An Infinity of Worlds; Will Kenny; 2022; MIT Press; 200 pgs, notes, glossary, index

This book deals with the biggest questions of all about the biggest thing of them all… the entire universe of universes, or if you prefer: life, the universe and everything.
These write-ups are really just my notes on what I have read. In many cases, the concepts are so large that my notes cannot do them justice and you must read the original source. This just such a case. This book contains some very large, deep ideas that I do not really understand and my notes cannot adequately reflect.
This book's index is quite poor. A pet peeve of mine.



Some basics the book discusses:

• The Copernican Principle is that the Earth has no special place in the cosmos. By extension,  no observer has a special place in the universe. This means the universe is homogeneous and isotropic (the same everywhere and in every direction).
• The universe is expanding from the Big Bang.
• The universe is only about 5% baryons (normal matter). The remaining mass is dark: either dark matter or dark energy. 
• Dark matter interacts with gravity. Dark energy does not.
• The book refers to galaxies here as "small scale structures", so you need to think really big.
• The evidence for dark energy is definitive.
• The current consensus is that the universe is  5% normal stuff,  26% invisible stuff (dark matter) and 69% invisible energy (dark energy). This is known as the "lambda cold dark matter" model or LCDM.
• As the universe expands, matter and radiation dilute. The vacuum contains energy… dark energy. But dark energy does not dilute, meaning that as the universe increases in size, the amount of dark energy increases too… violating conservation of energy, and driving expansion!

At this point, the book lays out a cosmic timeline beginning from a very hot and dense start 13.7 billion years ago, till now. Time starts in the Planck Era: from 10 to the -43 seconds to 10 to the -32 seconds. It goes on from there to 6 billion years ago when matter became less dense than energy and cosmic growth stopped decelerating and started accelerating. Which brings us to today.

It is very hard to summarize a book that deals largely in abstract ideas, but here goes.

• The universe is homogeneous and isotropic.
• The further out in space we look, the further back in time we see.
• The observable universe is finite in every direction,
• Things outside the observable universe are moving away from us faster than the speed of light.
• At about 300,000 years after the big bang, the universe became transparent. Prior to that time, we can see nothing but fog.
• The universe is about 13.7 BYO but it has a radius of about 40 BLY, because space has been expanding as well.
• Close examination of the Cosmic Microwave Background (CMB)  has yielded many insights into our universe's  history.
• A mystery is why the universe is geometrically flat to within 0,02 percent.  This is called the Flatness Problem.
• Space is expanding. The vacuum has energy. So as space expands, energy is created, and conservation of energy is violated!
• Inflation is the term used to describe the initial exponential increases in the volume of space. The doubling time of the universe, the time it takes for the scale of the universe to double (also called the Hubble Time), right now, is about 10 billion years. 
• The Higgs field is just the thing required to support inflation. That field was recently discovered.

The book goes on to describe how the effects of dark matter and dark energy  combine to explain the dynamic (i.e.: changing over time) nature of the universe. A good discussion of "symmetry breaking" is used to explain many of the ideas of physics. Symmetry breaking is the source of many physical arguments.  Imagine a sombrero with a marble on top. A tiny nudge will cause the marble to roll from its high energy point of instability and symmetry to a lower energy state of stability on the sombrero brim. Now the ball is free to move around on the brim. That is a broken symmetry, changing the new applicable laws of physics.

Another interesting idea about time is discussed. We see an infinite future, but a finite past. This is asymmetric. But if you use a logarithmic clock, time appears infinite in both directions.  

The descriptions of the many universe hypothesis is fun and intriguing, but as the author points out, these ideas are not scientific in that they are unfalsifiable.

The title of the book comes from Giordano Bruno who said, shortly before he was burned to death for heresy: "God is infinite so his universe must be too… He is glorified not in one, but in countless suns, not a single earth , a single world, but in a thousand thousand, I say an infinity of  worlds."

No discussion of the universe and how it came about is complete without mentioning the anthropic principle. One way to look at the multiverse is as a beer with many bubbles. Each bubble is a universe, possibly with its own physics laws. The multiverse, as implied by inflation, also addresses the fine tuning argument (the idea that the universe was made just for us, or vice versa).

In its simplest form, the anthropic principle is basically a tautology. The universe is what it is because we are here to observe it. I.e.: the universe implies us, and we imply the universe that made us.

There are three main objections to the anthropic principle. 1) It is not scientific because we are cut off from the other bubbles. 2) The anthropic principle does not say what parameters in each universe can change, and which cannot. 3) The idea is based on a narrow interpretation of "life"    

This book is very recent so it should represent our best ideas of how it all came to be. These are literally the largest ideas that the human mind has ever contemplated. Five thousand years ago, somebody speculated that we were super-duper special, and that we, and everything we see, was all created for us by a benevolent god. That idea is forgivable and a load of rubbish. As we have grown in knowledge, our specialness has all but disappeared. I find it utterly remarkable that we live in a time when we can ask these types questions  and actually get answers. Of course, most of those answers are negatives, often in the form of "close, but no cigar". The good news it that we are getting new data every day (e.g.: the James Webb telescope, CERN, LIGO, COBE, Chandra etc)), , and these data underscore our ignorance and drive us forward.          

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