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What is Dark Matter and Dark Energy?

Updated on July 20, 2014
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Introduction

(*Please refer to the Glossary at the end of the article for help with bold, italicized terms. There are quite a few throughout, but I wanted to make sure the reader received an in-depth explanation for each term.)

“Gravity - Take the earth away and the stone won't fall.”—Arthur Schopenhauer (1788-1860)[i]

This article covers gravity, dark energy, dark matter, black holes, and many other strange entities that, for the most part, are mysteries to cosmologists. Gravity is one of the four established forces of nature and dark energy may be the fifth, but physicists have yet to witness any of its properties. Indirect evidence of dark matter and energy was established with the launch of WMAP in 2001, and the data indicates most matter in the Universe is “dark,” or unseen. Based on these findings, what we see and experience accounts for only 4.6% of the total matter in the Universe. Some argue this means the Universe is either larger or older than physicists suspect, and that dark matter and energy are nothing more than a superfluous excuse.

As of late, scientists have discovered the elusive Higgs boson particle, captured clear images of a quasar, and snapped a time-lapse photo of the deepest image yet of other galaxies, indicating there are perhaps 200 billion or more of them in the Universe. The image reaches back 13.2 billion years in the history of a 13.8-billion-year-old Universe.

Discoveries are occurring at breakneck speed, which point to a new renaissance in physics. Sooner than we might think, scientists may come up with a theory to explain everything, and it could happen before the end of this century.

[i] Schopenhauer, Arthur. German Philosopher. 1788-1860.

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Gravity—The Weak Link

“We can lick gravity, but sometimes the paperwork is overwhelming”—Wernher von Braun[i]

Astronauts in orbit do not experience weightlessness. They actually experience gravity by falling toward Earth in a circular orbit and with such a force they fall at the same rate as Earth while it curves away beneath itself. Since there is no air resistance in space, this falling continues indefinitely, thereby producing a circular orbit.

Is gravity an illusion brought on by the apparent curvature of space-time? Gravitating bodies like planets and stars affect this curvature by existing within the three-dimensional framework.

Gravity is the attractive force between two masses yet is the weakest of the four forces of nature. It appears strong because of our interaction with it. (Refer to Figure #1 above for a visual Diagram.)

In addition to matter particles, there are other particles, called force carriers: for the electromagnetic force, it is the photon; for the weak nuclear force, W and Z bosons; for the strong nuclear force, gluon; and for gravity, the graviton, though proof of its existence has yet to surface. A massive particle, called the Higgs boson, also exists. It is responsible for the masses of all other particles. Some scientists believed all mass originated from this single particle, and in July 2012, physicists made this elusive discovery. It was the Holy Grail of particle physics and will be a major asset to scientists in that field. Because of this revelation, they will have the ability to reach into the very fabric of the Universe.

The electromagnetic force is associated with the electron, the strong nuclear force acts between protons and neutrons, the weak nuclear force is associated with the neutrino, and gravity plays a role in the interaction between electrons. These four basic forces of nature are found in all atoms and dictate interactions between individual particles.

Electricity is nothing more than an electric charge. The textbook definition indicates it is a fundamental entity of nature consisting of negative and positive kinds. Magnetism is the property of an object able to attract another.

Electromagnetic theory unifies electricity and magnetism. It is responsible for the structure of everyday matter from atoms to stars. The electromagnetic force is the combination of electricity, magnetism, and light. It governs the properties of electrons and light, also referred to as quantum electrodynamics (QED).

The weak force is responsible for mediating certain nuclear processes to include those reactions that make the Sun shine. It is responsible for radioactive decay and the production of radioactive waste. The weak force governs the properties of leptons and neutrinos. The combination of electromagnetic theory and the weak force unite as electroweak theory. To simplify, it describes electromagnetism and its weak interaction.

The strong force, referred to as the color force, or quantum chromodynamics, is responsible for binding quarks together to make hadrons and mesons. QCD provides the energy that fuels stars and makes them shine. It also gives power to hydrogen bombs. This force governs the properties of protons, neutrons, quarks, gluons, and the pi-meson. Unlike the other forces, it does not decrease in force with increasing distance between particles. The “colors” in QCD refer to the different classifications of quarks, not their actual hue.

The combination of the previous forces, minus a complete picture of gravity, produces the standard model of physics. This model is used to describe all known matter in the Universe except dark matter or dark energy.

