While reading following article you may wonder if this is science or philosophy (more likely it is philosophy of science). You are not alone. The "Landscape Multiverse" combines string theory and inflation to give us bubble universes in many dimensions. A number of physicists don't like the string landscape/multiverse idea. Leonard Susskind in 2006 said:
Why is it that so many physicists find these ideas alarming? Well, they do threaten physicists' fondest hope, the hope that some extraordinarily beautiful mathematical principle will be discovered: a principle that would completely and uniquely explain every detail of the laws of particle physics (and therefore nuclear, atomic, and chemical physics). The enormous Landscape of Possibilities inherent in our best theory seems to dash that hope.
What further worries many physicists is that the Landscape may be so rich that almost anything can be found: any combination of physical constants, particle masses, etc. This, they fear, would eliminate the predictive power of physics. Environmental facts are nothing more than environmental facts. They worry that if everything is possible, there will be no way to falsify the theory - or, more to the point, no way to confirm it. Is the danger real? We shall see.
In our exploration of multiverse idea, this however is unavoidable stop (before we gaze into wonderful world of quantum mechanics). Popular descriptions of the landscape seem to imply that the landscape exists because string theory gives different results depending on what geometry you choose to use for the "extra" dimensions, and that the landscape is basically supposed to be a collection of every conceivable way of folding up those extra dimensions.
Have you read article about distance measuring? We saw that Einstein introduced cosmological constant to make space static. He though gravity would lead space to contract and wanted to add something that would server as repulsive gravitation to make space static. Just as -1 and +1 will give you 0. Later on Hubble came with redshift and Einstein abandoned the whole idea. Nevertheless, by the end of 20th century this constant was back to headlines once we figure out space is expanding at accelerating pace. Scientists were also able to figure out the the numbers too. And since constant is the same everywhere and applies the same push to every cubic cm of space this leads to following: as space extended due to Big Bang process, distance between matter in universe extended too. Remember that matter provides gravity. As space continues to extend gravity provided by matter dilutes which in return gives more effect to repulsive gravity coming from cosmological constant (or dark energy if you want).
This is why dark energy continues to be dominant content of our Universe. Scientists express the cosmological constant’s value as a multiple of the so called Planck mass (about 2.17651x10–8 kg) per cubic Planck length (a cube that measures about 10–33 cm on each side and so has a volume of 10–99 cubic cm). In these units, the cosmological constant’s measured value is about 1.38 x 10–123.
Because of quantum uncertainty (I will cover Heisenberg uncertainty principle in next blog) and the jitters experienced by all quantum fields (quantum jitters are the incessant self-creation and self-annihilation of sub-atomic particles in empty space - or what we used to think was empty space), even empty space is full of microscopic activity. These quantum jitters harbor energy and they are everywhere. Since the cosmological constant is nothing but energy that fills the space, quantum field jitters provide a microscopic mechanism that generates a cosmological constant. How much energy is contained in these quantum jitters? When theorists calculated the answer, they got a ridiculous result: infinite amount of energy in every volume of space. This is related to Planck size and what happens if we get jitters below this size. And to describe them properly we require a framework that joins quantum mechanics and general relativity (this would shift the discussion to string theory). But scientist found more pragmatic response. They simply disregarded calculations for jitters on scales smaller than the Planck length. If you ignore jitters shorter than the Planck length, you're left with only a finite number, so the total energy they contribute to a region of empty space is also finite. Even so, energy levels calculated was just too high. Puzzle continued. Then, back in 1987, Steven Weinberg came up with very small value for cosmological constant; very small, but no zero. It was anthropic principle to get him to that conclusion.
