We Have No Idea: A Guide to the Unknown Universe

by Jorge Cham

But while thinking of space as a dynamic thing with physical properties and behaviors might explain weird phenomena like space bending and stretching, it only leads to more questions. For example, you might be tempted to say that what we used to call space should now be called physics goo (“phgoo”) but that this goo has to be in something, which we could now call space again. That would be clever, but as far as we know (which to date is not very far), the goo does not need to be in anything else. When it bends and curves, this is intrinsic bending that changes the relationships between parts of space, not the bending of the goo relative to some larger room that it fills. But just because our space goo doesn’t need to sit inside of something else doesn’t mean that it is not sitting inside something else. Perhaps what we call space is actually sitting inside some larger “superspace.”52 And perhaps that superspace is like an infinite emptiness, but we have no idea. Is it possible to have parts of the universe without space? In other words, if space is a goo, is it possible for there to be not-goo, or the absence of goo? The meaning of those concepts is not very clear because all of our physical laws assume the existence of space, so what laws could operate outside of space? We have no idea.

Scientists have done the equivalent of measuring triangles drawn in our three-dimensional universe by looking at a picture of the early universe (remember the cosmic microwave background from chapter 3?) and studying the spatial relationship between different points on that picture. And what they found was that the triangles they measured correspond to those of flat space. The other way in which we know that space is basically flat is by looking at the thing that causes space to curve in the first place: the energy in the universe. According to general relativity, there is a specific amount of energy in the universe (energy density, actually) that will cause space to bend in one direction or the other. It turns out that the amount of energy density that we can measure in our universe is exactly the right amount needed to cause the space that we can see to not bend at all (within a margin of error of 0.4 percent).

All the mass and energy in the universe is what gives space its curvature (remember that mass and energy distort space), and if we had just a little bit more mass and energy than we have right now, space would have curved one way. And if we had just a little bit less than we have right now, space would have curved the other way. But we seem to have just the right amount to make space perfectly flat as far as we can tell. In fact, the exact amount is about five hydrogen atoms per cubic meter of space. If we had had six hydrogen atoms per cubic meter of space, or four, the entire universe would have been a lot different (curvier and sexier, but different).

Time is how we relate the “now” we have now with the “now” we had before. Whatever we are feeling now is what we call the present, but the present is fleeting and ephemeral: there’s no way to savor it or stretch it as you might a tasty bite of chocolate cake. Every moment slips immediately from the intense experience of the present to a fading memory of the past.

This has some chilling consequences: because entropy only increases, eventually, very, very, very, very far in the future, the universe will reach some maximum amount of disorder, which goes by the cozy sounding name of “the heat death of the universe.” In this state, the whole universe will be at the same temperature, which means everything will be completely disordered, with no little useful pockets of ordered structure (like humans). Until then, creating local pockets of order by making compensating pockets of disorder is only possible because the universe has not yet reached maximum disorder, so there is still wiggle room.

Albert Einstein’s theory of relativity changed everything by tying together space and time into one concept: space-time.65 Einstein famously predicted that moving clocks run more slowly. If you take a trip to a nearby star by traveling close to the speed of light, you will experience less time than those left back on Earth. This doesn’t mean that you feel time moving slowly, like in The Matrix. It means that people and clocks back on Earth will measure more time passing than the clocks on your spaceship. We all experience time the same way (at the normal one-second-per-second rate), but our clocks disagree if we are moving at high speeds relative to each other.

The most important rule is that this speed limit has to apply to anyone measuring any speed from any point of view. When we say that nothing can be observed to go faster than the speed of light, we mean nothing, no matter what perspective you have on it. So let’s do a simple thought experiment. Suppose you are sitting on your couch and you turn on a flashlight. To you, the light from that flashlight is zooming away from you at the speed of light. But what if we strapped your couch to the top of a rocket and the rocket blasted away and started to move really fast? What happens now if you turn on your flashlight? If you point the flashlight toward the front of the rocket, does the light move at the speed of light plus the speed of the rocket? We’ll spend more time on these ideas in chapter 10, but the point is that in order for the light from that flashlight to appear to be moving at the speed of light to all observers (you in the rocket and the rest of us on Earth), something has to be different, and that something is time.

