Thursday, February 02, 2017

Is the Universe a Hologram?

The question might seem like nothing more than mental gymnastics, a thought-provoking but “out there” question meant to give college students something to discuss at 3am. However, work published last week in Physical Review Letters provides observational evidence that this could actually be the case.

“Was there a beginning of time? Does it make sense to ask: What were the physical laws before time and space exist as we now perceive them? If yes, is there observational evidence? What is [the] origin of structure (i.e. of galaxies, stars, planets etc.) in our universe? Is our universe really a hologram? Answering such questions was inspiration behind this work” says Kostas Skenderis, a team member from the University of Southampton in the United Kingdom.

Like the hologram on a passport or bank card that comes to life in the right light, holograms are flat, two-dimensional surfaces that produce three-dimensional images. Clearly, we see and experience the world in three dimensions, plus time, so it is a mind-bending exercise to consider how a hologram could describe the universe.

The idea was inspired by black holes, in particular the realization by physicists Gerard 't Hooft and Leonard Susskind in the 1990s that the physics near a black hole could be described as activity on the boundary of a two-dimensional surface. Fast forward a few years and physicist Juan Martin Maldacena showed that in string theory, a framework for exploring quantum gravity, you can describe the universe as a two-dimensional hologram with properties determined by the state of its boundary.

According to Skenderis, “This is a rather bold and dramatic proposal as it implies that one of the macroscopic dimensions that we perceive in everyday life and gravity itself are emergent phenomena. They do not exist at the fundamental level but they emerge under certain conditions. To give an analogy, temperature is not a fundamental property of atoms but it emerges as a useful description when many atoms interact with each other and reach equilibrium. Similarly, according to holography, space-time and gravity are emergent phenomena.”

This is a sketch of the timeline of the holographic Universe. Time runs from left to right. The far left denotes the holographic phase and the image is blurry because space and time are not yet well defined. At the end of this phase (denoted by the black and white fluctuating ellipse) the Universe enters a geometric phase, which can now be described by Einstein's equations. The cosmic microwave background was emitted about 375,000 years later. Patterns imprinted in it carry information about the very early Universe and seed the development of structures of stars and galaxies in the later Universe (far right).
Image Credit: Paul McFadden.

The concept of a holographic universe has been explored theoretically before, but no one has provided observational evidence of the possibility until now.

The story begins in the very early universe. At the beginning, all the energy that makes up the visible universe was contained in a single, tiny space. Traditional models predict that a fraction of a second after the big bang, our universe went through a short period of dramatic growth called inflation. The universe continued to expand after this but much more slowly. We can’t exactly look back at the early universe and see how it evolved, but we do have some clues. The cosmic microwave background (CMB) radiation is a kind of afterglow of the big bang that still permeates the universe and contains information about its early days. Measurements of this radiation and its fluctuations are a great match to predictions from the inflation model.

Several years ago, Skenderis was curious about how well predictions of the CMB from holographic models of the universe would match the measured values. After determining what these predictions are, he and colleagues compared them to CMB measurements. The results showed that the holographic model was viable, but existing CMB measurements were not precise enough to convince the researchers it was a solid match. A few years later the European Space Agency’s Planck mission released more precise measurements of the CMB than ever before, and it was time to really see whether the holographic models held up.

This new work brings together a theorist, two astrophysicists, and two phenomenologists—specialists in bridging the gap between theory and experiment—to test predictions against observations. The team compared the detailed data from Planck to predictions from the holographic models, and then to predictions from the inflationary models, which consider all three dimensions of space as being intrinsic to the fabric of the universe. It turns out that the predictions from both kinds of models of the very early universe match observations really well.

Next, the researchers explored whether, based on the data, one type of model is more likely to be correct than the other with a statistical technique called Bayesian inference. They also explored whether there are any constraints that rule out any of the models. The bottom line is that there is as much observational evidence for the holographic models as for the inflationary models, and neither one seems much more likely to be correct than the other.

This doesn’t mean that the universe is a hologram, but it means that we can’t rule it out. I’m not sure what all of the implications would be, but it would certainly provide a new framework for thinking about the structure and creation of the universe. And whether or not it turns out to be the case, the prospect will definitely fuel sci-fi movies and books, as well as 3am discussions, for years to come.

Kendra Redmond

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