The questions are as big as the universe and (almost) as old as time: Where did I come from, and why am I here? That may sound like a query for a philosopher, but if you crave a more scientific response, try asking a cosmologist.
This branch of physics is hard at work trying to decode the nature of reality by matching mathematical theories with a bevy of evidence. Today most cosmologists think that the universe was created during the big bang about 13.8 billion years ago, and it is expanding at an ever-increasing rate. The cosmos is woven into a fabric we call space-time, which is embroidered with a cosmic web of brilliant galaxies and invisible dark matter.
It sounds a little strange, but piles of pictures, experimental data and models compiled over decades can back up this description. And as new information gets added to the picture, cosmologists are considering even wilder ways to describe the universe—including some outlandish proposals that are nevertheless rooted in solid science:
The universe is a hologram
Look at a standard hologram, printed on a 2D surface, and you’ll see a 3D projection of the image. Decrease the size of the individual dots that make up the image, and the hologram gets sharper. In the 1990s, physicists realized that something like this could be happening with our universe.
Classical physics describes the fabric of space-time as a four-dimensional structure, with three dimensions of space and one of time. Einstein’s theory of general relativity says that, at its most basic level, this fabric should be smooth and continuous. But that was before quantum mechanics leapt onto the scene. While relativity is great at describing the universe on visible scales, quantum physics tells us all about the way things work on the level of atoms and subatomic particles. According to quantum theories, if you examine the fabric of space-time close enough, it should be made of teeny-tiny grains of information, each a hundred billion billion times smaller than a proton.
Stanford physicist Leonard Susskind and Nobel prize winner Gerard ‘t Hooft have each presented calculations showing what happens when you try to combine quantum and relativistic descriptions of space-time. They found that, mathematically speaking, the fabric should be a 2D surface, and the grains should act like the dots in a vast cosmic image, defining the “resolution” of our 3D universe. Quantum mechanics also tells us that these grains should experience random jitters that might occasionally blur the projection and thus be detectable. Last month, physicists at the U.S. Department of Energy’s Fermi National Accelerator Laboratory started collecting data with a highly sensitive arrangement of lasers and mirrors called the Holometer. This instrument is finely tuned to pick up miniscule motion in space-time and reveal whether it is in fact grainy at the smallest scale. The experiment should gather data for at least a year, so we may know soon enough if we’re living in a hologram.
The universe is a computer simulation
Just like the plot of the Matrix, you may be living in a highly advanced computer program and not even know it. Some version of this thinking has been debated since long before Keanu uttered his first “whoa”. Plato wondered if the world as we perceive it is an illusion, and modern mathematicians grapple with the reason math is universal—why is it that no matter when or where you look, 2 + 2 must always equal 4? Maybe because that is a fundamental part of the way the universe was coded.
In 2012, physicists at the University of Washington in Seattle said that if we do live in a digital simulation, there might be a way to find out. Standard computer models are based on a 3D grid, and sometimes the grid itself generates specific anomalies in the data. If the universe is a vast grid, the motions and distributions of high-energy particles called cosmic rays may reveal similar anomalies—a glitch in the Matrix—and give us a peek at the grid’s structure. A 2013 paper by MIT engineer Seth Lloyd builds the case for an intriguing spin on the concept: If space-time is made of quantum bits, the universe must be one giant quantum computer. Of course, both notions raise a troubling quandary: If the universe is a computer program, who or what wrote the code?
The universe is a black hole
Any “Astronomy 101” book will tell you that the universe burst into being during the big bang. But what existed before that point, and what triggered the explosion? A 2010 paper by Nikodem Poplawski, then at Indiana University, made the case that our universe was forged inside a really big black hole.
While Stephen Hawking keeps changing his mind, the popular definition of a black hole is a region of space-time so dense that, past a certain point, nothing can escape its gravitational pull. Black holes are born when dense packets of matter collapse in on themselves, such as during the deaths of especially hefty stars. Some versions of the equations that describe black holes go on to say that the compressed matter does not fully collapse into a point—or singularity—but instead bounces back, spewing out hot, scrambled matter.
Poplawski crunched the numbers and found that observations of the shape and composition of the universe match the mathematical picture of a black hole being born. The initial collapse would equal the big bang, and everything in and around us would be made from the cooled, rearranged components of that scrambled matter. Even better, the theory suggests that all the black holes in our universe may themselves be the gateways to alternate realities. So how do we test it? This model is based on black holes that spin, because that rotation is part of what prevents the original matter from fully collapsing. Poplawski says we should be able to see an echo of the spin inherited from our “parent” black hole in surveys of galaxies, with vast clusters moving in a slight, but potentially detectable, preferred direction.
The universe is a bubble in an ocean of universes
Another cosmic puzzle comes up when you consider what happened in the first slivers of a second after the big bang. Maps of relic light emitted shortly after the universe was born tell us that baby space-time grew exponentially in the blink of an eye before settling into a more sedate rate of expansion. This process, called inflation, is pretty popular among cosmologists, and it got a further boost this year with the potential (but still unconfirmed) discovery of ripples in space-time called gravitational waves, which would have been products of the rapid growth spurt.
If inflation is confirmed, some theorists would argue that we must live in a frothy sea of multiple universes. Some of the earliest models of inflation say that before the big bang, space-time contained what’s known as a false vacuum, a high-energy field devoid of matter and radiation that is inherently unstable. To reach a stable state, the vacuum began to bubble like a pot of boiling water. With each bubble, a new universe was born, giving rise to an endless multiverse.
The trouble with testing this idea is that the cosmos is ridiculously huge—the observable universe stretches for about 46 billion light years in all directions—and even our best telescopes can’t hope to peer at the surface of a bubble this big. One option, then, is to look for any evidence of our bubble universe colliding with another. Today our best maps of the big bang’s relic light do show an unusual cold spot in the sky that could be a “bruise” from bumping into a cosmic neighbor. Or it could be a statistical fluke. So a team of researchers led by Carroll Wainwright at the University of California, Santa Cruz, has been running computer models to figure out what other sorts of traces a bubbly collision would leave in the big bang’s echo.