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 this week in the Journal of High-Energy Physics.
Professor Hertog, whose work has been supported by the EU's European Research Council, first announced the new theory at a conference in Cambridge in July of last year, organised on the occasion of Professor Hawking's 75th birthday.
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.
Modern theories of the Big Bang predict that our local universe came into existence with a brief burst of inflation - in other words, during a tiny fraction of a second after the Big Bang itself, the universe expanded at a very fast, 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 wipe 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.
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.
In 2014 Hertog was awarded a 2 million euro ERC grant for his 5 year-long project on Holographic Quantum Cosmology.
"This kind of work is ambitious, high-risk, and it lies entirely in the realm of the curiosity-driven, fundamental sciences. Hence it fits in very well with the goals and the vision of the ERC. Moreover I felt my project would be a very exciting training ground for young students and postdocs interested in the interface between cosmology and high-energy physics. I used my ERC grant to set up a kind of school in theoretical cosmology which has proven to be a fertile and stimulating research environment to explore new ideas in this area."
Full interview with ERC grantee Thomas Hertog
Prof Thomas Hertog from KU Leuven and the late Professor Stephen Hawking from Cambridge University put forward a new theory of the origin of the universe. The paper, Hawking's last, now published in the Journal of High Energy Physics, is entitled 'A Smooth Exit from Eternal Inflation?'. It does away with the infinite multiverse and proposes a more rigorous framework for cosmology that predicts a simpler and finite universe. Professor Hertog's work has been supported by the European Research Council.
You are putting forward a new theory of the origin of the universe. What's wrong with the current one?
The prevailing theory of the big bang is called eternal inflation. It says that out of the Big Bang not only did our own universe arise, but also many other universes - the so called multiverse. You can picture the multiverse as a mosaic of pocket universes, somewhat like bubbles in boiling water. The laws of physics and chemistry can differ from one pocket universe to another. Some pocket universes contain stars and harbour life, others are nearly empty.
The problem with the prevailing theory is that it doesn't predict much about our own universe. If the scale of different pocket universes in the multiverse is large or even infinite as some suggest then anything is possible. Therefore the theory can't be properly tested. The key challenge facing modern fundamental cosmology is to turn the multiverse into a proper verifiable scientific framework. With our paper we take a step in this direction.
How does your new theory fix this and improve our understanding of the universe?
Our new theory reduces the vast multiverse to a much more manageable and smaller range of possible universes. This makes the theory more predictive and testable. Our model is based on string theory, a branch of theoretical physics that attempts to reconcile general relativity with quantum physics. In particular it makes use of the new concept of holography in string theory, 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.
When applied to cosmology, the holographic viewpoint implies that time evolution is emergent, not built in. In our theory the universe, which evolves in time, emerges from a timeless state at the big bang. In our paper we put forward a mathematical model for the state of the universe at its beginning. We then use this to predict what kind of universes can come into existence. We find our theory predicts the universe is finite and far simpler than the infinite fractal structure predicted by the old theory of eternal inflation.
If the old model of the multiverse had all these flaws, why did it prevail in the first place?
The reason the multiverse became popular, and to some extent appealing, is that it came along with the theory of cosmic inflation. This says that our universe expanded at an ever increasing rate in the earliest stages of its evolution. Inflation leads to a pattern of variations in the cosmic microwave background radiation - the afterglow of the big bang. ESA's Planck satellite has measured the background radiation in great detail and found a pattern of variations in agreement with what inflation predicts.
However inflation itself does not predict the details of this pattern, and eternal inflation with all its different pocket universes makes the situation far worse. You could view our work as a completion of the theory of inflation which sharpens its predictions. It explains how inflation started in the first place.
Do you think we will ever be able to test your theory?
Theories of the early universe can be tested by comparing their predictions to satellite observations, especially those of the cosmic microwave background radiation. The pattern of small temperature variations of the background radiation reaching us from different directions in the sky, as well as its polarization, provides us with a wealth of information about the earliest stages of the universe.
I plan to study the implications of our new theory for features of our universe on scales that are within reach of our space telescopes in greater detail in the coming months. Generally speaking, our theory predicts there is a contribution from gravitational waves generated during inflation to the pattern of cosmic microwave background variations. The observation of signatures of those gravitational waves from the Big Bang would be a smoking gun showing we are on the right track.
What does it really mean that the universe is finite and why can't it be infinite?
The key point is not so much its overall spatial size but the fact that we greatly reduce the variety of different regions or pocket universes. We predict the universe looks roughly the same everywhere whereas this was radically different in the old theory of eternal inflation.
Understanding the origin and the structure of the universe. Is it not too ambitious a challenge for a scientist?
We should be ambitious! We have come a long way in our understanding of the workings of the universe. Our goal remains to bring the study of our universe's origin entirely within the realm of the natural sciences. This means we want to develop theories of the universe that are both mathematically consistent and observationally testable.
Of course you might think there must be a fundamental limit to what we can observe, and hence to what we can know. Maybe there is, we simply don't know. But we should definitely try as hard as we can. Pure scientific inquiry is after all one of humankind's most ambitious and exciting missions.
Do you think that your theories can affect the way people view and behave in this world?
At the end of my ERC interview the panel asked me what I thought would be the broader implications of my project for astronomy in general and beyond. I replied that one of the goals of my project would be to use modern scientific methods to elucidate age-old questions like `why the universe is the way it is, and what is our place in the grand scheme' and that, in my opinion, astronomy and cosmology should not shy away from investigating these questions. These are fundamentally what this new paper is about.
What strikes me most about our theories of the cosmos is how they give us a unified understanding of reality. They show us that all of cosmological history is intimately connected. Our existence here on Earth is profoundly interwoven with what happened at the Big Bang. As an example, the minuscule variations in the temperature of the afterglow of the Big Bang are the seeds of the structures we observe in today's universe. Modern cosmology offers a kind of synthesis which strongly emphasizes we are very much part of a special cosmic evolution. The old Copernican worldview, which implied our existence and hence our actions are irrelevant in the grand scheme of things, is terribly outdated.
Why did you apply for an ERC grant and how did it help you in your research?
This kind of work is ambitious, high-risk, and it lies entirely in the realm of the curiosity driven, fundamental sciences. Hence it fits in very well with the goals and the vision of the ERC. Moreover I felt my project would be a very exciting training ground for young students and postdocs interested in the interface between cosmology and high-energy physics. I used my ERC grant to set up a kind of school in theoretical cosmology which has proven to be a fertile and stimulating research environment to explore new ideas in this area.
Project: Holographic Quantum Cosmology
Researcher: Thomas Hertog
Host Institution: KU Leuven
ERC funding: EUR 1 995 900 (Consolidator Grant)
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