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> I'm curious what % of matter in the Milky Way would be predicted to wind up in a black hole. I can't back this up, but my theory would be near 100% of all ma

• vlovich123 2y ago
  Objects in motion continue in motion. So without energy being created or destroyed, why would you expect a contraction instead of continuing expansion? Real closed loop systems are purely theoretical and the only one we know of is the Universe, so your statements about them are circular. One big unidirectional aspect of the universe is entropy - it’s a non-reversible state. Going to a previous state requires more energy than what you started with.
  • bArray 2y ago
    > Objects in motion continue in motion. So without energy being created or destroyed, why would you expect a contraction instead of continuing expansion? I think space is a little like an insanely low-friction version of water, i.e. a quantum foam [1] (or similar fluctuations caused by the structure of space-time). It may not be perceivable but it would mean that space could exert a drag in the same way water does. You could imagine it pulling out energy from a photon and red-shifting it, until eventually it just absorbed it. If that was true, it would also add a distance-based offset for all observations of the Universe and affects things like the Hubble constant. That would be just one way in which a contraction may occur - nothing exciting, just all the things travelling further away worn down by the substance they travel in. Then there would be several ways to bring it all back together into a big bang. > Real closed loop systems are purely theoretical and the only one we know of is the Universe, so your statements about them are circular. We have simulated closed-loop systems of many kinds, I think we can make educated guesses about how they may behave. As long as all state transitions undergo a reversible function, it should eventually return to its original state? > One big unidirectional aspect of the universe is entropy - it’s a non-reversible state. I don't think that's entirely true. Going by the big bang, the Universe started as one large point in space-time that exploded. Not long after things started to clump together into larger and larger particles/objects, there is currently nothing to make me believe that it will not create larger and larger clumps of matter, i.e. super massive black holes. > Going to a previous state requires more energy than what you started with. Then where did that energy go? Anyway, I understand this is highly theoretical and I'm not convincing anybody here, but it is nice to share some ideas. [1] https://en.wikipedia.org/wiki/Quantum_foam https://en.wikipedia.org/wiki/Quantum_foam [2] https://en.wikipedia.org/wiki/Hawking_radiation#:~:text=Hawking%20radiation%20is%20the%20theoretical,event%20horizon%2C%20it%20cannot%20escape https://en.wikipedia.org/wiki/Hawking_radiation#:~:text=Hawk....
    • vlovich123 2y ago
      > I think space is a little like an insanely low-friction version of water, i.e. a quantum foam [1] (or similar fluctuations caused by the structure of space-time). It may not be perceivable but it would mean that space could exert a drag in the same way water does. You could imagine it pulling out energy from a photon and red-shifting it, until eventually it just absorbed it. If that was true, it would also add a distance-based offset for all observations of the Universe and affects things like the Hubble constant As I understand it, expansion of the universe is expansion of the fabric of spacetime. It's not about objects just travelling away from each other. Thus the quantum foam friction analogy (even if true) wouldn't really apply. > We have simulated closed-loop systems of many kinds, I think we can make educated guesses about how they may behave. Don't mistake the incompleteness of our models for the truth. They are approximations for testing out ideas about the universe but that doesn't mean the approximations themselves exist. > As long as all state transitions undergo a reversible function, it should eventually return to its original state? But we know that non-reversible functions exist, both in terms of entropy and time irreversibility, & these functions happen all the time in our universe. > Then where did that energy go? Waste heat that can't be recovered to reverse entropy because it's already diffused throughout the universe & thus there's no local maximum to exploit. That's what the heat death of the universe refers to.
• pdonis 2y ago
  > If no energy is created or destroyed, whatever force is expanding the Universe will eventually un-expand it. This is not correct. First, there is no concept of "energy conservation" that applies to the universe as a whole the way you describe. Second, our best current model of the universe, based on the equations of General Relativity, says it will keep expanding forever. > you have a closed system with finite states Our best current model of the universe is that it is spatially infinite. So it would not have a finite number of possible states.
