Aug 9, 2020 04:37 PM
(This post was last modified: Aug 9, 2020 04:38 PM by C C.)
https://aeon.co/essays/our-cosmic-horizo...-than-ever
EXCERPTS (Kataie Mack): . . . The distance to our cosmic horizon is not, as you might expect, 13.8 billion light-years. As we discussed above, distances are weird in an expanding universe. Something that was 13.8 billion light-years away when its light started the journey toward us is much farther away now. If you factor all that in, that glowing plasma we see at the very edge of the observable universe is actually somewhere around 45 billion light-years away now. [...] Just because we can’t see things beyond our fiery horizon, it doesn’t mean that there’s nothing there. ... In fact, regions far enough beyond our horizon can even be considered to be separate, isolated universes of their own, for all practical purposes, since they can’t interact with ours.
But what if the universe isn’t just bigger than we perceive, circumstantially, but bigger than we even can perceive? What if it extends in every direction, and then some? [...] physicists have been wondering for years ... If there are more spatial dimensions, extending out in new directions we can’t perceive, that could help to explain some puzzling aspects of theoretical physics and the behaviour of gravity. ... Higher dimensions of space are also a requirement for string theory...
[...] what if an extra dimension can hide an entire universe? One hypothesis for the structure of our cosmos, developed in the early 2000s, suggests that we might live in a three-dimensional ‘brane’ (think: membrane) on the edge of a larger space with four spatial dimensions (plus time). Inside that higher-dimensional ‘bulk’ there could be another 3D brane, containing another universe, that might, from time to time, come crashing into our own. This theory’s originators called it the ‘ekpyrotic’ model of the cosmos, after a Greek term for conflagration - a nod to the fact that each cosmic collision would result in the fiery conditions of the Big Bang, and could explain the origin and eventual fate of our universe. In this model, the branes alternately move toward each other, collide and then move apart again, in an endless cycle, going from Big Bang, to expansion, to Big Crunch, and back to Big Bang. The patterns of structure we see in the cosmos today (distributions of galaxies and clusters) are, in this model, seeded by the interaction between the two branes in the slow collapse phase before the Bang.
While it might seem extravagant at first glance to postulate higher dimensions and new universes just to explain the Big Bang, there are good reasons why physicists take these ideas seriously [...] That background light we see right at the cosmic horizon, that afterglow of the Big Bang, tells us that a simple evolution from a singularity to the big beautiful universe we enjoy now just doesn’t make sense.
[...] The problem is that the Big Bang’s afterglow, what we refer to as the cosmic microwave background, is too perfect. To an absurd degree of precision (one part in 100,000), it looks the same, in every direction. Same colour (or, rather, frequency, since it’s microwave light), same spectrum, same intensity. The reason that’s a problem is because there’s no reason why two regions on opposite sides of the sky should match in that way. Even if everything started together, wrapped up in a singularity, the way it expanded outward should have introduced extreme differences in different parts of the early cosmos. Regions that are now far apart from each other in the expanding fireball stage of cosmic evolution never had a chance to come to an agreement on what temperature to be. The cosmic microwave background should be drastically different on one side of the sky than it is on the other.
The explanation that physicists settled on in the 1980s inserted a new chapter into our cosmic story. [....] From what we’ve seen so far, inflation seems to work well with our current paradigm, and even nicely explains the little one-part-in-100,000 fluctuations observed in the cosmic microwave background light. But we don’t have what anyone would call solid proof that it happened, nor can we say how or why it started, or what drove it. [...] If inflation did happen, the sequence of events that created our observable universe could have occurred time and time again in different parts of a much larger space, in a process called ‘eternal inflation’. The idea is that the larger background space is always inflating but, once in a while, in a small part of it, inflation stops; that bit of universe heats up, and normal cosmic expansion takes over. This would create a kind of multiverse, as little bubble universes, defined by those separate post-inflation regions, continually drop out of the inflating background. Each bubble universe would be separated from the others by that constantly expanding space, and would be incapable of interacting with each other. For the most part.
[...] As for the ekpyrotic model, it’s undergone a number of revisions over the years, and the current version doesn’t involve higher dimensions or cosmic collisions at all. In some ways, it looks more like inflation: driven not by the motion of branes but by the evolution of a scalar field, a species of space-filling energy field similar to what most physicists think fuelled cosmic inflation. ... Despite no longer requiring grand cosmic collisions, the ekpyrotic model still includes a transition between a collapsing universe and a Big Bang. In the new version, though, the collapse might be a relatively modest one, resulting in a little bit of compression before the sparking of the conflagration that starts the new cycle. If it cycles on and on forever that way, rather than endless pocket universes, our larger space would be one giant, ever-growing cosmos: expanding, taking a breath, and expanding again, over and over.
The cosmic horizon defining our observable universe is a hard limit. We can’t see beyond it, and unless our understanding of the structure of reality changes drastically, we can be confident we never will. The expansion of the cosmos is speeding up; anything beyond our horizon now will be carried away from us faster and faster, and its light will never be able to catch up. While we might never be able to say with certainty what lies beyond that border, what all the theories have in common is that our observable universe is part of a much, much larger space.
