One universe is not enough




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An illustration of multiple, independent universes, causally disconnected from each other in an ever-expanding cosmic ocean, is a reflection of the idea of ​​Multiverse. In a region where the Big Bang starts and inflation ends, the rate of expansion will drop, while inflation between two such regions will continue, separating them forever.Ozytive / Public domain

"The universe is all there is, was or will be." That seems like a reasonable explanation to make, is not it? It certainly corresponds to our conception of the word universe, which implies that this is all space and all matter and energy in it. We certainly live in the universe and can see a huge amount of it: about 46 billion light-years in all directions. After 13.8 billion years ago since the hot big bang and the fabric of space that expanded all that time, this is the absolute limit of how far away we can see.

But what lies behind that? Is there more universe like ours? The answer is yes, there should be. But there must be something more than that: a larger space-time structure with a huge, countless number of universes in it. If our best theories are correct, our one universe is not enough. This is why.

If you look further and further away, you look further and further into the past. The sooner you go, the hotter and closer, and less developed, the universe turns out to be. The earliest signals can even tell us what happened before the moments of the hot Big Bang.NASA / STScI / A. Feild (STScI)

Imagine that you went all the way back to the beginning of the universe as we know it: the beginning of the hot big bang. What would it look like? You would be immersed in a hot bath of particles, anti-particles and radiation. They would all be massless and therefore move at the speed of light because the Higgs have not yet given mass to the universe. And the temperature and energy of these particles would be extremely high: around 1028 K, give or take a bit. Everything that was energetically permitted would exist and particle collisions, including spontaneous creation and annihilation of particles / antiparticle pairs, would happen quickly, frequently and relentlessly.

The cosmic history of the whole known universe shows that we have the origin of all matter in it, and all the light, ultimately, until the end of inflation and the beginning of the hot big bang.E. Siegel / ESA and the Planck collaboration

From this point we normally turn the clock forward in time, we look at the universe, expand, cool, create more matter than antimatter and eventually form nuclei, atoms, gravity lumps, stars, galaxies and after enough time, human beings.

But what if we went the other way? What if we went backwards instead and asked where the circumstances that we call the hot big bang come from? We would come to a strange, almost extraterrestrial state of affairs: a period of cosmic inflation. Instead of the energy in the universe being bound into particles, antiparticles and radiation, it would instead be energy that is inherent in the fabric of the space itself. The effects of cosmic inflation are profound, but not always intuitive.

This diagram shows, to scale, how spacetime evolves / expands in equal time steps when your universe is dominated by matter, radiation or the energy inherent in space itself, the latter corresponding to an inflating, energy-inherent to space. – dominated universe.E. Siegel

If the universe has packaged all its energy in a form that is inherent to the space itself, it is still expanding, but it does not cool or become less dense as the post-Big Bang universe does. Instead it expands exponentially, which means that it doubles in size after a small, but a fixed amount of time passes: somewhere around 10-35 seconds. & nbsp; Therefore on time & nbsp; 10-34 seconds have passed, the universe has increased by a factor of 210or 1024. By the time & nbsp; 10-33 seconds pass, it has increased by 2100, or ~ 1030. And so on.

The most important realization of inflation is that the energy density remains constant. As the volume of the universe increases, the energy inherent in space in each region remains the same. As the universe blows up, it simply creates more and more universes that are still blowing up.

Inflation ensures that the space exponentially exponentially increases, which can very quickly ensure that an already existing curved or non-slippery space looks flat. If the universe is curved, it has a radius of curvature that is at least hundreds of times larger than what we can perceive.E. Siegel (L); Ned Wright's cosmology tutorial (R)

"So," you say, "it's still exactly the one Universe, right?"

Perhaps it is. The volume of the Universe increases enormously, relentlessly and without limits, but not indefinitely. This only happens until the inflation comes to an end. And when it ceases, all that energy inherent in space is turned into particles, antiparticles and radiation: the end of inflation coincides with the onset of the hot big bang.

The analogy of a ball that slides over a high surface is when inflation persists, while the structure of crumbling and releasing energy represents the conversion of energy into particles.E. Siegel

These are the basic principles of what inflation does. If we look at how this works physically, we can visualize inflation as a field: a ball that sits at the top of a hill. The hill must above all be flat on top, so that the ball can spend a lot of time there. The ball inevitably rolls down in the direction of the valley, but it has to roll slowly: only when the ball is on top of the flat part of the hill can inflation occur. When the ball rolls into the valley, inflation comes to an end, creating a universe that is filled with particles, anti-particles and radiation: it starts the hot big bang.

