meIt’s hard to describe the state of affairs in the universe when everything was compressed to a size slightly smaller than the period at the end of this sentence, because the concepts of time and space literally didn’t apply yet. But that challenge hasn’t stopped the pioneering theoretical astrophysicist, Dr. Laura Mersini-Houghton, of seeking knowledge at the edge of the known universe and beyond. In his new book, Before the Big BangMersini-Houghton recounts her early life in communist Albania, her career as she rose to prominence in the male-dominated field of astrophysics, and discusses her research on the multiverse that could fundamentally rewrite our understanding of reality.
Taken from Before The Big Bang: the origin of the universe and what lies beyond by Laura Mersini-Houghton. Published by Mariner Books. Copyright © 2022 by Laura Mersini-Houghton. All rights reserved.
Scientific investigations of problems such as the creation of the universe, which we cannot observe or reproduce or test in a laboratory, are similar to detective work in that they are based on both intuition and evidence. Like a detective, as the pieces of the puzzle begin to fit together, investigators can intuitively sense that the answer is close at hand. This was the feeling I had as Rich and tried to figure out how we could test our theory about the multiverse. Rationally, it seemed like a long shot, but intuitively, it seemed doable.
Finally, a possible solution hit me. I realized that the key to testing and validating this theory was hidden in quantum entanglement, because decoherence and entanglement were two sides of the same coin! It could rewind the creation story back to its roots in the quantum landscape, when our wave universe was entangled with others.
I already knew that the separation—the decoherence—of the branches of the universe’s wave function (which later become individual universes) was caused by their entanglement with the ambient bath of fluctuations. Now I was wondering if we could calculate and find any trace of this early entanglement imprinted in our sky today.
This may sound like a contradiction. How is it possible that our universe is still intertwined with every other universe all these eons after the Big Bang? Our universe must have separated from them in its quantum infancy. But while wrestling with these issues, I realized that it was possible to have a universe that had fallen apart long ago but also retained its childhood “dents” – minor changes in shape caused by interaction with other universes. survivors who had tangled with ours during the first few moments, like identifiable birthmarks. The scars of their initial entanglement should still be observable in our universe today.
The key was in the moment. Our wave universe was decohering at about the same time that the next stage, the particle universe, was going through its own cosmic inflation and coming into existence. Everything we see in our sky today was seeded from the primordial fluctuations produced in those early moments, which take place in the smallest of measurable units of time, much less than a second. In principle, at those moments, while the entanglement was erased, their signatures could have been stamped on the inflaton and its fluctuations. There was a possibility that the kind of scars he was imagining had formed during this brief period. And if they had, they should be visible in the skies.
Understanding how the scars formed from the entanglement is less complicated than you might imagine. I began by trying to create a mental image of the entanglement scars in our sky. I visualized all the surviving universes of the wave function branches of the universe, including our own, as a bunch of particles scattered throughout the quantum multiverse. Because they all contain mass and energy, they interact (attract) with each other gravitationally, just as Newton’s apple had its path of motion curved by interacting with Earth’s mass, thus guiding it toward the ground. However, the apple was also being attracted to the moon, the sun, all the other planets in our solar system, and all the stars in the universe. Earth’s mass has the strongest force, but that doesn’t mean these other forces don’t exist. The net effect that entanglement left on our sky is captured by the combined pull of other fledgling universes on our universe. Similar to the weak pull of stars on the famous apple, the entanglement signals in our universe today are incredibly small relative to the signals from cosmic inflation. But they are still there!
I admit it… I was excited at the very thought that I potentially had a way to glimpse beyond our horizon and before the Big Bang! Through my proposal to calculate and track the entanglement in our sky, I may well have found, for the first time, a way to test the multiverse. What excited me most about this idea was its potential to make possible what for centuries we thought was impossible: an observation window to peer into space and time beyond our universe in the multiverse. Our expanding universe provides the best cosmic laboratory to search for information about its infancy because everything we observe on a large scale in our universe today was also present at its beginning. The basic elements of our universe do not disappear over time; they simply scale back in size with the expansion of the universe.
And this is why I thought of using quantum entanglement as a litmus test for our theory: Quantum theory contains an almost sacred principle known as “unitarity,” which states that information about a system can never be lost. Unitarity is a law of conservation of information. It means that signs of previous quantum entanglement of our universe with the other surviving universes must still exist. Thus, despite decoherence, entanglement can never be erased from our universe’s memory; it is stored in its original DNA. Furthermore, these signs have been encoded in our sky since its infancy, from the moment the universe began as a wave across the landscape. The traces of this earlier entanglement would simply spread with the expansion of the universe as the universe became a much larger version of its child self.
I was worried that these signatures, which have been stretched out by inflation and the expansion of the universe, would be quite weak. But on the basis of unitarity, he believed that however faint they were, they were preserved somewhere in our sky in the form of local violations or deviations from the uniformity and homogeneity predicted by cosmic inflation.
Rich and I decided to compute the effect of quantum entanglement on our universe to find out if traces were left behind, then fast-forward them from infancy to the present and derive predictions about what kind of scars we should be looking for in our sky. . If we could identify where we need to look for them, we could test them by comparing them to real observations.
Rich and I started this research with the help of a physicist in Tokyo, Tomo Takahashi. I first met Tomo at UNC Chapel Hill in 2004 when we shared a year. He was a postdoc about to take a faculty position in Japan, and I had just arrived at UNC. We enjoyed interacting and I saw the high standards Tomo held for his work and his incredible attention to detail. I knew that he was familiar with the computer simulation program we needed to compare predictions based on our theory with actual data about matter and radiation signatures in the universe. In 2005 I called Tomo and he agreed to collaborate with us.
Rich, Tomo, and I decided that the best place to start our search was the CMB: the cosmic microwave background, the afterglow of the Big Bang. CMB is the oldest light in the universe, a universal “ether” that permeates the entire cosmos throughout its history. As such, it contains a kind of exclusive record of the first millisecond in the life of the universe. And this silent witness to creation still surrounds us today, making it an invaluable cosmic laboratory.
The energy of CMB photons in our current universe is quite low; their frequencies peak in the microwave range (160 gigahertz), just like the photons in the kitchen microwave when you heat food. Three major international scientific experiments, the COBE, WMAP, and Planck satellites (with a fourth on the way), dating from the 1990s to the present, have measured the CMB and its much weaker fluctuations with exquisite precision. We even come across CMB photons here on Earth. In fact, watching and listening to CMB used to be an everyday experience in the age of old TVs: when changing channels, the viewer experienced the CMB signal in the form of static: the fuzzy, buzzing gray and white specks that appeared on the TV. screen.
But if our universe started purely from energy, what can we see in CMB photons that gives us a nascent picture of the universe? Here, quantum theory, specifically the Heisenberg uncertainty principle, provides the answer. According to the uncertainty principle, quantum uncertainty, shown as fluctuations in the initial energy of inflation, is unavoidable. When the universe stops inflating, it suddenly fills with waves of quantum fluctuations of energy from the inflaton. The entire range of fluctuations, some with mass and some without, are known as density perturbations. The shortest waves of this spectrum, those that fit inside the universe, become photons or particles, depending on their mass (reflecting the phenomenon of wave-particle duality).
Tiny tremors in the fabric of the universe that induce weak waves or vibrations in the gravitational field, known as primordial gravitational waves, contain information about which particular pattern of inflation took place. They are incredibly small, one part in about ten billion of the strength of the CMB spectrum, and therefore much more difficult to observe. But they are preserved in the WBC.
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