The anthropic principle was proposed in Poland in 1973. It was proposed by Brandon Carter, who had the audacity to proclaim that humanity did indeed hold a special place in the Universe. Carter was not, however, claiming that the Universe was our own personal playground, made specifically with humanity in mind. The version of the anthropic principle that he proposed, which is now referred to as the Weak Anthropic Principle (WAP) stated only that by our very existence as carbon-based intelligent creatures, we impose a sort of selection effect on the Universe. For example, in a Universe where just one of the fundamental constants that govern nature was changed - say, the strength of gravity - we wouldn't be here to wonder why gravity is the strength it is. There is one arena in which we do play an absolutely indispensable role: our own observations. Because of this position, we must take into account of what statisticians call selection bias. For example, if you are interviewing a group of refugees who have endured astoundingly harsh conditions during their trek to safety, you might conclude that they are among the hardiest ethnicities on the planet. Yet, when you learn the devastating fact that you are speaking with less than 1 percent of those who started out, you realize that such a deduction is biased because only the phenomenally strong survived the journey. So, selection bias occurs when individuals or groups being compared are different. Two main factors that can contribute to selection bias are self selection, when the sample selects itself, and convenience sampling, when individuals are selected because they are easy to obtain. To help insure external validity, subjects in the study should be very similar to the population in which study results will be applied. Biased observations can launch you on meaningless quests to explain things that a broader, more representative view renders moot.
If we fail to take proper account of the impact such intrinsic limitations have on our observations, then, as in the examples above, we can draw wildly erroneous conclusions, including some that may impel us on fruitless journeys.
Perhaps more down to earth example. When you enter the shop with shoes and you ask for shoes fitting your number - you do not find unusual the fact they have it. You do not question whether there is deeper meaning to the fact they have exactly the shoes you want in size you carry. Once you learn they have whole range of numbers then all questions as to why your number was there - disappear. Just as it's no big surprise that among all the shoes in the shop there's at least one pair that fits you, so it's no big surprise that among all the planets in all the solar systems in all the galaxies there's at least one at the right distance from its host star to yield a climate conducive to our form of life. And it's on one of those planets, of course, that we live. We simply couldn't evolve or survive on the others. Simple as that. So, there is no fundamental reason why our planet is at specific distance from Sun. We just happen to be on one of planets where life could evolve which is at that distance and there is no need to search for any deeper meaning behind.
How does this translate to our universe? We know there are few specific and constant numbers in universe (mass of electron, EM force, gravitational constant, speed of light, etc). We do not know why they are the way they are, but key question is should we really care? If we apply the what we discussed before, to ask why the constants have their particular values is to ask the wrong kind of question. There is no law dictating their values; their values can and do vary across the multiverse. Our intrinsic selection bias ensures that we find ourselves in that part of the multiverse in which the constants have the values with which we’re familiar simply because we're unable to exist in the parts of the multiverse where the values are different. Why do we mention multiverse now? Simple; as in example with shoe shop having whole range of shoes, we are required to have whole range of different universes having different values for constants. This assumes:
- our universe is part of a multiverse
- from universe to universe in the multiverse, the constants take on a broad range of possible values
- for most variations of the constants away from the values we measure, life as we know it would fail to take hold
For many of nature’s constants, even modest variations would render life as we know it impossible. Make the gravitational constant stronger, and stars burn up too quickly for life on nearby planets to evolve. Make it weaker and galaxies don’t hold together. Make the electromagnetic force stronger, and hydrogen atoms repel each other too strongly to fuse and supply power to stars. But what about the cosmological constant? Does life’s existence depend on its value? This is the issue Steven Weinberg decided to address in 1987.
