Dynamical dark energy from an ultralight axion
Hoang Nhan Luu, Yu-Cheng Qiu, S.-H. Henry Tye
Abstract
Recently, the Dark Energy Survey Collaboration presented evidence that the equation of state <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>w</a:mi> </a:math> of the dark energy is varying or <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>w</c:mi> <c:mo>≃</c:mo> <c:mo>−</c:mo> <c:mn>0.948</c:mn> </c:math> if it is constant. In either case, the dark energy cannot be due to a cosmological constant alone. Here, we study an ultralight axion (or axionlike particle) with mass <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:msub> <e:mi>m</e:mi> <e:mi>ϕ</e:mi> </e:msub> <e:mo>≃</e:mo> <e:mn>2</e:mn> <e:mo>×</e:mo> <e:msup> <e:mn>10</e:mn> <e:mrow> <e:mo>−</e:mo> <e:mn>33</e:mn> </e:mrow> </e:msup> <e:mtext> </e:mtext> <e:mtext> </e:mtext> <e:mi>eV</e:mi> </e:math> that has properties that can explain the new <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi>w</g:mi> </g:math> measurement. In particular, <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mi>w</i:mi> <i:mo>≥</i:mo> <i:mo>−</i:mo> <i:mn>1</i:mn> </i:math> and a negative cosmological constant <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mi mathvariant="normal">Λ</k:mi> <k:mo><</k:mo> <k:mn>0</k:mn> </k:math> is preferred in this model. We also present a simple formula for <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"> <n:mi>w</n:mi> </n:math> for the model to ease data fitting.