Skip to comments.In a Multiverse, What Are the Odds?
Posted on 11/04/2014 1:05:26 AM PST by LibWhacker
If modern physics is to be believed, we shouldnt be here. The meager dose of energy infusing empty space, which at higher levels would rip the cosmos apart, is a trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion times tinier than theory predicts. And the minuscule mass of the Higgs boson, whose relative smallness allows big structures such as galaxies and humans to form, falls roughly 100 quadrillion times short of expectations. Dialing up either of these constants even a little would render the universe unlivable.
To account for our incredible luck, leading cosmologists like Alan Guth and Stephen Hawking envision our universe as one of countless bubbles in an eternally frothing sea. This infinite multiverse would contain universes with constants tuned to any and all possible values, including some outliers, like ours, that have just the right properties to support life. In this scenario, our good luck is inevitable: A peculiar, life-friendly bubble is all we could expect to observe.
Many physicists loathe the multivere hypothesis, deeming it a cop-out of infinite proportions. But as attempts to paint our universe as an inevitable, self-contained structure falter, the multiverse camp is growing.
The problem remains how to test the hypothesis. Proponents of the multiverse idea must show that, among the rare universes that support life, ours is statistically typical. The exact dose of vacuum energy, the precise mass of our underweight Higgs boson, and other anomalies must have high odds within the subset of habitable universes. If the properties of this universe still seem atypical even in the habitable subset, then the multiverse explanation fails.
But infinity sabotages statistical analysis. In an eternally inflating multiverse, where any bubble that can form does so infinitely many times, how do you measure typical?
Guth, a professor of physics at the Massachusetts Institute of Technology, resorts to freaks of nature to pose this measure problem. In a single universe, cows born with two heads are rarer than cows born with one head, he said. But in an infinitely branching multiverse, there are an infinite number of one-headed cows and an infinite number of two-headed cows. What happens to the ratio?
For years, the inability to calculate ratios of infinite quantities has prevented the multiverse hypothesis from making testable predictions about the properties of this universe. For the hypothesis to mature into a full-fledged theory of physics, the two-headed-cow question demands an answer.
As a junior researcher trying to explain the smoothness and flatness of the universe, Guth proposed in 1980 that a split second of exponential growth may have occurred at the start of the Big Bang. This would have ironed out any spatial variations as if they were wrinkles on the surface of an inflating balloon. The inflation hypothesis, though it is still being tested, gels with all available astrophysical data and is widely accepted by physicists.
Play this video
Katherine Taylor for Quanta Magazine
Video: MIT cosmologist Alan Guth, 67, discusses why two-headed cows are an important problem in an infinite multiverse.
In the years that followed, Guth and several other cosmologists reasoned that inflation would almost inevitably beget an infinite number of universes. Once inflation starts, it never stops completely, Guth explained. In a region where it does stop through a kind of decay that settles it into a stable state space and time gently swell into a universe like ours. Everywhere else, space-time continues to expand exponentially, bubbling forever.
Each disconnected space-time bubble grows under the influence of different initial conditions tied to decays of varying amounts of energy. Some bubbles expand and then contract, while others spawn endless streams of daughter universes. The scientists presumed that the eternally inflating multiverse would everywhere obey the conservation of energy, the speed of light, thermodynamics, general relativity and quantum mechanics. But the values of the constants coordinated by these laws were likely to vary randomly from bubble to bubble.
Paul Steinhardt, a theoretical physicist at Princeton University and one of the early contributors to the theory of eternal inflation, saw the multiverse as a fatal flaw in the reasoning he had helped advance, and he remains stridently anti-multiverse today. Our universe has a simple, natural structure, he said in September. The multiverse idea is baroque, unnatural, untestable and, in the end, dangerous to science and society.
Steinhardt and other critics believe the multiverse hypothesis leads science away from uniquely explaining the properties of nature. When deep questions about matter, space and time have been elegantly answered over the past century through ever more powerful theories, deeming the universes remaining unexplained properties random feels, to them, like giving up. On the other hand, randomness has sometimes been the answer to scientific questions, as when early astronomers searched in vain for order in the solar systems haphazard planetary orbits. As inflationary cosmology gains acceptance, more physicists are conceding that a multiverse of random universes might exist, just as there is a cosmos full of star systems arranged by chance and chaos.
