Taming the multiverse: Stephen Hawking's final theory about the big bang

Professor Stephen Hawking's final theory on the origin of the universe, which he worked on in collaboration with Professor Thomas Hertog from KU Leuven, has been published today in the Journal of High Energy Physics.

The theory, which was submitted for publication before Hawking's death earlier this year, is based on string theory and predicts the universe is finite and far simpler than many current theories about the big bang say.

Professor Hertog, whose work has been supported by the European Research Council, first announced the new theory at a conference at the University of Cambridge in July of last year, organised on the occasion of Professor Hawking's 75th birthday.

Modern theories of the big bang predict that our local universe came into existence with a brief burst of inflation -- in other words, a tiny fraction of a second after the big bang itself, the universe expanded at an exponential rate. It is widely believed, however, that once inflation starts, there are regions where it never stops. It is thought that quantum effects can keep inflation going forever in some regions of the universe so that globally, inflation is eternal. The observable part of our universe would then be just a hospitable pocket universe, a region in which inflation has ended and stars and galaxies formed.

"The usual theory of eternal inflation predicts that globally our universe is like an infinite fractal, with a mosaic of different pocket universes, separated by an inflating ocean," said Hawking in an interview last autumn. "The local laws of physics and chemistry can differ from one pocket universe to another, which together would form a multiverse. But I have never been a fan of the multiverse. If the scale of different universes in the multiverse is large or infinite the theory can't be tested. "

In their new paper, Hawking and Hertog say this account of eternal inflation as a theory of the big bang is wrong. "The problem with the usual account of eternal inflation is that it assumes an existing background universe that evolves according to Einstein's theory of general relativity and treats the quantum effects as small fluctuations around this," said Hertog. "However, the dynamics of eternal inflation wipes out the separation between classical and quantum physics. As a consequence, Einstein's theory breaks down in eternal inflation."

"We predict that our universe, on the largest scales, is reasonably smooth and globally finite. So it is not a fractal structure," said Hawking.

The theory of eternal inflation that Hawking and Hertog put forward is based on string theory: a branch of theoretical physics that attempts to reconcile gravity and general relativity with quantum physics, in part by describing the fundamental constituents of the universe as tiny vibrating strings. Their approach uses the string theory concept of holography, which postulates that the universe is a large and complex hologram: physical reality in certain 3D spaces can be mathematically reduced to 2D projections on a surface.

Hawking and Hertog developed a variation of this concept of holography to project out the time dimension in eternal inflation. This enabled them to describe eternal inflation without having to rely on Einstein' theory. In the new theory, eternal inflation is reduced to a timeless state defined on a spatial surface at the beginning of time.

"When we trace the evolution of our universe backwards in time, at some point we arrive at the threshold of eternal inflation, where our familiar notion of time ceases to have any meaning," said Hertog.

Hawking's earlier 'no boundary theory' predicted that if you go back in time to the beginning of the universe, the universe shrinks and closes off like a sphere, but this new theory represents a step away from the earlier work. "Now we're saying that there is a boundary in our past," said Hertog.

Hertog and Hawking used their new theory to derive more reliable predictions about the global structure of the universe. They predicted the universe that emerges from eternal inflation on the past boundary is finite and far simpler than the infinite fractal structure predicted by the old theory of eternal inflation.

Their results, if confirmed by further work, would have far-reaching implications for the multiverse paradigm. "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," said Hawking.

This makes the theory more predictive and testable.

Hertog now plans to study the implications of the new theory on smaller scales that are within reach of our space telescopes. He believes that primordial gravitational waves -- ripples in spacetime -- generated at the exit from eternal inflation constitute the most promising "smoking gun" to test the model. The expansion of our universe since the beginning means such gravitational waves would have very long wavelengths, outside the range of the current LIGO detectors. But they might be heard by the planned European space-based gravitational wave observatory, LISA, or seen in future experiments measuring the cosmic microwave background.