Formation of life is a complex process about which our understanding is in its earliest stages. Weinberg recognized it was hopeless to determine how one or another value of the cosmological constant directly impacts steps that breathe life into matter. Instead of giving up Weinberg had nice insight. He introduced a proxy for the formation of life: the formation of galaxies. Without galaxies, the formation of stars and planets would be compromised with a devastating impact on the chance that life might emerge. This was useful approach as it shifted the focus to determining the impact that cosmological constants of various sizes would have on galaxy formation and that was a problem Weinberg could solve. While precise details of galaxy formation are an active area of research, basics are known. A clump of matter forms here or there, and by virtue of being more dense than its surroundings, it exerts a greater gravitational pull on nearby matter and thus grows larger still (kind of snowball effect). The cycle continues feeding on itself to ultimately produce a swirling mass of gas and dust, from which stars and planets coalesce. Weinberg soon realized that a cosmological constant with large value would disrupt the clumping process (repulsive gravity would thwart galactic formation). He worked out the idea mathematically and found that a cosmological constant any larger than a few hundred times the current cosmological density of matter (a few protons per cubic meter) would disrupt the formation of galaxies. Math further shows the only universes that could have galaxies, and hence the only universes we could inhabit potentially, are ones in which the cosmological constant is no larger than Weinberg's limit, which in Planck units is about 10–121. This was the first time someone has come up with value of cosmological constant which is not infinite or absurdly large.
This brings us to another interesting point. If you imagine that cosmological constant can take values between 0 and 1 with increments between in Planck units you soon come up to at least 10124 universes. This is LARGE number. To get an idea how large, consider following:
- number of cells in your body - 1013
- number of seconds since Big Bang - 1018
- number of photons in observable universe - 1088
- number of different forms for extra dimensions in string theory - 10500
There are three possible alternatives from the anthropic principle;
- There exists one possible Universe "designed" with the goal of generating and sustaining "observers" (theological universe)
- Observers are necessary to bring the Universe into being (participatory universe)
- An ensemble of other different universes is necessary for the existence of our Universe (multiple universes)
The Inflationary Multiverse contains a vast, ever increasing number of bubble universes. The idea is that when inflationary cosmology and string theory are melded, the process of eternal inflation sprinkles string theory’s 10500 possible forms for the extra dimensions across the bubbles - one form for the extra dimensions per bubble universe - providing a cosmological framework that realizes all possibilities. By this reasoning, we live in that bubble whose extra dimensions yield a universe, cosmological constant and all, that's hospitable to our form of life and whose properties agree with observations.
In string theory, the range of possible universes is richer still. The shape of the extra dimensions determines the physical features within a given bubble universe, and so the possible "resting places" (various valleys) now represent the possible shapes the extra dimensions can take. To accommodate the all possible forms for these dimensions, the mountain terrain therefore needs a lush assortment of valleys, ledges, and outcroppings. Such landscape is called the string landscape.
The string landscape can be visualized schematically as a mountainous terrain in which different valleys represent different forms for the extra dimensions, and altitude represents the cosmological constant’s value. Picture above is just simplified view, but it suggests that universes with different forms for the extra dimensions are part of a connected terrain. And this is where process called quantum tunneling comes to life. Quantum tunneling refers to the quantum mechanical phenomenon where a particle tunnels through a barrier that it classically could not surmount (sort of passing through the wall).
Imagine an electron encountering a solid barrier, say a slab of steel which is thick, that classical physics predicts it can't penetrate. A hallmark of quantum mechanics is that the rigid classical notion of "can't penetrate" often translates into the softer quantum declaration of "has a small but nonzero probability of penetrating" (quantum mechanics makes everything probable, not matter how unlikely it is). This has been observed and confirmed for electrons in 2007 by researchers at Max Planck Institute. The reason is that the quantum jitters of a particle allow it, every so often, to suddenly materialize on the other side of an otherwise impervious barrier. The moment at which such quantum tunneling happens is random; the best we can do is predict the likelihood that it will take place. But the math says that if you wait long enough, penetration through just about any barrier will happen. And it does happen! If it didn't, the sun wouldn't shine: for hydrogen nuclei to get close enough to fuse, they must tunnel through the barrier created by the electromagnetic repulsion of their protons for example.
