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> If the Big Rip were to happen That's a really big If. Since we're well into the realm of fields-on-General Relativity in this paragraph, you'd have to add

> If the Big Rip were to happen That's a really big If. Since we're well into the realm of fields-on-General Relativity in this paragraph, you'd have to add one or more additional fields to provide a "fifth force" (thus, in these sorts of approaches quintessence) to overcome the other known fundamental interactions among the other fields of the Standard Model, of which there are more than three. The fourth, gravitation, is poorly modelled as a force when we are dealing with clearly relativistic gravitation, as is the case with [a] a black hole or [b] the appearance of lots of empty space around all sources of gravitation. Nobody has yet made a consistent interacting model of quintessence, where the interactions reflect those of the Standard Model or gravitation, so nobody can really do anything but handwave about what extra interactions from "fifth force" fields would do with respect to e.g. the Higgs mechanism. Additionally, I say fields because when one gets into the weeds of formalizing a quintessence field theory one ends up with self-interactions -- additional interactions in the new parts added to the Standard Model. Quintessence theories are interesting to probe the structure of theories (mostly quantum field theories in curved spacetime), but are really not interesting for studying the physically observed cosmos with our present level of understanding of the Hubble parameter, or the mechanisms that generate the "true" metric. Without these extra interactions, there is good reason to believe that there won't be a big rip: at smaller scales, stars, planets, people, pocket computers, molecules, atoms, are in no danger of being locally disturbed by the metric expansion of inter-galaxy-cluster space. > around the event horizons of black holes ("dead stars still burn") fizzing out Hawking radiation Hawking radiation appears surprisingly far from the event horizon, rather than near it[1] and for a black hole whose near-horizon exterior is gentle enough an environment to support any sort of structure (space-stations, say, and especially ones that can support life as we know it) the Hawking radiation is virtually always going to be colder than the cosmic microwave background measured in local coordinates by a near-horizon observer. If you want to be fanciful, you are much better off erecting a very large scaffold around a very large black hole, and then "mine" it with a bucket. [2] - -- [1] Unruh [2007], http://inspirehep.net/record/775859?ln=en http://inspirehep.net/record/775859?ln=en from which the DOI link will take you to the cc-by-nc-sa pdf. [2] Brown [2012], PRL preprint https://arxiv.org/abs/1207.3342 https://arxiv.org/abs/1207.3342 has a quick overview and decent bibliography notably Gibbons in [11].

at_a_remove 6y ago
I agree, hence the "if;" I tend to be very conservative about new forces introduced and the like, no matter how exciting. I remember the search for the "fifth and sixth" forces decades ago, as well as the more recent failure of the EM drive. Oh, I was not thinking of extracting energy via the Penrose process or anything similar. Rather, I was considering the need for baryons to create structure in a universe without many of them to go around. That'll be the next level of scarcity once you have switched to a civilization that only increments state changes as energy becomes available ... what do you build stuff out of? Baryon farming will be it. Personally, I rather doubt the math will work out. I think baryons will fall apart one way or another on a scale that will probably outstrip most black hole evaporation unless a civilization was very crafty about trying to find the black holes that were below the usual three solar masses and instead opt for the little ones. It's a fun thought experiment.
- raattgift 6y ago
  > baryons will fall apart on a [lonnnng] scale I doubt it. A more practical problem might be keeping baryons from falling into black holes or escaping to infinity (in which case, nobody will care if they decay or otherwise disintegrate eventually). There's evidence (from the solar system and from gravitationally bound multi-body systems outside it) that there is to all practical purposes no metric expansion of space within clusters of galaxies, which are the largest known gravitationally-bound system, nor any subsystem within such clusters. A hypothesis that involves the metric expansion (or a "fifth force" form thereof) disintegrating atomic nuclei is at this point very far-fetched. Instead, let's consider a "swiss cheese" cosmology where in the large galaxy clusters are homogeneously and isotropically embedded within a "cheese" of expanding Robertson-Walker spacetime. Each cluster can be represented by an eventual vacuole -- a hole in the cheese -- initially filled with gas and dust. They are "eventual" vacuoles because gravitational collapse will tend to condense the gas and dust into the centre. Mathematically, in the large these vacuoles can be represented as Lemaître-Tolman-Bondi-style collapsing dusts, where we can recurse, with subsystems like galaxies also behaving over long timescales as LTB collapsing dusts, likewise galactic nebulae and star systems. Everything inside a galaxy cluster tends to condense; and all galaxy clusters end up with more space between them over time. In solving "swiss cheese" cosmologies, or recursive LTB-style systems, we can employ an Israel-Darmois thin-shell junction as a boundry between the subsystems on which we can stitch together fractions of geodesics of matter and light travelling across the junction. For example, simplifying by among other things employing maximal spherical symmetries, an isotropically radiating star sends photons across the nearest junction into its isotropically radiating nearly spherical galaxy; the galaxy shines into its cluster across a junction; the cluster shines into the "cheese". We can study the flow of radiation out of the topmost LTB vacuole into the "cheese". We can also study the flow of thin wispy stuff -- the CMB, but also radiation, neutrinos, and cosmic rays from distant galaxy clusters -- from the cheese into the vacuole. For physically plausible arrangements of this sort of model, galaxy clusters barely change their mass-energy at late times (i.e., from ca. ten billion years ago to the practically infinite future), even though some gas (in the most general sense of baryons, photons, neutrinos, and so on) escapes them. Adding in colliding galaxy clusters (like Bullet) in small numbers doesn't really change this picture. The baryons and other gas that remain within the "topmost" thin-shell pretty inevitably condenses into ever larger sets of black holes, which merge into ever larger black holes at the centres of each vacuole. Hierarchical supermassive black hole mergers might keep these giant black holes always colder than what's left of the ever thinning, ever colder cosmic background, or at least they will gobble up practically all the baryons in today's "topmost" holes-in-the-cheese, save for the small amount that drifts out into the expanding cheese (too slowly to ever fall into a second vacuole, but fast enough to escape their parent vacuole forever). In the far far future -- ignoring the Boltzmann Brain problem -- baryons will probably only exist in the "cheese" at great distances from the ultramassive black holes that ate all the others. The Boltzmann Brain problem isn't so bad: very occasionally baryons will spontaneously appear very briefly before disappearing again. However, such fluctuations can generate implications for black hole evaporation.

