Dark Anything

Did I mention that I’m an atheist? Or rather, an agnostic on steroids. It’s not that I hate or despise or actively deny the existence of a god—I just don’t see or feel the need for one. But, with a gun to my head, I must admit that I just don’t know. And this state of unknowing, along with healthy dashes of doubt and skepticism, colors my tendency to disbelieve in anything I cannot prove or have logically demonstrated to me.

So, um, physics … The acceptance of and belief in what’s called “dark matter” is based on the motion of stars in a galaxy. We add up all the things we can see at galactic distances—that is, the things that shine brightly—and compare them to what we see in our own solar system and nearby detectable systems. The biggest things, the heaviest things, and the ones that shine most brightly are stars. Our Sun is about 99.9% of the mass of our local system. Not even Jupiter and Saturn compete as heavyweights. The rocky planets, the moons, the asteroids, and the icy comets are just a rounding error. That bright thing up there in the sky—the thing that can be seen from other systems in the Milky Way, if anyone is looking—is essentially the mass of our solar system. And we have no evidence that other stars, other bright things we can see, are any different.

Well and good. But add up all the bright things in visible galaxies, estimate their collective mass, and compare that mass and its computed gravitational effect to the observed motion of the galaxy’s stars—and you get an anomaly. The stars in the average galaxy, based on the mass holding them together as a system, should be rotating like wood chips swirling in a whirlpool. That is, the ones near the center should be circling faster than the ones on the periphery. But instead, we observe that they rotate like stars painted on a disk, with the outer ones keeping pace with, and so moving faster than, the inner ones. For that to happen, the galaxy must contain more mass—have a higher gravity attraction and greater bending of spacetime—than the sum of the mass of the bright things. From this is born “dark matter”—can’t see it but weighs a lot. Current candidates are “weakly interacting massive particles” (WIMPs), which would be really big subatomic particles that don’t show up in any reactions, and “massive cometary halo objects” (MACHOs), which would be big objects in the periphery of galaxies, like huge rogue planets or roving black holes, that just don’t shine.¹

The other issue in physics is the expansion rate of the universe. We know that the universe is expanding because we can use “standard candles”—certain periodically variable stars and certain types of exploding stars, both with a known brightness—to measure the distance to these galaxies. By knowing the actual brightness of these objects and comparing that with how bright they appear in our telescopes, we can estimate how far away they and their home galaxies are. In the same way, if you know that the light you see on a distant hill is from a 100-watt bulb, you can know that it’s closer than if it was from a 1,000-watt bulb, which would appear to be farther away.

But the other thing we see, aside from the apparent brightness of known stars, is that the light from all the stars in a particular galaxy, including these standard candles, is “red-shifted.” That is, instead of being, say, bluish-white—as we would expect to see in the light from a similar star in our own galaxy—the light from distant galaxies has a longer wavelength, more reddish in appearance. This is the Doppler effect. If the stars were coming toward us—and some galaxies, like Andromeda, are actually approaching the Milky Way—then the light waves would be compressed, more bluish, just as the sound waves of an approaching train’s horn are compressed and so appear higher in pitch. But stars that are going away shift redder, their light waves more stretched out, like the descending wail of a train that’s leaving you. And the farther away a galaxy is, the redder its apparent light. So from this, astronomers conclude that the universe is expanding.

Well, yes. It supposedly exploded from a single infinitely hot, infinitely dense point some 13.8 billion years ago. So, of course, it’s expanding. But based on observations of galactic red shifts at various distances, as well as from analysis of the cosmic microwave background radiation—the fading “echo” of the Big Bang—the expansion rate is apparently increasing. The momentum of the explosive expansion isn’t fading or even holding steady—it’s speeding up. Nothing in current physics explains this result, and so astronomers have proposed that the universe is influenced by something called “dark energy.” There is no good candidate for this energy, except perhaps some kind of “vacuum energy” that resides in and drives the expansion of empty space itself. But whatever it is, it’s vast.²

If you collect all the visible, bright matter we can see in the universe and compare it to our computation of the amounts of dark matter necessary to keep the galaxies spinning like they do, then dark matter makes up about 85 percent of the physical stuff in the universe—way more than the bright objects we can see and, supposedly, interact with.

And if you try to account for all that dark matter plus the dark energy necessary to create and drive the expansion of the known, observable universe, then about 69 percent of everything is dark energy, 26 percent is dark matter, and only 5 percent is the familiar, atomic matter that we can see as stars, planets, dust, gases … stuff. So, in short, we know very little, and have only vague conjectures and initial theories, about the vast majority of the universe we live in.

That’s either a shameful admission—or a vast opportunity.

In a blog last month, I mentioned a breakthrough discovery that gravity—which has long been conspicuously absent from the calculations of quantum mechanics, the science that deals with the invisible world of subatomic particles—may actually exist at and be measurable at the microscopic level. Einstein’s theories of relativity, dealing with the macro world of planets and stars, and quantum mechanics, governing the realm of the unbelievably small, were long thought to be mutually exclusive and irreconcilable. But if gravity is exerted by grains of sand and maybe by subatomic particles, then we may finally be in line to create a “Theory of Everything,” combining relativity and quantum physics—the goal of physicists since early in the twentieth century. That may eventually explain mysteries like dark matter and dark energy.

