Edwin Powell Hubble was an American astronomer who changed our understanding of our universe and our place in it. During a series of cold nights in October 1923, Hubble observed the night sky using the powerful Hooker telescope, looking for clues about the mysteries of our universe.
This is where he first spotted a star flaring up in a nova in the M31 nebula in the constellation of Andromeda. But there was something wrong. The astronomical object M31 appeared to be much further away than was believed possible, sitting a staggering 2.5 million light-years away.
This was strange because, at the time, astronomers had estimated that our galaxy was only about 200,000 light-years across. Using the help of his "computer" (someone tasked with examining photographic plates in order to measure and catalog the brightness of stars), the astronomer Henrietta Swan Leavitt, Hubble would later discover that M31 was a galaxy, which would eventually be dubbed the Andromeda Galaxy.
This discovery demonstrated that there were galaxies outside our own. Since then. astrophysicists have come to understand that there may potentially be trillions of galaxies in our universe.
It is no secret that our universe is unimaginably large, but just like there are many galaxies in our universe, could the same be true for the number of universes "out there"?
Welcome to the concept of the Multiverse.
You are probably familiar with the idea of a multiverse. Aside from time travel, it has to be one of science fiction's favorite tropes.
Though, at times it may seem like merely a fascinating plot device, like a lot of science fiction, some of the ideas behind it are based on real science. However, the Multiverse theory is not as straightforward as you think, and in some cases might be even stranger than in the movies.
Let's find out why.
All aboard for a journey to the edge of the universe
Before we dive a little deeper into the nuts and bolts of the idea of the Multiverse, let's first conduct a little thought experiment.
Imagine Elon Musk has given you the ability to live 1000 years into the future. Thanks to his Neuralink device, you have left your human body for an eternal robotic one.
Death is a thing of the past, and you can instantly 3D-print new robotic parts when things become faulty. With your unlimited life at hand, you decide to hop into an interstellar spaceship and journey to the edge of the universe.
After traveling billions of years, (or maybe less), you finally reach the end of the universe.
What do you find? Would you see that there is just much more universe to be discovered? Or, would you fly out of our own universe into another? Would you be surrounded by a host of other "bubble" universes, each just as big as our own? Or, depressingly, would you find absolutely nothing?
Some scientists believe our universe might just be the tip of the iceberg.
What exactly is the multiverse?
So, what is the Multiverse? A hint is somewhat in the name.
Think about the word "universe." When people use this term, they tend to refer to all that exists.
As the European Space Agency explains, "the Universe is everything we can touch, feel, sense, measure or detect. It includes living things, planets, stars, galaxies, dust clouds, light, and even time. Before the birth of the Universe, time, space, and matter did not exist."
Seems simple enough, however, this definition belies the true scale of things we are talking about here. We can probably never really be sure, but current estimates are that the observable universe has a radius of at least 46 billion light-years. (A light-year is the distance light travels in one year – about 9.46 trillion km.)
That's is unfathomably huge, and we can never hope to ever observe the full extent of our universe – let alone ever travel across it. For all intents and purposes, the universe will only ever be the extent of our knowledge of the cosmos (assuming there is something beyond the limits of our universe).
However, this has not stopped some scientists from thinking a little grander – there might just be more, a lot more, out there.
Though the idea of a multiverse is very controversial, at its core, it is a relatively simple concept to wrap your head around. In short, our entire universe is only a small part of a much larger number of universes. The additional universes are part of an unobservable area of space-time, with our universe just one of many of them. Some have speculated that each universe could also have its own separate laws of physics.
However, the concept of the multiverse is not a scientific theory in and of itself. Instead, it is a theoretical construct based on our current understanding of the laws of physics. If you have an inflationary universe governed by quantum physics, you may well be bound to wind up with a multiverse.
Under one interpretation of a multiverse, known as the "inflationary multiverse," the "Big Bang" didn't stop with just our universe. Our little piece of the cosmos was born, "budded off," so to speak, and then the expansion from the "Big Bang" just kept going, spawning other universes as it went.
If true, this process could continue forever, creating, for all intents and purposes, an infinite number of other universes outside of your own.
"Together, these cosmic islands form what scientists call a 'multiverse.' On each of these islands, the physical fundamentals of that universe—like the charges and masses of electrons and protons and the way space expands—could be different," the Smithsonian Magazine explains.
But the theory of a multiverse also has some big problems.
