A Turn to the Dark Side

By John McCormick

WHEN SOMEONE IS STILL INCAUTIOUS enough to ask me to talk about science, I like to focus on what isn’t known rather than what is known. For me the really interesting parts of science are the unknowns. The unknown is exciting. Once something is known, proven, and generally understood, it passes on from scientists to the engineers who often find a way to make something useful out of the discovery.

Because dark matter and dark energy are by far the biggest unknowns in science today, I like to talk about those, in part because dark energy is fairly easy to address. Unlike esoteric mathematical concepts that require addressing many other advanced topics such as quantum mechanics or the Higgs Field, the twin darks are actually simple to introduce.

The reason they are simple is that no one actually has a good handle on what the twin darks are. There are no complex details; therefore, dark energy is easy to explain to a lay audience. Dark energy is whatever it is that is causing the universe to expand. Dark energy is, in essence, whatever is causing a near universal repulsive force to act throughout the universe.

It may be some failure in the theory of gravity; for example, “universal” gravitation fails at the quantum level—perhaps it also fails at extreme distance.

Dark energy may possibly be some failure in our observation or analysis of observations, although that is very unlikely because so many observational results agree.

The most likely explanation, based on today’s knowledge, is that dark energy is probably some basic property of space itself that we had not seen before because it only shows up at the extreme distances we have recently been able to explore.

So why is there such interest in something we know so little about? Simply put, there is literally nothing bigger in the universe than dark energy.

The universe consists of:

I. Visible matter (mostly hydrogen or hydrogen nuclei) and electromagnetic energy (E=mc*c), which people are usually surprised to learn only makes up about five percent of everything. This part is easy to measure because it either is radiation, generates radiation, or reflects radiation (x-ray, gamma ray, radio, visible light, and so on). This is the universe we see at night.

II. Dark matter, a theoretical physical material rather than something directly observed (you can’t take a picture of it). However, the amount of dark matter in the universe can be calculated directly using Newton’s mechanics by looking at easily observed gravitational effects on objects (mostly the orbits of stars and galaxies) as well as gravitational lensing (bending of light) and subtracting effects caused by visible matter and energy. Dark matter is calculated to account for about 27 percent of the total mass-energy of the universe and was first hypothesized by Jan Oort in 1932, the same Dutch astronomer and radio astronomer for whom the Oort cometary cloud is named.

III. Dark energy. At this stage, even those who “hate” math can add five percent to 27 percent and realize there has to be something more in the universe to make up a total of 100 percent. That something more is what cosmologists call dark energy and this totally unknown “something” is calculated to make up about 68 percent of the universe. Other estimates for the amount of dark energy in the universe range as high as 75 percent, but all estimates are in that same range. An incredibly large part of everything making up the universe is completely unknown and wasn’t even suspected until the late 1990s.

Cosmologists have been trying to find direct evidence for dark energy since it was first proposed as the only explanation of some observations, but just very recently there have been some positive results from the search.

But before we get to evidence, I want to explain just why astrophysicists think there is such a thing, and if there is any obvious alternative.

The Expanding Universe

The short answer to why we think there is something we call dark energy is because the universe looks as if it is expanding. It has now been shown that objects in the universe are not just moving away from each other but are doing so at an accelerating pace.

In a cosmos generated by the Big Bang, matter was expelled from the point of origin at a great velocity that can be easily calculated. It is also relatively easy to calculate that the gravitational attraction between galaxies should have caused the expansion to at least slow down if not reverse by now, 13.7 billion years later.

At this point you may be wondering how we can know how much dark matter there is, and that it causes a calculated amount of attraction. The short answer is that dark matter’s effects can be seen over relatively short distances—on the order of magnitude of galaxies. Vastly simplified, dark energy was first seen in effects on the scale of the universe.

Einstein is known for many things besides the famous E=mc*c equation. In fact, that isn’t even the reason he was awarded the Nobel Prize. In his early work, he ran into a problem with the General Theory of Relativity because it seemed to imply that the universe would have already collapsed due to gravitational attraction, or would be expanding. The problem with that was simple—astronomers in those days believed the universe was unchanging, neither expanding nor contracting.

So, in 1917 Einstein introduced the cosmological constant to his equations to permit an adjustment in the theory to explain why the universe wasn’t changing. Later evidence concerning the nature of the universe during the ’20s and ’30s (most notably by Hubble) showed the universe was actually expanding. This evidence led Einstein to reject the gravitational constant and say it was the biggest mistake of his professional life.

