An artist's conception shows our cosmos amid other universes in the multiverse. (NASA / JPL-Caltech / R. Hurt (IPAC)
The multiverse may be a cool (and convenient) concept for comic books and superhero movies, but why do scientists take it seriously?
In a new book titled “The Allure of the Multiverse,” physicist Paul Halpern traces why many theorists have come to believe that longstanding scientific puzzles can be solved only if they allow for the existence of other universes outside our own — even if they have no firm evidence for such realms.
It’s easy to confuse the hypotheses with the hype, but Halpern says there’s a huge difference between the multiverse that physicists propose and the mystical realm that’s portrayed in movies like “Doctor Strange in the Multiverse of Madness.”
“Some people accuse scientists of trying to delve into science fiction if they even mention the multiverse,” Halpern says in the latest episode of the Fiction Science podcast. “But the type of science that people are doing when they talk about the multiverse is real science. It’s far-reaching science, but it’s real science. Scientists are not saying, ‘Hey, maybe we can meet another Spider-Man and attack Kingpin that way.'”
For decades, various physicists have theorized that even the slightest changes in the fundamental laws of nature would make it impossible for life to exist. This idea, also known as the “Fine-Tuned Universe” argument, suggests that the occurrence of life in the Universe is very sensitive to the values of certain fundamental physics. Alter any of these values (as the logic goes), and life would not exist, meaning we must be very fortunate to be here!
But can this really be the case, or is it possible that life can emerge under different physical constants, and we just don’t know it? This question was recently tackled by Luke A. Barnes, a postdoctoral researcher at the Sidney Institute for Astronomy (SIA) in Australia. In his recent book, A Fortunate Universe: Life in a Finely Tuned Cosmos, he and Sydney astrophysics professor Geraint F. Lewis argued that a fine-tuned Universe makes sense from a physics standpoint.
Could our Universe be part of a wider Multiverse? And could these other Universes support life? Credit: Jaime Salcido/EAGLE Collaboration
The Multiverse Theory, which states that there may be multiple or even an infinite number of Universes, is a time-honored concept in cosmology and theoretical physics. While the term goes back to the late 19th century, the scientific basis of this theory arose from quantum physics and the study of cosmological forces like black holes, singularities, and problems arising out of the Big Bang Theory.
One of the most burning questions when it comes to this theory is whether or not life could exist in multiple Universes. If indeed the laws of physics change from one Universe to the next, what could this mean for life itself? According to a new series of studies by a team of international researchers, it is possible that life could be common throughout the Multiverse (if it actually exists).
In honor of Dr. Stephen Hawking, the COSMOS center will be creating the most detailed 3D mapping effort of the Universe to date. Credit: BBC, Illus.: T.Reyes
Stephen Hawking is rightly seen as one of the most influential scientists of our time. In his time on this planet, the famed physicist, science communicator, author and luminary became a household name, synonymous with the likes of Einstein, Newton and Galileo. What is even more impressive is the fact that he managed to maintain his commitment to science, education and humanitarian efforts despite suffering from a slow, degenerative disease.
Even though Hawking recently passed away, his influence is still being felt. Shortly before his death, Hawking submitted a paper offering his final theory on the origins of the Universe. The paper, which was published earlier this week (on Wednesday, May 2nd), offers a new take on the Big Bang Theory that could revolutionize the way we think of the Universe, how it was created, and how it evolved.
The paper, titled “A smooth exit from eternal inflation?“, was published in the Journal of High Energy Physics. The theory was first announced at a conference at the University of Cambridge in July of last year, where Professor Thomas Hertog (a Belgian physicist at KU Leuven University) shared Hawking’s paper (which Hertog co-authored) on the occasion of his 75th birthday.
Stephen Hawking’s final theory on the Big Bang, submitted shortly before he passed away, was recently published. Credit: University of Cambridge
According to the current scientific consensus, all of the current and past matter in the Universe came into existence at the same time – roughly 13.8 billion years ago. At this time, all matter was compacted into a very small ball with infinite density and intense heat. Suddenly, this ball started to inflate at an exponential rate, and the Universe as we know it began.
However, it is widely believed that since this inflation started, quantum effects will keep it going forever in some regions of the Universe. This means that globally, the Universe’s inflation is eternal. In this respect, the observable part of our Universe (measuring 13.8 billion light-years in any direction) is just 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. 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 offer a new theory that predicts that the Universe is not an infinite fractal-like multiverse, but is finite and reasonably smooth. In short, they theorize that the eternal inflation, as part of the theory of the Big Bang, is wrong. As Hertog explained:
“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. 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.”
In contrast to this, Hawking and Hertog offer an explanation based on String Theory, a branch of theoretical physics that attempts to unify General Relativity with quantum physics. This theory was proposed to explain how gravity interacts with the three other fundamental forces of the Universe (weak and strong nuclear forces and electromagnetism), thus producing a Theory of Everything (ToE).
