Thanks @ZbigniewMotyka for your reply.

The paper-link on micro black holes is very interesting and hard linked with the aim of this RGquestion.

Good August.

ps: if you have some other references about this topic, please added to this #RGquestion.

--s.v.--

Black holes couldn't form between galaxies, other than possibly the primordial version if these exist at all. However, when black holes merge, as seen in the last few years by LIGO, theory says the should be given a significant recoil 'kick' due to the momentum carried away by the gravitational waves. This can exceed the escape velocity of the galaxy so yes, it is likely that there is a small population of such merger products between galaxies.

Thanks @GeorgeDishman for your very interesting reply. Implications are deep.

ps: if you have some references about this topic, please added to this #RGquestion.

---s.v---

Igael Azoulay thank you very much for your reply ....... This > is a very interesting element for me.

Thanks

ps: if you have some references about this topic, please added to this #RGquestion.

---s.v.---

Igael Azoulay many thanks for the NASA reference.

Have a nice August.

--sv--

Yes, during mergers of two galaxies each containing a supermassive black hole (SMBH), time dependent gravitational potential or gravitational waves kick may lead to a SMBH being ejected into the inter-galactic space. One such discovery has been reported: https://www.nasa.gov/feature/goddard/2017/gravitational-wave-kicks-monster-black-hole-out-of-galactic-core

Here are some old papers, the theory has been known for a long time and you may find the references in these or citations to them will be useful.

Article Black Hole Kicks as New Gravitational Wave Observables

Article Gravitational Recoil from Spinning Binary Black Hole Mergers

Article How Black Holes Get Their Kicks: Gravitational Radiation Rec...

ZM: what could stop her there and not made occur at the other side of the same galaxy. BH is small enough to go through galaxy like through very thin air.

Gravitational drag would have that effect to some extent.

https://en.wikipedia.org/wiki/Dynamical_friction

ZM: Actually, most of primordial BHs has to wonder between galaxies, this way, in my opinion. Therefore, they also may form the lacking Dark Matter. (As we can not know, how many of them exactly are there!)

A number of micro-lensing surveys have set an upper limit on the number, it is too small to explain dark matter.

https://en.wikipedia.org/wiki/Gravitational_microlensing#Microlensing_experiments

this link is useful

https://www.space.com › Science & Astronomy

regards

Patrick Das Gupta Thank you very much for the NASA link !!! Very useful.

--sv--

Most welcome, Salvatore Vicidomini@

Dear @Zbigniew Motyka

Dear Patrick,

PDG: .. the escape velocity associated with the merger ..

I think you meant ".. the escape velocity associated with the host galaxy ..".

Dear Zbigniew,

ZM: relativistic micro black holes [which] may be generated at powerful collisions of cosmic radiation

For a BH to survive for more than a few seconds, it has to have a mass

of the order of tonnes. The energy in cosmic rays could not form a stable BH.

ZM: and merge at the slow areas of their movement in clouds at peripheries of forming by them halos.

You said "relativistic" micro black holes but to form a cloud their velocity would have to be thermal, you can use the Virial Theorem to find the statistics.

GD: A number of micro-lensing surveys have set an upper limit on the number, it is too small to explain dark matter.

ZM: Again not in the case of uniform clouds of mBHs halos of spherical symmetry around each galaxy.

They would show up as individual events or be too small to be stable.

If GR is correct, there should be black-holes in our galaxy.

If there is no black-hole in universe, we should search for now gravity theory.

George Dishman , thanks very much. Yes, escape velocity associated with the host galaxy that forms from the merger.

Dear Wu Xinzhong,

WZ: If GR is correct, there should be black-holes in our galaxy.

There are a great number. The first to be found was Cygnus X-1. The central super-massive black hole is designated Sag A*.

Dear Zbigniew,

GD: For a BH to survive for more than a few seconds, it has to have a mass of the order of tonnes. The energy in cosmic rays could not form a stable BH. ZM: (1) Due to properties of Event Horizon, once it forms it never can vanish (as it always remains at the stage of forming itself for outer observer)

I think it would evaporate through Hawking Radiation which is described by Andrew Hamilton near the bottom of this page:

http://jila.colorado.edu/~ajsh/bh/hawk.html

"In the final second of its existence, the mini black hole radiates about 1000 tonnes of rest mass energy. Such an explosion is large by human standards, but modest by astronomical standards."

That means that in order to survive for 1 second, a small black hole would need a rest mass in excess of 1000 tonnes. That mass cannot be created by a cosmic ray.

