Bimetric gravity improves the fit to DESI BAO and eases the Hubble tension

We investigate whether the latest combination of the dark energy spectroscopic instrument (DESI) Year 3 data release (DR2) baryon acoustic oscillation measurements, cosmic microwave background (CMB) data ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mn>2018</a:mn> <a:mo>+</a:mo> <a:mi>ACT</a:mi> </a:mrow> </a:math> ), and Type Ia supernovae (SNe Ia) compilations (Pantheon+, Union3, and DES Y5) favor a dynamical dark energy component, and explore if such a scenario can simultaneously help resolve the Hubble tension. We contrast two frameworks: the widely used phenomenological <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:msub> <c:mrow> <c:mi>w</c:mi> </c:mrow> <c:mrow> <c:mn>0</c:mn> </c:mrow> </c:msub> <c:msub> <c:mrow> <c:mi>w</c:mi> </c:mrow> <c:mrow> <c:mi>a</c:mi> </c:mrow> </c:msub> </c:mrow> </c:math> cold dark matter (CDM) model, and bimetric gravity—a fundamental modification of general relativity that naturally gives rise to phantom dark energy. The <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:msub> <e:mi>w</e:mi> <e:mn>0</e:mn> </e:msub> <e:msub> <e:mi>w</e:mi> <e:mi>a</e:mi> </e:msub> <e:mi>CDM</e:mi> </e:math> model is moderately preferred over <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi mathvariant="normal">Λ</g:mi> <g:mi>CDM</g:mi> </g:math> , at the <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:mrow> <j:mn>2</j:mn> <j:mo>–</j:mo> <j:mn>4</j:mn> <j:mi>σ</j:mi> </j:mrow> </j:math> level, when fitting DESI DR2 + <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mrow> <l:mi>CMB</l:mi> <l:mo>+</l:mo> <l:mtext>SNe</l:mtext> </l:mrow> </l:math> Ia, but it exacerbates the Hubble tension. By comparison, bimetric gravity provides a modest improvement in fit quality, at the <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"> <n:mn>1</n:mn> <n:mi>σ</n:mi> </n:math> level, but, by inferring <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"> <p:msub> <p:mi>H</p:mi> <p:mn>0</p:mn> </p:msub> <p:mo>=</p:mo> <p:mn>69.0</p:mn> <p:mo>±</p:mo> <p:mn>0.4</p:mn> <p:mtext> </p:mtext> <p:mtext> </p:mtext> <p:mi>km</p:mi> <p:mo>/</p:mo> <p:mi mathvariant="normal">s</p:mi> <p:mo>/</p:mo> <p:mi>Mpc</p:mi> </p:math> , it partially eases the Hubble tension, from a <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"> <s:mn>5</s:mn> <s:mi>σ</s:mi> </s:math> discrepancy to a <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"> <u:mn>3.7</u:mn> <u:mi>σ</u:mi> </u:math> tension. Including locally calibrated SNe Ia brings the overall preference for the bimetric model over <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"> <w:mi mathvariant="normal">Λ</w:mi> <w:mi>CDM</w:mi> </w:math> to the <z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline"> <z:mn>2</z:mn> <z:mi>σ</z:mi> </z:math> level, comparable to that of the <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline"> <bb:msub> <bb:mi>w</bb:mi> <bb:mn>0</bb:mn> </bb:msub> <bb:msub> <bb:mi>w</bb:mi> <bb:mi>a</bb:mi> </bb:msub> <bb:mi>CDM</bb:mi> </bb:math> model when including the local SN Ia calibration.