If inflation were a classic field, you would get inflation as long as the field value remains large, but if it gets smaller by, for example, rolling in the valley, inflation would come to an end and the universe would heat up again.E. Siegel / Beyond the Galaxy

Again, so far, so good. We have one Universe, it blows up, inflation ends, we get the hot Big Bang and everyone is happy.

Until you remember an important warning that we have ignored so far: everything that exists physically, including all particles and fields, must be inherently quantum in nature. Quantum particles are strange intuition-oriented entities that do not always work like particles, but also have wave-like properties. And one of the most important things that quantum particles do, whenever you place one somewhere in the universe, is that their positions are no longer fixed, but rather described by probability distribution. And the longer you wait, the more the wave function describes where a particle is spreading.

As time passes, even the quantum wave function describing its position, even for a simple, single particle, will spread spontaneously over time. This happens for all quantum particles.Hans de Vries / Physics Quest

This is no big deal for a free electron in our universe today, but it's a huge deal for inflation! Imagine you are on top of this hill and slowly rolling into the valley. At the same time your position has the chance to spread out, and although there is a finite chance that you will get closer to the valley than usual, there is also a chance that you will climb the hill further than you started.

Now, here's the kicker: because your space expands and inflates, different spatial areas can make different things happen. Let us imagine that we are letting enough time pass that five different regions, each now the same size as the original region, now exist. What happens if we allow them to be randomly distributed?

If inflation is a quantum field, the field value spreads over time, with different regions of space assuming different realizations of the field value. In many regions, the field value will end up in the bottom of the valley, causing inflation to stop, but in much more inflation will continue, randomly far into the future.E. Siegel / Beyond The Galaxy

Inflation is ending in a few of them. Inflation is going on in other countries, but it looks like it is almost over. And in others it goes further, even more powerful than it would have been without spreading and rolling.

When you work out the probabilities for practically all viable inflation models, you notice that the amount of space where inflation occurs and does not end always increases with time. There will inevitably be regions where inflation ceases, and where that happens, you get a hot big bang. But outside each of those regions, there will be a place where inflation does not stop and the space continues to blow up there. With every new moment that comes with time, there is a finite chance that inflation will end, but an even greater chance that it will continue, further into the future.

This illustration shows regions where inflation will continue in the future (blue), and where it ends, giving rise to a big bang and a universe like ours (red X). Note that this may go back indefinitely, and we will never know, but once it ends in our region, we can not see the places beyond our horizon where it still blows up.E. Siegel / Beyond The Galaxy

What we end up with, is a space-time where inflation ends at any moment in a few regions and we get a hot big bang at the place where it happens. Each of these regions is surrounded by a wider spacetime that continues to blow up, with a few small spots in each of the inflation areas seeing the end of inflation and following a hot big bang. These different hot Big Bangs will each create their own observable Universe, just like ours, with a different starting point and different specific starting conditions for each region. They will be separated from each other by more inflating space, and no two universes will ever collide with each other or interact with each other.

This is where the concepts of eternal inflation, the multiverse and the existence of many disconnected universes come from.

Artistic impression of a Multiverse – where our universe is just one of many. These Universa will be separated from each other, have a slightly different, variable density and structure in them, and will start their Big Bangs at different times. They will never collide or interact, and instead will grow apart if they are embedded in a background of inflating space.Jaime Salcido / simulations by the EAGLE collaboration

If you accept that inflation is a phase that took place in the past of the universe prior to the hot big bang, and that the universe itself is naturally quantum in nature, the existence of a multiverse is inevitable. & Nbsp; Although we can not observe these other universes, we can observe an enormous amount of evidence for inflation, indirectly pointing to its inevitability. We can also see an enormous amount of evidence that the universe itself is quantum, even though we have no evidence that inflation itself behaves like a quantum field. If you merge these pieces, this unambiguously leads to the prediction that our universe would be only one of innumerable universes, all embedded in an ever-expanding, growing background. One universe is not enough. Although we can not detect it, the prediction of a multiverse is inevitable.