Gravity is the most mysterious and weakest of all the forces we experience but is the one that holds together the planets, stars, and galaxies throughout the Universe. The gravitational force keeps Earth’s atmosphere in place, not to mention one’s feet on the ground.

Why is gravity so weak when, to us, it appears so strong? A small magnet, for example, can overcome the entire gravitational pull of the entire mass of Earth by lifting a small nail off the ground. Many physicists believe gravity appears weak because, unlike the other forces, it is dispersed throughout higher dimensions.

Physicists are unsure how to combine gravity with the remaining forces. Concerning supersymmetry, most believe superstring theory is the candidate to combine all four forces into a theory of everything, or TOE. This ultimate theory would help scientists understand if there was a beginning to the present expansion of the Universe.

In string theory, or M-theory, the fundamental entities are not particles at all, but smaller string-like loops whose size is the Planck length, where incompatibility between general relativity and quantum physics arises. Different vibrations in the strings correspond to different elementary particles. All particles are aspects of a single underlying entity—the string. Because it is an extended object, the interaction between two particles occurs not at a point in space and instant in time, rather spread out over all of space and time. Strings do not occur in just the ordinary four dimensions of space-time, but in a space of up to 10 dimensions.

String theory awaits an ultimate interpretation to unify these forces and solve the biggest mystery in quantum physics. Discovery of the Higgs boson particle might help, but scientists have yet to find the right reference frame to comprehend all of its properties.

[i] Braun, Wernher von. Chicago Sun Times, Chicago, Illinois. July, 10 1958

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Dark Matters

“…not everything that can be counted counts, and not everything that counts can be counted”—William Bruce Cameron (Sign that hung in Einstein’s Princeton office)[i]

“Duct tape is like the force. It has a light side, a dark side, and it holds the universe together”—Carl Zwanzig[ii]

Normal observable matter, such as stars, planets, and galaxies, consists of only 4.6% of the total known matter in the entire Universe. The rest is “dark” and invisible to the naked eye. In other words, it is matter with no baryons. We would not be able to see or feel it since non-baryonic matter would pass right through any baryonic matter we see and feel. (See Figure #2 above for a visual Diagram.)

Cosmologists, with help from WMAP, determined dark energy is the unseen material responsible for 72.7% of the missing matter needed for the observed gravitational forces pushing galaxies and star clusters apart. It is the only viable explanation why visible matter appears so uniform and evenly distributed.

Quintessence is one hypothesis that explains dark energy and might determine a fifth force in nature. The gravity from dark energy repels, as opposed to normal matter and energy which attracts. This distributed energy spreads itself out in clumps throughout the entire Universe, much like cosmic background radiation. Gamma-ray bursts may provide evidence for it, but scientists continue studying the correlation. Theoretical physicist Nikodem Poplawski thinks they might be a type of Morse-code message from intelligent beings in another universe, but that is a topic for a later chapter.[iii]

Dark matter is responsible for the remaining 22.7% of matter in the Universe. This strange, unseen matter explains why galaxies and star clusters stay grouped together instead of flying apart. Without it, cosmologists believe a galaxy could never form. Gas would just dissipate rather than clumping together.

Dark matter is different from dark energy since they explain two different concepts regarding the structure of the Universe. Dark matter explains why galaxies and star clusters stay grouped together, and dark energy explains why these groups expand apart from each other and are so uniformly dispersed.

Dark matter and energy do not emit light or electromagnetic radiation and are invisible to the naked eye. They would reside in other dimensions as a reason for their elusiveness. Some evidence suggests scientists might discover a way to detect their presence by way of gravity without ever seeing either. (Indirect evidence is often used in science. For example, though astronomers cannot collect direct samples from stellar objects, by observing known characteristics of their spectra, they are able to infer their properties.)

There are a few non-baryonic candidates for dark matter and energy: black holes greater than or less than one magnitude, superstrings, brown dwarfs, old, burned-out white dwarfs, magnetic monopoles, and neutrinos to name a few. If combined, these entities might account for some of the absent matter in the Universe but not all.

Dark matter and energy are “dark” because the photon cannot travel across the void from a different dimension to this one. Such matter could be very different or very similar to matter in this dimension but stuck on a different dimensional wall. Objects on the other side of a dimensional fold will appear distant, even if they are less than a millimeter apart between realms. The light must travel the length of the Universe to the fold and back again. If the crease is billions of light years away, no light from the other side has yet reached the Milky Way since the Universe began.

