How likely is it that our universe will produce an Earth-like world?
Anywhere in the universe, we can see many similar objects, but each one is unique. Of all the galaxies, stars, and planets we know, no two are exactly the same, but each has its own unique history, nature, and composition. However, it is a compelling idea that given enough universes to work, eventually the particles in the universe will organize themselves in such a way that the same possibility—however unlikely—will occur many times at different times. Perhaps, given the concept of an infinite universe, there could be an infinite number of copies of every system we can imagine, including the Earth, in which each of us lives.
It was this idea, somewhere, probably an infinite number of copies of each of us, that gave rise to our modern concept of the multiverse. There may be different versions of us, and a small decision, outcome, or even a quantum measurement can lead to very different outcomes. While many people scoff at the idea that the multiverse is simply unscientific – after all, there is no way to see, test, or obtain information about any part of the universe beyond our finite observable universe – the truth is that the existence of the multiverse is rooted in science itself. In fact, if only two things are true:
The expansion of the universe that preceded and laid the great **, as we think, took place, and inflation, like all other realms in the universe, was essentially a quantum realm, following all the quantum rules that other quantum theories followed, then as a corollary of these ideas, the multiverse emerged. That's why physicists (although a few oppose it) overwhelmingly claim that the multiverse must exist.
credit: nasa/wmap science team
The story begins with the discovery of the expansion of the universe. Back in the twenties of the twentieth century, a large body of evidence showed that not only were the large number of spiral galaxies and elliptical galaxies in the sky actually entire galaxies, but that the farther such a galaxy was, the more systematically the light it emitted shifted to longer wavelengths. While a variety of explanations were initially proposed, they were all discarded by richer evidence until only one remained: that the universe itself is undergoing cosmic expansion, like a fermented piece of raisin bread in which bound objects like galaxies (such as raisins) are embedded in an expanding universe (like dough).
If today's universe is expanding, and the radiation in it is moving in the direction of longer wavelengths and lower energies, then this means that the universe of the past must have been smaller, denser, more uniform, and hotter. As long as any amount of matter and radiation is part of this expanding universe, the idea of a big ** will give rise to three clear and universal **:
a massive cosmic network of galaxies that grow, evolve, and aggregate over time;
A low-energy background blackbody radiation left over from the first formation of neutral atoms in the hot early universe;
The lightest elements – hydrogen, helium, lithium and their various isotopes – exist even in regions where stars have not yet formed, with a specific set of proportions.
credit: nasa/wmap science team
All three of these prophecies have been confirmed by observations, which is why the grand ** theory has taken hold supremacy as the main theory of the origin of our universe, while all other competing theories have been eliminated. However, the Big ** theory only describes what our universe looked like in its early stages; It does not explain why the universe has specific properties that we observe. In physics, if you know what the initial conditions of a system are and what the rules it follows, you can be extremely accurate** — within the limits of your computing power and the uncertainty inherent in the system — how it will evolve arbitrarily into the future.
Therefore, we can ask an important question: what initial conditions did the big ** need at the beginning to give us the universe we now observe? The answer is a bit surprising, but we found out:
There must be a maximum temperature that is significantly (at least about 1,000 times) below the Planck scale, where known laws of physics fail;
At the beginning of the universe, density fluctuated roughly the same at all scales (fluctuations on the small cosmic scale were a few percentage points smaller than on the larger cosmic scale);
The rate of expansion and the total density of matter and energy must be in almost perfect equilibrium: at least 30 significant figures are reached at the moment of the beginning of the great **, and the same initial conditions—the same temperature, density, and wave spectrum—must exist at all locations, even between two locations, between which it is impossible for a signal of the speed of light to pass the distance between them in the time elapsed after the great **;
The total entropy of the universe must have been much lower than it is today, trillions of times lower.
credit: e. siegel/beyond the galaxy
Whenever we are confronted with the question of initial conditions – basically, why our system started the way it necessarily started – we have only two options. We can resort to the agnostic, saying that things are the way they are because this is the only possible way (e.g., Lady Gaga's explanation, saying that it was only "born as such"), and we cannot further understand anything. However, we can also try a scientific approach: we can try to find a mechanism to build and create the conditions that we know we need. The second way is what physicists call "appealing to dynamics", where the mechanism we design must do three important things.
