The Goldilocks Universe
While we do not yet have any evidence of life beyond Earth, it’s already clear that many of its essential raw materials and building blocks are abundant in the universe. Right here in our own solar system, NASA has found persuasive evidence of oceans of liquid water on Mars in the ancient past, and scientists have long suspected that it may still be plentiful beneath the planet’s surface. And indeed, in August 2024, a team from the University of California, San Diego, announced that by using seismic activity to probe the Martian interior, they found evidence of liquid water buried six to twelve miles below the surface. And not just trace amounts; they estimate there’s enough groundwater to cover the planet’s surface to a depth of a mile. Satellites have also revealed water ice in the deep shadows inside craters on the Moon in the present day. Water is generally assumed to be essential to life.
Moreover, organic compounds are quite common in space. The Urey-Miller experiment of 1953 and hundreds of variations on it since then have demonstrated convincingly that many different organic compounds and an assortment of amino acids form spontaneously under conditions that are not improbable on an Earthlike planet. Those were all laboratory experiments, but now we have actual evidence from our own solar system. In July 2024, NASA announced that the Perseverance rover on Mars found a 3-foot-long rock that contains evidence of water, organic compounds and a chemical energy source, which they called “a potential biosignature”—in other words, the possible presence of microbial life on Mars in its ancient past. Meanwhile, Saturn’s largest moon, Titan, has copious amounts of methane in its atmosphere, and possibly oceans of water beneath a surface rich in methane ice, which could harbor life. Methane is an organic compound and a greenhouse gas, and scientists think that methane in the Earth’s early atmosphere kept the planet warm enough for life to evolve.
And then there are the comets, which are loaded with organic compounds. In 1999, NASA launched its “Stardust” spacecraft, which passed close to the comet known as Wild 2 and brought sample material from the tail back to Earth in 2006 for analysis. Project scientists analyzed the particles contained in the sample and discovered glycine, the simplest of the amino acids and an essential ingredient of life. Amino acids have been found inside of meteorites after they crashed to Earth but this was our first opportunity to gather material from a comet out in space. By some estimates, there may be as many as a trillion comets in the outer reaches of our solar system.
Finally, astronomers have recently begun finding Earthlike planets in relative proximity to us. We cannot yet detect planets orbiting the billions of distant stars in the Milky Way, but there are certainly thousands of them in our local neighborhood. It seems highly unlikely that all of the planets in our apparently homogenous galaxy just happen to be clumped together in such close proximity to Earth. So with a generous supply of planets and satellites, along with an abundance of the chemical building blocks for making life, it’s reasonable to assume that the right combination of materials and conditions must have occurred often throughout space and time. The universe is very fertile indeed.
How is it that it’s so accommodating to life? The British Astronomer Royal, Sir Martin Rees, has an intriguing answer. His book, Just Six Numbers, says that there are six critical variables, the values of which determined whether the universe would blossom and flourish, or be stillborn. If the value of any one of them fell even slightly outside a narrow range, we wouldn’t be here to contemplate it. Of these six numbers, two relate to basic forces, two determine the large-scale texture of the universe, and two fix the properties of space itself. The variables include the number of spatial dimensions, the strength of the “strong nuclear force” that binds the particles of an atomic nucleus together and determines how all atoms are made, and the strength of gravity.
So how do these variables affect the viability of our universe? First, take the number of spatial dimensions in the universe. In On the Origin of Time, Thomas Hertog, a physicist who worked with Stephen Hawking, says:
We live in a universe with three large dimensions of space. Is there anything special about three? There is. Adding just a single space dimension renders atoms and planetary orbits unstable. Earth would spiral into the sun instead of tracing out a stable orbit around it. Universes with five or more large space dimensions have even bigger problems. Worlds with only two space dimensions, on the other hand, may not provide enough room for complex systems to function… Three dimensions of space seems just right for life.
Or consider the effect of gravity. Rees explains that for life to emerge, one of the most critical elements is… elements. You need the periodic table, or at least most of it, to produce the complex chemistry that leads to and sustains life. Those elements are forged in the interiors of stars: No stars, no life. You also need time, and lots of it. Here on Earth, it took the better part of a billion years to go from basic chemistry to living organisms. And for three billion years after that, nothing more complex than a single-celled organism appeared.
Rees explains how different the universe would be if the force of gravity were only slightly more powerful than it actually is. A tiny increase in gravity would produce a universe with smaller stars, smaller galaxies, and smaller planets. Stars would be so closely packed together that any planets that emerged could not form stable orbits around them. As a result, planetary temperatures would seesaw wildly during the course of a “year,” which would make for a very inhospitable environment for life. More importantly, stellar lifetimes would be much shorter, which would not leave enough time for life to emerge and then evolve into complex forms.
On the other hand, if gravity were only a little weaker than its current value, stars could not form. It’s the mutual gravitational attraction of individual hydrogen molecules that causes the collapse of giant vacuous clouds into the nuclear furnaces that we call stars. Just slightly weaker gravity would result in matter being strewn across the expanding universe with nothing to hold it together. Without the gravitational collapse of those hydrogen gas clouds, there would be no stars to manufacture all of the heavy elements needed to facilitate the complex chemistry of life.
Through similar reasoning, Rees explains how the value of each of the remaining variables in our universe falls neatly into the narrow zone necessary for the universe to unfold without flying apart, for stars to form and churn out all the elements in the periodic table as they progress through the fusion cycle, then rip themselves apart in their death throes, thereby triggering the birth of new stars and the formation of solid planets like Earth, revolving in stable, nearly circular orbits around long-lived stars.
