We all have personal reasons to be thankful this Thursday, but among them should be the fact that we exist in the first place—after all, it easily might have been otherwise. After the big bang the universe could have turned into unrelieved chaos or monotonous simplicity if just a few things had gone differently.
The formation of the sun, Earth and all the species on it, especially intelligent life, required a parade of flukes. In particular, the constants of nature—such as c, the speed of light, and G, which denotes the force of gravity—seem to be fine-tuned for our existence. Just a slight variation in one of these values would render galaxies, stars, planets, life and even complex atoms like those that comprise your pumpkin pie impossible. Different constants might lead to a universe that flashed into existence for just a moment or one that expanded so rapidly after its birth that no elements beyond the simplest, hydrogen, could form.
So as you mull over what to be grateful for this Thanksgiving, here’s some food for thought:
- The Universe’s Expansion Rate: Not only is the universe expanding, but also at every moment that expansion is accelerating due to some unknown entity scientists call “dark energy.” The most popular explanation is that dark energy arises from the energy inherent in empty space, with its strength determined by the so-called “cosmological constant.” The trouble is, most calculations suggest the cosmological constant should be more than 120 orders of magnitude larger than it is. Somehow, it ended up incredibly small—lucky for us. If the constant were larger, space would have expanded so rapidly after the big bang that mass never would have had time to condense into galaxies and stars. And if the cosmological constant were smaller, the universe would have collapsed on itself long ago, again preventing the formation of galaxies and stars. The constant, in fact, is so precariously poised at the ideal value that a change of just one part in 1053 would be enough to ensure one of those alternate Thanksgiving-less scenarios came true.
- Stable Protons: The neutron is 1.00138 times heavier than the proton. This extra weight is what causes it to decay into a proton, electron and neutrino, and prohibits the proton, the lighter of the two, from decaying whatsoever. Indeed, many experimental efforts have failed to observe a proton decay despite extensive searches. This stability is lucky for us because if the ratio of proton and neutron masses were altered or even flipped, protons would decay into neutrons, leading to a universe without atoms.
- Stellar Fusion: When our universe was in its infancy, it contained mostly hydrogen and helium. It wasn’t until those elements coalesced to form stars that the alchemy of nuclear fusion inside stellar cores produced the more complex elements necessary to create life. Each step from hydrogen to heavier elements is extremely sensitive to the physics involved, specifically the strong and electromagnetic forces. The strong force, which binds atomic nuclei, must overpower the electromagnetic repulsion between the positively charged protons in the nucleus to create a stable atom. A decrease of more than 0.5 percent in the strength of the strong force, or a change of more than 4 percent in the strength of the electromagnetic force would ruin the chances of carbon—the building block of life—from being created inside stars.
- Goldilocks Stars: If gravity were a little stronger—that is, if the constant G was slightly larger—all stars would collapse into smaller red dwarfs (cool, low-mass stars), which are too cold to support planets that could bear Earth-like life. If G had been a little weaker, all stars would balloon up into blue giants (hot, high-mass stars), which burn too briefly for life to develop. A change in gravity by only one part in 1040 would have proved tragic for stars like the sun.
- A Balance of Laws: Scientists also observe a fine-tuning within the physical laws themselves—the rules, such as the laws of gravity and thermodynamics, that regulate the cosmos. Laws that were more complex than ours might lead to a universe so chaotic, every star and galaxy would appear to obey different standards. Intelligent life could never survive in such a cosmological mess. Whereas simpler laws might lead to a universe so straightforward and uniform that pure chance—like the merger of two simple, single-celled organisms, which scientists think gave rise to complex, eukaryotic life two billion years ago—would never occur. Our universe is poised delicately between these two extremes.
Such a finely tuned universe makes it easy to conclude that the universe is special, a particular and not-too-probable configuration perfectly suited for complexity such as stars, planets and human beings to arise. But this viewpoint directly opposes the Copernican principle, which argues that the universe and our place in it is far from special—it’s mediocre.
In response to this dilemma some cosmologists turn to the so-called Anthropic principle, which posits that the apparent fine-tuning for life is a selection bias: If it weren’t this way, we wouldn’t be here to observe it. Others turn to the multiverse—the idea that our cosmos is just one of many in which an infinite number of alternative laws are possible. The fact we find ourselves in this one, improbable as it is, is merely a result of the fact that we could only have come into being in such a universe as this. Regardless of how we choose to explain the apparent fine-tuning of the cosmos, it is certainly a reason to be thankful.