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Astronomy - The Universe

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    Study Guide Astronomy – The Universe 1. Beyond the Big Bang Theory The Big Bang theory does an excellent job of explaining how the universe evolved after about 30 seconds from its beginning. It helps us understand the expansion of the universe, the formation of atoms, and the cosmic background radiation we observe today. However, when scientists look even earlier in time, several important questions remain unanswered. These questions push us to go beyond the standard Big Bang model. 1. 1 The Communica Ɵ on (Horizon) Problem When we observe the universe on very large scales, it looks almost the same everywhere . The temperature, density, and overall structure are remarkably uniform. But here’s the problem: • Light travels at a finite speed . • In a standard Big Bang expansion, far - apart regions of the universe should never have had time to exchange information. So how did these distant regions “agree” to have the same properties? This mystery suggests that something unusual must have happened very early in the universe’s history to allow all regions to become uniform. 1. 2 The Origin of Galaxies Galaxies formed from tiny density fluctuations in the early universe. These fluctuations show up today as very small temperature differences in the cosmic background radiation — about 1 part in 100,000 (10 ⁻ ⁵ ) . The question is: • Why did these fluctuations exist at all? Pure randomness predicts a universe that is much smoother than what we observe. This means some physical process from an even earlier time must have created the initial unevenness that later grew into galaxies.

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    Study Guide 1. 3 Why Does Ma Ʃ er Exist at All? Modern physics shows a strong symmetry between matter and energy . Nature allows: • High - energy photons to turn into a matter – antimatter pair • Matter and antimatter to annihilate back into photons These reactions can go both ways , meaning no net matter should remain. Yet today: • The universe contains matter , not equal amounts of matter and antimatter. • For every 1 billion photons , only about one particle of matter remains. This tells us that, in the early universe, the laws of physics must have been slightly different , allowing matter to survive while antimatter disappeared. high - energy photon ↔ matter particle + antimatter particle matter particle + antimatter particle → high - energy photons high - energy photon ↔ matter particle + antimatter particle photons → matter + antimatter → photons + matter 1. 4 The Mystery of Dark Ma Ʃ er Astronomers observe much more gravity than visible matter can explain. Key observations: • Only about 10% or less of the universe’s matter emits detectable radiation. • The remaining 90% or more is invisible and called dark matter . Since all known forms of ordinary matter emit some radiation, dark matter must be made of unknown particles or structures not described by current physics.

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    Study Guide 1. 5 Why Are There Four Fundamental Forces? Nature has four forces : 1. Gravity – weakest force, acts over large distances 2. Electromagnetism – about 10 ⁴⁰ times stronger than gravity 3. Weak nuclear force – involved in radioactive decay and nuclear reactions 4. Strong nuclear force – holds protons and neutrons together in nuclei 1. 6 The Flatness Problem The Big Bang theory allows many possible geometries for the universe: • Open • Closed • Flat Observations show the universe is very close to flat . This is puzzling because: • Even a tiny deviation from flatness in the early universe should have grown larger over time. • Something must have forced the universe toward flatness very early on. 1.7 Grand Uni fi ed Theories (GUTs) To solve these problems, physicists developed Grand Unified Theories (GUTs) . These theories try to: • Unite the forces of nature • Explain the existence of many elementary particles (over 300 are known) Each force is carried by a particle: • Electromagnetism → photon • Weak force → Z particle • Strong force → gluons • Gravity → predicted graviton (not yet observed)

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    Study Guide 1.8 Force Uni fi ca Ɵ on • Electromagnetism and the weak force are already unified as the electroweak force • GUTs attempt to unite: o Electroweak force + strong force → superforce • The ultimate goal is a single force , including gravity, often called a Theory of Everything single unified force → superforce + gravity superforce → electroweak force + strong force electroweak force → electromagnetic force + weak force 1.9 The Earliest Moments of the Universe • Before 10 ⁻ ⁴ ³ seconds (the Planck time ), physics is unknown • After this time, the universe expanded and cooled • As energy dropped, forces split apart in a process called symmetry breaking Once separated, these forces cannot recombine under today’s conditions. 1.10 The In fl a Ɵ onary Universe At about 10 ⁻ ³ ⁵ seconds after the Big Bang, the universe experienced inflation : • A rapid expansion by a factor of about 10³ ⁰ • The universe grew from proton - size to grapefruit - size almost instantly This happened because: • The vacuum itself has energy • As the universe cooled, the vacuum underwent a phase transition , similar to steam condensing into water • The released energy caused rapid expansion

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    Study Guide 1.11 Why In fl a Ɵ on Ma Ʃ ers Inflation explains several mysteries: 1. Uniformity of the Universe Regions that were once close together were pushed far apart, explaining why distant regions look the same today. 2. Matter – Antimatter Asymmetry GUTs allow small differences in matter – photon interactions, leaving behind the tiny amount of matter we see today. 3. Flat Geometry Inflation stretches space so much that any curvature becomes negligible — making the universe appear flat. 1.12 In fl a Ɵ on and Dark Ma Ʃ er Inflation predicts: • The universe must have critical density • Observed matter (visible + dark) is only about 10% of this • Therefore, about 99% of the universe’s mass must be invisible

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    Study Guide 1.13 Dark Ma Ʃ er Candidates Possible forms of dark matter include: • WIMPs (Weakly Interacting Massive Particles) • Axions • Cosmic strings • Magnetic monopoles • Shadow matter Experiments are underway to determine which, if any, of these exist. 1.14 The Cosmological Constant Einstein introduced the cosmological constant to counter gravity and keep the universe static. Later: • The universe was found to be expanding • Einstein called this his “biggest blunder” Today: • The cosmological constant may explain accelerating expansion • It acts like a repulsive force • It may represent energy of empty space (vacuum energy) Future observations will determine its true value. 2. Observa Ɵ onal Cosmology Cosmology is the study of the entire universe — how it began, how it changes, and what it looks like today. Humans have wondered about the cosmos since ancient times, but our understanding has grown dramatically over the centuries.

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