at_a_remove 6y ago

I agree, hence the "if;" I tend to be very conservative about new forces introduced and the like, no matter how exciting. I remember the search for the "fifth and sixth" forces decades ago, as well as the more recent failure of the EM drive. Oh, I was not thinking of extracting energy via the Penrose process or anything similar. Rather, I was considering the need for baryons to create structure in a universe without many of them to go around. That'll be the next level of scarcity once you have switched to a civilization that only increments state changes as energy becomes available ... what do you build stuff out of? Baryon farming will be it. Personally, I rather doubt the math will work out. I think baryons will fall apart one way or another on a scale that will probably outstrip most black hole evaporation unless a civilization was very crafty about trying to find the black holes that were below the usual three solar masses and instead opt for the little ones. It's a fun thought experiment.

raattgift 6y ago
> baryons will fall apart on a [lonnnng] scale I doubt it. A more practical problem might be keeping baryons from falling into black holes or escaping to infinity (in which case, nobody will care if they decay or otherwise disintegrate eventually). There's evidence (from the solar system and from gravitationally bound multi-body systems outside it) that there is to all practical purposes no metric expansion of space within clusters of galaxies, which are the largest known gravitationally-bound system, nor any subsystem within such clusters. A hypothesis that involves the metric expansion (or a "fifth force" form thereof) disintegrating atomic nuclei is at this point very far-fetched. Instead, let's consider a "swiss cheese" cosmology where in the large galaxy clusters are homogeneously and isotropically embedded within a "cheese" of expanding Robertson-Walker spacetime. Each cluster can be represented by an eventual vacuole -- a hole in the cheese -- initially filled with gas and dust. They are "eventual" vacuoles because gravitational collapse will tend to condense the gas and dust into the centre. Mathematically, in the large these vacuoles can be represented as Lemaître-Tolman-Bondi-style collapsing dusts, where we can recurse, with subsystems like galaxies also behaving over long timescales as LTB collapsing dusts, likewise galactic nebulae and star systems. Everything inside a galaxy cluster tends to condense; and all galaxy clusters end up with more space between them over time. In solving "swiss cheese" cosmologies, or recursive LTB-style systems, we can employ an Israel-Darmois thin-shell junction as a boundry between the subsystems on which we can stitch together fractions of geodesics of matter and light travelling across the junction. For example, simplifying by among other things employing maximal spherical symmetries, an isotropically radiating star sends photons across the nearest junction into its isotropically radiating nearly spherical galaxy; the galaxy shines into its cluster across a junction; the cluster shines into the "cheese". We can study the flow of radiation out of the topmost LTB vacuole into the "cheese". We can also study the flow of thin wispy stuff -- the CMB, but also radiation, neutrinos, and cosmic rays from distant galaxy clusters -- from the cheese into the vacuole. For physically plausible arrangements of this sort of model, galaxy clusters barely change their mass-energy at late times (i.e., from ca. ten billion years ago to the practically infinite future), even though some gas (in the most general sense of baryons, photons, neutrinos, and so on) escapes them. Adding in colliding galaxy clusters (like Bullet) in small numbers doesn't really change this picture. The baryons and other gas that remain within the "topmost" thin-shell pretty inevitably condenses into ever larger sets of black holes, which merge into ever larger black holes at the centres of each vacuole. Hierarchical supermassive black hole mergers might keep these giant black holes always colder than what's left of the ever thinning, ever colder cosmic background, or at least they will gobble up practically all the baryons in today's "topmost" holes-in-the-cheese, save for the small amount that drifts out into the expanding cheese (too slowly to ever fall into a second vacuole, but fast enough to escape their parent vacuole forever). In the far far future -- ignoring the Boltzmann Brain problem -- baryons will probably only exist in the "cheese" at great distances from the ultramassive black holes that ate all the others. The Boltzmann Brain problem isn't so bad: very occasionally baryons will spontaneously appear very briefly before disappearing again. However, such fluctuations can generate implications for black hole evaporation.