It’s hard to swallow the idea that different people can experience time differently because we like to think that there’s an absolute true history of the universe. We imagine that in principle someone could write down a single (very, very long and mostly superboring) story about everything that happened in the universe so far. If this existed, then everyone could check it against their experience, and barring honest mistakes and fuzzy vision, the story would agree with what people saw. But Einstein’s relativity makes it clear that everything is relative, and even the description of events in the universe depends on who is recording that description.

Some physicists are convinced that the “arrow” of time is determined by the rule that entropy has to increase or that the direction of time is the same thing as the direction of increased entropy. But if that’s true, what happens when the universe reaches maximum entropy? In such a universe, everything will be in equilibrium and no order can be created. Will time stop at that point or have no meaning? Some philosophers speculate that at this moment the arrow of time and the law of increasing entropy could reverse themselves, leading the universe to shrink back to a tiny singularity. But this is more in the category of late-night herb-inspired speculation than actual scientific prediction.

Our brain creates a three-dimensional model of the world inside our head because that is what has proven useful for survival on Earth. That doesn’t mean that we are capable of perceiving the full nature of our environment. On the contrary, we are shockingly blind to features of our universe that may be irrelevant to daily survival but are crucial to understanding the fundamental nature of reality.

If there is a fourth spatial dimension (other than time), why do we never see it? Well, we know that it has to be mostly irrelevant and useless to our survival in order to explain why we can’t control or perceive our motion in that dimension. We also know that if it was a linear dimension like the other (regular) dimensions, we would probably have noticed by now. Even if we can perceive in only three dimensions, we would notice things appearing and disappearing if they move toward and away from us in this other dimension. So we can be pretty confident that there’s no fourth spatial dimension that is like the other three. If there is a fourth dimension, it has to be sneaky in some way that makes it hard for us to see it. One possibility is that all the force and matter particles we know about simply can’t move through these extra dimensions of space. This would prevent objects from sliding in the fourth dimension and prevent energy (via force particles like photons) from dispersing into those additional dimensions. Could these impenetrable dimensions exist? Yes, but if they are truly impenetrable by any known particles, then we have little chance of discovering them or exploring them.

So what happens if there are two extra loop dimensions one centimeter in size and only gravity can spread through those dimensions, not other forces? For objects less than one centimeter apart, the force of gravity would dilute into the extra dimensions and go down in strength very quickly. For objects greater than one centimeter apart, the extra dimensions wouldn’t play a role. This would explain why gravity feels so weak to us: it is actually just as strong as the other forces for short distances, but once you go farther than one centimeter, most of it has already been diluted in the other dimensions.

After much work in the past few years, physicists were able to measure how the force of gravity changes with distance at a scale of one millimeter. They found that, at least down to one-millimeter distances, the force of gravity still behaves just as it does for large scales. This doesn’t mean that extra dimensions don’t exist. It just means that if they exist, they are smaller than one millimeter in size.

Well, if the particle is moving in the extra dimensions, it means it has momentum in these other dimensions, which means it has extra energy. But since the particle is not moving in our dimensions, we would experience that extra energy as extra mass (remember that mass and energy are the same according to Einstein). In other words, you would notice if a particle was moving in extra dimensions because it would be heavier than a particle that wasn’t. This is how we can use particle colliders to detect extra dimensions. If we smash particles together, and one day we see a particle that looks, for example, exactly like an electron (same charge, same spin, etc.) except it is much heavier, we could reasonably suspect that it is actually an electron that is also moving in other dimensions.

Along the way, though, some fun candidates have emerged. One of them is string theory, which suggests that the universe is not built out of zero-dimensional point particles but instead is constructed from tiny one-dimensional strings—not tiny like minimarshmallow tiny, but tiny like 10−35 meters tiny. The theory says that these strings can vibrate in lots of ways, and each vibrational mode corresponds to a different particle. When you look at the strings from far enough away (a resolution of only 10−20 meters) they look like point particles because you can’t see their true stringy nature. One feature of this theory is that the math that describes it is much simpler and more natural if you have additional spatial dimensions. There are different flavors of string theory, and each predicts a different number of dimensions in our universe. Superstring theory prefers to work in a universe that has ten spatial dimensions. Bosonic string theory likes a universe with twenty-six dimensions.