Isn’t it a rather amazing stroke of luck for us that each of these six variables happen to have just the right value? Like we won some cosmic lottery? We seem to live in a “Goldilocks” universe, where all the key ingredients are “just right.” Some people view the universe as being so uniquely hospitable to life that it must have been consciously designed by a Creator, but Rees’ intriguing suggestion is that it could simply be a case of elementary probability. Through inductive reasoning, he concludes that it’s only surprisingly good luck if ours is the only universe that has ever existed and yet is still perfectly “tuned” for life to arise. On the other hand, if there were millions, or billions, or trillions of universes, then the probability that some of those universes would be conducive to life (and humanity) approaches certainty. Furthermore, as physicist Alan Lightman points out, conditions in our universe have not always been conducive to life. It may be a Goldilocks universe now, but it hasn’t always been, nor will it always be that way. We just happen to exist during the comfortable middle age of our universe, sandwiched between its hellish beginning in the super-heated afterglow of the Big Bang, and its frozen future, when the continuing expansion of the universe will eventually cause it to succumb to what physicists call heat death, when all life will be extinguished. It’s not really surprising then that we just happen to be alive during the only age of the universe in which it is possible to be alive. So perhaps we did, in effect, win a cosmic lottery: with trillions of chances to win, our number—or rather, our six numbers—eventually came up.
Before going any further, I need to point out that the idea of multiple universes remains entirely hypothetical. It is the only topic in this book that is not yet supported by any direct observations or data. But even though it is hypothetical, it is not illogical. Furthermore, if at some future date it were proven true, it would be the most dramatic discovery in history, one that would forever change our perspective on our importance in the cosmos, or lack thereof, which is why I have included this chapter in this part of the book.
I should also acknowledge that some people would argue that the very idea of multiple universes is self-contradictory: if the universe is everything that exists, then there cannot be more than one of it. That’s reasonable. But when physicists and cosmologists talk about multiple universes, they define our universe to be everything that unfolded from the Big Bang approximately 13.8 billion years ago. There may well have been countless other Big Bangs that initiated other “parallel” universes, physically adjacent to ours perhaps, but not interacting with it. In that case, it’s entirely possible that there is an infinite number of self-contained universes, each one unaware of all the others. Myriad universes may have come and gone long before the Big Bang initiated ours, and others may exist side-by-side with us even now. An infinite number may yet be born in the future. Ours may be one small flower in an infinite garden.
This idea of parallel universes is not new; writers and philosophers have mused about the possibility for centuries. In 1997, a science fiction writer named Michael Moorcock coined the term “multiverse” to describe the set of all of the universes that ever have or ever will exist. Rees proposes that our universe is just one instance of that multiverse, and that most of the other universes are probably inhospitable to life; we just happen to inhabit one of the rare ones that are ideally suited to our existence.
But if we are to accept this potentially infinite number of universes, it would be helpful to have a mechanism for explaining their origin, and some way of verifying that the mechanism exists. Otherwise, it’s not science, only conjecture. As it happens, several ideas have been proposed that are at least theoretically testable, if not now, then in the future.
One of the most popular candidates was proposed by theoretical physicist André Linde. The description of it in Wikipedia sounds as though the multiverse is like sea foam caused by the crashing of a wave on the ocean shore and our universe is one single bubble:
The bubble universe concept involves creation of universes from the quantum foam of a "parent universe." On very small scales, the foam is frothing due to energy fluctuations. These fluctuations may create tiny bubbles and wormholes. If the energy fluctuation is not very large, a tiny bubble universe may form, experience some expansion like an inflating balloon, and then contract and disappear from existence. However, if the energy fluctuation is greater than a particular critical value, a tiny bubble universe forms from the parent universe, experiences long-term expansion, and allows matter and large-scale galactic structures to form.
It sounds like a giant, cosmic popcorn popper: while some of the kernels never seem to pop, others burst open, one after another, until the popper is overflowing with newly created popcorn, and each kernel is a self-contained universe in its own right. The kernels abut each other, but they remain separate and distinct.
But there’s another interesting feature of this concept: you could compare the propagation of universes in the multiverse to the propagation of life on Earth: perhaps a cosmic version of natural selection is operating on the individual universes in the multiverse. Hertog says that at the beginning of time:
The rules of physics transmute in the primeval universe, in a process of random variation and selection akin to Darwinian evolution, with particle species, forces, and, we will argue, even time fading away into the big bang.
The result is that those universes with the attributes that favor continuous unfolding for many eons (such as ours) have, as a result, sufficient time to spawn other universes. In effect, they “survive long enough to reproduce,” which is what drives biological evolution on Earth. Other universes, those with one or more of Rees’ six numbers falling outside the critical ranges, may in effect be stillborn or fail to thrive; as a result, they wink out of existence without ever getting the chance to produce offspring universes. That would be the cosmic equivalent of a human child being born with a genetic defect that guarantees an early death. In this analogy, Rees’ six numbers act as a kind of genome for each universe in the multiverse. The expected result of this cosmic version of natural selection over the long term would be an ever-increasing number of thriving universes like ours in the population of the multiverse.
That’s all conjecture, of course, but here’s something that’s not: In addition to the fact that Linde’s “bubble universe” hypothesis is completely compatible with the now widely accepted theory of cosmic inflation associated with the Big Bang, one of its other interesting aspects is that it allows for endless variability in the values of Rees’ six numbers. And that variability multiplies exponentially the likelihood that the multiverse as a whole would be hospitable to life.
In summary, it’s possible that our universe may be just one of myriad inhabitable oases in an infinity of space and time. If so, then all of the attributes, both good and bad, of our beautiful home planet can be easily seen as the result of ordinary probability, rather than the conscious design of an intelligent deity.
This essay is excerpted from It’s Only Natural: How I Learned to Embrace Science and Let Go of Religion.
Neil McNamara spent more than three decades working in IT in the federal government. He is retired and lives in Virginia.