How Can We Detect Dark Energy? (Cosmology / Astronomy)

Summary:

  • Garoffolo and colleagues built an estimator, by combining luminosity distance measurements from Gravitational waves (GW) and Supernova sources (SN) , for the direct detection of the signal of dark energy fluctuations, that does not rely on non-gravitational interactions between dark energy (DE) and known particles.
  • They mentioned that this signal can not be mimicked by other effects and would provide convincing evidence for the existence of the DE field or modifications of the laws of gravity.
  • In addition, in order to detect this signal, very precise measurements of SN/GW events are required. Alongwith, the higher number of events, to deal with possible systematic effects.
  • Finally, they demonstrated that this signal would be directly detected by future SN surveys and space-based interferometers, if one decreases the statistical error on each measure.

Over the last decades, a variety of cosmological data have confirmed ΛCDM as the standard model of cosmology. It provides understanding of the Big Bang cosmology, inflation, the matter-antimatter asymmetry in the universe, the nature of dark energy, etc. Despite its successes, the physical nature of its main components still eludes us. In particular, understanding whether cosmic acceleration is sourced by a cosmological constant, Λ, or rather by dynamical dark energy (DE) or modifications of the laws of gravity (MG) is one of the main science drivers of upcoming cosmological missions. In the presence of DE/MG, the dynamical degrees of freedom of the theory change, generally with the appearance of a new scalar field to which we broadly refer as the “DE field”. The latter, leaves imprints not only on the dynamics of the Universe, but also on the clustering and growth of large-scale cosmological structures.

The detection of gravitational waves (GW) has opened a new observational window onto our Universe, promising to offer complementary probes to shed light on cosmic expansion. GW events at cosmological distances can be used as “standard sirens” (a nod to “standard candles”) for measuring the expansion rate of the universe. This recent approach is complementary to measuring the luminosity distance of standard candles, like Type-Ia supernovae (SN).

On the homogeneous and isotropic background, luminosity distances depend only on redshift, leading to the standard distance-redshift relation. Inhomogeneities in the Universe induce a dependence of the distances on direction. Fluctuations in the EM luminosity distance constitute an important probe for cosmology.

Previous papers showed that, in presence of DE/MG, the gravitational wave (GW) luminosity distance generally differs from the one traced by electromagnetic (EM) signals, both at the unperturbed, background level and in its large-scale fluctuations. Importantly, fluctuations in the EM luminosity distance are affected by the DE field only indirectly while, linearized fluctuations of the GW luminosity distance contain contributions directly proportional to the clustering of the DE field.

Now, considering this, Garoffolo and colleagues built an estimator, by combining luminosity distance measurements from GW and SN sources, for the direct detection of the signal of DE clustering (or imprint of the DE fluctuations), that does not rely on non-gravitational interactions between DE and known particles. They mentioned that this signal can not be mimicked by other effects and would provide convincing evidence for the existence of the DE field or modifications of the laws of gravity.

“If DE does not directly couple to known particles through nongravitational interactions, ours is a promising method to pursue its direct detection.” told Alice Garoffolo, first author of the study, “Even if the DE clustering signal is below cosmic variance, any detection of our joint estimator would be a convincing proof of a running Planck mass.”


In addition, their proposed approach allows to probe the DE field at cosmological scales, far from sources that can hide its presence by means of screening mechanisms. (Note: Screening mechanisms were first categorized by Austin Joyce and colleagues into three broad classes: mechanisms which become active in regions of high Newtonian potential, those in which first derivatives become important, and those for which second derivatives are important. For more, please refer their paper.)

But, on the other hand, it has been suggested that, one should leverage as much as possible on the precision of the measurement; for instance, given the number of SN/GW events (of order 106, at least in the higher redshift bins) that can be observed with future SN surveys and space-based interferometers, a detection would be possible, if one decreases the statistical error on each measure, according to table I below.

© Alice et al.

Finally, they considered an ideal case for their estimates: the number of events needed for a detection might be higher to deal with possible systematic effects. This suggested that future facilities might have to develop new technologies and observational strategies to meet these detection goals.

“We leave it to future work to determine whether a detection of the signal we propose can be aided by studying additional MG models, synergies with large scale structure surveys or considering different sources of GW/EM signals.”

— concluded authors of the study

Reference: Alice Garoffolo, Marco Raveri, Alessandra Silvestri, Gianmassimo Tasinato, Carmelita Carbone, Daniele Bertacca, and Sabino Matarrese, “Detecting dark energy fluctuations with gravitational waves”, Phys. Rev. D 103, 083506 – Published 12 April 2021. https://doi.org/10.1103/PhysRevD.103.083506 Link to paper


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