For example, it doesn't predict anything we have observed and can't explain without it, and it also doesn't predict anything definitive we can go and look for. In essence, it is purely theoretical – we can never really hope to prove it quantitatively.
This is important, as the scientific method relies on both ideas and observed measurements. But, there are some clues that might help support the hypothesis.
The "Big Bang" might be the first clue
You are probably very familiar with the concept of the "Big Bang."
Astronomers know the universe is expanding, they can measure the distance of galaxies from us, and how fast they appear to be moving away. The farther away they are, the faster they appear to recede, which General Relativity tells us means the universe is expanding. And if the universe is expanding, that means it was smaller and denser in the past, as well as more uniform and hotter. This leads us to the Big Bang, which has been postulated to have occurred some 13.8 billion years ago.
However, what happened before the Big Bang? If we go back far enough in time, we find that there are things that can be observed in the universe that the Big Bang can't explain.
So, astronomers have come up with another theory - cosmic inflation. This tells us that before the Big Bang, space was filled with energy. That energy caused space to expand very rapidly. When inflation ended, the energy was converted into matter and energy, which then led to the Big Bang.
If that is how our universe came into existence, could a similar process have formed others? Maybe many more?
The universe may have siblings
This is where Hollywood takes most of its script ideas. The supporting theory of the multiverse centers around the idea that multiple universes follow the physics of quantum mechanics. Dubbed the daughter universe theory, this is an idea driven by the laws of probability. A fun thought experiment would be to imagine all of the choices that you made today. Now sit back and think about just one of those choices.
Why did you make that choice? What other choices could you have made? With a potentially infinite amount of options out there, the daughter theory postulates that within the Multiverse, there are infinite versions of yourself, each making different choices. In one universe, you could have a different job, blue hair, be born in a different country, and so on.
In essence, this theory suggests that all possible outcomes do occur – just in different universes. Spooky.
Perhaps we are just living in a simulation?
We have been down this rabbit hole before. Aside from talking about multiple universes, people love to discuss the idea that we are living in a simulation.
Swedish philosopher Nick Bostrom published a paper called The Simulation Argument, that would go on to question the nature of our reality. A controversial idea, Bostrom's simulation theory centers around three assumptions, at least one of which needs to be true:
1. All human-like civilizations in the universe go extinct before they develop the technological capacity to create simulated realities;
2. If any civilizations do reach the phase of technological maturity where they can create simulated realities, none of them will bother to run simulations; or
3. Advanced civilizations would have the ability to create many, many simulations, and that means there are far more simulated worlds than non-simulated ones.
Bostrom argued that although we can’t know for certain which of these options is true, they’re all possible, and the third option might be the most probable outcome.
If you want to learn more about simulation theory, be sure to stop by here.
However, some people have taken this idea even further. What if our universe is currently being simulated alongside many other universes? The thinking goes that, if our future ancestors do possess the power to simulate our universe, what is stopping them from simulating multiple universes at once? The mind boggles.
There might not be parallel universes out there
As previously mentioned, one popular idea to come out of multiverse theory is the concept of parallel universes. However, most astrophysicists are not on board the parallel universe train.
In one of many examples, Astrophysicist Ethan Siegal has been vocal about the limitations of the theory. He does believe that space-time could possibly go on forever. But, are there alternate realities out there similar to our own? Not so much.
According to Siegal, "Even setting aside issues that there may be an infinite number of possible values for fundamental constants, particles, and interactions, and even setting aside interpretation issues such as whether the many-worlds-interpretation actually describes our physical reality."
"The fact of the matter is that the number of possible outcomes rises so quickly — so much faster than merely exponentially — that unless inflation has been occurring for a truly infinite amount of time, there are no parallel universes identical to this one."
The Multiverse hypothesis is popular, but we may never be able to prove it
Multiverse theory appears everywhere in popular entertainment and even plays a role in the epic finale of the Marvel Infinity Saga. You see it in video games, like the Final Fantasy series, and, of course, the legendary game Half-Life.
In reality, there are plenty of researchers who are considering or have considered the theory, like the late Stephen Hawking. Though, his idea of a multiverse is much simpler. In Hawking's final published study, he stated, "We are not down to a single, unique universe, but our findings imply a significant reduction of the multiverse, to a much smaller range of possible universes.”
While we may never be able to actually test the hypothesis to any satisfactory level, it is a fun thought experiment nonetheless. Perhaps one day we'll discover a way to find and explore them.