34.1 Cosmology and Particle Physics

The universe is constantly expanding, with galaxies moving away from each other at increasing speeds. Hubble's law quantifies this relationship, while general relativity provides the framework for understanding cosmic expansion on a large scale.

The Big Bang theory proposes the universe began as a singularity 13.8 billion years ago. Evidence supporting this includes the observed expansion, cosmic microwave background radiation, and the abundance of light elements in the universe.

The Expanding Universe and the Big Bang

Expansion of the universe

When astronomers observe distant galaxies, they find that nearly all of them are moving away from us. The farther away a galaxy is, the faster it recedes. This pattern is the basis of Hubble's law.

• Hubble's law quantifies the relationship between a galaxy's distance and how fast it's moving away:
  • $v = H_0 \times d$, where $v$ is the recessional velocity, $H_0$ is the Hubble constant, and $d$ is the distance to the galaxy
  • The Hubble constant is estimated at roughly 70 km/s/Mpc (kilometers per second per megaparsec), which tells you the current expansion rate of the universe
• This doesn't mean galaxies are flying through space away from a central point. Space itself is stretching, carrying galaxies along with it. That's why more distant galaxies recede faster: there's more expanding space between us and them.
• Note: some nearby galaxies, like Andromeda, are actually moving toward the Milky Way because local gravitational attraction can overpower the expansion at short distances.
• General relativity provides the mathematical framework for modeling this expansion on cosmic scales. Einstein's field equations describe how the distribution of mass and energy determines the geometry and evolution of spacetime.

Big Bang theory evidence

The Big Bang theory proposes that the universe began as a singularity approximately 13.8 billion years ago and has been expanding and cooling ever since. In its earliest moments, the universe was extraordinarily hot and dense. As it expanded, it cooled enough for fundamental particles to form, and eventually for light elements like hydrogen and helium to assemble.

Three major lines of evidence support the Big Bang:

• Observed expansion (redshift): Light from distant galaxies is shifted toward longer, redder wavelengths. This redshift is direct evidence that space is expanding, consistent with Hubble's law.
• Cosmic microwave background (CMB) radiation: The CMB is leftover thermal radiation from about 380,000 years after the Big Bang, when the universe cooled enough for atoms to form and light to travel freely. It has since cooled to about 2.7 Kelvin and is nearly uniform in all directions, which supports the idea of a hot, dense early universe that expanded rapidly.
• Abundance of light elements: The observed ratios of hydrogen (~75%), helium (~25%), and trace lithium in the universe closely match predictions from Big Bang nucleosynthesis, the process by which light nuclei formed in the first few minutes after the Big Bang.

Inflation theory extends the standard Big Bang model by proposing a brief period of extremely rapid expansion in the universe's first fraction of a second. This helps explain why the CMB is so uniform (the horizon problem) and why the universe appears geometrically flat (the flatness problem).

Matter, Antimatter, and the Universe

Matter vs antimatter asymmetry

Every fundamental particle has an antimatter counterpart with the same mass but opposite charge and quantum numbers. For example, the electron (negative charge) has the positron (positive charge) as its antimatter partner.

When a particle meets its antiparticle, they annihilate each other, converting their combined mass into energy according to $E = mc^2$. This process also works in reverse: enough energy can produce a particle-antiparticle pair.

The Big Bang should have created equal amounts of matter and antimatter. Yet the observable universe is overwhelmingly made of matter: stars, planets, and galaxies are all matter, with very little antimatter detected. This puzzle is called the baryon asymmetry problem.

The leading explanation is that some slight asymmetry in the laws of physics favored matter over antimatter by a tiny margin in the early universe. For roughly every billion antimatter particles, there were a billion and one matter particles. After mutual annihilation wiped out the matched pairs, that tiny surplus of matter became everything we see today. Researchers are investigating mechanisms like CP violation (a difference in how matter and antimatter behave under certain transformations) to explain this, but the full answer remains an open question.

Modern Cosmology and Particle Physics

The boundary between cosmology and particle physics blurs at the highest energies. Understanding the smallest particles helps explain the largest structures in the universe.

• The Standard Model of particle physics describes the fundamental particles (quarks, leptons, and force-carrying bosons) and three of the four fundamental forces (electromagnetic, weak, and strong). It does not include gravity.
• Quantum field theory provides the mathematical framework underlying the Standard Model, treating particles as excitations of underlying fields.
• Dark matter is a hypothesized form of matter that doesn't emit or absorb light but exerts gravitational influence. Evidence includes the rotation curves of galaxies, which spin faster than visible matter alone can explain. Dark matter is estimated to make up about 27% of the universe's total energy content.
• Dark energy is a separate concept: it's the unknown cause of the universe's accelerating expansion, discovered in 1998 through observations of distant supernovae. It accounts for roughly 68% of the universe's energy content. Ordinary matter makes up only about 5%.
• Supersymmetry (SUSY) is a theoretical extension of the Standard Model that predicts a heavier "superpartner" for every known particle. If confirmed, it could help explain dark matter and resolve some mathematical issues in the Standard Model. So far, no superpartners have been detected at particle accelerators like the Large Hadron Collider.