To put it simply, this theory describes the fundamental constituents of the Universe as tiny, one-dimensional vibrating strings. Hawking and Hertog’s approach uses the holography concept of string theory, which postulates that the Universe is a large and complex hologram. In this theory, physical reality in certain 3D spaces can be mathematically reduced to 2D projections on a surface.
This illustration shows the evolution of the Universe, from the Big Bang on the left, to modern times on the right. Image: NASA
Together, Hawking and Hertog developed a variation of this concept to project out the dimension of time in eternal inflation. This enabled them to describe eternal inflation without having to rely on General Relativity, thus reducing inflation to a timeless state defined on a spatial surface at the beginning of time. In this respect, the new theory represents a change from Hawking’s earlier work on “no boundary theory”.
Also known as the Hartle and Hawking No Bounary Proposal, this theory viewed the Universe like a quantum particle – assigning it a wave function that described all possible Universes. This theory also predicted that if you go back in time to the beginning of the Universe, it would shrink and close off like a sphere. Lastly, it predicted that the Universe would eventually stop expanding and collapse in on itself.
As Hertog explains, this new theory is a departure from that earlier work:
“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. Now we’re saying that there is a boundary in our past.”
Using this theory, Hawking and Hertog were able to derive more reliable predictions about the global structure of the Universe. In addition, a Universe predicted to emerge from eternal inflation on the past boundary is also finite and much simpler. Last, but not least, the theory is more predictive and testable than the infinite Multiverse predicted by the old theory of eternal inflation.
Artist’s impression of merging binary black holes. Credit: LIGO/A. Simonnet.
“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. In theory, a finite and smooth Universe is one we can observe (at least locally) and will be governed by physical laws that we are already familiar with. Compared to an infinite number of Universes governed by different physical laws, it certainly simplifies the math!
Looking ahead, Hertog plans to study the implications of this theory on smaller scales using data obtained by space telescopes about the local Universe. In addition, he hopes to take advantage of recent studies concerning gravitational waves (GWs) and the many events that have been detected. Essentially, Hertog believes that primordial GWs generated at the exit from eternal inflation are the most promising means to test the model.
Due to the expansion of our Universe since the Big Bang, these GWs would have very long wavelengths, ones which are outside the normal range of the Laser Interferometry Gravitational-Wave Observatory‘s (LIGO) or Virgo‘s detectors. However, the Laser Interferometry Space Antenna (LISA) – an ESA-led plan for a space-based gravitational wave observatory – and other future experiments may be capable of measuring them.
Even though he is longer with us, Hawking’s final theory could be his profound contribution to science. If future research should prove him correct, then Hawking will have resolved one of the most daunting problems in modern astrophysics and cosmology. Just one more achievement from a man who spent his life changing how people think about the Universe!
An artist's impression of parallel universes. Credit: VisionGfx on deviantArt.com
Since the 1960s, astronomers have been aware of the electromagnetic background radiation that pervades the Universe. Known as the Cosmic Microwave Background, this radiation is the oldest light in the Universe and what is left over from the Big Bang. By 2004, astronomers also became aware that a large region within the CMB appeared to be colder than its surroundings.
Known as the “CMB Cold Spot”, scientists have puzzled over this anomaly for years, with explanations ranging from a data artifact to it being caused by a supervoid. According to a new study conducted by a team of scientists from Durham University, the presence of a supervoid has been ruled out. This conclusion once again opens the door to more exotic explanations – like the existence of a parallel Universe!
The Cold Spot is one of several anomalies that astronomers have been studying since the first maps of CMB were created using data from the Wilkinson Microwave Anisotropy Probe (WMAP). These anomalies are regions in the CMB that fall beneath the average background temperature of 2.73 degrees above absolute zero (-270.43 °C; -460.17 °F). In the case of the Cold Spot, the area is just 0.00015° colder than its surroundings.
Map of the cosmic microwave background (CMB) sky produced by the Planck satellite. The Cold Spot is shown in the inset, with coordinates and the temperature difference in the scale at the bottom. Credit: ESA/Durham University.
And yet, this temperature difference is enough that the Cold Spot has become something of a thorn in the hip of standard models of cosmology. Previously, the smart money appeared to be on it being caused by a supervoid – and area of space measuring billions of light years across which contained few galaxies. To test this theory, the Durham team conducted a survey of the galaxies in the region.
This technique, which measures the extent to which visible light coming from an object is shifted towards the red end of the spectrum, has been the standard method for determining the distance to other galaxies for over a century. For the sake of their study, the Durham team relied on data from the Anglo-Australian Telescope to conduct a survey where they measured the redshifts of 7,000 nearby galaxies.
Based on this high-fidelity dataset, the researchers found no evidence that the Cold Spot corresponded to a relative lack of galaxies. In other words, there was no indication that the region is a supervoid. The results of their study will be published in the Monthly Notices of the Royal Astronomical Society (MNRAS) under the title “Evidence Against a Supervoid Causing the CMB Cold Spot“.
As Ruari Mackenzie – a postdoctoral student in the Dept. of Physics at Durham University, a member of the Center for Extragalactic Astronomy, and the lead author on the paper – explained in an RAS press release:
“The voids we have detected cannot explain the Cold Spot under standard cosmology. There is the possibility that some non-standard model could be proposed to link the two in the future but our data place powerful constraints on any attempt to do that.”