So far, the experiments measuring gravity at short distances have seen nothing unexpected, and the LHC has not discovered any black holes or particles moving in other dimensions. In other words, we have no evidence that this string-theory picture of the world is correct or that gravity moves in extra dimensions. So far, we really have no idea how many spatial dimensions there are in our universe.

There is little doubt that this speed limit is real. The physics that describes it—relativity—has been tested repeatedly to very high precision. It is a basic principle woven into the fabric of modern physics theories. If this speed limit was not a fact of life, we would almost certainly have noticed by now. No matter what you do, who you know, or what you are, you cannot go faster than 300 million meters per second.

Now we have three conflicting reports: Bertha sees the light hit both targets at once; you see one of the targets get hit first; and Larry, who is probably surprised to find you out in space doing physics experiments, sees the other target get hit first. And you’re all correct! Not only do we have to accept that there is a top speed in the universe, but we have to give up the idea that events happen at the same time for everyone everywhere. No longer can we even assume the very reasonable-sounding idea that there is a single agreed-upon description of what happens in our universe. It all depends on which pet you ask!

In 1887 two scientists named Michelson and Morley performed an experiment somewhat similar to our hypothetical hamster situation (albeit without the hamster). They shot a beam of light and split it into two perpendicular directions. Then they measured if the two resulting beams took the same amount of time to bounce off a mirror and return to their starting point. Like Bertha the hamster, they found that light took the same amount of time to travel in any direction. And because the Earth is moving at some unknown speed relative to the rest of the universe, they concluded that the speed of light is always the same no matter how fast or slow your relative motion is. From this, we can conclude that nothing can go faster than light because it would result in situations where causality is broken (like Larry seeing the photon hit the target before it leaves the flashlight). And breaking causality is not a minor thing even for first-time offenders. The universe tends to take it pretty seriously.

The question of why the universe is causal is very difficult even to discuss, not to mention answer in a satisfactory way. Causality is built so deeply into our pattern of thinking that we can’t just step outside of it and consider a universe without it. We can’t use logic and reasoning to consider a universe without logic and where reasoning is impossible or inappropriate. This is certainly a deep mystery, and since science assumes causality and logic, it is possibly a question beyond the power of science to answer. It may be one that we never solve, or it could be tied inextricably to the thorny questions of consciousness.

The more tractable question is: Why this particular maximum speed? None of our theories gives any reason for choosing one value over another. A causal universe with a faster speed of light would be less local than ours; a causal universe with a slower speed of light would be hyperlocal. But each of those universes would still work, and any setting of the speed of light is allowed in physics. It just so happens that we have measured it in our universe and found it to be 300 million meters per second: very fast compared to human experience but very slow compared to the distances one has to travel to move between the stars or the galaxies.

When it comes to the future, predictions are difficult. We can extrapolate based on 14 billion years of history; that seems like a solid game to play, but it implicitly relies on the assumption that the universe will keep working the same way in the future that it has in the past. That is pure assumption—we know that the universe has had multiple radically different periods in its past (pre–Big Bang, Big Bang inflation, current era of expansion) so predicting that the universe will not change in the future smacks of overconfidence.

If you ever wondered where the aurora borealis or aurora australis (i.e., the northern and southern lights) come from, they are the glow that comes from the stream of cosmic rays diverted by the Earth’s magnetic field to the North and South poles.

The highest-energy particle we have seen hit the Earth clocked in at over 1020 eV, which is almost two million times more energetic than the LHC’s fastest particles. The record-setting space particle was going so fast that physicists nicknamed it the Oh-My-God particle. And when jaded physicists start sounding like flabbergasted teenagers, you know they are impressed.

But here is the mind-blowing fact about particles this high on the energy spectrum: we don’t know anything in the universe that is capable of making such high-energy particles. That’s right, we are being bombarded by millions of extremely high-energy particles on a daily basis, and we have no idea what could be creating them. If you ask astrophysicists75 to estimate what the highest speed a particle anywhere in space could ever have (based on what we know right now), they will (a) thank you for asking them such a cool question, (b) come up with crazy situations like particles surfing on exploding supernovas or black holes swinging particles around like slingshots, and (c) still come up short. Based on all the things we know about in the universe right now, the highest energy a particle could have in space is about 1017 eV, which is still more than a thousand times less energetic than the ones hitting the Earth every day.

