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> BB's really are a problem to modern physics/cosmology No, they really aren't. They have never been observed. They are incompatible with present-day observa

> BB's really are a problem to modern physics/cosmology No, they really aren't. They have never been observed. They are incompatible with present-day observations, and so as far as we can tell probably they have never existed in the history of our observable universe. They thus are a diagnostic on approaches to cosmology which have conditions favourable to BBs in approximately present times (or in the past), essentially killing any theory that does not suppress them. They also imperil some theories of the far future which rely on fluctuation theory to produce a whole new universe (some decay-of-false-vacuum models, for instance). Very short lived and spatially tiny low-entropy configurations are enormously enormously ENORMOUSLY more likely than large low-entropy configuration that persist. Boltzmann brains describe a very short lived (much less than seconds) fluctuation into a very low entropy configuration, and are thus much more likely than a fluctuation producing Boltzmann being who can survive in outer space for at least minutes, let alone a galaxy in which a thinking being evolved and personally experienced many years of life, or a universe with many such galaxies. This also means that because of the scale and history of the universe that we have direct experience of, one cannot safely draw a physical-theory parallel between our universe's projected high-entropy far future and our low-entropy past more distant than the dense low-entropy phases about which we have evidence (from the surface of last scattering to baryon species and abundances, ...). Multiverses are not needed in this analysis, and don't really help one way or another. If you figure out how some other space in a multiverse configuration interacts with ours (and prove it experimentally or observationally), you will have mostly just changed our understanding of our universe and its (quasi-)local physics, particularly thermodynamics.

meroes 3y ago
Just because whole new universes might fluctuate from spacetime patches, doesn't mean the non-fluctuating patches wont be in heat death forever in the far future. In so far as we can talk about multiple universes in such a picture, "ours" will be in heat death and some daughter spacetimes will not be for some duration. To say we can't predict more than 14b.y. in to the future seems at odds with most theoreticians and I've seen many predict much further with mainstream theories. For BBs, yes the "tiny" ones will be exponentially more likely. But it seems like the 'medium to large' BBs in the heat death spacetimes will outnumber natural observers still, unless there are more spacetimes out of heat death comparatively. Some kind of measure of pre-heat death spacetimes vs far-into heat death spacetimes is needed to theoretically match our observation of no BBs. This is a multiverse. A certain kind of multiverse does this, one where the odds to fluctuate new universes is high enough to outpace the ones in terminal heat death. I mean, we talk about inflation with little hope of the verification you talk about in your last paragraph. This is barely anything beyond inflation, it just sets the proper measure/odds among all the details to match observations. To say it does nothing, what of Susskind's video then? I'm just paraphrasing him. Carroll and Susskind both find it useful to talk of BB's, and use them to weigh on which theories to accept. That's all I was saying. Whether one has actually existed, don't know. Inflation might be so pervasive it makes BBs infinitesimal.
- raattgift 3y ago
  Sorry, assume I've read standard textbooks and Ellis-Maartens-Mccallum, and explain to me how inflation controls the spatially flat FLRW chart density of low-entropy fluctuations in the quasi de Sitter +CC phase (ideally beginning with the "dark era", or even the earlier black hole era if you want to deal in nonequilibrium entropy explicitly, rather than just assuming heat death). (I don't know where you got "14b.y." from; we still have the local group (including Milkdromeda) making new stars that soon from now; it's a cosmological eyeblink). > This is barely anything beyond inflation, it just sets the proper measure/odds among the details to match observations "Barely anything" seems to be doing a lot of heavy lifting there, but perhaps I'm missing something simple. See at the end below. > For BBs, yes the "tiny" ones will be exponentially more likely. Well, yes, statmech. The problem is that if you allow non-"tiny" low entropy states you get them even during the matter-dominated era, so our sky should be sufficiently full of Boltzmann-russell-teapots to show up in studies of high-redshift quasars (and H II). I do not understand the rest of that paragraph. > That's all I was saying. If that was all you were saying, I'd agree. I used the word "diagnostic" in the comment you replied to. But you also write: > Inflation might be so pervasive it makes BBs infinitesimal ??? > ... Susskind ... Are you trying to summarize stochastic eternal inflation? Or is this something out of his recent investigations of Sachdev-Ye-Kitaev? I haven't looked at his DSSYK stuff because AdS results aren't interesting to me. ETA (just for me, i had to put this somewhere to find it later): Brandenberger et al 1992 "The Entropy of the Gravitational Field", https://arxiv.org/abs/gr-qc/9208009v1 https://arxiv.org/abs/gr-qc/9208009v1

meroes 3y ago

Just because whole new universes might fluctuate from spacetime patches, doesn't mean the non-fluctuating patches wont be in heat death forever in the far future. In so far as we can talk about multiple universes in such a picture, "ours" will be in heat death and some daughter spacetimes will not be for some duration. To say we can't predict more than 14b.y. in to the future seems at odds with most theoreticians and I've seen many predict much further with mainstream theories. For BBs, yes the "tiny" ones will be exponentially more likely. But it seems like the 'medium to large' BBs in the heat death spacetimes will outnumber natural observers still, unless there are more spacetimes out of heat death comparatively. Some kind of measure of pre-heat death spacetimes vs far-into heat death spacetimes is needed to theoretically match our observation of no BBs. This is a multiverse. A certain kind of multiverse does this, one where the odds to fluctuate new universes is high enough to outpace the ones in terminal heat death. I mean, we talk about inflation with little hope of the verification you talk about in your last paragraph. This is barely anything beyond inflation, it just sets the proper measure/odds among all the details to match observations. To say it does nothing, what of Susskind's video then? I'm just paraphrasing him. Carroll and Susskind both find it useful to talk of BB's, and use them to weigh on which theories to accept. That's all I was saying. Whether one has actually existed, don't know. Inflation might be so pervasive it makes BBs infinitesimal.

raattgift 3y ago
Sorry, assume I've read standard textbooks and Ellis-Maartens-Mccallum, and explain to me how inflation controls the spatially flat FLRW chart density of low-entropy fluctuations in the quasi de Sitter +CC phase (ideally beginning with the "dark era", or even the earlier black hole era if you want to deal in nonequilibrium entropy explicitly, rather than just assuming heat death). (I don't know where you got "14b.y." from; we still have the local group (including Milkdromeda) making new stars that soon from now; it's a cosmological eyeblink). > This is barely anything beyond inflation, it just sets the proper measure/odds among the details to match observations "Barely anything" seems to be doing a lot of heavy lifting there, but perhaps I'm missing something simple. See at the end below. > For BBs, yes the "tiny" ones will be exponentially more likely. Well, yes, statmech. The problem is that if you allow non-"tiny" low entropy states you get them even during the matter-dominated era, so our sky should be sufficiently full of Boltzmann-russell-teapots to show up in studies of high-redshift quasars (and H II). I do not understand the rest of that paragraph. > That's all I was saying. If that was all you were saying, I'd agree. I used the word "diagnostic" in the comment you replied to. But you also write: > Inflation might be so pervasive it makes BBs infinitesimal ??? > ... Susskind ... Are you trying to summarize stochastic eternal inflation? Or is this something out of his recent investigations of Sachdev-Ye-Kitaev? I haven't looked at his DSSYK stuff because AdS results aren't interesting to me. ETA (just for me, i had to put this somewhere to find it later): Brandenberger et al 1992 "The Entropy of the Gravitational Field", https://arxiv.org/abs/gr-qc/9208009v1 https://arxiv.org/abs/gr-qc/9208009v1