The Cosmological Constant in Modern Physics

        Albert Einstein’s General Relativity improved upon the gravitational theory of Isaac Newton, but, like Newton, he was at a loss to explain why the universe did not contract from gravity and collapse in on itself. With no force to counteract gravity, all matter would remain in a constant state of clumping together, and there would be no reason why the universe should appear so diverse and expansive. Einstein believed in a static universe, one that was neither expanding nor contracting. In order to avoid an infinite limit in his field equations and to make his equations engender a static reality, he inserted a fudge factor into his equations, the variable lambda, providing the exact outward force necessary to stabilize the universe. He made no hypothesis for what this “cosmological constant” represented on a physical level, but the apparent stability of the universe necessitated its presence in the equations. Little more than a decade later, the astronomer Edwin Hubble discovered that the universe was not static at all. In fact, virtually all observable star systems were moving away from each other.
        If the observable universe is expanding and all of the matter in the universe is attracting other matter through gravity, it seems that the expansion would have to be in the process of slowing down. Surprisingly, the opposite is true. Several studies in the last two decades have confirmed that the observable universe is accelerating in its expansion. The question then remains: what is causing the expansion? What is the physical embodiment of lambda? For some physicists, the cosmological constant needs no explanation. They seek only to find lamda’s exact value, while leaving it as an abstract figure. Astrophysicist Jim Peebles, for example, wrote in 1983 that the cosmological constant was “ugly” and “an addition,” but this did not stop him from declaring that “the universe might have put in bells and whistles—a cosmological constant.” Nor has the arbitrary nature of lamda stopped the many other notable commenters in the years since Peebles published his own work. The nineteen nineties saw the publication of several pro-lamda papers, with titles including “WHY A COSMOLOGICAL CONSTANT SEEMS INEVITABLE,” “The Observational Case for a Low-Density Universe With a Non-Zero Cosmological Constant,” and “The Cosmological Constant is Back.”
        Many physicists, however, rightly felt that the cosmological constant must arise from some sort of physical phenomenon. The answers they offered were dark matter and dark energy, invisible hypothetical substances which must fill space and account for more than 90% of the mass in the universe. Perhaps because of the aesthetically pleasing epithet, dark matter has captured the public imagination and rapidly become a popularly accepted explanation for universal expansion. The fervor over dark matter has reached such a fever pitch that Lisa Randall, who the reader will recall from the last chapter, has even written a book titled Dark Matter and the Dinosaurs, which claims that a spinning disk of dark matter flinging asteroids around was inadvertently responsible for the extinction of the dinosaurs. Far-fetched as that may sound, the book was a bestseller. There is, however, convincing evidence to suggest that the universe is genuinely “missing mass,” that visible matter fails to account for many of the gravitational effects we can observe in distant galaxies. “Dark matter” does provide a quick, simple solution to this mystery, but ultimately it is an arbitrary one, and it complicates our picture of the universe by adding yet another form of fundamental matter. Without direct physical evidence, it is ludicrous to propose that the universe is filled to the brim with invisible particles, and few physicists would be willing to admit that this proposition is functionally identical to the classic aether theories which they regard as facile and antiquated. The invisibility of dark matter particles renders their existence or nonexistence inherently unprovable. It is like postulating the existence of a ghost and claiming that, since you can’t see it, you have no proof that it isn’t there.
        Until someone can verify the existence of dark matter particles, we have no choice but to attribute the missing mass to the field of space itself.

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