Inflationary cosmology and the multiverse
Inflationary cosmology is the theory that suggests that for a few nanoseconds after the Big Bang the baby cosmos swelled “like a balloon blown up by Dumbo”, repeatedly doubling in size, before slowing down to the more moderate rate of growth it continues to enjoy today [1].
- The theory explains why the universe appears basically the same in all directions, and does away with the idea that it has a centre. We now know that there are 350 billion observable galaxies in the universe, all moving away from us at breakneck speed, and we're pretty sure that the universe extends significantly beyond that, and is possibly infinite.
- Inflationary cosmology not only creates a universe that is massively bigger than the one we can see. It provides a mechanism for creating an infinite number of universes, or in other words, eternal inflation.
- So it's not only the universe that we are in – the one that extends way beyond the visible horizon and has countless galaxies, billions and billions of them – that's relevant here, but there's also the possibility that it might be just one piece of a much bigger thing that we call the multiverse, in which ours is but one universe among billions, each going through cycles of big bang after big bang after big bang and no scenario, no matter how bizarre, is ruled out.
- Whilst this complicates the comparatively simple idea of a single universe with a known beginning and an anticipated end, it explains some things that were previously inexplicable – in particular, how the universe developed to have precisely the right, finely calibrated conditions to support life on Earth.
- Under these scenarios, you end up possibly suggesting that our existence is inevitable, says Professor Brian Cox. "You even end up not having to explain the beginning – and you don't have to explain why our universe appears quite special in certain respects, such as the strengths of the forces and the amounts of dark energy in it. If you just look at our universe, it appears beautifully tuned for life to emerge. But in eternal inflation multiverses, every possibility happens, so every possible universe gets made in reality."
- "There's already very good evidence for inflation," Cox goes on. "The way the galaxies are spread out in the universe and the way the cosmic microwave background radiation is distributed, all those things fit very well. Inflation works very well. And the better inflation works, the more you start to trust the predictions of the theory."
- And as evidence for inflationary cosmology slowly accretes, the likelihood that the multiverse exists gets better and better, he says. At this point in time it remains a speculative idea, but, crucially, one for which supporting data is at least theoretically possible.
- "If there are, say, multiple pocket universes, then they could interact, particularly early on in the history of this universe, and that may leave imprints in the cosmic microwave background. Another thing that would give support to these ideas is the discovery of extra dimensions in this universe" - referring to the concepts inherent in string theory, which holds that instead of just four dimensions, the cosmos actually comprises 10 or 24, depending on the theoretical variant involved. "As a theory, string theory fits neatly with the inflationary multiverse."
- Which all makes us look even more insignificant than we already appear to be, situated as we are on a planet which is but a speck in the cosmos.
[1] This material is drawn from an article in the Sydney Morning Herald on 24 July 2016, entitled "Brian Cox on life, universe and the big bang" at http://www.smh.com.au/national/brian-cox-on-life-universe-and-the-big-bang-20160719-gq8srn.html The citations are in each case from Brian Cox.
These ideas are also the subject of elaboration in the Brian Cox documentary Life of a Universe, Part 1 - Creation at https://www.youtube.com/watch?v=Or2Itbzxo6A and Part 2 - End of days at https://www.youtube.com/watch?v=j-ZhS9Tya-8 including a Brian Greene interview or two on the subject just for good measure.
These ideas are also the subject of elaboration in the Brian Cox documentary Life of a Universe, Part 1 - Creation at https://www.youtube.com/watch?v=Or2Itbzxo6A and Part 2 - End of days at https://www.youtube.com/watch?v=j-ZhS9Tya-8 including a Brian Greene interview or two on the subject just for good measure.
Back to basics
Brian Cox having revivified this subject in his usual animated fashion in conjunction with Brian Greene, perhaps we would do well to refresh our memories with Greene’s erudite and perceptive analysis of the same issues in The Hidden Reality – Parallel Universes, and the Deep Laws of the Cosmos published, Knopf, 2011. What follows is a précis of Greene’s analysis. I take responsibility for any errors in the abbreviation process. Page numbers are cited following each segment. The interpolations in the form of headings are my own. The two paragraph explanatory interpolation in italics is from another source.
If the big bang theory is correct, space should now be uniformly filled with remnant photons from the creation event, streaming every which way, whose vibrational frequencies are determined by how much the universe has expanded and cooled during the billions of years since they were released: the cosmic background microwave radiation 39
Shortly after “the beginning”, whatever that may have happened to be, there was a brief burst of enormously fast and ever-quickening expansion, called inflationary expansion.. A common temperature was established before the regions were rapidly driven apart.
