Astronomy - The Universe - Quick Guides | Astronomy

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Study GuideAstronomy–The Universe1. Beyond the Big Bang TheoryTheBig Bang theorydoes an excellent job of explaining how the universe evolved after about30secondsfrom its beginning. It helps us understand the expansion of the universe, the formation ofatoms, and the cosmic background radiation we observe today.However, when scientists lookeven earlierin time, several important questions remain unanswered.These questions push us to gobeyondthe standard Big Bang model.1.1The CommunicaƟon (Horizon) ProblemWhen we observe the universe on very large scales, it looks almost thesame everywhere. Thetemperature, density, and overall structure are remarkably uniform.But here’s the problem:•Light travels at afinite speed.•In a standard Big Bang expansion,far-apart regionsof the universe should never have hadtime to exchange information.So how did these distant regions “agree” to have the same properties?This mystery suggests that something unusual must have happenedvery earlyin the universe’shistory to allow all regions to become uniform.1.2 The Origin of GalaxiesGalaxies formed from tinydensity fluctuationsin the early universe. These fluctuations show uptoday as very small temperature differences in the cosmic background radiation—about1 part in100,000 (10⁻⁵).The question is:•Why did these fluctuations exist at all?Pure randomness predicts a universe that ismuch smootherthan what we observe. This meanssomephysical process from an even earlier timemust have created the initial unevenness thatlater grew into galaxies.

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Study Guide1.3 Why Does MaƩer Exist at All?Modern physics shows a strongsymmetry between matter and energy.Nature allows:•High-energy photons to turn into amatter–antimatter pair•Matter and antimatter to annihilate back intophotonsThese reactions can goboth ways, meaning no net matter should remain.Yet today:•The universe containsmatter, not equal amounts of matter and antimatter.•For every1 billion photons, only aboutone particle of matterremains.This tells us that, in the early universe, the laws of physics must have beenslightly different,allowing matter to survive while antimatter disappeared.high-energy photon↔matter particle + antimatter particlematter particle + antimatter particle→high-energy photonshigh-energy photon↔matter particle + antimatter particlephotons→matter + antimatter→photons + matter1.4 The Mystery of Dark MaƩerAstronomers observe much more gravity than visible matter can explain.Key observations:•Only about10% or lessof the universe’s matter emits detectable radiation.•The remaining90% or moreis invisible and calleddark matter.Since all known forms of ordinary matter emit some radiation, dark matter must be made ofunknownparticles or structuresnot described by current physics.

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Study Guide1.5 Why Are There Four Fundamental Forces?Nature hasfour forces:1.Gravity–weakest force, acts over large distances2.Electromagnetism–about 10⁴⁰times stronger than gravity3.Weak nuclear force–involved in radioactive decay and nuclear reactions4.Strong nuclear force–holds protons and neutrons together in nuclei1.6 The Flatness ProblemThe Big Bang theory allows many possiblegeometriesfor the universe:•Open•Closed•FlatObservations show the universe isvery 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 haveforced the universe toward flatnessvery early on.1.7Grand Unified Theories (GUTs)To solve these problems, physicists developedGrand Unified Theories (GUTs).These theories try to:•Unite theforces of nature•Explain the existence of manyelementary particles(over 300 are known)Each force is carried by a particle:•Electromagnetism→photon•Weak force→Z particle•Strong force→gluons•Gravity→predictedgraviton(not yet observed)

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Study Guide1.8Force UnificaƟon•Electromagnetism and the weak force are already unified as theelectroweak force•GUTs attempt to unite:oElectroweak force + strong force→superforce•The ultimate goal is asingle force, including gravity, often called aTheory of Everythingsingle unified force→superforce + gravitysuperforce→electroweak force + strong forceelectroweak force→electromagnetic force + weak force1.9The Earliest Moments of the Universe•Before10⁻⁴³ seconds(thePlanck time), physics is unknown•After this time, the universe expanded and cooled•As energy dropped, forcessplit apartin a process calledsymmetry breakingOnce separated, these forcescannot recombineunder today’s conditions.1.10The InflaƟonary UniverseAt about10⁻³⁵secondsafter the Big Bang, the universe experiencedinflation:•A rapid expansion by a factor of about10³⁰•The universe grew fromproton-size to grapefruit-sizealmost instantlyThis happened because:•The vacuum itself hasenergy•As the universe cooled, the vacuum underwent aphase transition, similar to steamcondensing into water•The released energy caused rapid expansion

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Study Guide1.11Why InflaƟon MaƩersInflation explains several mysteries:1. Uniformity of the UniverseRegions that were once close together were pushed far apart, explaining why distant regions look thesame today.2. Matter–Antimatter AsymmetryGUTs allow small differences in matter–photon interactions, leaving behind the tiny amount of matterwe see today.3. Flat GeometryInflation stretches space so much that any curvature becomes negligible—making the universeappear flat.1.12InflaƟon and Dark MaƩerInflation predicts:•The universe must havecritical density•Observed matter (visible + dark) is only about10%of this•Therefore,about 99% of the universe’s mass must be invisible

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Study Guide1.13Dark MaƩer CandidatesPossible forms of dark matter include:•WIMPs(Weakly Interacting Massive Particles)•Axions•Cosmic strings•Magnetic monopoles•Shadow matterExperiments are underway to determine which, if any, of these exist.1.14The Cosmological ConstantEinstein introduced thecosmological constantto 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 explainaccelerating expansion•It acts like arepulsive force•It may represent energy of empty space (vacuum energy)Future observations will determine its true value.2. ObservaƟonal CosmologyCosmology is the study of theentire universe—how it began, how it changes, and what it looks liketoday. Humans have wondered about the cosmos since ancient times, but our understanding hasgrown dramatically over the centuries.

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