DESI Dark Secrets
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arXiv
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| Format: | Preprint |
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2025
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| _version_ | 1866908967008993280 |
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| author | Abreu, Matilde L. Turner, Michael S. |
| author_facet | Abreu, Matilde L. Turner, Michael S. |
| contents | The first and second year results of DESI provide consistent evidence that dark energy may not be quantum vacuum energy ($Λ$). If true, this would be an extraordinary development in the 25-year quest to understand cosmic acceleration. We find that the best-fit DESI $w_0w_a$ models for dark energy, which underpin the DESI claim, have unusual behavior: they achieve a maximum energy density around $z\simeq 0.5 $ and rapidly decrease before and after. We show that this could be explained by the fact that the $w_0w_a$ parameterization is limited in its ability to model dark energy as it only allows four generic behaviors: monotonically increasing or decreasing, or with a maximum or minimum. In turn, $w=-1$ can only be achieved at a minimum or maximum of the dark energy (for $1+w_0, w_a \not= 0$). $w_0w_a$ is a one-parameter characterization of scalar-field models, and cannot represent them to the precision needed for the DESI results. We explore models where the dark energy is a rolling scalar-field characterized by one dimensionless parameter $β$, which, in the limit of $β\rightarrow 0$ reduces to $Λ$CDM. None of these models fit the DESI data significantly better than $Λ$CDM or as well as the best-fit DESI $w_0w_a$ models. We also examine the supernovae data from Pantheon+ that strengthen the DESI claims for evolving dark energy. The combination of DESI, CMB (Planck) and SNe data favor a 95% credible interval $β= 0.27 - 1.03$, providing some evidence for a scalar-field explanation for dark energy. While the DESI data prefer $w_0w_a$ to a scalar field, the SNe data prefer a scalar field to $w_0w_a$, and together they favor a $w_0w_a$ model. We also point out that the unusual behavior of the best-fit DESI $w_0w_a$ models could arise due to the matter density not varying as expected or an unaccounted for component of energy density in the Universe. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2502_08876 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | DESI Dark Secrets Abreu, Matilde L. Turner, Michael S. Cosmology and Nongalactic Astrophysics The first and second year results of DESI provide consistent evidence that dark energy may not be quantum vacuum energy ($Λ$). If true, this would be an extraordinary development in the 25-year quest to understand cosmic acceleration. We find that the best-fit DESI $w_0w_a$ models for dark energy, which underpin the DESI claim, have unusual behavior: they achieve a maximum energy density around $z\simeq 0.5 $ and rapidly decrease before and after. We show that this could be explained by the fact that the $w_0w_a$ parameterization is limited in its ability to model dark energy as it only allows four generic behaviors: monotonically increasing or decreasing, or with a maximum or minimum. In turn, $w=-1$ can only be achieved at a minimum or maximum of the dark energy (for $1+w_0, w_a \not= 0$). $w_0w_a$ is a one-parameter characterization of scalar-field models, and cannot represent them to the precision needed for the DESI results. We explore models where the dark energy is a rolling scalar-field characterized by one dimensionless parameter $β$, which, in the limit of $β\rightarrow 0$ reduces to $Λ$CDM. None of these models fit the DESI data significantly better than $Λ$CDM or as well as the best-fit DESI $w_0w_a$ models. We also examine the supernovae data from Pantheon+ that strengthen the DESI claims for evolving dark energy. The combination of DESI, CMB (Planck) and SNe data favor a 95% credible interval $β= 0.27 - 1.03$, providing some evidence for a scalar-field explanation for dark energy. While the DESI data prefer $w_0w_a$ to a scalar field, the SNe data prefer a scalar field to $w_0w_a$, and together they favor a $w_0w_a$ model. We also point out that the unusual behavior of the best-fit DESI $w_0w_a$ models could arise due to the matter density not varying as expected or an unaccounted for component of energy density in the Universe. |
| title | DESI Dark Secrets |
| topic | Cosmology and Nongalactic Astrophysics |
| url | https://arxiv.org/abs/2502.08876 |