Dark Energy

FAS Astronomers Blog, Volume 30, Number 1.

In a previous article, I explored the Standard Model of Particle Physics and discussed what ordinary matter is made of. It can be a bit confusing because of the different ways to look at it.

• Particles are either Fermions (those subject to Fermi-Dirac statistics) and Bosons (those subject to Bose-Einstein statistics).
• Matter (Fermions) is divided into Hadrons (particles subject to the strong nuclear force) and Leptons (particles that are not).
• Hadrons are composed of Quarks (or antiquarks).
• Baryons, which are Fermions, are particles composed of three quarks (or three antiquarks).
• Mesons, which are Bosons, are particles composed of a quark and antiquark pair.

Matter is made up of atoms, which are composed of protons, neutrons, and electrons. Protons and neutrons are baryons composed of three quarks (proton: up, up, and down quarks and neutron: up, down, and down quarks). Electrons are leptons and considered to be fundamental and are not composed of any other particles. Most of the atomic mass comes from the protons and neutrons, which again are baryons.

Therefore, most of the matter we see is composed of baryons, so astronomers call all ordinary (i.e., visible) matter baryonic matter. However, baryonic matter isn’t all there is. Dark Matter appears to dominate the universe and possibly provided the scaffolding upon which galaxies were built. But dark matter is not the only mysterious component of the universe.

Gravity is all around us as the Earth constantly pulls us toward the ground. The first complete theory of gravity was published in 1687 by Isaac Newton with his Mathematical Principles of Natural Philosophy (Principia). Newton viewed gravity as a force acting between objects. You might remember his law of universal gravitation from high school, F = Gm₁m₂/d², where F is the force of gravity, m₁ and m₂ are the mass of two objects, d is the distance between the objects, and G is the gravitational constant.

Later in 1915, Albert Einstein expanded on Newton’s picture of gravity with his General Theory of Relativity. Einstein treated gravity as something that warps/curves space-time. His equations are not as simple as Newton’s. They look something like this: Rmn – ½ gmn R = -k Tmn. Physicist John Wheeler is given credit for summarizing them as “matter (mass) tells space-time how to curve and space-time tells matter (mass) how to move.”

Today, most astronomers believe that the universe began with something called the Big Bang and it has been expanding ever since. Astronomer Fred Hoyle coined the term “Big Bang” back in 1949 and it was meant to discredit the theory that the universe began at some point in the past. However, the term caught on and has stuck with us ever since.

Einstein’s General Theory of Relativity led Willem de Sitter, Alexander Friedmann, and Georges Lemaitre to conclude that the universe is expanding (or at least is not static). Observations by Edwin Hubble, along with Vesto Slipher and Milton Humason, showed that most galaxies are moving away from us, further evidence for an expanding universe. In the 1960s, Arno Penzias and Robert Wilson detected the Cosmic Microwave Background (CMB), which is the remnant radiation left over from the time when light first began to spread throughout the universe, some 380,000 years after the Big Bang.

Initially, Einstein rejected the notion that the universe was expanding, so much so, that he added a cosmological constant (Λ) to his equations of relativity, Rmn – ½ gmnR – Λgmn = -kTmn. The constant was designed to balance the perceived expansion and preserve a stable universe. He later relented after hearing of Hubble’s results and is alleged to have called the cosmological constant his worst blunder.

The universe is full of mass (both visible and not visible), so one would think that gravity is causing the rate of expansion to slow down.

In 1998, two groups, the Supernova Cosmology Project led by Saul Perlmutter and the High-z Supernova Team led by Brian Schmidt and Adam Riess, decided to check this out. Both teams measured the rate of expansion by observing type 1a supernovae far out in the universe. A type 1a supernova is the explosion of a white dwarf star after it has absorbed material from a companion star and reached a critical mass called the Chandrasekhar limit. Because the star explodes at a known mass, its intrinsic brightness is known, and its distance can be determined.

The two teams carried out observations of supernovae in galaxies far out into the universe. They found that the observed supernovae were dimmer than expected and, as such, further away than expected. The universe was not slowing down, it was accelerating. Something was overcoming gravity and pushing galaxies away from each other at an ever-increasing rate. In 2011, Perlmutter, Schmidt, and Riess received the Nobel Prize in Physics for their discovery.

So, if the Universe is expanding, what is driving the expansion? Like dark matter, astronomers just don’t know, so it has been given the name “Dark Energy”. It does seem, however, that the universe was slowing down for a time, but after some 8 to 9 billion years, dark energy kicked in and the acceleration began. The density of the universe provides a possible explanation for this. Just after the Big Bang, radiation dominated the universe. However, as the universe cooled, matter (both ordinary and dark) came to dominate. Then as the universe continued to expand, the density of matter became less and less. After 8 or 9 billion years, dark energy took over and now drives the acceleration of the universe’s expansion.

Dark Energy might be associated with the cosmological constant (Λ), and maybe Einstein was correct all along. Although, if the cosmological constant is real, it is not something that results in a stable universe. The generally accepted theory of the universe (i.e., the standard model of the universe) is now referred to as the Lambda Cold Dark Matter (ΛCDM) model. It implies that dark energy is a constant and doesn’t vary over time (at least over the last 5 billion years or so).

An alternative thought is that dark energy does vary over time and is closer to something called Quintessence. Quintessence is a medieval term for aether, which was the ancient Greek’s “fifth element” beyond earth, air, fire, and water.

Although we don’t know what dark energy is, when we measure the full extent of the universe, thanks to Einstein’s E = mc², astronomers think that it is around 68% dark energy, 27% dark matter and only 5% baryonic matter. Note that these percentages change slightly as more precise measurements are taken, but the main point is that the stuff we can see is a very small percentage of the stuff that is there.