Gorkavyi N. proposed that gravitational wave background detected by NANOGrav may have produced from merging stellar mass black holes (SBHs) during Big Crunch. His model not only explained all the observed features of the GWB very well but also unlike all other GWB models, it predicted a sharp decrease in the GWB amplitude. If confirmed by future observations this may be the first evidence of big crunch..
What are the most powerful known sources of gravitational radiation? Yeah, merging black holes (BH). In 2015, the LIGO group discovered gravitational waves of ∼ 10² Hz caused by the merging of black holes with stellar masses (SBH up to 100 solar masses). These gravitational waves can be detected by variations in the radio pulses from millisecond pulsars.
Recently, the NANOGrav consortium announced the possible discovery of an isotropic stochastic background of gravitational waves (GWB) with an amplitude of h ∼ 10¯15 and a frequency of f ∼ 10¯8 Hz using observations of millisecond pulsars. Several hypothesis have been put forth by researchers in order to explain the origin of GWB such as current binary SMBHs, collapsing fluctuations in the early stages of the Big Bang, the break of a cosmic string etc.
Now, Gorkavyi N. proposed a new hypothesis in which he assumed that the the GWB is relic radiation left over from the merging stellar mass black holes (SBHs) during Big Crunch (refer fig 1.(b)) at z ∼ 1010 (or this parameter ‘z’, satisfies the model of cyclic universe.)
According to him, the spectrum of GWB discovered by NANOGrav and PPTA will not be described by Planck’s law or any other law related to thermal equilibrium. The GWB should reflect the observed distribution of black holes (SBHs and SMBHs).
With the help of the model, he found that the frequency of relic gravitational waves coincides with the frequency of gravitational waves observed by NANOGrav and PPTA.
“Decrease in 1010 times the frequency of these relic gravitational waves is associated with the expansion of the Universe. We believe that the NANOGrav and the PPTA data support the hypothesis that black holes can make up the bulk of dark matter and that a significant portion of the observed black holes came from the past cycle of the oscillating Universe.”— Gorkavyi N., author of the study.
His model not only explained all the observed features of the GWB very well but also unlike all other GWB models, it predicted a sharp decrease in the GWB amplitude at frequencies f > 3.5 ∗ 10¯8 Hz, reflecting the deficit of SBHs with masses < 4 M.
For more: N. Gorkavyi, Gravitational wave background discovered by NANOGrav as evidence of a cyclic universe, New Astronomy, Volume 91, 2022, 101698, ISSN 1384-1076, https://doi.org/10.1016/j.newast.2021.101698. (https://www.sciencedirect.com/science/article/pii/S1384107621001214)
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