Atoms may hum a tune from grand cosmic symphony

Researchers playing with a cloud of ultracold atoms uncovered behavior that bears a striking resemblance to the universe in microcosm. Their work, which forges new connections between atomic physics and the sudden expansion of the early universe, will be published in Physical Review X and highlighted by Physics.

"From the atomic physics perspective, the experiment is beautifully described by existing theory," says Stephen Eckel, an atomic physicist at the National Institute of Standards and Technology (NIST) and the lead author of the new paper. "But even more striking is how that theory connects with cosmology."

In several sets of experiments, Eckel and his colleagues rapidly expanded the size of a doughnut-shaped cloud of atoms, taking snapshots during the process. The growth happens so fast that the cloud is left humming, and a related hum may have appeared on cosmic scales during the rapid expansion of the early universe -- an epoch that cosmologists refer to as the period of inflation.

The work brought together experts in atomic physics and gravity, and the authors say it is a testament to the versatility of the Bose-Einstein condensate (BEC) -- an ultracold cloud of atoms that can be described as a single quantum object -- as a platform for testing ideas from other areas of physics.

"Maybe this will one day inform future models of cosmology," Eckel says. "Or vice versa. Maybe there will be a model of cosmology that's difficult to solve but that you could simulate using a cold atomic gas."

It's not the first time that researchers have connected BECs and cosmology. Prior studies mimicked black holes and searched for analogs of the radiation predicted to pour forth from their shadowy boundaries. The new experiments focus instead on the BEC's response to a rapid expansion, a process that suggests several analogies to what may have happened during the period of inflation.

The first and most direct analogy involves the way that waves travel through an expanding medium. Such a situation doesn't arise often in physics, but it happened during inflation on a grand scale. During that expansion, space itself stretched any waves to much larger sizes and stole energy from them through a process known as Hubble friction.

In one set of experiments, researchers spotted analogous features in their cloud of atoms. They imprinted a sound wave onto their cloud -- alternating regions of more atoms and fewer atoms around the ring, like a wave in the early universe -- and watched it disperse during expansion. Unsurprisingly, the sound wave stretched out, but its amplitude also decreased. The math revealed that this damping looked just like Hubble friction, and the behavior was captured well by calculations and numerical simulations.

"It's like we're hitting the BEC with a hammer," says Gretchen Campbell, the NIST co-director of the Joint Quantum Institute (JQI) and a coauthor of the paper, "and it's sort of shocking to me that these simulations so nicely replicate what's going on."

In a second set of experiments, the team uncovered another, more speculative analogy. For these tests they left the BEC free of any sound waves but provoked the same expansion, watching the BEC slosh back and forth until it relaxed.

In a way, that relaxation also resembled inflation. Some of the energy that drove the expansion of the universe ultimately ended up creating all of the matter and light around us. And although there are many theories for how this happened, cosmologists aren't exactly sure how that leftover energy got converted into all the stuff we see today.

In the BEC, the energy of the expansion was quickly transferred to things like sound waves traveling around the ring. Some early guesses for why this was happening looked promising, but they fell short of predicting the energy transfer accurately. So the team turned to numerical simulations that could capture a more complete picture of the physics.

What emerged was a complicated account of the energy conversion: After the expansion stopped, atoms at the outer edge of the ring hit their new, expanded boundary and got reflected back toward the center of the cloud. There, they interfered with atoms still traveling outward, creating a zone in the middle where almost no atoms could live. Atoms on either side of this inhospitable area had mismatched quantum properties, like two neighboring clocks that are out of sync.

The situation was highly unstable and eventually collapsed, leading to the creation of vortices throughout the cloud. These vortices, or little quantum whirlpools, would break apart and generate sound waves that ran around the ring, like the particles and radiation left over after inflation. Some vortices even escaped from the edge of the BEC, creating an imbalance that left the cloud rotating.

