The Book of Universes: Exploring the Limits of the Cosmos
The possibility of the existence of multiple universes is a hot topic among cosmologists, and John D. Barrow, one of the best popular science writers in the universe, and The Book of Universes is a match made in the stars.
We are observing our universe at a special period in its history. There are only certain intervals of time when life of any sort in a universe is possible, and astronomy can only be practiced during that habitable period. Like a tree in a forest needing an observer to hear it fall, for a universe to exist, it too needs an observer: the cosmologist. For any universe to be observed by a cosmologist, that universe must have expanded enough to link time and space, and must have expanded at a critical rate. If that universe instead expanded too fast, galaxies wouldn’t form. If it expanded too slowly, it would condense into black holes instead of stars. For a universe to be observed, it must be both big enough and old enough to have spread out the building blocks of life (that provide the conditions necessary for a cosmologist to evolve).
It seems as if cosmologists prefer nothing better than making models and refuting models, and taking models’ constants and turning them into variables and varying those variables to their limits, and beyond. Note that cosmologists are not only interested in the structure and history of our universe but also the possible other universes, universes that not only might be but also might be—now.
Our biases in interpreting our universe come from where we are situated. Situated here on Earth, the tilt of the Earth at 23.5 degrees leads to viewing a different variety of stars at different latitudes in the night sky. Viewing the night sky at one particular latitude at a particular time can lead to preconceptions and interpretations of the where we are in the grand scheme of things.
Aristotle’s and Ptolemy’s view of the universe were Earth-centric. The sun revolved around the Earth. The motion of the planets (not) circling the Earth had to be explained by complex movements called epicycles. A poor model is noticed by the effort that occurs when having to incorporate a new fact that doesn’t fit the model. If you have to keep adding details to a theory to explain every new fact that doesn’t fit you end up with a theory with too many exceptions and too little explanatory power. The Copernican revolution, the paradigm shift to a Sun-centered solar system simplified the planets’ motions. But as what we knew expanded, what we knew we didn’t know expanded as well.
The pre-19th century view that the universe was static and unchanging. In the 19th century the belief that Newton’s laws of motion that applied on Earth also applied to the universe, and lead to the same belief about the laws of thermodynamics. Thermodynamics states that entropy (energy dissipation) always increases. The consequence of entropy lead to the belief in future heat death for the universe. The universe suddenly became dynamic, and if the universe has a future, it must also have had a past, and in that past, a beginning.
It is still an open question whether the universe was created at a finite time in the past (something tremendous must have happened at a finite time in the past, but too bad, we can’t see earlier than a couple hundred million years after the “big bang”). And it is still an open question as to whether the universe is fated to stagnation and heat death in the distant future.
These are some of the questions asked by the author. Did the universe have just one start and one end, or is it cyclic? If the universe cycled would all cycles be exactly the same? Is the universe expanding or contracting or oscillating between the two? If expanding (as the evidence shows) then is that expansion accelerating, decelerating, or some combination, varying over time? These questions present a gallery of possibilities.
Einstein formulated laws of nature so that all observers in the universe would see the same laws wherever they were and however they were moving. To do this, space and gravity became interrelated. The author quotes John Archibald Wheeler, a collaborator of Einstein’s on that particular point. “Matter tells space how to curve. Space tells matter how to move.” Given that space may be curved, then what is the shape of the universe? If the universe were curved (curved closed, and with circumference not greater than the speed of light times the age of the universe) then it would be possible for a light ray to circumnavigate the universe, go around it and come back again, and (possibly) do it more than once. And if so, we could (theoretically) see before the universe’s creation!
The universe appears to us flat and steady state rather than static, on average roughly the same everywhere, so new matter appears to be added at the same rate as it is diluted by the universe’s expansion. But why is the universe when viewed locally “lumpy,” denser in some places with more galaxies and not the same everywhere? When considered in the larger scale, the lumpiness in our universe smoothes to such an extent that the smoothness becomes extreme. How can an infinite universe be so uniformly filled with matter, and how did it get that way?
There is a possibility that the universe does not have the same curvature everywhere—that the curvature might vary across the universe. And given the size of the universe is greater then we can see, there could be global features of the universe that do not show up locally. If so, there are more possibilities:
• The expansion of the universe may not be simultaneous everywhere. There is a possibility that the universe expands at different rates in different directions.