When I heard about eternal inflation in 1986, it made me sick to my stomach, said John Donoghue, a physicist at the University of Massachusetts, Amherst. But when I thought about it more, it made sense.
One for the Multiverse
The multiverse hypothesis gained considerable traction in 1987, when the Nobel laureate Steven Weinberg used it to predict the infinitesimal amount of energy infusing the vacuum of empty space, a number known as the cosmological constant, denoted by the Greek letter lambda. Vacuum energy is gravitationally repulsive, meaning it causes space-time to stretch apart. Consequently, a universe with a positive value for lambda; expands faster and faster, in fact, as the amount of empty space grows toward a future as a matter-free void. Universes with negative lambda; eventually contract in a big crunch.
Physicists had not yet measured the value of lambda; in our universe in 1987, but the relatively sedate rate of cosmic expansion indicated that its value was close to zero. This flew in the face of quantum mechanical calculations suggesting lambda; should be enormous, implying a density of vacuum energy so large it would tear atoms apart. Somehow, it seemed our universe was greatly diluted.
Weinberg turned to a concept called anthropic selection in response to the continued failure to find a microscopic explanation of the smallness of the cosmological constant, as he wrote in Physical Review Letters (PRL). He posited that life forms, from which observers of universes are drawn, require the existence of galaxies. The only values of lambda; that can be observed are therefore those that allow the universe to expand slowly enough for matter to clump together into galaxies. In his PRL paper, Weinberg reported the maximum possible value of lambda; in a universe that has galaxies. It was a multiverse-generated prediction of the most likely density of vacuum energy to be observed, given that observers must exist to observe it.
Is Nature Unnatural? Decades of confounding experiments have physicists considering a startling possibility: The universe might not make sense.
At Multiverse Impasse, a New Theory of Scale Mass and length may not be fundamental properties of nature, according to new ideas bubbling out of the multiverse.
Waiting for the Revolution An interview with the Nobel Prize-winning physicist David J. Gross.
A decade later, astronomers discovered that the expansion of the cosmos was accelerating at a rate that pegged at 10-123 (in units of Planck energy density). A value of exactly zero might have implied an unknown symmetry in the laws of quantum mechanics an explanation without a multiverse. But this absurdly tiny value of the cosmological constant appeared random. And it fell strikingly close to Weinbergs prediction.
It was a tremendous success, and very influential, said Matthew Kleban, a multiverse theorist at New York University. The prediction seemed to show that the multiverse could have explanatory power after all.
Close on the heels of Weinbergs success, Donoghue and colleagues used the same anthropic approach to calculate the range of possible values for the mass of the Higgs boson. The Higgs doles out mass to other elementary particles, and these interactions dial its mass up or down in a feedback effect. This feedback would be expected to yield a mass for the Higgs that is far larger than its observed value, making its mass appear to have been reduced by accidental cancellations between the effects of all the individual particles. Donoghues group argued that this accidentally tiny Higgs was to be expected, given anthropic selection: If the Higgs boson were just five times heavier, then complex, life-engendering elements like carbon could not arise. Thus, a universe with much heavier Higgs particles could never be observed.
Until recently, the leading explanation for the smallness of the Higgs mass was a theory called supersymmetry, but the simplest versions of the theory have failed extensive tests at the Large Hadron Collider near Geneva. Although new alternatives have been proposed, many particle physicists who considered the multiverse unscientific just a few years ago are now grudgingly opening up to the idea. I wish it would go away, said Nathan Seiberg, a professor of physics at the Institute for Advanced Study in Princeton, N.J., who contributed to supersymmetry in the 1980s. But you have to face the facts.
However, even as the impetus for a predictive multiverse theory has increased, researchers have realized that the predictions by Weinberg and others were too naive. Weinberg estimated the largest lambda; compatible with the formation of galaxies, but that was before astronomers discovered mini dwarf galaxies that could form in universes in which lambda; is 1,000 times larger. These more prevalent universes can also contain observers, making our universe seem atypical among observable universes. On the other hand, dwarf galaxies presumably contain fewer observers than full-size ones, and universes with only dwarf galaxies would therefore have lower odds of being observed.
Researchers realized it wasnt enough to differentiate between observable and unobservable bubbles. To accurately predict the expected properties of our universe, they needed to weight the likelihood of observing certain bubbles according to the number of observers they contained. Enter the measure problem.
