORISTS IN THE ABSENCE OF NEW DATA?