

A two-dimensional surface (a de Sitter geometry) representing an accelerating universe, containing the cosmic microwave background and the large scale structure of the universe.

Credit: Ana Achúcarro, Sebastián Céspedes, Anne-Christine Davis, and Gonzalo A. Palma

The Holographic Principle was first conceived in the 90s by physicists Gerard 't Hooft and Leonard Susskind, while they were trying to solve a paradox about black holes. The question on their minds was relatively simple: what happens to the matter consumed by a spacey abyss?

To understand this problem, let’s first picture dropping an ice cube in a glass of lemonade: When you drop the cube in the glass, the total volume inside the glass is now equal to the volume of the lemonade plus the volume of the ice. The surface area, on the other hand, is slightly greater than what it was before, but not equivalent to the surface area of the lemonade and the cube. Black holes work in the opposite way. For a black hole, all the information stored inside them can be represented in surface area (2 dimensions), not in volume (3 dimensions)



This is also how a hologram works, which takes 3 dimensions of information, and stores it on a 2D surface. For example, when “holographic Tupac” performed at Coachella in 2012, all three dimensions were represented in the image, but it was still a just 2D projection. Susskind and ‘t Hooft reconciled that even though objects are physically consumed by black holes, all of their information remains imprinted on the event horizon as a hologram.

Now, let’s take that idea, and apply it to everything that has existed previously, exists now, and will exist in the future.

In 1998, theorist Juan Martin Maldacena provided more credibility to this theory in what's referred to as the AdS/CFT Correspondence. This describes a theoretical universe with no curvature, where gravity exists as a fifth dimension. If this kind of universe can be holographic, physicists argue that ours could be as well. This mind-bending idea shook the world of physics because it eloquently combines relativity and quantum mechanics; encapsulating contradictory physics of objects at both cosmic and subatomic scales.

The holographic principle has been used to explain cosmic inflation , a pivotal period of time in the early universe where our cosmos expanded at quizzically breakneck speeds. Researchers aren’t quite sure why the universe suddenly inflated so quickly; but under certain conditions, physicists found that a lower-dimensional universe could accommodate this rapid expansion . Their findings mean our universe could have started holographic, then morphed into its 3D self.





Map of the Cosmic microwave background measured by the Wilkinson Microwave Anisotropy Probe. Credit: NASA





While the math is appealing, many take issue with the practicality of holographic theory. In the search for concrete evidence, an international team of scientists from the Netherlands, Spain, UK, and Chile investigated what kinds of detectable signals could be produced during cosmic inflation, and how those can be used to assess the validity of the holographic principle.

During cosmic inflation, the universe became populated with quantum fluctuations, particles that temporarily appear out of empty space. “The inflationary universe had the right characteristics to make these fluctuations the seed of everything we observe today from the Cosmic Microwave Background , galaxies, stars, planets to cosmologists.”, said the researchers. These quantum fluctuations are what causes the irregular distribution of galaxies in the universe.

Through interactions with scalar fields, these fluctuations create distinct signals that can oscillate. To tell whether the universe is holographic, researchers took a look at the different intricate ways these signals could oscillate. If the signals are underdamped, meaning they move back and forth before reaching a point of equilibrium, then we can’t be living in a hologram. This is because if we’re living in a holographic universe, every signal we see must have a counterpart in a lower dimensional world. An underdamped signal can’t have such a counterpart.

If we see underdamped signals, then we can conclude that we’re not living in a hologram!

So, how do we search for such a signal? In the coming years, the Simons Observatory will be scanning the cosmic microwave background looking for information about cosmic inflation. Additionally, surveys like the Large Synoptic Sky Telescope, Euclide and SPHEREx will be looking at the Large Scale Structure of the universe, creating a 3D map of how galaxies in our vicinity are dispersed. These projects will not only teach us about the evolution of our universe, but also whether the universe we see is truly the one that exists.

So is the universe a hologram? Probably not, but soon we can prove it.





The world as we perceive might not actually exist. It could just be a hologram. This statement sounds more like a thought you've had at 3 am than a scientific theory, but the existence of a holographic universe is more possible than you think. Some theoretical work shows that the universe could exist in a dimension lower than the one our minds perceive. The debate over what’s called “the holographic principle” has largely remained theoretical, but new research inshows how scientists can resolve this cosmological conundrum.