Introduction
You live in a universe where ninety-five percent of all material is completely invisible to you and all modern instruments. Traditional astronomy reveals that stars, planets, and gas clouds account for only a tiny fraction of cosmic mass, leaving the vast majority hidden in the form of dark matter and dark energy. When you look at the rotation speeds of distant galaxies or the large-scale acceleration of space, ordinary atoms fail to explain the observed physics by a wide margin.
Ninety-five percent of the universe consists of invisible dark matter and dark energy, and solving this cosmic mystery reveals the ultimate structure and destiny of space.
Dan Hooper guides you through this profound puzzle in Dark Cosmos. You journey from classical astronomy to the strange realms of quantum mechanics, particle physics, string theory, and modern cosmology. This book addresses several central questions about reality:
- What holds spinning galaxies together if visible stars do not have enough mass?
- Why is the expansion of the universe accelerating instead of slowing down?
- What elementary particles could make up the invisible halo around you?
- How do extra spatial dimensions and string theory reshape your understanding of gravity?
1. Evidence for Dark Matter
Astronomical observations of galaxy rotations and gravitational lensing prove that galaxies contain massive halos of invisible matter.
Approximately 95 percent of our Universe's mass and energy exists in an invisible form. The large-scale structure and evolution of our Universe requires about twenty times more mass and energy than we see.
Galactic Rotation Curves
You find that the visible world contains familiar objects, but galaxies rotate much faster than visible matter allows. Of this invisible majority, roughly one-third is dark matter and two-thirds is dark energy.
In 1950, Vera Rubin presented her master's thesis results on galaxy clustering to the American Astronomical Society, where her unorthodox conclusions were initially dismissed before being vindicated decades later, culminating in their 1970 Andromeda Galaxy paper. By observing the velocities of individual hydrogen-emitting stars within galaxies using the Doppler effect of light, Rubin and W. Kent Ford demonstrated that stars in galaxies like Andromeda move at speeds requiring up to ten times more mass than what is visible, indicating that roughly 90 percent of the galaxy consists of dark matter.
The Limits of Ordinary Matter
You must look beyond standard atomic components to explain what holds these spinning systems together. Calculations regarding the relative abundances of light chemical elements like hydrogen, lithium, helium, and deuterium formed in the early universe show that the total amount of matter made of protons and neutrons is far too small to account for the dark matter observed today.
Ralph Alpher, Hans Bethe, and George Gamow published their famous 1948 paper calculating light element abundances, validating the Big Bang theory.
2. Quantum Physics and Neutrinos
Foundations of Quantum Mechanics
You uncover the foundation of modern particle physics when you study how light behaves. In 1905, Albert Einstein explained the photoelectric effect by proposing that light itself consists of discrete packets called quanta, later named photons.
Experiments showed that electric current is induced by light only if its frequency exceeds a specific threshold, regardless of light intensity. Einstein argued that individual light quanta carry energy proportional to their frequency, proving light exhibits both particle and wave properties.
Solar-powered calculators and experiments where light striking a metal plate frees electrons to flow around an electrical circuit demonstrate this principle.
Quantum objects exist as probabilistic states rather than definite locations, and observation causes their wave function to collapse into a single reality. Unlike classical objects, quantum particles possess multiple simultaneous positions and velocities, existing as wavy clouds of probability. Erwin Schrödinger illustrated this with his famous cat paradox, where an unobserved system remains in a mixed state until measurement triggers the collapse of the wave function.
Erwin Schrödinger's cat paradox involves a sealed chamber with a radioactive material, a Geiger counter, and a cat-killing device.
Neutrinos and Their Limitations
Quantum mechanics established the foundation of modern particle physics, while neutrino discoveries revealed that hot dark matter cannot explain cosmic structures.
Detectors like the Homestake Mine, Super-Kamiokande, and the Sudbury Neutrino Observatory found far fewer electron neutrinos arriving from the Sun than predicted. This deficit occurs because neutrinos transform from one type into another during flight, a quantum phenomenon that requires them to have non-zero mass.
The Super-Kamiokande experiment utilized a 50,000-ton tank of ultra-pure water buried half a mile underground and 13,000 detectors to record these rare events. In 1968, the first detection of solar neutrinos occurred in the Homestake Mine. In 1998, Super-Kamiokande announced compelling evidence of neutrino transformations and atmospheric neutrino oscillation. On June 18, 2001, the Sudbury Neutrino Observatory confirmed solar neutrino oscillation.
Cosmological structure formation requires slow-moving cold dark matter to clump together early and seed galaxies. Because neutrinos travel at near-light speeds as hot dark matter, computer simulations prove they cannot form the large-scale structures observed in the universe. Neutrinos are too light and fast, meaning any structures they form would arise far too late to match the actual universe, eliminating them as the primary solution for dark matter despite tens of millions of relic neutrinos existing in every cubic meter of space today.