Whether that space contains a multiverse of bubbles, each with different physical laws; whether it’s part of an ever-growing cosmos of which we are only one part, in one cycle; or whether space extends outward in directions we can’t conceive, we currently just don’t know. But we’re seeking clues... (MORE - details)
EXCERPTS (Kataie Mack): . . . The distance to our cosmic horizon is not, as you might expect, 13.8 billion light-years. As we discussed above, distances are weird in an expanding universe. Something that was 13.8 billion light-years away when its light started the journey toward us is much farther away now. If you factor all that in, that glowing plasma we see at the very edge of the observable universe is actually somewhere around 45 billion light-years away now. [...] Just because we can’t see things beyond our fiery horizon, it doesn’t mean that there’s nothing there. ... In fact, regions far enough beyond our horizon can even be considered to be separate, isolated universes of their own, for all practical purposes, since they can’t interact with ours.
But what if the universe isn’t just bigger than we perceive, circumstantially, but bigger than we even can perceive? What if it extends in every direction, and then some? [...] physicists have been wondering for years ... If there are more spatial dimensions, extending out in new directions we can’t perceive, that could help to explain some puzzling aspects of theoretical physics and the behaviour of gravity. ... Higher dimensions of space are also a requirement for string theory...
[...] what if an extra dimension can hide an entire universe? One hypothesis for the structure of our cosmos, developed in the early 2000s, suggests that we might live in a three-dimensional ‘brane’ (think: membrane) on the edge of a larger space with four spatial dimensions (plus time). Inside that higher-dimensional ‘bulk’ there could be another 3D brane, containing another universe, that might, from time to time, come crashing into our own. This theory’s originators called it the ‘ekpyrotic’ model of the cosmos, after a Greek term for conflagration - a nod to the fact that each cosmic collision would result in the fiery conditions of the Big Bang, and could explain the origin and eventual fate of our universe. In this model, the branes alternately move toward each other, collide and then move apart again, in an endless cycle, going from Big Bang, to expansion, to Big Crunch, and back to Big Bang. The patterns of structure we see in the cosmos today (distributions of galaxies and clusters) are, in this model, seeded by the interaction between the two branes in the slow collapse phase before the Bang.
While it might seem extravagant at first glance to postulate higher dimensions and new universes just to explain the Big Bang, there are good reasons why physicists take these ideas seriously [...] That background light we see right at the cosmic horizon, that afterglow of the Big Bang, tells us that a simple evolution from a singularity to the big beautiful universe we enjoy now just doesn’t make sense.
[...] The problem is that the Big Bang’s afterglow, what we refer to as the cosmic microwave background, is too perfect. To an absurd degree of precision (one part in 100,000), it looks the same, in every direction. Same colour (or, rather, frequency, since it’s microwave light), same spectrum, same intensity. The reason that’s a problem is because there’s no reason why two regions on opposite sides of the sky should match in that way. Even if everything started together, wrapped up in a singularity, the way it expanded outward should have introduced extreme differences in different parts of the early cosmos. Regions that are now far apart from each other in the expanding fireball stage of cosmic evolution never had a chance to come to an agreement on what temperature to be. The cosmic microwave background should be drastically different on one side of the sky than it is on the other.
The explanation that physicists settled on in the 1980s inserted a new chapter into our cosmic story. [....] From what we’ve seen so far, inflation seems to work well with our current paradigm, and even nicely explains the little one-part-in-100,000 fluctuations observed in the cosmic microwave background light. But we don’t have what anyone would call solid proof that it happened, nor can we say how or why it started, or what drove it. [...] If inflation did happen, the sequence of events that created our observable universe could have occurred time and time again in different parts of a much larger space, in a process called ‘eternal inflation’. The idea is that the larger background space is always inflating but, once in a while, in a small part of it, inflation stops; that bit of universe heats up, and normal cosmic expansion takes over. This would create a kind of multiverse, as little bubble universes, defined by those separate post-inflation regions, continually drop out of the inflating background. Each bubble universe would be separated from the others by that constantly expanding space, and would be incapable of interacting with each other. For the most part.
[...] As for the ekpyrotic model, it’s undergone a number of revisions over the years, and the current version doesn’t involve higher dimensions or cosmic collisions at all. In some ways, it looks more like inflation: driven not by the motion of branes but by the evolution of a scalar field, a species of space-filling energy field similar to what most physicists think fuelled cosmic inflation. ... Despite no longer requiring grand cosmic collisions, the ekpyrotic model still includes a transition between a collapsing universe and a Big Bang. In the new version, though, the collapse might be a relatively modest one, resulting in a little bit of compression before the sparking of the conflagration that starts the new cycle. If it cycles on and on forever that way, rather than endless pocket universes, our larger space would be one giant, ever-growing cosmos: expanding, taking a breath, and expanding again, over and over.
The cosmic horizon defining our observable universe is a hard limit. We can’t see beyond it, and unless our understanding of the structure of reality changes drastically, we can be confident we never will. The expansion of the cosmos is speeding up; anything beyond our horizon now will be carried away from us faster and faster, and its light will never be able to catch up. While we might never be able to say with certainty what lies beyond that border, what all the theories have in common is that our observable universe is part of a much, much larger space.
Whether that space contains a multiverse of bubbles, each with different physical laws; whether it’s part of an ever-growing cosmos of which we are only one part, in one cycle; or whether space extends outward in directions we can’t conceive, we currently just don’t know. But we’re seeking clues... (MORE - details)
