Why Do We Live in a Huge, Yet Finite, Expanding Universe?
Editor's Note: With excitement about the upcoming Fox series Cosmos approaching a peak, we asked our resident physicist, Rob Sheldon, to comment on the question: Why do we live in a huge, yet finite, expanding universe, and not a smaller or infinite or static one? Is there something about the former that is more suited to the development of life? Dr. Sheldon received his PhD from the University of Maryland, College Park. After appointments at the University of Bern in Switzerland, Boston University, and the University of Alabama in Huntsville, he is currently consulting with NASA's Marshall Space Flight Center.
Philosophers from the time of Plato and Epicurus have wanted a static universe, infinite in extent, infinite in time. As Augustine tried to explain circa 400 AD, this was one of the big stumbling blocks preventing Christianity from being taken seriously. As far as I can tell, this prompted the only joke Augustine ever wrote: "I do not answer as others do when asked, 'What was God doing before He created the world?' by saying 'He was creating Hell for those who ask.'" So it should not be too surprising that when the medieval synthesis fell apart, one of the first things the scientific consensus decided upon, was a reversion to the static and infinite universe.
This position was challenged by "Olbers' Paradox" c. 1823. If there are an infinite number of stars, then a star will cover every spot in the sky, and the night sky should appear white. The consensus answer was either "finite stars" in an infinite space, or "dust obscures" the more distant stars.
Newton also discovered a paradox, that in infinite time, all the stars should clump together under the force of mutual attractive gravity. Descartes filled the heavens with "ether," which took the place of gravity, but had the advantage of not clumping. Newton's solution was to have the stars so smoothly distributed that no clumping could start. (And occasionally God would stir.)
Einstein also wanted a static and eternal universe, but his solutions for gravity couldn't be solved by stirring the pudding. They would clump no matter what distribution one began with. He solved the problem by reinvoking Descartes's ether, now called "the Cosmological Constant" -- an anti-gravity or pressure term filling space-time. It countered the force of gravity and gave an eternal but static universe.
Then along came a Belgian monk, Lemaître, who showed Einstein that an initial expansion, a "Big Bang" -- Hoyle's derogatory term -- would overcome any clumping without an ether. Lemaître's solution to the non-linear Einstein equations was undoubtedly inspired by his Christian faith. This also seemed to fit the redshift data that Hubble was getting for his galaxy spectra. Einstein wrote that this "cosmological constant" was his greatest mistake, and adopted Lemaître's Big Bang.
Hoyle was not happy, and continued to question the redshift data. He hypothesized other interpretations of the red shift -- "tired light" -- and worked on a "Steady State" universe model. It was infinite in time, infinite in extent, and also had an expanding red shift. But as the galaxies moved further apart, the vacuum would spontaneously make hydrogen atoms, which then clumped into galaxies and filled in the spaces. But Hoyle's model meant that no matter how far back in time you looked, the universe was always the same. This is not what astronomers discovered, however, since the early universe had quasars and Seyfert galaxies. But the real clincher came with the discovery of the 3-degree blackbody radiation, which was predicted for the Big Bang, but inexplicable under the Steady State. The discoverers were awarded a Nobel Prize, and Hoyle was not.
Then about twenty years ago, cosmology modelers started trying to duplicate the delicate network of galaxies in their computer codes of the Big Bang. It wasn't working very well. The holes in the "galaxy lace" were bigger in reality than in their codes. Either the galaxies were clumping sooner than expected, or something about space-time was expanding more than they expected. Particle theorists suggested "dark matter" caused early clumping, and cosmologists brought back Einstein's "cosmological constant" antigravity to inflate the holes. Today this model is known as "the Standard Model" or Lambda-CDM where "lambda" is a Greek letter used to represent Einstein's cosmological constant, and CDM is "cold dark matter." For the sake of press releases, lambda is often called "dark energy."
Then by having one dial for contracting galaxies -- CDM -- and another dial for expanding galaxies -- lambda -- you can get anything you want at Alice's restaurant, including static, eternal universes.
Life is just as possible in a small universe, a big universe, an infinite universe, as in our own. There is no a priori reason to think that size has anything to do with the impossible presence of life.
A closed universe (that ultimately collapses into the Big Crunch), an open universe (that expands into the void forever), or a flat universe (that comes to rest in infinite time), also makes no difference to impossible life, since multiplying an impossibility by infinite time does not make it possible.
Now if you hold the mistaken belief that life is only possible where there are probablistic resources to stumble over it, then you might think that the bigger the universe, the more probability resources for life. But life is not just an improbable event; it is a collection of improbable events. It is a concentration of information. So if there is an information threshold for life, it is simultaneously an information density threshold. So let us suppose that a universe was 10 times bigger, and contained 10 times the information. The information density would remain exactly the same, and therefore the probability of life would remain unchanged.
Okay, what happens to this argument when you have an expanding universe? Well, by all accounts, information is some function of matter, so as the matter expands and lowers its density, so does the information density. Therefore an expanding universe might have a "peak" in information density as it cooled enough to allow "spontaneous symmetry breaking" to store information in the matter, (i.e., a searing hot universe can't hold much information) but ultimately the information density will decline and with it the chances for life.
But of course, in all these cases, life is far, far more improbable than any number of Boltzmann brains, or other unlikely accidents of nature. Let me repeat this for emphasis. Information cannot be created or destroyed (this was Hawking's confession, remember?) so the spontaneous generation of life is impossible, independent of the size or age of the universe. So if we want the universe to produce life, we must assume that the information for life in our universe was placed there, and that means for "concentrating" the information into life were also placed there. This is called front loading. Having made this necessary modification of the model, does a bigger universe help, and does an expanding universe help?
Well, I have argued that there are means for "concentrating information," but since most of that information is in DNA, the speed of DNA transport is much, much slower than the speed of the universe's expansion. So this difference in speed means that most of the information in the universe is inaccessible or "unconcentratable" shortly after it forms. That in turn means the information density peaks very early and then declines, no matter how large the universe is.
If we take our universe as an example, the Milky Way galaxy is thought to be 12 billion years old, and the nearest big galaxy is the Andromeda galaxy 2.5 million light-years away. The two galaxies separated in the first billion years of the universe, and information transfer between them stopped maybe 11 billion years ago. So whatever mechanism for information concentration exists, the available pool of information halved after 1 billion years.
In this model, rather than make it easier for life, cosmological expansion is the one thing that is guaranteed to make life more difficult.
Sean Carroll, and any number of other "true believers" in Darwin, like to think that the scant 200 million years between the Hadean "Late Bombardment" (~3.85Ga) and the first cyanobacterial stromatolites (3.65Ga) are sufficient to spontaneously generate life. So they argue that all we need is a universe that has the "right conditions" for 200 million years and we'd have life. (Fred Hoyle famously argued that the flaw in this logic is that it makes spontaneous generation so easy, he should be able to duplicate it in 15 minutes in a test-tube in the lab.) Ignoring Fred, the true believers say "it happened" so it must be likely, and since it happened in our expanding universe, then this must be the ideal universe for it to happen in. The circularity is breathtaking. But accomplished mathematical physicists like Paul Davies spout similar sorts of platitudes, so evidently faith trumps physics.
On the other hand, if you were the designer, and you preloaded the information into an expanding universe, then that window of peak information density could be exactly the way you wanted it to happen.