ZM: (2) Even if relativistic mBH could decay (what is impossible due to 1) they are relativistic, so they may decay after arbitrarily long time.

They take the same amount of proper time. GD:You said "relativistic" micro black holes but to form a cloud their velocity would have to be thermal. ZM: Relativistic mBHs in our region of universe are becoming thermal on its (this region) peryphery.

To become part of a cloud, each would need to be moving slowly relative to the average motion of the cloud.

ZM: However we do not see evidences of such decaying anywhere.

They would not be visible beyond a few light years, but I agree, it is very unlikely that they exist at all.

GD: A number of micro-lensing surveys have set an upper limit on the number, it is too small to explain dark matter. ZM: There can not be micro-lensing effect in dense cloud of such particles in the halo.

Each individual would cause a lensing event if the mass was high enough. The surveys using the LMC failed to see any but there is a lower mass limit. The short life of low mass holes however gives you a minimum so there is only a band of mass where they might be possible.

Zbigniew, as with any proposal like this, they only way to back it up is with an observation of a small black hole. Surveys like MACHO would be a possible method but so far none have been found so astronomers will continue to think they don't exist.

ZM: That is all the trouble. Hawking radiation timing concerns your proper time. However, far away from event horizon (EH) any finite proper (time) interval starting from EH is equal to infinity for an outer observer.

Hawking radiation doesn't start from the event horizon.

http://backreaction.blogspot.com/2015/12/hawking-radiation-is-not-produced-at.html

Aleš Kralj thank you very much for your contribution to this question.

The underlying LINK in this question about the theoretical extragalactic BH is the possible match with the exotic dark matter and with the total matter of Universe ...

Dear researchers and professors in this recent ''submitted'' paper there are some interesting alternative view about the exotic matter .... I hope useful

............

Fractal universe and cosmic acceleration in a Lemaître-Tolman-Bondi scenario. Leonardo Cosmai, Giuseppe Fanizza, Francesco Sylos Labini, Luciano Pietronero, Luigi Tedesco (Submitted on 15 Oct 2018) - Preprint Fractal universe and cosmic acceleration in a Lema\^itre-Tol...

Early-type Host Galaxies of Type Ia Supernovae. II. Evidence for Luminosity Evolution in Supernova Cosmology.-- Yijung Kang, Young-Wook Lee, Young-Lo Kim, Chul Chung, Chang Hee Ree(Submitted on 10 Dec 2019) -- |arXiv:1912.04903| |arXiv:1912.04903v1|

The most direct and strongest evidence for the presence of dark energy is provided by the measurement of galaxy distances using type Ia supernovae (SNe Ia). This result is based on the assumption that the corrected brightness of SN Ia through the empirical standardization would not evolve with look-back time. Recent studies have shown, however, that the standardized brightness of SN Ia is correlated with host morphology, host mass, and local star formation rate, suggesting a possible correlation with stellar population property. In order to understand the origin of these correlations, we have continued our spectroscopic observations to cover most of the reported nearby early-type host galaxies. From high-quality (signal-to-noise ratio ~175) spectra, we obtained the most direct and reliable estimates of population age and metallicity for these host galaxies. We find a significant correlation between SN luminosity (after the standardization) and stellar population age at a 99.5% confidence level. As such, this is the most direct and stringent test ever made for the luminosity evolution of SN Ia. Based on this result, we further show that the previously reported correlations with host morphology, host mass, and local star formation rate are most likely originated from the difference in population age. This indicates that the light-curve fitters used by the SNe Ia community are not quite capable of correcting for the population age effect, which would inevitably cause a serious systematic bias with look-back time. Notably, taken at face values, a significant fraction of the Hubble residual used in the discovery of the dark energy appears to be affected by the luminosity evolution. We argue, therefore, that this systematic bias must be considered in detail in SN cosmology before proceeding to the details of the dark energy.

................ Strikingly, the comparison of our evolution curves with SN data in Figures 16 and 17 shows that the luminosity evolution can mimic a significant fraction of the Hubble residual (the difference in standardized SN brightness) used in the discovery and inference of the dark energy.6

This would indicate that the empirical standardization of SN Ia brightness practiced by the SNe Ia community is not quite capable of correcting for this effect of luminosity evolution. In order to put this result on a firmer refined basis, further observations (for more host sample, for the local property at the site of SN) are definitely required. Nevertheless, until then, this work presents some serious evidence for the luminosity evolution in SN cosmology.

The future direction of SN cosmology, therefore, should investigate this systematic bias before proceeding to the details of the dark energy.