Measuring the Multiverse
Guth and other scientists sought a measure to gauge the odds of observing different kinds of universes. This would allow them to make predictions about the assortment of fundamental constants in this universe, all of which should have reasonably high odds of being observed. The scientists early attempts involved constructing mathematical models of eternal inflation and calculating the statistical distribution of observable bubbles based on how many of each type arose in a given time interval. But with time serving as the measure, the final tally of universes at the end depended on how the scientists defined time in the first place. Berkeley physicist Raphael Bousso, 43, extrapolated from the physics of black holes to devise a novel way of measuring the multiverse, one that successfully explains many of our universes features.
Courtesy of Raphael Bousso
Berkeley physicist Raphael Bousso, 43, extrapolated from the physics of black holes to devise a novel way of measuring the multiverse, one that successfully explains many of our universes features.
People were getting wildly different answers depending on which random cutoff rule they chose, said Raphael Bousso, a theoretical physicist at the University of California, Berkeley.
Alex Vilenkin, director of the Institute of Cosmology at Tufts University in Medford, Mass., has proposed and discarded several multiverse measures during the last two decades, looking for one that would transcend his arbitrary assumptions. Two years ago, he and Jaume Garriga of the University of Barcelona in Spain proposed a measure in the form of an immortal watcher who soars through the multiverse counting events, such as the number of observers. The frequencies of events are then converted to probabilities, thus solving the measure problem. But the proposal assumes the impossible up front: The watcher miraculously survives crunching bubbles, like an avatar in a video game dying and bouncing back to life.
In 2011, Guth and Vitaly Vanchurin, now of the University of Minnesota Duluth, imagined a finite sample space, a randomly selected slice of space-time within the infinite multiverse. As the sample space expands, approaching but never reaching infinite size, it cuts through bubble universes encountering events, such as proton formations, star formations or intergalactic wars. The events are logged in a hypothetical databank until the sampling ends. The relative frequency of different events translates into probabilities and thus provides a predictive power. Anything that can happen will happen, but not with equal probability, Guth said.
Still, beyond the strangeness of immortal watchers and imaginary databanks, both of these approaches necessitate arbitrary choices about which events should serve as proxies for life, and thus for observations of universes to be counted and converted into probabilities. Protons seem necessary for life; space wars do not but do observers require stars, or is this too limited a concept of life? With either measure, choices can be made so that the odds stack in favor of our inhabiting a universe like ours. The degree of speculation raises doubts.
The Causal Diamond
Bousso first encountered the measure problem in the 1990s as a graduate student working with Stephen Hawking, the doyen of black hole physics. Black holes prove there is no such thing as an omniscient measurer, because someone inside a black holes event horizon, beyond which no light can escape, has access to different information and events from someone outside, and vice versa. Bousso and other black hole specialists came to think such a rule must be more general, he said, precluding solutions to the measure problem along the lines of the immortal watcher. Physics is universal, so weve got to formulate what an observer can, in principle, measure.
This insight led Bousso to develop a multiverse measure that removes infinity from the equation altogether. Instead of looking at all of space-time, he homes in on a finite patch of the multiverse called a causal diamond, representing the largest swath accessible to a single observer traveling from the beginning of time to the end of time. The finite boundaries of a causal diamond are formed by the intersection of two cones of light, like the dispersing rays from a pair of flashlights pointed toward each other in the dark. One cone points outward from the moment matter was created after a Big Bang the earliest conceivable birth of an observer and the other aims backward from the farthest reach of our future horizon, the moment when the causal diamond becomes an empty, timeless void and the observer can no longer access information linking cause to effect. The infinite multiverse can be divided into regions called causal diamonds that range from large and rare with many observers, left, to small and common with few observers, right. In this scenario, causal diamonds like ours should be large enough to give rise to many observers but small enough to be relatively common.
Olena Shmahalo / Quanta Magazine, source: Raphael Bousso, Roni Harnik, Graham Kribs and Gilad Perez
The infinite multiverse can be divided into finite regions called causal diamonds that range from large and rare with many observers (left) to small and common with few observers (right). In this scenario, causal diamonds like ours should be large enough to give rise to many observers but small enough to be relatively common.