Computer simulations depict the evolution of matter distribution in space from the early universe to modern galaxy clusters.
3. Supersymmetry and Dark Matter Candidates
Supersymmetry predicts heavy superpartner particles like neutralinos, which serve as leading cold dark matter candidates.
Symmetry and Superpartners
Supersymmetry posits a grand symmetry linking matter particles and force-carrying particles, proposing a heavy superpartner for every Standard Model particle. Developed in the early 1970s, supersymmetry connects fermions and bosons. Because superpartners have not been observed in accelerators, the symmetry must be broken, making them much heavier than their Standard Model counterparts.
The electron has a superpartner called the selectron, the photon has the photino, and quarks have squarks. R-parity is a symmetry ensuring proton stability and the existence of a stable, lightest superpartner particle that acts as a viable dark matter candidate. R-parity requires superpartners to be created or destroyed only in pairs, preventing the lightest superpartner from decaying. The lightest neutralinos among these serve as prominent dark matter candidates.
Analogy of mixing four dog breeds like the German shepherd, beagle, golden retriever, and toy poodle illustrates how neutralinos can be complex mixed combinations of photinos, zinos, and Higgsinos. Protons exhibit a minimum observed survival time of $1,000,000,000,000,000,000,000,000,000,000,000$ years in a Universe that is $14$ billion years old.
Detecting Dark Matter
Direct detection experiments operating deep underground and near absolute zero attempt to register rare collisions between dark matter particles and atomic nuclei. To eliminate cosmic ray noise, experiments place sensitive silicon and germanium detectors far underground, cooling them close to absolute zero to reduce thermal internal motion.
The Cryogenic Dark Matter Search, known as CDMS, operates at the bottom of the Soudan Mine almost half a mile underground. The CDMS detector is maintained at one-hundredth of one degree Kelvin, far below the equivalent room temperature of $290$ degrees Kelvin or $65$ degrees Fahrenheit. In $2006$, CDMS reached the highest level of sensitivity among worldwide dark matter experiments.
Indirect detection methods search for the secondary products of dark matter particle annihilations occurring in galactic halos or trapped inside the Sun. When dark matter particles annihilate, they produce gamma rays, antimatter, and neutrinos. Neutrinos from annihilations trapped in the solar core can escape the Sun and be detected by massive Antarctic telescopes.
The IceCube neutrino telescope deploys deep within the Antarctic ice using $5,000$ detectors across a half-mile volume. Light detectors beneath the Antarctic ice reach a maximum depth of $2,450$ meters in IceCube.
4. String Theory and Extra Dimensions
Quantum physics describes the microscopic world of particles, while general relativity describes macroscopic gravity and space-time curvature. Attempting to combine them using standard particle physics tools yields infinite probabilities instead of reasonable ones between zero and one. You encounter a fundamental incompatibility when you apply both theories together.
String theory requires ten spatial-temporal dimensions and gives rise to Kaluza-Klein particles that can act as stable dark matter.
Ten Dimensions
String theory posits that fundamental particles are different modes of vibration of tiny strings. It overcame the tachyon problem by introducing supersymmetry, and its mathematics requires precisely dimensions.
Edwin A. Abbott's book Flatland is used as an analogy to illustrate how beings living in a lower-dimensional slice fail to perceive larger hidden dimensions.
Kaluza-Klein States
The Theodor Kaluza and Oskar Klein theory proposed an extra dimension wrapped up in a tiny circle. Because of the law of conservation of momentum, particles moving through this extra dimension retain their momentum and cannot slow down to become ordinary particles.
The classic arcade game Asteroids is used as an analogy where flying off one edge of the screen makes the ship reappear on the opposite side.
5. Cosmic Geometry and Dark Energy
The geometry of the universe depends on whether its total density of matter and energy exceeds a critical value. If density exceeds this threshold, the universe is closed. If density is lower, the universe is open and infinite. If density equals the critical value, the universe is flat.
Precision cosmology reveals that the universe is flat and its expansion is accelerating due to a dominant component called dark energy.
The critical value separating open and closed universes is one gram per hundred trillion cubic kilometers.
The Accelerating Universe
The BOOMERANG balloon experiment made a billion measurements of the cosmic microwave background around the South Pole. On April 26 2000, the project announced that temperature fluctuations measured about three-quarters of one degree of arc across. This prevalent size matched predictions for a flat universe, a finding later confirmed by WMAP.
Observations of distant type Ia supernovae revealed that cosmic expansion is accelerating rather than slowing down. These objects act as standard candles because they have a consistent mass limit when they explode. That limit is 1.4 times the mass of our Sun, known as the Chandrasekhar limit.
Teams like the Supernova Cosmology Project and the High-Z Supernova Search Team studied distant type Ia supernovae using powerful telescopes like Keck, Cerro Tololo, and Hubble. Some of these observed supernovae exploded seven billion years ago.