The difficulty with nailing down where these cosmic rays are coming from is that the Earth is a pretty big target. Even though millions of them hit the Earth on a daily basis, actually placing a detector and catching them at the right time is tricky. We said earlier that hundreds of these hit the Earth every second, and we didn’t lie, but the Earth is a very big place. So the more relevant number is how many cosmic rays hit an area the size of a typical detector, which is counted in square kilometers. Particles at LHC energies (1013 eV) arrive at Earth at a rate of one thousand per square kilometer per second. Particles at absurd energies (1018 eV) arrive more rarely, at a rate of one per square kilometer per year. But the prize jewels, particles above 1020 eV, are much rarer. They arrive at a rate of roughly one per square kilometer per millennium

But even though space seems very clear and empty to us, for an electrically charged high-energy particle it is actually like making your way through a crowded train station. The light that makes up the baby picture of the universe, the cosmic microwave background, fills the universe with a kind of photonic fog. Cosmic rays interact with this fog and get slowed down fairly quickly. A particle at 1021 eV can only go for a few million light-years before it gets slowed down to energies below 1019 eV or so.

When a super-high-energy particle hits the top of the atmosphere, it (thankfully) doesn’t make it all the way down to the Earth’s surface without banging into a lot of air and gas molecules. When a 1020 eV particle first hits a molecule in the atmosphere, it breaks up into two particles each with half that energy. Those two particles then hit other molecules, creating four particles with a quarter the energy, and so on. Eventually, you get trillions of particles with 109 eV of energy washing over the surface of the Earth in a flash. This shower of particles is typically about a kilometer or two wide and consists mostly of high-energy photons (gamma rays), electrons, positrons, and muons. Such a wide and powerful shower is how we know that a super-high-energy particle hit the Earth.

When two particles get close enough to each other, they don’t actually touch, because they don’t actually have surfaces. Instead, you can think of their quantum mechanical features as merging and the two particles as disappearing into another form of energy, in most cases a photon. From this energy, other kinds of particles can emerge, depending on the amount of energy you smooshed together. This is exactly what happens when we smash particles at the Large Hadron Collider to create new kinds of particles from ordinary everyday particles.

The other thing to keep in mind is that when particles interact (or smoosh, as it were), certain things are conserved. For example, we have observed that electric charges are never created out of nothing and that they are never destroyed. The total electric charge of the particles before and after the smooshing has to be the same. Why is that? We don’t know. We don’t understand why these rules apply; we simply see these patterns in experiments and incorporate the rules into our theories.

Maybe we have it all wrong. What if there are equal amounts of matter and antimatter in the universe, but they’re all separated into different regions? The Earth and its immediate neighborhood are definitely made of matter, but what if there are other neighborhoods out there made of antimatter? Matter and antimatter are so similar that we can’t tell if a distant star is made of matter or antimatter just by looking at the light that comes from it. Both types of star would have the same nuclear reactions and generate photons the same way with the same energies.

Astronomers have expanded this search, looking for entire solar systems made of antimatter in our galaxy. So far, we have not seen the bright flashes of photons you would expect to find at the interface between the matter and antimatter regions. They have even considered the possibility of entire galaxies made of antimatter. But if any existed, we would see the space between the matter galaxies and the antimatter galaxies light up from the annihilation of particles streaming from both types of galaxies. Currently, astronomers have pushed this technique far enough that they are confident that our entire cluster of galaxies is all made of matter.

Does every particle have an antiparticle? So far, every particle that has electric charge has a distinct antiparticle. But the answer is not so clear for neutral particles. For example, there is no distinct antiversion of the photon (which has no charge), i.e., an antiphoton. Some would say that the photon is its own antiparticle, which seems more like avoiding the question than answering it (i.e., does being your own best friend mean you have no friends?).

Up until recently, it would have been the subject of pure speculation to talk about the early moments of the universe. After all, how do you study something that happened 14 billion years ago? And more important, how do you do experiments to verify your theories? It’s not like we can rerun the Big Bang for our scientific convenience. Fortunately for us, the Big Bang left a big mess. There are all kinds of clues and bits of rubble for us to analyze in detail. And in the last half century, our technology, mathematics, and physics theories have progressed to the point where we have started to move the question of what happened during the Big Bang to the scientific category. We can test theories about the Big Bang as long as they make predictions about things we can find in the rubble; that counts as a prediction even if the events happened a long time ago.