Just a word about Einstein; he did great with math but when he let metaphysics into his calculations it caused problems. What else can you call it when you remember he famously said “God doesn’t play dice?”

What Einstein later saw as his biggest blunder has more recently been shown to be one of his greatest triumphs, and his only mistake was merely a minor error in the magnitude and sign of the cosmological constant.

Why do we think the universe is expanding?

Back in the 1930s, Edwin Hubble showed by observation of the spectra of stars that they were all moving away, not just from us but also from each other, hence that the universe was expanding. This was actually predicted by Lemaitre by extrapolating the General Theory of Relativity, but Hubble expanded on the theory and conducted a series of observations that provided the first solid evidence of an expanding universe.

There were some weird theories (such as tired light) proposed to get around the red shift seen in the spectra of all stars, but eventually it became a basic part of cosmological theory that the universe was, in truth, expanding. To be more precise, because each galaxy appears to be moving away from every other galaxy, it is now common to analogize the universe likeraisins in bread dough, where the raisins are galaxies and the dough is space itself that is expanding, causing every raisin to move away from every other raisin.

How do we know there is something we refer to as dark matter? [Right, dark matter map in galaxy cluster Abell 1689, taken by Hubble Space Telescope.]

The simplest answer makes it obvious to anyone who has abasic grasp of Newton’s law of gravity. The people responsible for the “discovery” of dark matter were measuring the orbital velocity of stars making up the Great Nebula in Andromeda (M31).

What they expected to find was that the farther from the center a star was, the slower it would be moving in its orbit, just like the planets in the solar system. Because the gravitational pull of the sun (and central core of a spiral galaxy) decreases with distance (actually the square of the distance), the velocity needed to remain in a stable orbit decreases the farther you are from the center of a large mass.

But what they found instead was that stars close to the center of the galaxy were moving at almost exactly the same speed as those furthest away in the galactic arms. This was later confirmed for other spiral galaxies.

Unless we are prepared to throw out Newton’s law of gravity that works so well in every other test and observation, the only explanation for the equal velocities is if the pull of gravity doesn’t decrease as you get farther from the center of the galaxy, and that means there must be much more matter scattered relatively evenly throughout the galaxy than we can see in the form of stars. In other words, to explain the orbits of stars in spiral galaxies we have to assume there is some vast quantity of invisible, or dark, matter.

Dark Energy

Between 1998–2000, two groups of cosmologists looking at entirely unrelated sorts of data found that their results indicated, in one case, that the universe was expanding and accelerating and, in the other, that there was far too little known energy to account for the structure of the universe as shown by cosmic background radiation maps.

A specific kind of supernova, specifically a Type 1a, was the subject of study of the first group, which found the supernova were too dim to match the expected magnitudes. Type 1a supernovas are the spectacular white dwarf binary system supernovas where the smaller star shrinks to the point where a teaspoon full of star matter would mass five tons and begin to suck matter from the larger companion.

When the Chandrasekhar limit is reached, the star explodes so the light output and decay curve is always the same, providing a standard candle for distant galaxies where Cepheid variables (used by Hubble) are no longer visible.

The other group (“Science,” January 30, 1998, p. 651) looked at the background radiation of the universe and saw that the fabric of the universe was too flat to be explained on the basis of the known energy (mass and energy being the same thing).

Although the results of those observations and later analyses of those and other sets of data provided very convincing evidence of some previously unknown repulsive force, what we call dark energy, which acts as if every part of space has a small but predictable repulsive force, there was no direct observation of the force.

Physical Evidence

That brings us up to today. Just the past few weeks have seen the first publication of data providing direct evidence of dark energy.

This July, the Cornell University Library pre-publication server carried a report titled “Physical Evidence for Dark Energy.” Although the evidence is strong, it is, unfortunately, too esoteric to explain easily and involves a statistically significant correlation “between luminous red galaxies from the Sloan Digital Sky Survey and the cosmic microwave background temperature maps from the Wilkinson Microwave Anisotropy Probe.” Numbers beyond that that can be explained by known effects.

The data analysis first had to account for the Integrated Sachs-Wolfe (ISW) effect—gravitational redshift of photons in the cosmic microwave background (CMB) radiation—and the Sunyaev-Zel’dovich (SZ) effect (a CMB signature modified by high-energy electrons in galaxies the radiation passes through). Neither ISW or SZ effects are part of dark energy; they are merely the two known effects that had to be accounted for before deciding there is remaining evidence for the presence of dark energy.

