Beradze and Gogberashvili considered the possibility that LIGO events GW190521, GW190425 and GW190814 may have emerged from the mirror world binaries.
According to them, to form binaries similar to GW190521, GW190425 and GW190814, the component masses of which lie in the upper and lower mass gaps, hierarchical mergers of very rare systems are required.
They argued that such scenarios are order of magnitude more probable in mirror world, where star formation begins earlier and matter density can exceed 5 times the ordinary matter density.
Moreover, M-World is dominated by helium stars which evolve faster and create compact objects earlier.
So, in M-World hierarchical mergers are more probable and second-generation compact objects (remnants of first generation mergers) are formed with higher rate.
In May 2019, Advanced LIGO/VIRGO detected the signal GW 190521, which was radiated by the coalescence of two massive progenitor black holes (BHs) (91.4 M and 66.8 M). The most massive final blackhole (at 157.9 M), the very first “Intermediate Mass Black Hole” ever detected. It is located about 15 billion light-years away. Yeah, so, so far. The biggest surprise of GW190521 was that the primary BHs with massed 91 and 67 M lie within the mass gap produced by pair-instability supernova processes. There are also other unexpected GW signals like GW 190425 and GW190814 which astronomers believe that, they were likely radiated by the coalescence of objects with masses 2 & 1.4 M, 23.2 BH and 2.59 NS, respectively. Due to lack of objects discovered with these properties, we don’t have a definite theory for their formation mechanism for now.
Now, Beradze and Gogberashvili considered the possibility that LIGO events GW190521, GW190425 and GW190814 may have emerged from the mirror world binaries. According to them, to form binaries similar to GW190521, GW190425 and GW190814, the component masses of which lie in the upper and lower mass gaps, hierarchical mergers of very rare systems are required. They argued that such scenarios are order of magnitude more probable in mirror world, where star formation begins earlier and matter density can exceed 5 times the ordinary matter density. Their research paper recently appeared on Journal Monthly Notices of the Royal Astronomical Society.
Mirror World (M-World) was introduced to restore left-right symmetry of nature, suggesting that each Standard Model particle has its mirror partner with opposite chirality. The fundamental reason for existence of mirror partners has been first revealed in 1956 by Lee and Yang. Later, based on this idea, theory of M-World was introduced, stating that mirror particles are invisible for ordinary observers and vice versa. Only way for the interaction between these two worlds is gravity. So, gravitational waves (GW) radiated by mirror matter can be sensed by an ordinary observer.
If M-World really exists, it was created by the Big Bang along the ordinary universe. But its temperature, T′, must be lower than the temperature of our world, T. This requirement emerges from the fact that mirror particles, having similar cosmological abundance, also contribute into the Hubble expansion rate and they should not violate the Big Bang Nucleosynthesis (BBN) bound. This could be achieved if mirror and ordinary worlds are reheated asymmetrically after inflationary epoch.
Due to some factors, evolution of mirror stars can be somehow different from ordinary stars. Unlike our universe, in M-World with the lower temperature, T′ < T , all the processes occur earlier at higher redshifts. This means that the star formation rate, depending on the temperature ratio, will peak earlier at z ∼ 10, corresponding to the lookback time in our world. This implies that mirror BHs and NSs have more time to pick up mass and to create binaries in the area covered by the LIGO observations.
As M-World is several times colder, at ordinary BBN epoch the universe expansion rate is completely determined by ordinary world itself. So, M-World contribution into the ordinary light element production is negligible. In contrary, in the M-World nucleosynthesis epoch, the contribution of ordinary matter scales as x¯4 and plays a crucial role. It was shown that, for x ≲ 0.3, the mirror helium mass fraction can reach 75 − 80%. Thus, M-World is dominated by mirror helium and mirror stars are mostly He-stars.
Evolution of He-stars should be similar to ordinary stars, when latter have converted most of hydrogen into helium and formed a helium core. During the process of gravitational collapse of protogalaxy, it fragments into hydrogen clouds, which then cools and collapses until the opacity of the system becomes so high that the gas prefers to fragments into protostars. This is a way how first stars (Pop. III stars) in the Universe are formed. The lack of metals for that time, makes cooling process less efficient within clouds. So, their fragmentation could produce only high mass stars.
In the He-dominated world, the cooling process inside primordial clouds should have also a lower efficiency and mirror stars are formed even more massive. We know that higher is the mass of the ordinary star, the shorter is its life, as it burns out fuel faster. Increasing the initial helium abundance of a star, corresponds to the increase of the mean molecular weight, and correspondingly in both luminosity and effective temperature, that leads to the shorter lifetime. For instance, 10M⊙ star with 70% initial He content has the evolution timescale ∼ 10 times faster than the star with ordinary He abundance (24%).
At first, Beradze & Gogberashvili in their paper, considered possible M-World origin of the black holes (BHs) event GW190521. In principle, BH-BH mergers, which account for the most amount of LIGO events, should not have optical counterparts, so they can be originated from both normal (Pop III) stars and mirror ones. However, BH binaries of mirror origin merely amplifies chance of these BH-BH mergers. As the microphysics of mirror stars is similar to that of ordinary stars, they probably also are subject to pair instability and produce the mass gap for intermediate mass BHs. However, stars in M-World are born with higher initial mass, compared to ordinary stars, they evolve faster and higher quantity of massive BHs are formed in short period of time. Adding the fact that the mirror matter density is ∼ 5 times the ordinary matter density, collisions of BHs formed by mirror stars are more frequent, increasing merger rate naturally. As a consequence, formation of intermediate mass BHs is easier in M-World, that could be a good interpretation for the heavy components of GW190521. Also, BHs formed in the mirror matter environment, can increase in mass by accretion of mirror matter that has higher abundance compared to ordinary matter.
Another consequence of the M-World scenario, can be explanation of lower mass gap compact objects of the events GW190425 and GW190814. NS-NS or BH-NS mergers, in case they contain normal neutron stars (NSs), both should be typically accompanied by GRB and optical afterglows. However, neither GW190425 nor GW190814 had such associations. In their previous paper, Beradze et al. suggested that this could indicate to their mirror origin, i.e. as merger of mirror NSs in which case no optical counterpart should be expected. This can be not completely true in the presence of neutron-mirror neutron transitions, which can be rather fast process. Due to this effect, cores of normal matter can be formed inside the mirror NS, which can make their merger also optically observable though perhaps more faint.
So, the fact that ”heavy” NSs are not detected through electromagnetic spectrum but are observed through gravitational radiation, may be indication that they exist in the mirror world. As discussed in paper, in order to form a GW190425-like binary NS system, ultra-tight binary with NS and massive He-star is required, that is more easily achieved in mirror world, as M-World is inhabited mostly by He-stars. The formation of GW190814-like systems is also challenging for current theories and their abundance is expected to be extremely low. However, in M-World the abundance of matter exceeds ∼ 5 times the abundance of ordinary matter and stars in MWorld evolve a way faster.
This increases the probability of hierarchical mergers may by an order of magnitude, and the formation of GW190814-like systems is more common.— concluded authors of the study
Reference: Revaz Beradze, Merab Gogberashvili, Unexpected LIGO events and the mirror world, Monthly Notices of the Royal Astronomical Society, Volume 503, Issue 2, May 2021, Pages 2882–2886, https://doi.org/10.1093/mnras/stab685
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