Our Place in the Universe and Time Scales - Study Notes

Course Goals and Outcomes

  • Scientific reasoning skills that can be applied in a variety of fields.

  • Ability to describe several significant discoveries throughout the history of astronomy.

  • Ability to explain how measurements of various properties of astronomical objects are made.

  • Conversational understanding of modern astronomy to interpret and explain press releases of new discoveries in astronomy.

  • A desire to learn more about the cosmos.

Instructional Team and Contacts

  • Instructor: Prof. Ben Bromley (ben.bromley@utah.edu)

    • Office: LSSB W3226

    • Office Hours: Wed 10-11am (in person; more TBA)

    • Please email with requests for meetings outside scheduled office hours

  • TA: Amanda Alves

    • TA office hours: TBA

    • Additional team office hours to be announced during weeks of exams

Course Operations and Tools

  • Canvas: home base for everything; follow the Modules.

  • Smartworks: homework platform ("homework" + "process of science").

  • In-class activities: lecture + in-class activities; mostly on Canvas.

  • Attendance: encouraged, not required.

Course Assessments

  • Reading Quizzes: due before class; 15%

  • In-Class Activities: 15%

  • Homework: per chapter; 30% total points possible

  • Exams: two midterms and a final; 12.5% for each midterm and 15% for the (cumulative) final; a higher final score replaces the lowest midterm score.

  • Note: if you miss an exam, other arrangements apply (as described in the course materials).

About the Instructor: Background and Research Focus

  • Professional path (high-level): from back East; parent; astrophysicist; professor; occasionally in need of support from family and friends; The U.

  • Research interests and activities:

    • Young Pluto simulated debris disks

    • Cosmic voids

    • Planet formation and orbital dynamics (planets, stars)

    • Other topics: black holes, cosmic voids, black hole shadows

  • Recent work on voids spans unimaginable distances and times; uses numbers and computer codes; daily grind; occasional childhood reference (age 6) appears in slides.

  • Real goal for students: understand what this is all about.

Semester Scope and Topics

  • This semester covers:

    • Solar System: Earth, Moon, Sun

    • Stars: star formation and evolution (including black holes)

    • Galaxies including massive black holes

    • Large-scale structure of the Universe with modern mysteries (dark matter/dark energy)

    • Do we have company? (search for other civilizations or planetary systems as a broader question)

Reading for the Week

  • Reading: Understanding Our Universe (Palen & Blumenthal)

  • Sections: 1.1 and 1.2 (Our place in the universe and the process of science)

  • Today’s focus: time, length, and speed

Time Scales: A Feel for Time and Deep Time

  • Time as a concept and a feel for how long things take.

  • Deep time: how old is old? (things in our lives, the Earth, the Universe?)

  • Look for inferences from surrounding objects and data.

Deep Time: Earth and Universe Ages

  • Geological evidence for Earth’s age includes: geological stratification, fossils, ocean salinity, and other data.

  • Lord Kelvin’s view: Earth was molten; cooling time estimated ~100 million years.

  • Modern radiometric dating of meteorites supports Earth’s age ≈
    4.6\times 10^9\ \mathrm{yr}

  • Universe age is known to be ≈
    13.8\times 10^9\ \mathrm{yr}

  • These ages frame deep time and are connected to lookback times in astronomy.

Distance and Time Scales: Units and Conversions

  • Common distance scales used in class:

    • 1 au = 150 million km

    • 1 ly = 63,000 au

  • Common distance units: au, ly, pc, kpc, Mpc

  • Distances within the Milky Way and nearby structures are often expressed in ly or pc; the diameter of the Milky Way ≈ 100,000 ly.

  • 1 kpc = 10^3 pc; 1 Mpc = 10^6 pc.

  • Milky Way-related distances often given in kpc and ly (example: 8 kpc ≈ 8,000 pc ≈ 20,000 ly).

  • Example distances in the lecture:

    • Milky Way diameter: about 100,000 ly

    • Center of the Milky Way: ~20,000 ly away; ~8 kpc

    • Nearby galaxy separation: ~2 million ly

    • 1 Mpc = 1,000,000 pc

Speed and Time Concepts in Astronomy

  • Speed of light:
    c = 3\times 10^5\ \mathrm{km/s}

  • Distances can be expressed as travel times at given speeds (e.g., light travel time).

  • Example notes from the lecture:

    • Moon: 1.3 light-seconds away

    • Sun to Neptune: about 8 light-minutes to reach Neptune and 4.2 years (as some slides show) to illustrate enormous distances in different contexts

    • Nearest star Proxima Centauri: about 4 light-years away

    • The Sun to Earth light travel time is a baseline measure (eight minutes is a common reference in astronomy)

  • The fastest speed known is the speed of light; discussions about faster-than-light travel and time travel are treated as theoretical or not supported by current physics.

Lookback Time and the Edge of the Observable Universe

  • Lookback time: light from distant regions takes time to reach us, so we see them as they were in the past.

  • The observable universe has an age of about 13.8 billion years; this implies a finite lookback time for the most distant observable light.

  • The cosmic lookback helps explain Olber’s paradox (see below).

Olber’s Paradox: Why is the Night Sky Dark?