Sorry about that, but it is a lot worse with the math. When looking for something that no one knew existed until a decade ago, the evidence is almost certainly going to be mostly indirect or derived from analysis of signals which are of a very low magnitude.

Another publication a month earlier, titled “Cosmology: Hydrogen wisps reveal dark energy,” “Nature” June 13, 2013, reported that traces of hydrogen measured over enormous regions of space have been used as a yardstick to actually measure the magnitude and map the location of dark energy.

The Baryon Oscillation Spectroscopic Survey (BOSS) is a study that maps out the distribution of red galaxies and quasars. The study was designed to span the galactic distance (andhence the age) of the universe between the nearby (easily observed) portion of the universe, and the extreme outer edge—regions already mapped.

One of the more surprising things about the expansion and acceleration is that this acceleration only began about seven billion years ago. For half the life of the universe, all space was expanding, but the expansion wasn’t accelerating as it is today. [Illustration of the concept of baryon acoustic oscillations, left, which are imprinted in the early universe. Courtesy of Chris Blake and Sam Moorfield.]

BOSS is unusual as astronomical research goes because it looks at sound patterns. Now you might be about to argue that sound doesn’t travel through a vacuum (in space no one can hear you scream) but at the time of the Big Bang and for about the first 330,000 years after point zero, the universe was compact enough that sound waves could travel through the matter, and at that point the pattern of the waves were impressed on the matter that eventually became galaxies.

The BOSS survey took that information and added to those results data gathered from observing the light from quasars (black holes at the center of many galaxies) acting as a searchlight to shine through the hydrogen gas that permeates the space between galaxies (empty space is actually quite crowded). The pattern of the hydrogen gas density was found by measuring the amount of light absorbed.

These two completely unrelated studies provide physical evidence of dark energy in distant parts of the universe.

Local Dark Energy

Much closer to home, researchers are trying to measure dark energy by bouncing laser beams off the moon and are very close to doing so.

But you may have noticed that I haven’t actually described what dark energy is. The reason is simple.

I don’t know.

I don’t feel in the least embarrassed to admit that because neither does anyone else.

(Perhaps I should point out that no one knows what gravity or time are either, although we think we have a handle on mass with the discovery of the Higgs particle. So, despite the advances of the past century, physicists still have a few minor problems to work on.)

Quantum mechanics supports the existence of a force inherent in space itself, but this force as predicted would be far too powerful, vastly too powerful, to be dark energy.

In 2010, Claudia de Rham, a cosmologist at Case Western Reserve University, proposed a complex theory regarding a purely theoretical graviton particle spread throughout space that could/would absorb a lot of the energy which quantum theory predicts is inherent in space itself. This particle, if it exists, would weaken the repulsive energy enough that the excess force would account for the observed magnitude of dark energy.

A graviton is the as-yet-undetected particle that “conducts” the force of gravity, just as photons carry electromagnetic force. Because photons are massless, EM force has an unlimited range. The weak nuclear force is carried by massive (not big, just having mass), as opposed to massless, particles and has a very short range.

Gravity, of course, has a very long, possibly unlimited, range but a graviton could have an extremely small but still non-zero mass.

(Just a random thought of my own which I’ve had over the years: Could it be that the universe is expanding at an accelerated rate because gravity weakens dramatically at really extreme ranges, making gravity slightly less than “universal” because it fails both at extreme long distance and at extremely short quantum distances? The fact that the acceleration didn’t begin until the universe had expanded for seven billion years also means it didn’t occur until the distances got really big.)

Unlike dark energy models that can theoretically be measured in deep space but can’t be experimented with directly, a slightly massive gravity particle could theoretically affect the gravity between Earth and the moon just barely enough to be measured as a variation in the moon’s orbit.

In fact, the amount of precession predicted to occur in the moon’s orbit due to the theoretical “fat” (massive) gravitons is just one order of magnitude beyond today’s ability to precisely measure the moon-Earth distance by bouncing laser beams off the mirrors that were left on the moon.

One order of magnitude is so close when looking at such measurements that relatively minor improvements in equipment (such as generating a shorter light pulse) or experimental technique are likely to make the effect measurable, therefore making it possible to have yet a third direct proof of the existence of dark energy—a major discovery about the biggest thing in the universe. 

John McCormick is a trained physicist, science/technology journalist, and widely-published author with more than 17,000 bylines to his credit. He is a member of The National Press Club and the AAAS. His full bibliography can be accessed online.