• The beginning of the universe in some places may lag far behind that late beginners might be able to see the beginning. An indicator that there is indeed a beginning is the measurable, uniform cosmic microwave background radiation.
• There might even be, in extreme cases, regions of the universe were there was no beginning!
All the accepted varieties of theories of our universe began at a finite time in the past, at infinite density, yet none of the math for the models of those theories match with what we observe now. There is no math for inside a singularity of infinite density where mathematically time and space are destroyed.
Each step in understanding the universe arrives only at the dependence of having a breakthrough theory. In one such theory, there is no need to equate infinite density with the beginning of the universe. Where Einstein’s theory fails to give a full description at T = 10e-42 seconds, the unit of Planck time, the theory of quantum gravity states that a “bounce” occurs before the singularity and gravity becomes repulsive. The theory of quantum gravity is called the quantum version of general relativity. (For more on quantum gravity, see Once Before Time: A Whole Story of the Universe by Martin Bojowald, Knopf, 2010.)
There is a growing body of astronomical evidence that the laws of nature were different 10 billion years ago. Astronomical sources of natural radio waves (quasars) were not equally prevalent in the universe at all times. The ongoing search for changes in constants of nature if found, may be indicators of dimensions we can’t see.
John D. Barrow asks more questions:
Does the universe rotate?
What if the gravitational constant weren’t constant but varied with space and time?
Is time travel possible, and if so, could one live forever by exploiting the circularity of time?
Why is there more matter than anti-matter?
Why are there a billion photons for every proton?
The question “Why are there a billion photons for every proton?” when asked in the second half of the 1970s lead to where particle physicists’ theories merged with cosmologists’. The answer hinged on evidence that the strength of particle interaction changes as the temperature of the environment increases. From this, the Grand Unified Theory (GUT) came about which proposed that protons decay into quarks and anti-quarks, and that quarks and anti-quarks did not decay at the same rate. The GUT theory answers both the matter-anti-matter imbalance and also accounted for an excess of photons. The larger implication of GUT is that the matter in the universe doesn’t depend on how the universe began.
It appears as though random processes have determined vital aspects of our universe, the matter anti-matter balance, the density of atomic matter, the strength and orientation of magnetic fields. Randomness and uncertainty will likely be true for any universe, though the author has the hope that perhaps we are like before, just one good idea away from a solution.
The Grand Unified Theory also raised a question by implying the existence of a new particle, the magnetic monopole, and not just its existence but existence in large numbers that we just don’t see. Where did they go? The magnetic monopole’s absence needed a new theory. This new theory was based on evidence of an accelerated expansion in the early stages of the universe. Inflationary theory states that the universe expanded and the expansion was followed by a deceleration. The mismatches between the acceleration and deceleration prevented the creation of monopoles.
For every mystery solved a new one is introduced. The Inflationary theory implies a universe of expanding bubbles, and not just one universe, but many universes. According to the theory beyond string theory, M-theory (see The Shape of Inner Space: String Theory and the Geometry of the Universe’s Hidden Dimensions, by Shing-Tung Yau and Steve Nadis, Basic Books, 2010), there could be 10e500 possible self-consistent universes! How can cosmologist cope with the possibility of 10e500 universes?
The three concluding chapters of The Book of Universes have evocative titles, Post-Modern Universes, Fringe Universes, and Runaway Universes, and in them even more imaginative theories are explored. One of these is the possibility that we might be living in a simulated universe created by an all-powerful physicist, a grand designer. If so, there would be a high likelihood that this grand designer would also be having problems with complexity just as we, its simulations have, and so introduce flaws through approximations of reality (that just might be detectable through careful observation).
Another is a model called the “no boundary” state model for the origin of the universe, which removes time and instead follows the universe’ temperature. In the no boundary model, the universe appears to create itself and though it has a past it has no beginning! The author also provides a small diversion on the concept of infinity in the search for other universes, which if the reader is interested, should read John D. Barrow’s The Infinite Book: A Short Guide to the Boundless, Timeless and Endless, Vintage, 2006.
This reviewer will leave the reader with one last mystery from The Book of Universes. The universe we know, the one that exists, is much stranger than anyone could have imagined. From 1988 evidence began to show that our universe was not just expanding but that its expansion was accelerating, and the acceleration began as recently as 5 billion years ago. The mystery is not that the acceleration is being driven by something called dark energy, or that dark energy comprises 72% of all the energy in the universe! The mystery is: What dark energy is has no explanation!