It must replicate the model it is trying to replace – in this case the hot big ** – that produces every success. These early building blocks must have to come out of whatever mechanism we propose.
It must explain the key observational fact that the big ** cannot explain: the initial conditions for the origin of the universe. These problems, those that cannot be explained in the big ** theory, must be explained by any new ideas.
It has to make something new different from the original (big) theory, and these have to lead in some way to an observable, testable, or measurable result.
The only theory we have that satisfies these three criteria is the theory of cosmic inflation, which has been unprecedentedly successful in all three areas.
credit: e. siegel (l); ned wright’s cosmology tutorial (r)
The basic idea of inflation theory is that before the universe was hot, dense, and full of matter and radiation, it was in a state dominated by the massive energy inherent in space itself: some kind of field or vacuum energy. However, unlike today's dark energy, which had a very small energy density (equivalent to about one proton per cubic meter of space), the energy density during inflation was enormous: about 1025 times that of today's dark energy. According to Einstein's theory of general relativity, it is the energy density that determines the rate of expansion, which means that during inflation, the rate of expansion is not only incredibly large, but also inexorable: as space continues to expand, the rate of expansion remains enormous.
This behaves very differently from the universe as we know it today. In an expanding universe full of matter and radiation, the volume increases while the number of particles remains the same, so the density decreases. Since energy density is related to the rate of expansion, the rate of expansion of the universe slows down over time.
But if the form of energy density is inherent to the space itself, then the energy density will remain the same over time and the rate of expansion will remain the same. The result is what we call exponential inflation, where after a short period of time, the size of the universe doubles, after that time, it doubles again, and so on. In an extremely short period of time – a fraction of a second – an area initially smaller than the smallest subatomic particle can be stretched to be larger than the entire visible universe today.
credit: e. siegel/beyond the galaxy
During inflation, the universe – regardless of its nature at the beginning of inflation – is stretched to a massive scale. This accomplishes a ton of things in the process, including:
stretching the observable universe, regardless of its initial curvature, makes it indistinguishable from flat;
take any initial conditions that exist in the areas that begin to expand, and stretch them so that they are now uniform throughout the visible universe;
take any quanta that existed in the region before the inflation, and quickly move them away from each other, reaching an arbitrarily low density;
produces tiny quantum fluctuations and stretches them throughout the universe so that they are almost the same on all distance scales, but on smaller scales (when inflation is coming to an end);
converts all "inflationary" field energy into matter and radiation, but only allows matter and radiation to reach maximum temperatures well below the Planck scale (but comparable to the inflationary energy scale);
A spectrum of density and temperature fluctuations is created that exists on a much larger scale than the cosmic horizon, and is adiabatic (constant entropy) rather than isothermal (constant temperature) everywhere.
This ultimately satisfies all three conditions we need to consider replacing the old theory with a new one. Inflation reproduces the success of non-inflationary thermal hyper**, provides a mechanism to explain the initial conditions of large**, and makes a series of new ** different from the onset of non-inflation. From the 90s of the 20th century until today, the inflation scenario is consistent with the observations and is different from the non-inflationary thermal maximum.
credit: e. siegel
Based on the properties that the universe possesses as we observe it today, in order to reproduce what we see, a minimum of expansion must occur in the past. This further suggests that in order to be successful, the skyrocket must meet certain conditions: those conditions that produce the **and after-the-fact** that we just mentioned. Perhaps the simplest and easiest model to understand is to think of it as a hill where as long as you are on the top of the mountain, you expand, but once you roll to the valley below, the expansion ends and converts energy into matter and radiation.
If you do, you'll find that your hill has a certain shape, or what physicists call "potential," to succeed in these areas that others don't have at all. The key to getting the required amount of swelling has to do with the summit: it needs to maintain a sufficiently flat shape over a sufficiently large area. To put it simply, if you think of the swelling field as a ball on top of a mountain, it needs to roll slowly for most of the time the swelling lasts, and only picks up speed and rolls quickly when it enters the valley, which is what causes the swelling to end. As scientists, we have quantified how slow the expansion needs to roll, which allows us to understand the shape needed for this potential. As long as the top is flat enough, inflation can serve as a viable solution to the origin of the universe.
credit: e. siegel/beyond the galaxy
So, what does the concept of the multiverse play a role in? This has to do with one aspect where we can't take the ball and mountain analogy too seriously: in fact, it's a purely classic view. The universe, at least in our understanding, is not purely classical, but quantum in nature. This means that, by its very nature, inflation, like all fields we know, should also be a quantum field. The quantum properties of the field tell us that many of its properties cannot be precisely determined, but rather have a probability distribution. And, like all time-dependent quantum systems, the longer the elapsed time, the greater the probability distribution will spread.
