What conditions would you say are ‘congenial to life’? For physicist Robert Jaffe and colleagues at MIT, the phrase refers to places where stable forms of hydrogen, carbon and oxygen can exist. Jaffe explains why:
“If you don’t have a stable entity with the chemistry of hydrogen, you’re not going to have hydrocarbons, or complex carbohydrates, and you’re not going to have life. The same goes for carbon and oxygen. Beyond those three we felt the rest is detail.”
It’s an important issue in Jaffe’s work because he wants to see whether other universes could harbor life. We know that slight changes to the laws of physics would disrupt the evolution of the universe we live in. The strong nuclear force, for example, could have been just a bit stronger, or weaker, and stars would have been able to produce few of the elements needed to build planets. Remove the electromagnetic force and light would not exist, nor would atoms and chemical bonds.
Nudging Nature’s Parameters
Run through the constants of nature and you’ll find many that have to show precise values for life as we know it to have formed. Thus the idea that there may be not one but many universes, each with its own laws, and the thought that we happen to occupy a universe where the conditions that make life a possibility managed to fall into place.
Anthropic reasoning like this — things have to be this way because otherwise we couldn’t be here to think about all this — suggests that multitudes of universes exist, a multiverse in which almost all the universes would be devoid of life and, indeed, matter as we know it. Jaffe is interested in finding out whether universes with different physical laws might not be so inhospitable to life after all. His team focused on universes with nuclear and electromagnetic forces that allow atoms to exist. Another stipulation: Universes that allowed stable forms of hydrogen, carbon and oxygen.
Then it became a matter of playing with nature’s building blocks. Take quarks: In our universe, the ‘down’ quark is roughly twice as heavy as the ‘up’ quark, so that neutrons are 0.1 percent heavier than protons. Jaffe’s team lightened up the down quark so that protons were up to one percent heavier than neutrons. According to this modeling, hydrogen would no longer be stable, but the heavier isotopes deuterium and tritium would be. Carbon-14 could exist and so would a form of oxygen. It’s a different universe than ours, but the models say life could emerge in it.
Other quark variations, including one where the ‘up’ and ‘strange’ quarks have roughly the same mass, unlike in our universe, produced atomic nuclei made up of neutrons and a hyperon called the ‘sigma minus,’ which would replace protons. The fact that we have a reasonable understanding about quark interactions makes them useful for studies of this kind, but changing other physical laws is even trickier business.
Into a ‘Weakless’ Universe
Nonetheless, Lawrence Berkeley National Laboratory researchers have modeled universes that lack one of the four fundamental forces of ours. Without the weak force big bang nucleosynthesis — turning groups of four protons into helium 4 nuclei of two protons and two neutrons — would not have been possible. But when the team at LNBL removed the weak nuclear force in their models, they were able to tweak the other three forces to compensate. Stable elements could form in this universe as well.
Note what’s happening here. Rather than changing a single constant, the LBNL researchers tweaked several. After all, in a multiverse that can keep spewing out universe after universe, all combinations would seem to be possible and you can keep trying until you get it right. This Scientific American article by Alejandro Jenkins (MIT) and Gilad Perez (now at the Weizmann Institute) gets into the specifics:
In the weakless universe, the usual fusing of protons to form helium would be impossible, because it requires that two of the protons convert into neutrons. But other pathways could exist for the creation of the elements. For example, our universe contains overwhelmingly more matter than antimatter, but a small adjustment to the parameter that controls this asymmetry is enough to ensure that the big bang nucleosynthesis would leave behind a substantial amount of deuterium nuclei. Deuterium, also known as hydrogen 2, is the isotope of hydrogen whose nucleus contains a neutron in addition to the usual proton. Stars could then shine by fusing a proton and a deuterium nucleus to make a helium 3 (two protons and one neutron) nucleus.
But would these stars be anything like what we are familiar with? The article continues:
Such weakless stars would be colder and smaller than the stars in our own universe. According to computer simulations by astrophysicist Adam Burrows of Princeton University, they could burn for about seven billion years—about the current age of our sun—and radiate energy at a rate that would be a few percent of that of the sun.
A Strange But Living Universe
A strange place, this ‘weakless’ universe. Supernova explosions of the kind that synthesize and distribute heavy elements in our universe would not occur, at least not from the same causes, but a different kind of supernova caused by accretion rather than gravitational collapse would be possible, allowing elements to seed interstellar space. A planet like ours circling one of the weakless stars would need to be six times closer to the Sun to stay habitable. And check this out:
Weakless Earths would be significantly different from our own Earth in other ways. In our world, plate tectonics and volcanic activity are powered by the radioactive decay of uranium and thorium deep within Earth. Without these heavy elements, a typical weakless Earth might have a comparatively boring and featureless geology—except if gravitational processes provided an alternative source of heating, as happens on some moons of Saturn and Jupiter.
Chemistry, on the other hand, would be very similar to that of our world. One difference would be that the periodic table would stop at iron, except for extremely small traces of other elements. But this limitation should not prevent life-forms similar to the ones we know from evolving. Thus, even a universe with just three fundamental forces could be congenial to life.
Accounting for the Cosmological Constant
Still tantalizing is the cosmological constant, a measure of the amount of energy found in empty space. The discovery of the continuing acceleration of the universe’s expansion has brought ‘dark energy’ into the picture, implying a cosmological constant that is positive as well as minute, allowing the universe to form structure. It’s a constant that seems fine-tuned to a remarkable degree, and as the article notes, “…the methods our teams have applied to the weak nuclear force and to the masses of quarks seem to fail in this case, because it seems impossible to find congenial universes in which the cosmological constant is substantially larger than the value we observe. Within a multiverse, the vast majority of universes could have cosmological constants incompatible with the formation of any structure.”
All of this is almost joyously theoretical, basing itself on a theory of inflation that conceives of small pockets of spacetime that inflate so rapidly that it is impossible to travel between them. Inflation is highly regarded but not definitively understood, but different values for the constants of nature in the universes it produces seem like a reasonable conjecture. And the cosmological constant itself is an example of fine-tuning on such a scale that it may require the existence of a multiverse to give us a rational explanation for how we lucked into this one.