Unlike the analogy to Hubble friction, the complicated story of how sloshing atoms can create dozens of quantum whirlpools may bear no resemblance to what goes on during and after inflation. But Ted Jacobson, a coauthor of the new paper and a physics professor at the University of Maryland specializing in black holes, says that his interaction with atomic physicists yielded benefits outside these technical results.

"What I learned from them, and from thinking so much about an experiment like that, are new ways to think about the cosmology problem," Jacobson says. "And they learned to think about aspects of the BEC that they would never have thought about before. Whether those are useful or important remains to be seen, but it was certainly stimulating."

Eckel echoes the same thought. "Ted got me to think about the processes in BECs differently," he says, "and any time you approach a problem and you can see it from a different perspective, it gives you a better chance of actually solving that problem."

Future experiments may study the complicated transfer of energy during expansion more closely, or even search for further cosmological analogies. "The nice thing is that from these results, we now know how to design experiments in the future to target the different effects that we hope to see," Campbell says. "And as theorists come up with models, it does give us a testbed where we could actually study those models and see what happens."

The new paper included contributions from two coauthors not mentioned in the text: Avinash Kumar, a graduate student at JQI; and Ian Spielman, a JQI Fellow and NIST physicist.

An expanding cloud of atoms could offer insight into unanswered cosmological questions

Attosecond physics: Molecules brilliantly illuminated

A new high-power laser system generates ultrashort pulses of light covering a large share of the mid-infrared spectrum.

Molecules are the building blocks of life. Like all other organisms, we are made of them. They control our biorhythm, and they can also reflect our state of health. Researchers led by Ferenc Krausz at the Laboratory for Attosecond Physics (LAP) -- a joint venture between Ludwig-Maximilians-Universität (LMU) and the Max Planck Institute for Quantum Optics (MPQ) in Munich -- want to use brilliant infrared light to study molecular disease markers in much greater detail, for example to facilitate early stage cancer diagnosis. The team has developed a powerful femtosecond light source which emits at wavelengths between 1.6 and 10.2 micrometers. This instrument should make it possible to detect organic molecules present in extremely low concentrations in blood or aspirated air.

Myriads of molecules react in highly specific ways to light of certain wavelengths in the mid-infrared region. By absorbing particular wavelengths, each type of molecule in a sample imprints a specific signature on the transmitted beam, which serves as a molecular fingerprint. With a source of broadband mid-infrared light one detects the fingerprints of many molecular structures at once -- in a sample of blood or aspirated air, for example. If the sample contains marker molecules that are associated with specific disease states, these too will reveal their presence in the spectrum of the transmitted infrared light.

LAP physicists have now constructed such a light source, which covers the wavelengths between 1.6 and 10.2 microns. The laser system exhibits watt-level average output power, and is well focusable which results in a highly brilliant infrared light source. This feature enhances the ability to detect molecules present in extremely low concentrations. In addition, the laser can produce trains of femtosecond pulses [a femtosecond is a millionth of a billionth of a second (10-15 sec)], which makes it possible to carry out time-resolved as well as low-noise and highly- precise measurements.

At present, infrared spectroscopy is often based on the use of incoherent light, which provides coverage of the whole mid-infrared region. However, the relatively low brilliance of the beam produced by incoherent sources markedly reduces the ability to detect very weak molecular fingerprints. Synchrotron radiation produced in particle accelerators can alternatively be used, but such facilities are in short supply and are extremely expensive. However, laser-based methods can generate even brighter beams than synchrotrons do. The physicists at LAP have now succeeded in building a coherent light source which produces brilliant laser light over a broad spectral region in the infrared range. That used to be the major drawback of laser sources Moreover, the new system has a much smaller footprint (and is far less costly) than a synchrotron: it fits on a large table.

"Of course, there is still a long way to go until we can diagnose cancer at much early stage than at present. We need a better understanding of disease markers and we have to design an efficient way to quantify them, for example," says Marcus Seidel, one of the researchers involved in the project. "But now having significantly improved light sources available, we can begin to tackle these issues." Moreover, the new laser system will find applications in areas beyond the biosciences. After all, the precise observation of molecules and their transformations is at the core of both chemistry and physics too.