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An illustration of multiple, independent universes, causally disconnected from each other in an ever-expanding cosmic ocean, is a reflection of the idea of ​​Multiverse. In a region where the Big Bang starts and inflation ends, the rate of expansion will drop, while inflation between two such regions will continue, separating them forever.Ozytive / Public domain

"The universe is all there is, was or will be." That seems like a reasonable explanation to make, is not it? It certainly corresponds to our conception of the word universe, which implies that this is all space and all matter and energy in it. We certainly live in the universe and can see a huge amount of it: about 46 billion light-years in all directions. After 13.8 billion years ago since the hot big bang and the tissue of the expanding space for all that time, this is the absolute limit of how far away we can see.

But what lies behind that? Is there more universe like ours? The answer is yes, there should be. But there should even be something more than that: a larger space-time structure with an enormous, innumerable number of universes embedded in it. If our best theories are correct, our one universe is not enough. This is why.

If you look further and further away, you look further and further into the past. The sooner you go, the hotter and closer, and less developed, the universe turns out to be. The earliest signals can even tell us what happened before the moments of the hot Big Bang.NASA / STScI / A. Feild (STScI)

Imagine that you went all the way back to the beginning of the universe as we know it: the beginning of the hot big bang. What would it look like? You would be immersed in a hot bath of particles, anti-particles and radiation. They would all be massless and therefore move at the speed of light because the Higgs have not yet given mass to the universe. And the temperature and energy of these particles would be extremely high: around 1028 K, give or take a bit. Everything that was energetically permitted would exist, and particle collisions – including spontaneous creation and annihilation of particles / antiparticle pairs – would happen quickly, frequently and relentlessly.

The cosmic history of the whole known universe shows that we have the origin of all matter in it, and all the light, ultimately, until the end of inflation and the beginning of the hot big bang.E. Siegel / ESA and the Planck collaboration

From this point we normally turn the clock forward in time, we look at the universe, expand, cool, create more matter than antimatter and eventually form nuclei, atoms, gravity lumps, stars, galaxies and after enough time, human beings.

But what if we went the other way? What if we went backwards instead and asked where the circumstances that we call the hot big bang come from? We would come to a strange, almost extraterrestrial state of affairs: a period of cosmic inflation. Instead of the energy in the universe being bound into particles, antiparticles and radiation, it would instead be energy that is inherent in the fabric of the space itself. The effects of cosmic inflation are profound, but not always intuitive.

This diagram shows, to scale, how spacetime evolves / expands in equal time steps when your universe is dominated by matter, radiation or the energy inherent in space itself, the latter corresponding to an inflating, energy-inherent to space. – dominated universe.E. Siegel

If the universe has packaged all its energy in a form that is inherent to the space itself, it is still expanding, but it does not cool or become less dense as the post-Big Bang universe does. Instead it expands exponentially, which means that it doubles in size after a small, but a fixed amount of time passes: somewhere around 10-35 seconds. Therefore, by the time 10-34 seconds have passed, the universe has increased by a factor of 210or 1024. By the time 10-33 seconds pass, it has increased by 2100, or ~ 1030. And so on.

The most important realization of inflation is that the energy density remains constant. As the volume of the universe increases, the energy inherent in space in each region remains the same. As the universe blows up, it simply creates more and more universes that are still blowing up.

Inflation ensures that the space exponentially exponentially increases, which can very quickly ensure that an already existing curved or non-slippery space looks flat. If the universe is curved, it has a radius of curvature that is at least hundreds of times larger than what we can perceive.E. Siegel (L); Ned Wright's cosmology tutorial (R)

"So what," you say, "it's still exactly the one Universe, right?"

Perhaps it is. The volume of the Universe increases enormously, relentlessly and without limits, but not indefinitely. This only happens until the inflation comes to an end. And when it ceases, all that energy inherent in space is turned into particles, antiparticles and radiation: the end of inflation coincides with the onset of the hot big bang.

The analogy of a ball that slides over a high surface is when inflation persists, while the structure of crumbling and releasing energy represents the conversion of energy into particles.E. Siegel

These are the basic principles of what inflation does. If we look at how this works physically, we can visualize inflation as a field: a ball that sits at the top of a hill. The hill must above all be flat on top, so that the ball can spend a lot of time there. The ball inevitably rolls down in the direction of the valley, but it has to roll slowly: only when the ball is on top of the flat part of the hill can inflation occur. When the ball rolls into the valley, inflation comes to an end, creating a universe that is filled with particles, anti-particles and radiation: it starts the hot big bang.