What if gravity—the bending of spacetime according to the amount of available mass—exists and could be measured not just at the level of stars and planets, but in grains of dust, atoms of dispersed gases, and subatomic particles flying through “empty” space? The effects would be small, vanishingly small. After all, the effects of gravity are subtle. It’s the weakest of the four known and fundamental forces.³ The mass of the entire Earth has such a weak gravity field—an acceleration of 9.8 meters per second squared toward its center from the planet’s surface—that you can overcome it briefly simply by jumping and more permanently by firing off a chemical rocket.

But over vast distances, the distances between galaxies? We know that the bending of spacetime caused by a gravity field creates a “time dilation.” That is, time slows down under the influence of gravity. A clock on the surface of the Earth ticks more slowly than one out in interplanetary or interstellar space. It’s not just that the clock’s mechanism is retarded by the force of gravity, but time itself as the clock measures it has slowed. And a clock positioned near the event horizon of a black hole is slowed so much that the passage of hours it records would register as years to an astronaut orbiting outside the black hole’s effective gravity well.⁴

So … If dust, gases, and subatomic particles hanging about in intergalactic space have actual gravity effects—bending spacetime for anything small enough that passes near them, like a moving photon—might not that effect the timing, the energy level, of the photons themselves? This is a conjecture, similar to but not exactly the same as the “tired light” hypothesis described in the second footnote below.

I told you I was an agnostic, and I write a kind of science fantasy—not real but hypothetical and, hopefully, barely plausible. As a skeptic, I don’t accept that our current knowledge of anything is actually final.

Sir Isaac Newton refined the observations and theories of the ancient Greek and Babylonian astronomers. Then Albert Einstein refined the observations and mathematics of Newton. And then Niels Bohr and the quantum physicists threw it all into a cocked hat at the subatomic level. Edwin Hubble observed that the light of distant galaxies was red-shifted and so moving away from us, and that the universe was expanding. Cosmologists then used this expansion to wind it back to a single point some 13.8 billion years ago, and the decay of the cosmic microwave background as measured by Arno Penzias and Robert Wilson seemed to confirm that dating. Then astronomers figured that the universe is actually larger than it could have grown in that time, and so Alan Guth proposed inflation—that at the moment of the Big Bang, the universe expanded exponentially fast, faster than light speed—to finally arrive at the observable scale. Now we have dark matter and dark energy—unknown and, so far, undetectable “things” and “forces”—to explain anomalies in our observations that don’t fit our theories.

As I said before, these are exciting times. But the whole thing may eventually get tossed into a cocked hat. The Big Bang is, after all, just another creation story, like God moving across the waters and separating the light from the dark, or Great Raven dropping a wing feather upon the Earth to create everything. We yearn to know. And when we don’t, we keep piling theory on theory, until eventually we come upon a universe made up of things and forces we can’t know and can only imagine.

We ain’t done yet.

1. By now it’s generally accepted that our galaxy—and every other one we can study at close range—include a massive black hole at their center. These invisible objects have a mass on the order of a million suns, give or take. The one at the center of the Milky Way is about 4.3 million suns. So, why doesn’t that constitute “dark matter” all by itself? Well, our galaxy has over 100 billion stars; so the central black hole is not even one percent of galactic mass—kind of the situation of all the rocky planets, moons, and asteroids in the solar system.

2. An alternate theory has been proposed to this red-shifted Doppler effect—one that would discount the continuous expansion of the universe in the first place. Suppose light that travels long distances just “gets tired.” Suppose that photons traveling so far—not just between the Sun and the Earth, or from nearby stars to the Earth, but the billions of light years between galaxies—tend to lose energy, so that their wavelength becomes increasingly longer and redder. But there doesn’t seem to be any mechanism driving this effect. Nothing in the vacuum of space exists to “sap the energy” of a moving photon. And just as a body in motion tends to stay in motion—according to the first law of Sir Isaac Newton—until and unless some outside force acts upon it, so light waves should retain their energy unless they interact with something.
    It is part of the General Theory of Relativity that objects in deep gravity fields—spacetime that is bent sharply enough—experience time dilation, as described above in the main text. A physical object in the gravity field of a black hole and approaching its singularity or one moving at light speed—if it could ever attain that velocity—would experience the complete stoppage of time. So presumably a photon, which has no mass, experiences a timeless, changeless existence. But might it not also, at that speed, because it is moving through the dilation, lose just a bit of its energy, become just a little red-shifted, and more so the longer it travels? Conventional physics and cosmology say not, but our observations and theories are continually evolving.

3. The strong nuclear force, which holds quarks together to form protons and neutrons; the weak nuclear force, which holds these particles together in atomic nuclei; the electromagnetic force, which holds electrons in their atomic “orbits” or shells and accounts for the joining together of atoms into molecules, among other effects; and gravity, which bends spacetime around large masses and holds planets, stars, solar systems, and galaxies together.

4. This was one of the effects correctly described—not all of them were!—in the 2014 movie Interstellar.