Yale's latest work expanding the reach of quantum information science is actually a game of quantum pitch and catch

In a new study published April 23 in the journal Nature Physics, Yale researchers "pitch" a qubit -- a tiny bit of quantum data -- from one physical point in a microwave cavity to a separate point in a different cavity. It is the first time an end-to-end quantum transmission has been done on demand and represents the first of two Yale experiments involving "pitch-and-catch" technologies that will be published this year.

Quantum computing offers the possibility of computation speeds that are orders of magnitude faster than today's supercomputers. Yale researchers are at the forefront of efforts to develop the first fully useful quantum computers, and have done pioneering work in quantum computing with superconducting circuits.

But in order for a quantum computer to run more complex algorithms, it will need more processing power, just as a classical computer does. To do that, qubits must be interfaced with each other -- which is why a "pitch and catch" capability would come in handy.

"Our approach is to use a quantum network to connect many qubits together in independent modules," said Christopher Axline, a Yale graduate student and co-lead author of the new study. "The strategy is similar to clustering computers together on a local area network."

Axline works in the Yale lab of Robert Schoelkopf, the study's principal investigator. The other co-lead authors of the study are Yale graduate student Luke Burkhart and former Yale postdoctoral associate Wolfgang Pfaff, who is now at Microsoft.

Previous work by the researchers enabled them to pitch a qubit, while preserving its information. Now they're able to catch the information, as well.

"You might think catching our flying qubit would be a straightforward extension of our other work, but it actually requires some careful treatment," Burkhart said. "It meant varying how quickly, and at what frequency, the information is released. If we open the floodgates and let energy flow out as quickly as possible, it will overwhelm the catcher."

Instead, the researchers carefully shape their pitch-and-catch over time, so that both ends of the transaction are in sync.

Another first for the experiment is the use of the cavities -- in addition to the qubit itself -- as the memory for the system. "Much of the research in our lab and at the Yale Quantum Institute focuses on how to take advantage of cavity modes for quantum information processing," Axline said. "Superconducting cavities are the most secure places we can store quantum information, and even more important, cavities are flexible as to the form of the stored information."

This quantum game of pitch and catch also includes quantum entanglement, a key concept in quantum physics and a requirement in any quantum algorithm. In this instance, it means the pitcher is pitching and not pitching, simultaneously.

"We entangle the states between the pitcher and the catcher," Burkhart said. "This remote entanglement will be crucial in quantum networks."

The 'missing link' in conducting molecules, butadiene -- solved Researchers have solved a long-standing puzzle in the dynamics of the smallest linear polyene, butadiene

Linear polyenes are hydrocarbon chains with unusual optical and electrical properties. They have become a paradigm for studying photoisomerization -- when molecular structures rearrange from absorbing light -- because of their straightforward molecular structure, potential for electrical conductivity, and role in vision. Understanding how these molecules simultaneously rearrange through photoisomerization could advance materials science research by enabling artificial vision and producing wires from plastic, and new photovoltaic technologies.

Trans 1,3-butadiene, the smallest polyene, has challenged researchers over the past 40 years because of its complex excited-state electronic structure and its ultrafast (femtosecond, 10-15 s) dynamics. Butadiene remains the "missing link" between ethylene (C2H4,), which has only one double bond, and longer linear polyenes with three or more double bonds.

Now, an experimental team headed by Albert Stolow at the University of Ottawa and the National Research Council of Canada has solved trans 1,3-butadiene's electronic-structural dynamics. The researchers recently reported their findings in The Journal of Chemical Physics, from AIP Publishing.

Stolow's group developed an ultrafast laser spectroscopy called time-resolved photoelectron-photoion coincidence spectroscopy (TRPEPICO) to conduct this research. The method involves a femtosecond pump-probe process wherein an emitted photoelectron is measured as a function of time. The photoelectron spectrum and angular distribution is sensitive to the electronic and structural dynamics of molecules. Over the past 20 years, Stolow has applied his method to a broad range of problems, including the ultraviolet stability of DNA bases and intramolecular proton transfer.