Einstein’s equations dictate how the universe expands by the force of expanding gravity. Regions of space had ample time before the burst to come to the same temperature. Billions of years ago just around the time of the big bang, space was momentarily permeated by an enormous and uniform negative pressure (tension). 44
The viability of the inflationary scenario thus depends on whether its proposed modification to the standard big bang theory can emerge from Einstein’s maths, under which curved spacetime is the source of gravity. Under Einstein’s theory, gravity arises not only from an object’s mass (and energy) but also from its pressure, and in certain circumstances, pressure can be negative, then giving rise to repulsive gravity. So by infusing space with a uniform energy, a cosmological constant not only endows the spatial fabric with a uniform energy determined by the constant’s value, but also fills space with a negative pressure, and negative pressure yields repulsive gravity. 46
Repulsive gravity, due to negative pressure, will cause the universe to expand, regular mass-energy will cause universe to contract. Einstein introduced the so-called cosmological constant to balance the contraction of the universe so to make it static.
In physics “negative pressure” is called tension. Ordinary positive pressure pushes outwards. Negative pressure simply pulls inwards. By including pressure in the source for gravity, Einstein opened the possibility that a system with very large negative pressure could have zero or even negative gravitational mass that would repel instead of attract.
Negative pressure derives from quantum fields, and fields carry energy. A field’s value can vary from place to place, but should it be constant, taking the same value everywhere, it would fill space with the same energy at every point. Such uniform field configurations fill space not only with uniform energy but also with uniform negative pressure. 47, 49
A uniform field sucks inwards. This means that a uniform field results in negative pressure, and if there’s a field – the hypothetical inflaton field – that has a uniform value throughout a region of space, it will fill that region not only with energy but also with negative pressure, meaning a repulsive gravity which drives an ever-quickening expansion of space. As with a uniform field, a cosmological constant’s uniform negative pressure also yields repulsive gravity.
When the inflaton is at its zenith high up on its potential energy curve, it fills space with a large potential energy and negative pressure. As it rolls down the slope it releases its potential energy by its value throughout space. As its value decreases, the energy and negative pressure it harbours decreases and condenses into a uniform bath of particles that fills space. Energy conversion to particles results in a huge, uniform spatial expanse filled with stars and galaxies. In inflation, a uniform temperature across space is inevitable. 53
Regions with slightly more energy (that is, via E= mc2, regions with slightly more mass) exerted a slightly stronger gravitational pull, attracting more particles from their surroundings and growing larger, evolving into galaxies and the stars within them. The very existence of galaxies, stars, planets and life itself derives from microscopic quantum uncertainty amplified by inflationary expansion. 60
The quantum field will also undergo quantum uncertainty. Quantum jitters act like tremors. An inflaton field’s uniform energy and negative pressure generate repulsive gravity. The region the inflationary field permeates expands at a fantastic rate, and once inflation begins it never ends.
The quilted variety emerges if space is infinite. In a quilted universe, conditions necessarily repeat across space, yielding parallel worlds. Inflation emerges from eternal inflationary expansion.
Fields may settle down to have different values in different universes. Because fields can settle down to different values in different bubbles, the universes in the Inflationary Multiverse can have different physical features even though the universes are all governed by the same fundamental physical laws. 62
Because of their fundamental differences, the Quilted and Inflationary Multiverses might appear unrelated, but the parallel universes arising from inflationary generate their quilted cousins.
Each of the bubble universes appears to have finite spatial extent when viewed from the outside, but infinite spatial extent when viewed from the inside. Infinite spatial extent is just what we need for quilted parallel universes. The outsider and insider have vastly different conceptions of time. So what appears as endless time to an outsider appears as endless space at each moment of time to an insider. 66
Brian Cox having revivified this subject in his usual animated fashion in conjunction with Brian Greene, perhaps we would do well to refresh our memories with Greene’s erudite and perceptive analysis of the same issues in The Hidden Reality – Parallel Universes, and the Deep Laws of the Cosmos published, Knopf, 2011. What follows is a précis of Greene’s analysis. I take responsibility for any errors in the abbreviation process. Page numbers are cited following each segment. The interpolations in the form of headings are my own. The two paragraph explanatory interpolation in italics is from another source.
- Inflation is a cosmological framework but not a specific theory. The essence of the Inflationary Multiverse is that eternal cosmological inflation yields an enormous network of bubble universes, of which our universe would be one. 309
- Inflationary cosmology modifies the big bang theory by inserting an intense burst of enormously fast expansion during the universe’s earliest moments. The most convincing version of the theory also yields a vast collection of parallel universes, radically transforming the complexion of reality. 37
- The CMBR
If the big bang theory is correct, space should now be uniformly filled with remnant photons from the creation event, streaming every which way, whose vibrational frequencies are determined by how much the universe has expanded and cooled during the billions of years since they were released: the cosmic background microwave radiation 39
- The horizon problem – solved!