  • bArray 2y ago
    > This is not correct. First, there is no concept of "energy conservation" that applies to the universe as a whole the way you describe. If the concept of energy conservation does not apply to the entire Universe, then we must ask where the additional energy created comes from or is removed. I'm not aware of evidence to suggest that it is not conserved. If there was something like this, it seems like something that could be exploited. > Second, our best current model of the universe, based on the equations of General Relativity, says it will keep expanding forever. I'm aware that's what is currently supported, but the collapsing Universe followed by the expanding Universe loop ties everything up nicely (cyclic model [1]). I'm also aware that this is essentially based on the second law of thermodynamics [2], but I think as long as all processes a system undergoes are fully reversible, it should in theory be able to reverse. It would be like observing a binary counter: 0000 is the start, then we observe numbers like 0110 and 1011 and say "see, entropy is increasing indefinitely", but eventually we tick over to 1111, then 0000. Of course if any system variable changes irreversibly (i.e. space expands infinitely), then it would not work. But given there is ongoing debate about the Hubble constant [3] and I think there is still room to believe a cyclic model could be plausible. [1] https://en.wikipedia.org/wiki/Cyclic_model https://en.wikipedia.org/wiki/Cyclic_model [2] https://en.wikipedia.org/wiki/Second_law_of_thermodynamics https://en.wikipedia.org/wiki/Second_law_of_thermodynamics [3] https://en.wikipedia.org/wiki/Hubble's_law#Hubble_tension https://en.wikipedia.org/wiki/Hubble's_law#Hubble_tension
    • raattgift 2y ago
      Here's something for you to think about. Light from sources at cosmological distances is redshifted, and we have several excellent lines of evidence for that, notably https://en.wikipedia.org/wiki/Lyman-alpha_forest https://en.wikipedia.org/wiki/Lyman-alpha_forest which clearly shows that the emitted light was redshifted progressively as it reached intervening clouds of hydrogen on its way to our spectrographs. Energy is directly proportional to wavelength. For light, E = hf, where h is Planck's constant. It's inversely proportional to wavelength, E = (hc)/λ, where c is the speed of light and λ is the wavelength. Redshifting means longer wavelengths, so less energy at the point of detection compared to the point of emission. Energy conservation is local. At large scales, photons lose energy. Where does it go? After you've thought about that on your own for a bit, compare your thinking with working physical cosmologist Sean Carroll's blog entry https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ https://www.preposterousuniverse.com/blog/2010/02/22/energy-... The connection to what I wrote above is that General Relativity guarantees that at every single point in a general curved spacetime there is a small patch of (quasi-)static flat spacetime. That patch can be ultramicroscopic deep within a black hole, or very large (in human terms) in interplanetary space in our solar system. In that flat patch energy conservation holds, as Carroll described. Outside that patch, we can see light redshift or blueshift through spacetime; the same energy shifts happen to massive particles like cosmic ray electrons, protons and neutrinos, too. It also happens to gravitational waves. Event horizons break time-reversal symmetry. Nothing comes back from the other side. Generically, in the presence of a relativistic quantum field (like the 17 in the standard model of particle physics), an event horizon radiates greybody radiation comparable to a blackbody with a temperature inversely proportional to the horizon area. To save words, we just use that temperature (and go back to a calculation of the exact spectrum when greater accuracy is required). Black hole event horizons are really cold. Cosmological event horizons are ridiculously cold. And really it's the apparent horizon that has a measurable temperature. There may not be an event horizon in finite time, but in many circumstances the apparent horizon is indistinguishable by experiment. We have evidence of matter crossing to the other side of these horizons (e.g. from tidal disruption events and black hole/neutron star collisions); we have decent upper limits on horizon temperatures for a small handful of black hole horizons. (And someday we will have precision laboratory evidence of Unruh event horizon temperatures). If the universe undergoes recollapse, black holes will tend to merge into ever bigger black holes as galaxy clusters freely-fall closer together (and then interact with each other, merging into gargantuan elliptical things with silly large velocity dispersions generating all sorts of collision & merger opportunities). So you need some unknown extra step to undo the time-reversal invariance violation of black hole horizons. Have any ideas? The bright side is that black holes hold so much entropy that you aren't in any danger of violating the second law of thermodynamics via recollapse, unless your idea does that accidentally. The entropy is easy to see in a Boltzmann sense. Given the "no hair" conjecture, a black hole is fully described by a small number of parameters. Those parameters are a macrostate. A microstate is a configuration of things that fell into the black hole imparting its mass and spin and charge(s)). Entropy is proportional to the log of all the possible microstates for that macrostate. As more matter goes into a black hole, its entropy skyrockets. (The increase in BH entropy absolutely dwarfs the decrease in vacuum entropy; since hard vacuum is a macrostate, and every reasonable volume of hard vacuum is totally substitutable for every other similar volume of hard vacuum, there's a lot of entropy there too. As we fill the vacuum with ejected stars, radiation, high-metallicity dust, and so on as we squash galaxy clusters together during recollapse, that's a loss of entropy. But still nowhere near as low an entropy as the original cold gas clouds that were around after the cosmic microwave background formed. So your idea that recollapse means low entropy is on shaky ground, especially if you are taking your "binary counter ticks back to zero" idea seriously. You can take some comfort that lots of very smart people who can do the maths and have decent physical intuitions (Penrose comes to mind) have struggled with entropy in a cyclic cosmology. Finally, the Hubble tension at its very strongest changes the age of the the oldest galaxies from almost 14 billion years to almost double that, not whether it the universe is expanding faster now than it was between one and four billion years ago. It's not something on which you want to rest a theory of cosmic recollapse.