Bousso is not interested in what goes on outside the causal diamond, where infinitely variable, endlessly recursive events are unknowable, in the same way that information about what goes on outside a black hole cannot be accessed by the poor soul trapped inside. If one accepts that the finite diamond, being all anyone can ever measure, is also all there is, Bousso said, then there is indeed no longer a measure problem.
In 2006, Bousso realized that his causal-diamond measure lent itself to an evenhanded way of predicting the expected value of the cosmological constant. Causal diamonds with smaller values of lambda; would produce more entropy a quantity related to disorder, or degradation of energy and Bousso postulated that entropy could serve as a proxy for complexity and thus for the presence of observers. Unlike other ways of counting observers, entropy can be calculated using trusted thermodynamic equations. With this approach, Bousso said, comparing universes is no more exotic than comparing pools of water to roomfuls of air.
Using astrophysical data, Bousso and his collaborators Roni Harnik, Graham Kribs and Gilad Perez calculated the overall rate of entropy production in our universe, which primarily comes from light scattering off cosmic dust. The calculation predicted a statistical range of expected values of lambda;. The known value, 10-123, rests just left of the median. We honestly didnt see it coming, Bousso said. Its really nice, because the prediction is very robust.
Bousso and his collaborators causal-diamond measure has now racked up a number of successes. It offers a solution to a mystery of cosmology called the why now? problem, which asks why we happen to live at a time when the effects of matter and vacuum energy are comparable, so that the expansion of the universe recently switched from slowing down (signifying a matter-dominated epoch) to speeding up (a vacuum energy-dominated epoch). Boussos theory suggests it is only natural that we find ourselves at this juncture. The most entropy is produced, and therefore the most observers exist, when universes contain equal parts vacuum energy and matter.
In 2010 Harnik and Bousso used their idea to explain the flatness of the universe and the amount of infrared radiation emitted by cosmic dust. Last year, Bousso and his Berkeley colleague Lawrence Hall reported that observers made of protons and neutrons, like us, will live in universes where the amount of ordinary matter and dark matter are comparable, as is the case here.
Right now the causal patch looks really good, Bousso said. A lot of things work out unexpectedly well, and I do not know of other measures that come anywhere close to reproducing these successes or featuring comparable successes.
The causal-diamond measure falls short in a few ways, however. It does not gauge the probabilities of universes with negative values of the cosmological constant. And its predictions depend sensitively on assumptions about the early universe, at the inception of the future-pointing light cone. But researchers in the field recognize its promise. By sidestepping the infinities underlying the measure problem, the causal diamond is an oasis of finitude into which we can sink our teeth, said Andreas Albrecht, a theoretical physicist at the University of California, Davis, and one of the early architects of inflation.
Kleban, who like Bousso began his career as a black hole specialist, said the idea of a causal patch such as an entropy-producing diamond is bound to be an ingredient of the final solution to the measure problem. He, Guth, Vilenkin and many other physicists consider it a powerful and compelling approach, but they continue to work on their own measures of the multiverse. Few consider the problem to be solved.
Every measure involves many assumptions, beyond merely that the multiverse exists. Predictions of the expected range of constants like lambda; and the Higgs mass always speculate that bubbles tend to have larger constants. And each prediction of one constant keeps all the others fixed at their value in this universe; when all the parameters vary at once, the calculations become too difficult. Clearly, this is a work in progress.
The multiverse is regarded either as an open question or off the wall, Guth said. But ultimately, if the multiverse does become a standard part of science, it will be on the basis that its the most plausible explanation of the fine-tunings that we see in nature.
Perhaps these multiverse theorists have chosen a Sisyphean task. Perhaps they will never settle the two-headed-cow question. Some researchers are taking a different route to testing the multiverse. Rather than rifle through the infinite possibilities of the equations, they are scanning the finite sky for the ultimate Hail Mary pass the faint tremor from an ancient bubble collision.
Part two of this series, exploring efforts to detect colliding bubble universes, will appear on Monday, Nov. 10.
All we are is dust in the wind, dude.
I find it amusing that scientists keep searching to find the smallest particle that can account for the mass necessary to make their formulas work properly.
The missing ingredient is not a minuscule nano-particle, but the greatest one of all.... GOD. There scientists will find the missing ingredient that surpasses all understanding and makes all the equations balance!
It comes down to there must be a creator, and we are but one
instance of a multiverse of creation. We think we are so smart, but the more we learn, the more we realize we really don’t know much at all.