If inflation were a classic field, you would get inflation as long as the field value remains large, but if it gets smaller by, for example, rolling in the valley, inflation would come to an end and the universe would heat up again.E. Siegel / Beyond the Galaxy

Again, so far, so good. We have one Universe, it blows up, inflation ends, we get the hot Big Bang and everyone is happy.

Until you remember an important warning that we have ignored so far: everything that exists physically, including all particles and fields, must be inherently quantum in nature. Quantum particles are strange intuition-oriented entities that do not always work like particles, but also have wave-like properties. And one of the most important things that quantum particles do, whenever you place one somewhere in the universe, is that their positions are no longer fixed, but rather described by probability distribution. And the longer you wait, the more the wave function describes where a particle is spreading.

As time passes, even the quantum wave function describing its position, even for a simple, single particle, will spread spontaneously over time. This happens for all quantum particles.Hans de Vries / Physics Quest

This is no big deal for a free electron in our universe today, but it's a huge deal for inflation! Imagine you are on top of this hill and slowly rolling into the valley. At the same time your position has the chance to spread out, and although there is a finite chance that you will get closer to the valley than usual, there is also a chance that you will climb the hill further than you started.

Now, here's the kicker: because your space expands and inflates, different spatial areas can make different things happen. Let us imagine that we are letting enough time pass that five different regions, each now the same size as the original region, now exist. What happens if we allow them to be randomly distributed?

If inflation is a quantum field, the field value spreads over time, with different regions of space assuming different realizations of the field value. In many regions, the field value will end up in the bottom of the valley, causing inflation to stop, but in much more inflation will continue, randomly far into the future.E. Siegel / Beyond The Galaxy

Inflation is ending in a few of them. Inflation continues in other countries, but it looks like it is almost over. And in others it goes further, even more powerful than it would be without spreading and rolling all the way.

When you work out the probabilities for practically all viable inflation models, you notice that the amount of space where inflation occurs and does not end always increases with time. There will inevitably be regions where inflation ceases, and where that happens, you get a hot big bang. But outside each of those regions, there will be a place where inflation does not stop and the space continues to blow up there. With every new moment that comes with time, there is a finite chance that inflation will end, but an even greater chance that it will continue, further into the future.

This illustration shows regions where inflation will continue in the future (blue), and where it ends, giving rise to a big bang and a universe like ours (red X). Note that this may go back indefinitely, and we will never know, but once it ends in our region, we can not see the places beyond our horizon where it still blows up.E. Siegel / Beyond The Galaxy

What we end up with, is a space-time where inflation ends at any moment in a few regions and we get a hot big bang at the place where it happens. Each of these regions is surrounded by a wider spacetime that continues to blow up, with a few small spots in each of the inflation areas seeing the end of inflation and following a hot big bang. These different hot Big Bangs will each create their own observable Universe, just like ours, with a different starting point and different specific starting conditions for each region. They will be separated from each other by more inflating space, and no two universes will ever collide with each other or interact with each other.

This is where the concepts of eternal inflation, the multiverse and the existence of many disconnected universes come from.

Artistic impression of a Multiverse – where our universe is just one of many. These Universa will be separated from each other, have a slightly different, variable density and structure in them, and will start their Big Bangs at different times. They will never collide or interact, and instead will grow apart if they are embedded in a background of inflating space.Jaime Salcido / simulations by the EAGLE collaboration

If you accept that inflation is a stage that took place in the past of the universe prior to the hot big bang, and that the universe itself is naturally quantum in nature, the existence of a multiverse is inevitable. Although we can not observe these other universes, we can observe an enormous amount of evidence for inflation, indirectly pointing to its inevitability. We can also see an enormous amount of evidence that the universe itself is quantum, even though we have no evidence that inflation itself behaves like a quantum field. If you merge these pieces, this unambiguously leads to the prediction that our universe would be only one of innumerable universes, all embedded in an ever-expanding, growing background. One universe is not enough. Although we can not detect it, the prediction of a multiverse is inevitable.


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