Shortly after “the beginning”, whatever that may have happened to be, there was a brief burst of enormously fast and ever-quickening expansion, called inflationary expansion.. A common temperature was established before the regions were rapidly driven apart.
- Enter Einstein
Einstein’s equations dictate how the universe expands by the force of expanding gravity. Regions of space had ample time before the burst to come to the same temperature. Billions of years ago just around the time of the big bang, space was momentarily permeated by an enormous and uniform negative pressure (tension). 44
- Negative pressure = repulsive gravity
The viability of the inflationary scenario thus depends on whether its proposed modification to the standard big bang theory can emerge from Einstein’s maths, under which curved spacetime is the source of gravity. Under Einstein’s theory, gravity arises not only from an object’s mass (and energy) but also from its pressure, and in certain circumstances, pressure can be negative, then giving rise to repulsive gravity. So by infusing space with a uniform energy, a cosmological constant not only endows the spatial fabric with a uniform energy determined by the constant’s value, but also fills space with a negative pressure, and negative pressure yields repulsive gravity. 46
- Thus:
Repulsive gravity, due to negative pressure, will cause the universe to expand, regular mass-energy will cause universe to contract. Einstein introduced the so-called cosmological constant to balance the contraction of the universe so to make it static.
In physics “negative pressure” is called tension. Ordinary positive pressure pushes outwards. Negative pressure simply pulls inwards. By including pressure in the source for gravity, Einstein opened the possibility that a system with very large negative pressure could have zero or even negative gravitational mass that would repel instead of attract.
- A uniform field such as the hypothetical inflaton field results in negative pressure and repulsive gravity
Negative pressure derives from quantum fields, and fields carry energy. A field’s value can vary from place to place, but should it be constant, taking the same value everywhere, it would fill space with the same energy at every point. Such uniform field configurations fill space not only with uniform energy but also with uniform negative pressure. 47, 49
A uniform field sucks inwards. This means that a uniform field results in negative pressure, and if there’s a field – the hypothetical inflaton field – that has a uniform value throughout a region of space, it will fill that region not only with energy but also with negative pressure, meaning a repulsive gravity which drives an ever-quickening expansion of space. As with a uniform field, a cosmological constant’s uniform negative pressure also yields repulsive gravity.
- As the inflaton’s value diminishes, its pent up energy is released in the form of particles which devolve into stars and galaxies
When the inflaton is at its zenith high up on its potential energy curve, it fills space with a large potential energy and negative pressure. As it rolls down the slope it releases its potential energy by its value throughout space. As its value decreases, the energy and negative pressure it harbours decreases and condenses into a uniform bath of particles that fills space. Energy conversion to particles results in a huge, uniform spatial expanse filled with stars and galaxies. In inflation, a uniform temperature across space is inevitable. 53
- Quantum uncertainty amplified by inflationary expansion also explains the existence of stars, galaxies etc
Regions with slightly more energy (that is, via E= mc2, regions with slightly more mass) exerted a slightly stronger gravitational pull, attracting more particles from their surroundings and growing larger, evolving into galaxies and the stars within them. The very existence of galaxies, stars, planets and life itself derives from microscopic quantum uncertainty amplified by inflationary expansion. 60
- The inflationary field has a value at each point in space.
The quantum field will also undergo quantum uncertainty. Quantum jitters act like tremors. An inflaton field’s uniform energy and negative pressure generate repulsive gravity. The region the inflationary field permeates expands at a fantastic rate, and once inflation begins it never ends.
- The Inflationary Multiverse arises when bubble universes continually form within an ever-expanding spatial environment permeated by a high valued inflationary field.
The quilted variety emerges if space is infinite. In a quilted universe, conditions necessarily repeat across space, yielding parallel worlds. Inflation emerges from eternal inflationary expansion.
- Our universe is but one of many going through the same experience throughout the cosmos.
- In the Inflationary Multiverse, the member universes are sharply divided. Each is a hole in the cosmic cheese, separated from the others by domains in which the inflaton field’s value remains high.
Fields may settle down to have different values in different universes. Because fields can settle down to different values in different bubbles, the universes in the Inflationary Multiverse can have different physical features even though the universes are all governed by the same fundamental physical laws. 62
Because of their fundamental differences, the Quilted and Inflationary Multiverses might appear unrelated, but the parallel universes arising from inflationary generate their quilted cousins.
- Different perspectives as to time
Each of the bubble universes appears to have finite spatial extent when viewed from the outside, but infinite spatial extent when viewed from the inside. Infinite spatial extent is just what we need for quilted parallel universes. The outsider and insider have vastly different conceptions of time. So what appears as endless time to an outsider appears as endless space at each moment of time to an insider. 66