The idea for the Big Bang came in the early parts of the twentieth century when scientists discovered that all the galaxies we could see were moving away from us, which meant that the universe was expanding. Cosmologists tried to make sense of this observation by playing with Einstein’s new equations for general relativity, which describe how space and time and gravity work, and found that these equations could easily describe an expanding universe. But they also found something odd. If you project that expansion backward in time as far back as possible, then the equations predict something that is almost totally alien to our intuition: the entire universe contained in a single point, a singularity, where the mass is enormous, the volume zero, the density infinite, and the parking impossible

The predictions of general relativity are expected to fail when masses get so dense that quantum mechanical effects become important. Like during the early moments of the universe when things really were squished down into incredibly small spaces. Sometimes you can’t take a theory all the way to its logical conclusion. Imagine if you measured how quickly your cats were growing over time and then tried to extrapolate their growth backward in time. If you only went by size, you might end up with the prediction that your pets were once infinitesimal kitten singularities or, if you ignore the physical boundaries completely, that they once had negative size. That would be . . . catastrophic.

Since we don’t have a quantum theory of relativity, we don’t really know how to calculate or predict what was happening in the very early universe. This means that the picture of the Big Bang starting with a singularity is probably not accurate; in those early moments quantum gravity effects dominated, but we have no idea how to describe them.

This is the “observable universe.” Everything you can see has to be inside a sphere centered at your head whose radius is the distance that light can have traveled since the universe was born. If a point on the surface of that sphere sent you a photon at the earliest possible moment, it will only be arriving now; that is what defines the edge of our vision. Light from stars, planets, and kittens outside that sphere will not yet have reached us, so no telescope can see them. Even a superbright supernova or a giant planet-size pink kitten would be invisible to us if it was outside that sphere. Oddly enough, this concept has returned us to our ancient place as the center of the observable universe except that we are each at the center of our own observable universes!

So if all the things in the universe started from a tiny but finite quantum dot and are simply moving through space away from the Big Bang, our horizon should expand faster than the stars and kittens of the universe can move away from us, giving us a longer and longer view. Very quickly, if not already, our horizon would be larger than the entire universe. What would that look like? When our horizon is bigger than the universe, it means that we can see beyond the point where there are no more stars (or were no stars, since what we see happened a long time ago). We’d be looking at a spot that had nothing: an end to the stars. But in every direction we look, we see no end of stars. The universe is still bigger than our horizon, even though 14 billion years have passed since the beginning of the universe. Clearly, there is something not quite correct about this idea that everything in the universe started from a small blob and simply moved outward through static space.

We can measure the temperature of the CMB photons that are hitting the Earth on one side and compare that to the temperature of the photons hitting the Earth from the other side. And what we find is a little surprising: the temperature is the same (about 2.73 K) no matter which direction you look! It seems unlikely that we are standing at the exact center of a microwave-reheated universe, so we can only conclude from our measurement that the entire universe is at the same even temperature. That is, the universe is more like a warm bath that’s been sitting there invitingly for a while rather than a freshly nuked pastry.

In its early days, the universe was much hotter and denser than it is today. Back then, the universe was too hot even for atoms to form, leaving all matter in a state of floating ions called plasma. Electrons whizzed around freely, having too much energy and too much fun to be committed to a single positive nucleus. But as the universe cooled, there was a brief period when this ceased to be true: the temperature dropped enough that the charged plasma turned into neutral gas, and electrons started to orbit around protons to form atoms and elements. During this transition, the universe went from being opaque to being transparent. Back in the plasma phase, photons couldn’t get very far without bumping into the freely moving electrons and ions. But once the electrons and protons (and neutrons) formed neutral atoms, it was much rarer for photons to interact with them, so the photons could move around more freely. To the photons, the foggy universe suddenly became crystal clear. And because the universe has mostly gotten cooler since then, most of those photons are still flying along untouched. These are the photons that we detect when we measure the cosmic microwave background radiation, and the curious thing is that the temperature of these photons seems to be the same everywhere.

So the universe is too big and too smooth for it to have come from a Big Bang in which everything simply moved through space starting from a small blob. If we had written this book thirty years ago, this might be one of the great mysteries. Today, there does exist a compelling but totally crazy-sounding explanation. Are you ready? What if, a few moments after the universe was created, there was a period of about 0.00000000000000000000000000000001 seconds in which the fabric of space-time itself expanded by a factor of about 10,000,000,000,000,000,000,000,000—at a rate faster than the speed of light?