"We've shown over many years that our approach works and have provided lots of examples," Stolow said. He previously studied under John C. Polanyi and Yuan T. Lee, two Nobel Prize winners who researched molecular collision dynamics.

"Many of us thought that if we could understand ethylene, the basic building block, we would be able to understand the longer linear polyenes," Stolow said. "But butadiene is the 'missing link.' It didn't seem to behave like either case."

Stolow's team discovered that trans 1,3-butadiene behaves, simultaneously, like both ethylene and longer polyenes. Specifically, there is an ultrafast competition between ethylenelike dynamics and polyenelike dynamics.

The research team's experimental results were independently modeled and confirmed computationally by Todd J. Martínez's research team. Martinez is a researcher and professor of chemistry at Stanford University, who specializes in molecular quantum dynamics. Michael S. Schuurman of the NRC, a theorist specializing in quantum dynamics, also helped confirm this work.

"This collaboration is key. We each independently came up with the same results," Stolow said. "Dramatic technical advances in both experiment and theory have allowed us to finally solve the long-standing puzzle of electronic dynamics in butadiene, the 'missing link' of polyene photophysics."

The elusive quantum mechanical phenomenon of entanglement has now been made a reality in objects almost macroscopic in size

This is an illustration of the 15-micrometre-wide drumheads prepared on silicon chips used in the experiment. The drumheads vibrate at a high ultrasound frequency, and the peculiar quantum state predicted by Einstein was created from the vibrations.

Perhaps the strangest prediction of quantum theory is entanglement, a phenomenon whereby two distant objects become intertwined in a manner that defies both classical physics and a "common-sense" understanding of reality. In 1935, Albert Einstein expressed his concern over this concept, referring to it as "spooky action at a distance."

Nowadays, entanglement is considered a cornerstone of quantum mechanics, and it is the key resource for a host of potentially transformative quantum technologies. Entanglement is, however, extremely fragile, and it has previously been observed only in microscopic systems such as light or atoms, and recently in superconducting electric circuits.

In work recently published in Nature, a team led by Prof. Mika Sillanpää at Aalto University in Finland has shown that entanglement of massive objects can be generated and detected.

The researchers managed to bring the motions of two individual vibrating drumheads -- fabricated from metallic aluminium on a silicon chip -- into an entangled quantum state. The objects in the experiment are truly massive and macroscopic compared to the atomic scale: the circular drumheads have a diametre similar to the width of a thin human hair.

The team also included scientists from the University of New South Wales Canberra in Australia, the University of Chicago, and the University of Jyväskylä in Finland. The approach taken in the experiment was based on a theoretical innovation developed by Dr. Matt Woolley at UNSW and Prof. Aashish Clerk, now at the University of Chicago.

'The vibrating bodies are made to interact via a superconducting microwave circuit. The electromagnetic fields in the circuit are used to absorb all thermal disturbances and to leave behind only the quantum mechanical vibrations,' says Mika Sillanpää, describing the experimental setup.

Eliminating all forms of noise is crucial for the experiments, which is why they have to be conducted at extremely low temperatures near absolute zero, at -273 °C. Remarkably, the experimental approach allows the unusual state of entanglement to persist for long periods of time, in this case up to half an hour.

'These measurements are challenging but extremely fascinating. In the future, we will attempt to teleport the mechanical vibrations. In quantum teleportation, properties of physical bodies can be transmitted across arbitrary distances using the channel of "spooky action at a distance",' explains Dr. Caspar Ockeloen-Korppi, the lead author on the work, who also performed the measurements.

The results demonstrate that it is now possible to have control over large mechanical objects in which exotic quantum states can be generated and stabilized. Not only does this achievement open doors for new kinds of quantum technologies and sensors, it can also enable studies of fundamental physics in, for example, the poorly understood interplay of gravity and quantum mechanics.

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