One interpretation of dark matter and energy relies on a matter of perspective and perception. The reason we see normal, three-dimensional matter is due to the repulsion of electrons that exist on the outer shells of atoms. This is what makes objects appear solid and gives them apparent substance. In principle, no matter is really solid. Dark matter may be similar to normal matter, but have no electric charge. Because of this, it may interact with only a handful of atoms within any three-dimensional substance. Like gravity, dark matter and energy may be spread throughout multiple dimensions.

[i] Cameron, William Bruce. Informal Sociology: A Causal Introduction to Sociological Thinking. New York: Random House, 1963, p. 4.

[ii] Zwanzig, Carl. (unsourced)

[iii] Than, Ker. “Every Black Hole Contains Another Universe?" National Geographic Daily News. (Apr 9, 2010.)

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Heavy Fuel

“The crux…is that the vast majority of the mass of the universe seems to be missing.”—William J. Broad[i]

There are some stars scattered throughout the Universe so dense that one contains more mass than the Sun and all planets in the Solar System combined. Such exceedingly dense celestial objects are neutron stars and pulsars. They are very small and very heavy since they are in the process of collapsing in on themselves. Super-dense stars like these are unable to withstand the overwhelming force of their gravity. Their ultimate fate will be either a black hole or cold, dead star.

If a massive star is one and a half times or less the mass of the Sun, it will stop contracting and become a white dwarf. Stars of less than 1.44 solar masses end up as these densely gaseous entities. Their radius is a few thousand miles with a density hundreds of tons per cubic inch.

In the distant future, the Sun will cool and become a white dwarf. In about five or six billion years, it will swell into a red giant and then, over time, exhaust its fuel until succumbing to this fate. By the time this happens, the Andromeda Galaxy will have merged with the Milky Way. Even so, the likelihood of the Sun colliding with any other star is remote based on the vast distances between each.

If a normal, main-sequence star is too massive to die a white dwarf, it will become either a neutron star, pulsar, or black hole, which forms from what remains after a star exhausts its fuel supply or explodes. If a star implodes after supernova, it will become a black hole; an empty point of infinite density with zero dimensions.

A white dwarf harbors a similar mass to that of the Sun but is about the size of Earth. It is a dying star that used to be a small-to-medium-sized stellar object or a main-sequence star at first between one and ten solar masses. As these objects begin to exhaust fuel supplies, they shrink to become a white dwarf and are very small, very hot, and very dense. They are 200,000 times as dense as Earth, or hundreds of tons per cubic inch, with surface temperatures of 180,000 degrees, which is sixteen times as hot as the Sun.

A neutron star has a radius of only 10 miles and density hundreds of millions of tons per cubic inch. The entire mass of a mountain squeezed into a cubic centimeter is the density of a neutron star. They form of main-sequence stars with four or more solar masses that supernova, and only if leftover masses are less than 3.2. Stars that end up with more than 3.2 solar masses after supernova forever collapse into black holes. Pulsars are nothing more than magnetized, spinning neutron stars.

Main-sequence stars with original masses greater than 20 times that of the Sun before supernova may become a black hole. If the remaining core of such a star is more than 3.2 times the mass of the Sun after supernova, it will collapse under the weight of itself into a point of infinite density. Some explode in the form of supernovae, reduce their masses, and overcome the Chandrasekhar limit, which is a star left with 1.44 solar masses. In other words, they avoid gravitational collapse. Those left with above 1.44 solar masses instead leave behind either neutron stars or, those a little heavier, black holes.

The Universe is a strange place indeed. We are only beginning to learn just how strange. If quantum theory wins out, it may become stranger yet.

[i] Broad, William J., “If Theory is Right, Most of the Universe is Still ‘Missing.’” New York Times, New York, New York. (Sept. 11, 1984): Science.

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Glossary

Black Hole A point in space of infinite gravity where nothing can escape, not even light. A black hole is observable by way of its event horizon, or outer perimeter. Some physicists and cosmologists theorize black holes are gateways to other parts of the same Universe or are portals to another universe altogether. The laws of particle physics start to break down at the singularity of a black hole. Nothingness could be at the heart of it, or it may sprout into a distinct parallel universe separate from our own. A black hole may be the only conceivable wormhole, or gateway from one part of the Universe to another.

Bosons Particles associated with the transmission of each force of nature. Those forces include the weak nuclear force, strong nuclear force, electromagnetic force, and gravity.