  • The paradox asks why the night sky is not bright like the daytime sky if the Universe is infinite and static.

  • The resolution (as discussed in the lecture): light from the most distant regions has not had enough time to reach us due to the finite age of the Universe and the expansion of space; lookback time limits the sky’s brightness at night.

The Place in the Universe: A Hierarchy of Scales

  • Key structural levels:

    • Earth is a small planet orbiting the Sun (one of many stars in the Milky Way).

    • The Sun orbits the center of the Milky Way.

    • There is evidence that many other stars have planets.

    • The Milky Way is a spiral galaxy; part of the Local Group (~50 galaxies).

    • The Local Group is part of the Virgo Supercluster.

    • The Virgo Supercluster is part of the Laniakea Supercluster.

    • The Universe is about 13.8 billion years old.

The Local and Large-Scale Structure: Laniakea and Friends

  • Laniakea Supercluster is the large-scale structure containing the Milky Way and many galaxy clusters.

  • The slide map shows several groups and clusters: Pavo-INDUS, Centaurus, Virgo, Hydra, Canes II, etc., illustrating the cosmic web.

  • The Local Group sits within the Virgo Supercluster, which sits within Laniakea.

Vocabulary and Key Units (Condensed)

  • Universe: everything; the entire space and time.

  • Observable Universe: that portion we can access experimentally.

  • Galaxy: gravitationally bound system of stars, gas, dust, and dark matter.

  • Solar System: star(s) plus planets, moons, comets, asteroids, etc.

  • Massive: contains a lot of matter, not necessarily large in size.

  • Distances: cm, km, ly, pc; Time: s, d, yr, Myr, Gyr; Speed: km/s, c = speed of light.

Chapter Context and Course Text

  • Chapter 1: Our Place in the Universe; 4th Edition by Stacy Palen and George Blumenthal.

  • Slides prepared by Windsor Morgan; Copyright 2021 W. W. Norton & Company, Inc.

Illustrative Timeline: Universe History Graphics

  • The lecture includes a schematic timeline showing events from inflation and gravitational waves to the Cosmic Microwave Background and the formation of neutral hydrogen, etc., with approximate times:

    • 0.01 s: proton formation

    • 3 min: nuclear fusion begins/ends in the early universe

    • 380,000 years: decoupling and release of the Cosmic Microwave Background

    • 13.8 billion years: present age of the visible universe

  • This timeline helps connect deep time to observable astronomy.

Why We Study Astronomy: Cosmic Perspective

  • We study because it helps us understand our place, our connection to the universe, and our origins.

  • The Cosmic Neighborhood: Earth, Solar System, Milky Way, Local Group, Virgo Supercluster, and Laniakea as a progressively larger context.

Reading and Preparation Notes

  • For Thursday: Read Chapter 1.1–1.2 (Our Place in the Universe).

  • Take the Reading Quiz: Sections 1.1–1.2.

  • Check the Modules on Canvas for updates.

  • In-class activity: starting point survey will be available on Canvas.

Checkpoint Questions (Sample)

  • Checkpoint Question 1 (What an Astronomer Sees): Which of the following ranks the sizes of the listed objects from smallest to largest in the correct order?

    • A. Earth, Milky Way, Solar System, Universe

    • B. Solar System, Local Group, Sun, Virgo Supercluster

    • C. Solar System, Milky Way, Virgo Supercluster, Laniakea Supercluster

    • D. Earth, Solar System, Sun, Virgo Supercluster

  • Checkpoint Question 2: Which of the following is a measure of distance?

    • A. 1 light-day

    • B. 1 day

Final Preparatory Notes for the Week

  • The course emphasizes using light-year scales to interpret astronomical distances and looking at the cosmic neighborhood from the Earth’s perspective.

  • The In-class activities and readings are designed to connect to the broader questions of astronomy, including the origin and evolution of cosmic structures and the processes that shape them.

Life on Earth and Evolution (Contextual Side Note)

  • The lecture includes a brief mention of the early land-dwelling life, Ichthyostega, and the possible oldest dinosaur Nyasasaurus parringtoni (~243 Myr), illustrating deep-time biological context for the age-scale discussions.

Quick References (Selected from Slides)

  • Universe age: 13.8\times 10^9\ \mathrm{yr}

  • Earth age: 4.6\times 10^9\ \mathrm{yr}

  • 1 au: 1\ \mathrm{au} = 1.5\times 10^8\ \mathrm{km}

  • 1 ly: 1\ \mathrm{ly} = 6.3\times 10^4\ \mathrm{au}

  • Milky Way diameter: \sim 1\times 10^5\ \mathrm{ly}

  • Center of Milky Way distance: \sim 2\times 10^4\ \mathrm{ly} (≈ 8\ \mathrm{kpc})

  • 1 Mpc: 1\ \mathrm{Mpc} = 10^6\ \mathrm{pc}

  • Speed of light: c = 3\times 10^5\ \mathrm{km/s}

  • Moon distance: 1.3\ \text{light-seconds}

  • Sun–Neptune light travel context: 8\ \text{light-minutes}

  • Nearest star Proxima Centauri: \sim 4\ \text{light-years}

  • 13.8 Gyr lookback and 13.8 Gyr present universe context is a central theme throughout the week.