Same principle can be applied to our bubble universe, but theory shows that we get bubbles within bubbles then. The result is a more intricate version of the Swiss cheese multiverse we found in earlier encounter with eternal inflation. In that version, we had two types of regions: the "cheesy" ones that were undergoing inflationary expansion and the "holes" that weren't which represented separated universes. That was a direct reflection of the simplified landscape with a single mountain whose base we assumed to be at sea level. The richer string theory landscape, as the one seen on picture showing string landscape, with its sundry peaks and valleys corresponding to different values of the cosmological constant, gives rise to the many different regions. This in return has bubbles inside of bubbles inside of bubbles. Ultimately, the relentless series of quantum tunnelings through the mountainous string landscape realizes every possible form for the extra dimensions in one or another bubble universe. This is called Landscape Multiverse. When the string landscape combines with eternal inflation, all possible forms for the extra dimensions, including those with such a small cosmological constant, are brought to life. And according to this line of thought, it is in one of those bubbles that we live in.
What about all other features? Cosmological constant is just one. What about other constants of Nature? Researchers surveying the string landscape have found that these numbers, just like the cosmological constant, also vary from place to place, and hence - at least in our current understanding of string theory - are not uniquely determined.
Objects that have always been beyond our cosmic horizon are objects that we have never observed and never will observe; conversely, they have never observed us, and never will. Objects that at some time in the past were within our cosmic horizon but have been dragged beyond it by spatial expansion are objects that we once could see but never will again. Yet we can agree that such objects are as real as anything tangible, and so are the realms they inhabit.
When quantum mechanics invokes probability waves, its impressive ability to describe things we can measure, such as the behavior of atoms and subatomic particles, compels us to embrace the ethereal reality it posits. When general relativity predicts the existence of places we can't observe, its phenomenal successes in describing those things we can observe, such as the motion of planets and the trajectory of light, compels us to take the predictions seriously. So for confidence in a theory to grow we don't require that all of its features be verifiable; a robust and varied assortment of confirmed predictions is enough. The Brane, Cyclic, and Landscape Multiverses are based on string theory, so they suffer multiple uncertainties. Remarkable as string theory may be, rich as its mathematical structure may have become, the dearth of testable predictions, and the concomitant absence of contact with observations or experiments, relegates it to the realm of scientific speculation.
Currently, the best information about the primordial universe comes from the CMB. As stated before, collision will produce inhomogeneities in the early stages of cosmology inside our bubble, which are then imprinted as temperature and polarization fluctuations of the CMB. One can look for these fingerprints of a bubble collision in data from the WMAP or Planck satellites.
Since a collision affects only a portion of our bubble interior, and because the colliding bubbles are nearly spherical, the signal is confined to a disc on the CMB sky. Imagine now two merging soap bubbles; their intersection is a ring. The effect of the collision inside the disc is very broad because it has been stretched by inflation. In addition, there might be a jump in the temperature at the boundary of the disc.
In 2010, Scientics have run models of such collision and found few interesting facts. Existence of a temperature discontinuity at the boundary of the disc greatly increases our ability to make a detection. However, they did not find any circular temperature discontinuities in the WMAP data. BUT! They did find four features in the WMAP data that are better explained by the bubble collision hypothesis than by the standard hypothesis of fluctuations.
One of the features identified is the famous Cold Spot, which has been claimed as evidence for a number of theories including textures, voids, primordial inhomogeneities, and various other candidates. Note that some recent work, however, has called into question the statistical significance of this cold spot.
While identifying the four features consistent with being bubble collisions was an exciting result, these features are on the edge of sensitivity thresholds, and so should be considered only as a hint that there might be bubble collisions to find in future data analysis. One of many dilemmas facing physicists is that humans are very good at cherry-picking patterns in the data that may just be coincidence. However, the team's algorithm is much harder to fool, imposing very strict rules on whether the data fits a pattern or whether the pattern is down to chance.
The good news is that we can do much more with data from the Planck satellite, which has better resolution and lower noise than the WMAP experiment. Launched in 2009, the Planck satellite is probing the entire sky at microwave wavelengths from 0.35 mm to one cm. By measuring the CMB at these wavelengths, Planck has already provided (and yet will) an unprecedented view of the sky and new insights into existing theories.
Credits: Brian Greene, Leonard Susskind, Delia Schwartz-Perlov, Matt Johnson, ESA, Wikipedia, Encyclopedia Britannica