Brown Dwarf A failed star. It harbors both planetary and solar attributes though is neither. A brown dwarf is something between what has the characteristics of both but can be classified as either.

Chandrasekhar Limit Developed by Nobel-prize-winning physicist Subrahmanyan Chandrasekhar in 1930. It is the maximum amount of mass allowed for a stable white dwarf star, or 1.44 solar masses. Any more and it would collapse into a black hole or become a neutron star.

Dark Energy Hidden energy thought to exist in higher dimensions. It accounts for 72.8% of the missing matter in the Universe. (Dark matter accounts for the missing 22.7%, leaving only 4.6% for observable matter.) Dark energy is the driving force behind the expansion of visible matter in the Universe and its overall uniformity. Dark energy has a different function, though it too should be found in hidden dimensions.

Dark Matter Hidden matter thought to exist in higher dimensions. It accounts for 22.7% of the missing matter in the Universe. (Dark energy accounts for 72.8 %, the remaining 4.6% normal, observable matter.) Dark matter is responsible for the clumping together of visible matter, dark energy for the expansion and overall uniformity of it.

Electromagnetic Force A force of nature that interacts between electrically charged particles, otherwise known as electromagnetism. One of the four basic forces of nature that explains electromagnetic fields. In layman’s terms, the study of the relationship between lightening and magnets.

Electromagnetic Theory Study of the electromagnetic force as one of the four forces of nature.

Force Carriers In quantum mechanics, particles that are bundles of energy of various fields. There is a force carrier for each type of elementary particle. Described as quanta, or an electron field for electrons, photons, and more.

General Relativity Albert Einstein’s theory of the association between gravity and geometry, developed in 1916. It is a proven theory accepted by physicists to explain gravity as a geometric property of curved space and time, or space-time. Time is a required component of the association between objects and their gravitational influence on space. You cannot measure gravity without time. If GPS satellites did not take into account general relativity, they would not work properly. Special relativity, on the other hand, deals more with properties of the speed of light. See also special relativity.

Gluons Elementary particles that act like “glue” by binding quarks together within hadrons. (Quarks are the building blocks of hadrons.) Gluons are bosons without mass and have a neutral vector.

Gravitational Force The weakest and most mysterious of the four forces of nature. The graviton, or particle related to the force of gravity, has yet to be verified since it is thought to exist in hidden dimensions. Physicists believe this is why it appears so weak when compared to the other three forces though it feels strong to us. For example, a small magnet can act against the entire force of gravity by lifting a paperclip off the ground.

Graviton Virtual particle of gravity yet to be quantized. Physicists believe the graviton is spread throughout multiple dimensions and, as a result, gravity appears as the weakest force of nature.

Gravity Weakest of the four fundamental forces of nature to include the strong nuclear force, weak nuclear force, and electromagnetic force. Gravity appears strong because of our interaction with it, but a single magnet can lift a small nail off the ground.

Hadrons A baryon or meson-composite particle made up of bound quarks. Protons and neutrons are the most common examples of hadrons.

Higgs Boson Also called the God Particle. In 2012, scientists discovered this elusive particle, which should be responsible for holding the entire Universe together. It is a massive elementary particle with zero charge and helps explain the masses of all other particles. Its discovery is considered the Holy Grail of particle physics.

Leptons An electron, muon, or neutrino not part of the strong nuclear force, only the weak.

Magnetic Monopoles A hypothetical, one-dimensional concept in particle physics, but superstring and grand-unified theories predict their existence. The idea is to separate either a distinct north or south-pole property of a magnet into isolated monopoles after cutting one in half. A bar magnet retains its own set of both properties after being divided, each with a distinct north and south pole still intact, so scientists, as of yet, are unable to separate the forces. Quantum mechanics might provide physicists with the ability to do so in the future.

Mesons Concerning the strong force of nature, they are hadronic elementary particles composed of a quark and an anti-quark. Their existence is unstable, lasting just a fraction of a second. Charged mesons decay to form electrons and neutrinos, uncharged ones decay to photons.

M-theory (Magic, Matrix, or Mystery Theory) An extension of string theory that allows for the existence of up to 11 hidden dimensions, or 10 spatial and one of time. Some versions allow for up to 26, but the math tends to break down above 10. Each dimension exists on its own membrane, or brane, and is an atom’s length apart. The reason we are unable to see these dimensions is because one would have to travel the length of the Universe and back along the dimensional fold to witness anything above three dimensions. Physicists argue these higher dimensions could be similar to ours but may be more dynamic with peculiar properties. Our three-dimensional bodies could not exist in or witness any higher dimension.