The Nature of Dark Energy
Dark energy acts as a counter-gravity force driving the acceleration of the universe. Unlike ordinary matter, its density remains constant as space expands. This mysterious component eventually overtook matter to dominate cosmic evolution.
Our universe consists of distinct components:
- dark energy comprises 70 percent of the total density;
- dark matter and ordinary matter make up the remaining 30 percent;
- atoms form only 4 percent of the universe.
Science magazine declared the discovery of the accelerating expansion and dark energy as the breakthrough of the year in 1998.
6. Cosmic Inflation and the Multiverse
The early Universe presented three profound puzzles. Cosmologists struggled to explain why magnetic monopoles were missing and why density was so close to the critical flatness value. That near-flatness requires extreme initial fine-tuning, specifically a precision of 10 to the 60th power for the density at one second after the Big Bang.
Researchers also wondered how distant regions of the cosmic microwave background shared the same temperature despite being causally disconnected.
The theory of cosmic inflation resolves the monopole, flatness, and horizon problems through a brief period of ultra-fast early expansion.
Alexei Starobinsky and later Alan Guth proposed inflation. This process causes the size of the Universe to double nearly once every fraction of a second.
During inflation, the size of the Universe doubles every 10 to the -32 seconds. The typical duration of the period of inflation in most models lasts 10 to the -30 seconds. The Universe grew by a minimum factor of 10 to the 25th power during this interval.
Scalar fields drive this mechanism, which dilutes magnetic monopoles, flattens spatial curvature, and places previously connected regions into causal contact before accelerating them apart.
Eternal Inflation
Mathematical models of inflation suggest that inflation never truly ends everywhere. This process results in eternal inflation and a practically infinite number of separate universes.
While inflation stops in most places, small fractions of space continue to expand rapidly and give rise to massive new volumes. This dynamic produces an endless network of causally disconnected universes that may possess different laws of physics, containing parent universes and baby universes.
You can observe a relic of the early universe at home:
- tune your television between channels;
- observe the static or snow appearing on your screen to detect a few percent of the cosmic background from the Big Bang.
7. Future Horizons in Cosmology
You face a decisive era for particle physics and cosmology. Researchers test whether dark matter actually exists or if gravity simply behaves differently on large scales.
Advanced particle accelerators like the Large Hadron Collider provide the near-term experimental path to discovering dark matter particles.
Testing Alternatives
Mordechai Milgrom proposed modified Newtonian dynamics in 1983. This approach suggests that anomalous galactic rotations stem from altered gravitational laws under tiny accelerations rather than invisible halos.
Jacob Bekenstein proposed a relativistic version of modified Newtonian dynamics in 2004. Low-surface-brightness galaxies served as a primary toy model for this research.
This alternative approach successfully predicts galactic rotation curves without dark matter. It struggles with galaxy cluster masses and requires extra matter to match cosmic microwave background data.
The Large Hadron Collider
The Large Hadron Collider represents the primary near-term experimental avenue for discovering dark matter particles and resolving particle physics mysteries. The facility sits in a circular tunnel 17 miles in length and 100 yards beneath Geneva, Switzerland.
Proton beams circulate through the machine at 99.999999 percent the speed of light. The instrument achieves a maximum energy of 14 trillion electron volts per collision.
These high-energy collisions aim to produce massive particles predicted by supersymmetry, such as neutralinos. Previous experiments like the Tevatron measured particles like the top quark at a mass equivalent of 175 giga-electron volts. Meanwhile, W and Z bosons measure between 80 and 90 giga-electron volts.
10 Key Ideas
- Approximately 95 percent of our Universe's mass and energy exists in an invisible form known as dark matter and dark energy.
- By observing the velocities of individual hydrogen-emitting stars within galaxies, Vera Rubin and W. Kent Ford demonstrated that roughly 90 percent of galaxies consist of invisible dark matter.
- Quantum mechanics established the foundation of modern particle physics, while neutrino discoveries revealed that hot dark matter cannot explain cosmic structures.
- Supersymmetry predicts heavy superpartner particles like neutralinos, which serve as leading cold dark matter candidates.
- String theory requires ten spatial-temporal dimensions and gives rise to Kaluza-Klein particles that can act as stable dark matter.
- Precision cosmology reveals that the universe is flat and its expansion is accelerating due to a dominant component called dark energy.
- Dark energy acts as a counter-gravity force driving the acceleration of the universe while comprising 70 percent of the total density.
- The theory of cosmic inflation resolves the monopole, flatness, and horizon problems through a brief period of ultra-fast early expansion.
- Mathematical models of inflation suggest that inflation never truly ends everywhere, resulting in eternal inflation and a practically infinite number of separate universes.
- The Large Hadron Collider represents the primary near-term experimental avenue for discovering dark matter particles and resolving particle physics mysteries.