The 3-D galaxy distribution in the foreground of the CMB Cold Spot, where each point is a galaxy. Credit: Durham University.
Specifically, the Durham team found that the Cold Spot region could be split into smaller voids, each of which were surrounded by clusters of galaxies. This distribution was consistent with a control field the survey chose for the study, both of which exhibited the same “soap bubble” structure. The question therefore arises: if the Cold Spot is not the result of a void or a relative lack of galaxies, what is causing it?
This is where the more exotic explanations come in, which emphasize that the Cold Spot may be due to something that exists outside the standard model of cosmology. As Tom Shanks, a Professor with the Dept.of Physics at Durham and a co-author of the study, explained:
“Perhaps the most exciting of these is that the Cold Spot was caused by a collision between our universe and another bubble Universe. If further, more detailed, analysis of CMB data proves this to be the case then the Cold Spot might be taken as the first evidence for the multiverse – and billions of other Universes may exist like our own.”
Multiverse Theory, which was first proposed by philosopher and psychologist William James, states that there may be multiple or an even infinite number of Universes that exist parallel to our own. Between these Universes exists the entirety of existence and all cosmological phenomena – i.e. space, time, matter, energy, and all of the physical laws that bind them.
Whereas it is often treated as a philosophical concept, the theory arose in part from the study of cosmological forces, like black holes and problems arising from the Big Bang Theory. In addition, variations on multiverse theory have been suggested as potential resolutions to theories that go beyond the Standard Model of particle physics – such as String Theory and M-theory.
Another variation – the Many-Worlds interpretation – has also been offered as a possible resolution for the wavefunction of subatomic particles. Essentially, it states that all possible outcomes in quantum mechanics exist in alternate universes, and there really is no such thing as “wavefunction collapse’. Could it therefore be argued that an alternate or parallel Universe is too close to our own, and thus responsible for the anomalies we see in the CMB?
As explanations go, it certainly is exciting, if perhaps a bit fantastic? And the Durham team is not prepared to rule out that the Cold Spot could be the result fluctuations that can be explained by the standard model of cosmology. Right now, the only thing that can be said definitively is that the Cold Spot cannot be explained by something as straightforward as a supervoid and the absence of galaxies.
And in the meantime, additional surveys and experiments need to be conducted. Otherwise, this mystery may become a real sticking point for cosmology!
Could our Universe be part of a wider Multiverse? And could these other Universes support life? Credit: Jaime Salcido/EAGLE Collaboration
If you’re a fan of science fiction or fantasy then chances are, at some point, you’ve read a book, seen a movie, or watched a series that explored the concept of multiple universes. The idea being that within this thing we call time and space, there are other dimensions where reality differs from our own, sometimes slightly, sometimes radically. Interestingly enough, this idea is not restricted to fiction and fantasy.
In science, this is known as the Multiverse Theory, which states that there may be multiple or even an infinite number of universes (including the universe we consistently experience) that together comprise everything that exists: the entirety of space, time, matter, and energy as well as the physical laws and constants that describe them. In this context, multiple universes are often referred to as parallel universes because they exist alongside our own.
The term was coined in 1895 by the American philosopher and psychologist William James. However, the scientific basis of it arose from the study of cosmological forces like black holes and problems arising out of the Big Bang theory. For example, within black holes it is believed that a singularity exists – a point at which all physical laws cease – and where it becomes impossible to predict physical behavior.
Beyond this point, it is possible that there may be an entirely new set of physical laws, or just slightly different versions of the ones that we know, and that a different universe might exist. Theories like cosmic inflation support this idea, stating that countless universes emerged from the same primordial vacuum after the Big Bang, and that the universe as we know it is just what is observable to us.
Max Tegmark’s taxonomy of universes sums up the different theories on multiple universes. IN this model, there are four levels that classify all major schools on thought on the subject.
In Level One, different universes are arranged one on top of the other in what is called Hubble Volumes, all having the same physical laws and constants. Though each will likely differ from our own in terms of distribution of matter, there will eventually be Hubble volumes with similar, and even identical, configurations to our own.
In Level Two, universes with different physical constants exist and the multiverse as a whole is stretching and will continue to do so forever, but some regions of space stop stretching and form distinct bubbles, like gas pockets in a loaf of rising bread.
In Level Three, known as the Many Worlds Interpretation of Quantum Mechanics, observations cannot be predicted absolutely but a range of possible observations exist, each one corresponding to a different universe. Level Four, aka.the Ultimate Ensemble devised by Tegmark himself, considers as equally real all universes that can be defined by mathematical structures. In other words, universes with the same or different constants may exist.
If you’d like more info on the Multiverse, check out some Recent Innovations about the Concept of Universe, and here’s a link to an article about the Size of the Universe.
We’ve also recorded an entire episode of Astronomy Cast all about Multiverses. Listen here, Episode 166: Multiverses.