This expansion of space was very dramatic: the universe got bigger by a factor of more than 1025 in less than 10−30 seconds. After inflation ended, the universe kept expanding, first at a much slower rate and then more recently at a faster rate due to dark energy. Now the observable universe has a bit of a chance to catch up because it’s still expanding at the speed of light. But how much of the universe is still way beyond the observable universe for us to see? We have no idea, but that’s the topic for the next chapter.

Solving the smooth-photon problem means finding a way for those early photons (the ones coming from different ends of the universe) to have mixed so they could even out in temperature; this can happen only if—at some time in the distant past—those photons were much closer to one another than the current rate of expansion predicts. Inflation solves this problem by saying that the photons were indeed closer together at some point before the rapid expansion of space-time. Before inflation, the universe was small enough that there was time for all those photons to get to know each other and equilibrate, thus getting to the same temperature. Once inflation happened, those photons got pulled apart to distances that to us seem impossibly far for them to have the same temperature. It just seems to us in the present day that they are too far apart to have talked to one another, but before inflation, they were plenty close together.

How, you might ask, can we verify something that happened 14 billion years ago? Well, the theory of inflation predicts specific signatures in the tiny ripplings of the cosmic microwave background that we should see today, and some of those signatures seem to be present in experimental measurements of the CMB. Of course, this doesn’t mean that we know inflation is real because there are other theories that also predict such wiggles, but it lends weight to it. In fact, this is also how we know that the universe started about 14 billion years ago. From those ripples, we can estimate the proportions of matter, dark matter, and dark energy in our universe, and we can combine them in a model with the rate of expansion of the universe. This model tells us the age of the universe.

What caused the Big Bang? And what happened before it? This question made sense when we thought about the Big Bang as a specific moment when the universe was a tiny dot, the clocks all read t = 0, and things started explosively from that first instant. But now we have replaced the tiny dot with a fuzzy quantum blob (maybe small, maybe infinite), and the explosion has been replaced with inflation followed by dark-energy-fueled expansion. So the question still has meaning, but we have to first rephrase it in our new context. Instead of asking what came before the Big Bang, we should ask: Where did the quantum inflating blob come from? Did that blob lead inevitably to a universe like ours, or could it have been different? Could the blob happen again? Has it happened before? The answer is that, as usual, we have no idea.

Not every question has a satisfying answer because not every question is well posed. That might be the case for questions such as “What happens after you die?” because it depends on whether there is still a “you” after “you” die. Similarly, the question of “Why doesn’t my cat love me?” might be ill posed because we don’t even know if cats can love. Even crisp mathematical questions can fall in this category. Stephen Hawking has suggested that asking “What came before the Big Bang?” is like asking “What is north of the North Pole?” At the North Pole, every direction you walk points south, and there is no more northness. This is just a feature of the geometry of the Earth. If space-time was created at the moment of the Big Bang, then it’s possible that the geometry of space-time means that there is no satisfactory answer to the question of what came before (i.e., there is no “before”).

As far as we can see, the universe seems to follow physical laws, and so even the creation of the Big Bang should be describable in such terms. But it’s possible that from our vantage point inside space-time we don’t have access to the information necessary to learn what came before it. Such a cataclysmic event might have destroyed any information about what happened before, leaving no evidence for us to discover. That is very unsatisfying, but there is no rule that all of science’s answers will make us feel good.

Another possibility is that the weird stuff with negative pressure expands rapidly, and as it expands, it creates more of this weird stuff. And even though the weird stuff decays into normal matter, it’s possible that it doesn’t decay fast enough. If more new weird stuff is created faster than it can decay into normal matter, then the result is that the universe will continue to inflate forever. Some parts of it will decay, but this will be overwhelmed by the creation of new inflationary stuff, which, if this theory is true, is continuing to inflate right now. What happens in those spots where it decays? Each spot represents the end of the Big Bang in that part of space and the start of a slowly expanding universe of normal matter. Each of these spots can form a “pocket universe” just like the one around us. Because the inflation continues forever, multiple universes are constantly being created. If inflation continues to create space faster than light can travel through it, then the inflationary stuff between the pocket universes will grow too quickly to allow these universes to ever interact with one another.