Dynamical dark energy from an ultralight axion

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>&lt;</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.