Neutrinos Neutral elementary particles that almost never interact with normal matter. Associated particles are electrons, muons, and taus. Italian for little neutron.

Neutrons Subatomic particles similar to protons but with no electric charge. Found in all atomic nuclei except hydrogen.

Neutron Star A very small, extremely heavy star with a radius of 10 miles and density weighing in at millions of tons per cubic inch. A teaspoon from one would weigh more than Mount Everest! They form from a collapsing star that explodes with a leftover mass less than 3.2 times of the Sun.

Non-Baryonic Matter not composed of normal, everyday material. Matter we are unable to observe or measure. Dark energy and dark matter are examples of non-baryonic matter. They have no neutrons, protons, or electrons. See also exotic matter.

Photon An elementary particle of light and electromagnetic radiation. The force carrier for the electromagnetic force.

Pi-Meson Also called a pion. It is a meson that holds together the nucleus of a particle and is one of the three fundamental particles defining the strong force of nature.

Planck Length According to the uncertainty principle, the shortest possible length of measurement, or 1.616199(97) × 10−35.

Proton A subatomic particle in an atomic nucleus with a positive electric charge, in equal magnitude to the opposing electron.

Pulsar A rapidly spinning, magnetized neutron star emitting regular pulses of radio waves. Scientists may discover they are perfect universal timekeepers relative to all observers. Some believe they are neutron stars manipulated by intelligent beings for the purpose of interstellar mobility.

Quantum Chromodynamics (QCD) Regarding theoretical physics and quantum field theory, the study of the strong nuclear force, more specifically the study of interactions between quarks and gluons. These particles are each assigned a quantum number, referred to as a color.

Quantum Electrodynamics (QED) To do with quantum field theory and study of the electromagnetic force. Studies how light and matter interacts through electrically charged particles and photons.

Quarks Theoretical elementary particles with a small electric charge. They are fundamental constituents of matter that combine to form hadrons, or protons and neutrons.

Quasar It is suggested a quasar might be a candidate for a white hole since it expels matter rather than pulling it in. A black hole is just the opposite. These objects appear more toward the far reaches of space, and some physicists think they lurk at the centers of galaxies. They are bright blue entities, the brightest objects in the Universe in the transitory stage of becoming a new galaxy. The Milky Way may have once been a quasar. One can emit 1,000 times more energy than an entire developed galaxy.

Quintessence A theoretical fifth force of nature existing in hidden dimensions and responsible for observations of an accelerating Universe. See also dark energy.

String Theory Its goal is to reconcile incompatible aspects of quantum mechanics and general relativity into a theory of everything. See also M-theory.

Strong Nuclear Force (Color Force) The force between two or more nucleons that binds together protons, neutrons, and atomic nuclei. One of the four known forces of nature.

Supernova An exploding star that exhausted its fuel supply. The heavy elements left over are the basic building blocks of life throughout the Universe. Without supernovae, we would not be here. Carbon-based life forms would never have appeared without the heavy elements they spew into the cosmos.

Superstring Theory A version of string theory dealing with supersymmetric particles. It attempts to model particles and forces of nature after vibrating strings. Each type of particle or force can be recognized by its distinct “note” in higher dimensions. See also supersymmetry and M-theory.

Theory Of Everything (TOE) Would allow scientists to determine the properties of all known physical phenomena. All experimental outcomes could then be predicted. A complete theory of everything would solve the mystery of the graviton, dark energy, and dark matter to combine all forces of nature. Decades may pass before scientists develop one, but discovery of the Higgs boson particle should lend critical data to the theory.

Weak Nuclear Force (Weak Force) As one of the four fundamental forces of nature, it is responsible for the radioactive decay of subatomic particles. Based on the theoretical exchange of W and Z bosons.

White Dwarf A star that swells to the size of a red giant then gradually shrinks to the size of Earth instead of exploding as a supernova or becoming a black hole. Its solar mass remains the same, yet its density is hundreds of tons per cubic inch. In the distant future, the Sun will become a white dwarf.

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    • Anne Harrison profile image

      Anne Harrison 4 years ago from Australia

      Great hub, the glossary at the end was particularly useful

    • Beata Stasak profile image

      Beata Stasak 4 years ago from Western Australia

      Very interesting, thank you for sharing:)