In other words, inflation is not rolling a spiky ball down a hill. Instead, what really rolls down the hill is a quantum probability wave function that can take all sorts of permissible values.
But as the ball rolls along the hillside, the universe is experiencing cosmic expansion, which means it expands exponentially in all three dimensions. If we take a cube of 1 1 1 and call it "our universe", then we can see that this cube expands during inflation. If it takes a short time for this cube to double in size, then it becomes a 2 2 2 cube, which requires 8 original cubes to fill. Letting the same amount of time pass, it turns into a 4 4 4 cube that requires 64 original cubes to fill. Let this time go through again, it is a cube of 8 8 8 and the volume is 512. After about 100 "doublings", we will have a universe with about 1090 primordial cubes, or a universe that has expanded in volume by about 1090 times.
credit: e. siegel/beyond the galaxy
That's the problem. If inflation is a quantum field, and the quantum field spreads over time, what happens when the "quantum ball" on the top of the mountain slowly rolls along the flat part of the mountain?
The answer is that the part of the wave function that propagates towards the end of the valley is more likely to roll into the valley itself. In these areas, the inflation is likely to end soon, where the field energy will be converted into matter and radiation, and what we know as the thermal great** will follow. The shape of this region on the boundary may be irregular, but certain regions seem to describe a part of the observable universe that we can see and enter. As long as there is enough inflation to reproduce the observational successes we see in the universe, it seems like a good description of our own cosmic history.
But what about the part of the wave function near the top of the mountain? Inflation there lasts longer, and we can think of these areas as being outside of areas where inflation ends quickly. What does this mean, as far as these areas are concerned:
The skyrocket is over, and the big ** will follow;
Compared to those countries where inflation persists, continues unabated, and even ends elsewhere?
credit: e. siegel/beyond the galaxy
When you work out the math formula for getting enough expansion before the hot big ** happens, that's where science tells us the multiverse is almost inevitable. We must require the universe to undergo enough expansion so that our universe can exist with the properties we observe. We also know that inflation has certainly persisted longer outside of the regions where it ended.
Now we're going to ask the big question, "What is the relative size of these areas?" "If we compare the following regions:
Inflation ends at a certain point;
For those areas where inflation has not ended after this period;
We found that the latter region, where the expansion continued, was exponentially larger (and still growing over time) compared to where the expansion ended and where the heat was large with it. Moreover, this disparity in size continues to worsen over time. Even if there are infinite number of areas where the surge ends, there will be more infinite areas where the boom continues. In addition, the various regions of the universe at the end of the universe – where the thermal boom occurs – will be disconnected by cause and effect, further separated by more expansive regions of space.
To put it simply, if every hot big ** takes place in a "bubble" universe, then these bubbles will never collide. Over time, what we end up with is more and more disconnected bubbles, separated by an eternally expanding space.
credit: ozytive/public domain
This is the multiverse, and why scientists have accepted it as the default. We have overwhelming evidence that the big ** is hot, and that the big ** starts with a series of conditions that have no factual explanation. If we add an explanation to it – the expansion of the universe – then the expanding space-time that forms and causes the big ** will make its own novel set**. Many of these are confirmed by observation, but others that are not observable are still present as a consequence of inflation.
One of them is that there are countless universes that are made up of disconnected regions, each with its own thermo-large**, and when you add them together, they make up the multiverse as we know it. This doesn't necessarily mean that different universes have different rules, laws, or fundamental constants, or that all the possible quantum outcomes you can imagine are happening elsewhere in the multiverse. It doesn't even have to mean that the multiverse is physically real, as it's a ** that we can't verify, verify, or falsify. But if that's if:
The inflation theory is a good one, and the data proves it;
Our universe is quantum in nature, and all the evidence suggests that it is;
Then the universe is inevitable. You may not like it, and you may really not like the idea of some physicists abusing it, but until a better, viable alternative boom comes along—until this alternative boom has cleared the same three theoretical hurdles—the multiverse will be here to stay.