Pratyusava Baral
Gravitational waves from supermassive black hole binary mergers along with an electromagnetic counterpart has the potential to shed `light' on the nature of dark energy in the intermediate redshift regime. An accurate measurement of dark energy parameters at intermediate redshift is extremely essential to improve our understanding of dark energy, and to possibly resolve couple of tensions involving cosmological parameters. We present a Fisher matrix forecast analysis in the context of eLISA to predict the errors for three different cases: the non-interacting dark energy with constant and evolving equation of state (EoS), and interacting dark sectors with a generalized parametrization. In all three cases, we perform the analysis for two separate formalisms, namely, the standard EoS formalism and the model-independent null diagnostics using Om parametrization for a wide range of fiducial values in both phantom and non-phantom regions, in order to make a comparative analysis between the prospects of these two diagnostics in LISA. Our analysis reveals that it is wiser and more effective to probe null diagnostics instead of the standard EoS parameters for any possible signature of dark energy at intermediate redshift measurements like LISA.
The discrepancy is primarily attributed to incorrect value of the EoS parameters. Thus more accurate direct measurements of EoS parameters are necessary. All measurements till date are from local Universe. The gravitational waves (GWs) emitted from supermassive black hole binary (SMBHB) mergers detected at Laser Interferometer Space Antenna (LISA) give us a unique probe to study the middle Universe. LISA is a space base GW mission expected to fly by 2030s. The observations that we are taking about are primarily luminosity distance (a measure of cosmic distance) vs redshift (a proxy for time). The redshift measurements shall be primarily from spectroscopic or photometric observations in electromagnetic telescopes due to host galaxy mergers. However, there exists another problem. The errors in density parameters effect the values of the EoS parameters. To solve this problem Varun Sahni and his collaborators introduced a new novel null parameterization technique called Om, which can measure the EoS without reference to density parameters. This parameter actually depends on four redshift points which effectively eliminate the density parameters. This new parameterization technique when applied to present datasets of the local Universe show no deviation from ΛCDM. This motivates why we should probe middle redshifts.
Since we do not have measurements from LISA, a simple Fisher matrix analysis is presented. We have used three models to study the EoS parameters.
• 1 parameter model - Matter EoS is fixed at 0; Dark energy EoS has one free parameter.
• 2 parameter model - Matter EoS is fixed at 0; Dark energy EoS has two free parameters.
• 3 parameter model – Matter EoS is a free parameter. Dark energy EoS has two free parameters.
This three models have been studied using both standard and Om parameters. For all cases the errors in case of Om parameters are less than that of standard parameters by at least one order of magnitude. Thus, Om parameters are an excellent tool to probe the middle redshift region using GW observations with electromagnetic counterparts at LISA. For more details refer to arXiv: 2005.0Astrobiology1[astro-ph].