We certainly are living in interesting times when it comes to physics and astronomy. The times are so interesting, in fact, and so changing (with new data coming in all the time from space telescopes and particle accelerators and neutrino detectors buried in mine shafts, etc) that I wish dates would be added to quotes from scientists. Some of the scientists quoted in the above article (Guth, for example) have expressed doubts about their own particular theories based on new data from refined measurements of the cosmic background radiation. When anyone is quoted in one of these articles, whenever possible I would like to see the date the statement was made.
It's just another bit of the Universe.
Other Universes - if they existed - would have to be strictly orthogonal to our Universe, and thus be undetectable.
Whenever we detect something - whatever we detect is by definition part of our Universe.
If/when we find different phase spaces, areas of reduced or enhanced dimensionality, multiply connected space - whatever weird and wonderful things are out there - we're merely discovering that our Universe is more complex and strange than we originally thought.
Other Universes cannot be detected. If you can detect them then they weren't 'other' Universes in the first place.
There is no observation you can make that can show the existence of another Universe. If you can observe it, it's in our Universe.
The theory of a 'MultiVerse' of multiple Universes is not only non-disprovable: it is also strictly non-provable. This makes it a uniquely pointless concept.
Nothing exploded and created everything. No faith required there.
The Universe's parameters have to be exactly fine-tuned to allow for the existence of life. The fact that they have been so fine-tuned argues for the existence of a Creator.
Multiverse theory is not scientific: it's a figleaf.
ping for later
Exactly! I am no physicist, nor mathematician (for sure), but it seems to me the problem they have is they are trying their best to look for an explanation that does not include the Creator, whether they realize it or not. I suspect that is the source of their frustration.
If you want to see an atheist’s head explode, point out that the multiverse theory implies an infinity of universes with god(s), and an infinity without ... and ask, which one do you think we live in, and why?
Increased coffee Ping.
Ha! That’s a wonderful, truly sci-fi concept.
Maybe there could be some Universes made by God, and some rubbish ones that just turned up by themselves.
Anyone requiring "immortal watchers", "two headed cows", and "eternal inflation" to describe their theories, is certainly "moving in the opposite direction". Keep in mind, that during "inflation", the laws of physics DON'T apply.
They claim that the entire universe is expanding. I’ve always wondered “Expanding into what?”
Assume for the moment that the universe started with a big bang, expanded to some size that allowed planets, galaxies etc, and continues to expand.
If the universe is really expanding then wouldn’t all of it be expanding more or less uniformly? That is, as the distance between galaxies expands the distance between the stars in those galaxies also expands and the distance between the planets orbiting those stars also expands etc etc etc down to the distance between the atoms in our bodies also expands and those very atoms expand! Likewise the distance measurable by our most accurate tools also expands because those very tools are expanding. So universal expansion is unmeasureable and undetectable.
Going backwards in time would bring us to the point where the galaxies (in comparison to us today) would be microscopic as their atoms are even smaller. The “point” source would have to be a fully formed and functioning universe that, to it’s inhabitants, is indistinguishable from the universe today (relative sizes do not change as everything gets bigger).
This is not to say that the galaxies cannot be moving one in relation to the other.
If it was expanding, at some point there would have to be an observable limit to the universe beyond which you could not go. But if you can see the limit, then you can see beyond the limit, which means that it’s not really a limit after all, as whatever the universe is expanding into, is already in the universe.
So, it seems to me that the galaxies moving away from each other is not a sign of universal expansion but of something else. And I’d expect that we’d find some cases of galaxies moving towards one another (colliding) which we do.
...What are the odds , indeed...
What are the odds that the Church would celebrate the “Alteration of Mass” every Sunday morning....and what are the odds that you turn your back to this fact...and what are the odds if the total sum of all Mankind’s theoretical hyperbole is zero...
What are the odds that the Earth is flat and what are the odds that the speed of light as well as the sound barrier can never be broken...
...But do not be discouraged my friends , just keep at it for a time , for time is why you are here.
“Than the theory predicts”
Then the theory is wrong and must be trashed.
Truth: there is one universe. Talk of “multiverses” is just more of the insanity into which this country has descended.
The article points out that many physicists don’t like the multiverse theory and wish it would go away. The problem is they can’t rule it out at this point.
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