The Expanding Universe Study Pack

Kibin's free study pack on The Expanding Universe includes a 3-section study guide, 8 quiz questions, 10 flashcards, and 1 open-ended Explain review question. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.

Last updated May 21, 2026

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The Expanding Universe Study Guide

Trace the evidence behind one of astronomy's most profound discoveries — from the redshift of galactic light and Hubble's Law (v = H₀ × d) to the cosmological principle and dark energy's role in accelerating expansion. This pack also resolves Olbers' paradox and clarifies why space itself stretches rather than galaxies racing outward from a central point.

Key Takeaways

  • •The universe is expanding, meaning the space between galaxies is stretching over time, a conclusion supported by the redshift of light from distant galaxies.
  • •Hubble's Law states that a galaxy's recession velocity is proportional to its distance from us, expressed as v = H₀ × d, where H₀ is the Hubble constant (approximately 70 km/s/Mpc).
  • •Cosmic expansion does not mean galaxies move through space from a central point; rather, space itself stretches uniformly, so every observer in the universe sees other galaxies receding.
  • •The observable universe has a finite age of approximately 13.8 billion years, and the cosmic horizon — the farthest distance from which light has had time to reach us — defines the limit of what we can observe.
  • •The fate of the universe depends on the interplay between the gravitational pull of matter and the accelerating effect of dark energy, which current evidence suggests will drive indefinite expansion.
  • •The cosmological principle — that the universe is homogeneous and isotropic on large scales — underpins all standard models of cosmic expansion.
  • •Olbers' paradox, the question of why the night sky is dark despite an infinite-seeming universe of stars, is resolved by the finite age and expansion of the universe, which limits the number of photons that can reach us.

Evidence That the Universe Is Expanding

The case for an expanding universe rests on a specific, measurable signal in the light arriving from distant galaxies: nearly all of them show a shift toward longer, redder wavelengths compared to what their constituent elements would emit in a laboratory on Earth.

Cosmological Redshift and What It Measures

  • •When a source of light moves away from an observer, the wavelengths of its emitted photons are stretched — a phenomenon called the Doppler effect for nearby sources and cosmological redshift for objects embedded in expanding space.
  • •Edwin Hubble and Milton Humason systematically measured the spectra of dozens of galaxies in the 1920s and confirmed that virtually all galaxies outside the Local Group show this redshift, meaning they are receding.
  • •The degree of redshift is quantified as z = (λ_observed − λ_emitted) / λ_emitted; larger z values correspond to greater recession speeds and, at cosmological distances, to greater distances.

Hubble's Law: Recession Velocity and Distance

  • •Hubble discovered that recession velocity (v) is directly proportional to a galaxy's distance (d): v = H₀ × d, where H₀ is the Hubble constant.
  • •The currently accepted value of H₀ is approximately 70 km/s per megaparsec (Mpc), meaning a galaxy 1 Mpc away recedes at ~70 km/s, while one 10 Mpc away recedes at ~700 km/s.
  • •This linear relationship is the observational signature of uniform, large-scale expansion — not a local clustering effect.

The Nature of Cosmic Expansion

A common misconception is that galaxies are flying outward through pre-existing, static space from some central explosion point; the modern picture is more subtle and has profound implications for how we interpret distances and positions in the universe.

Space Itself Is Stretching

  • •Cosmic expansion is the stretching of the metric of space — the fabric of spacetime described by Einstein's general relativity — rather than the motion of galaxies through a fixed background.
  • •Because space stretches everywhere simultaneously, there is no privileged center of expansion; every galaxy sees every other galaxy receding, exactly as predicted by a uniform expansion.
  • •A useful analogy is dots drawn on the surface of a balloon being inflated: every dot moves away from every other dot, and no dot is 'the center.'

The Cosmological Principle

  • •The cosmological principle holds that the universe is homogeneous (the same average density everywhere) and isotropic (the same in all directions) when viewed on sufficiently large scales, roughly above 300 million light-years.
  • •This principle justifies applying a single mathematical model — the Friedmann–Lemaître–Robertson–Walker (FLRW) metric — to the universe as a whole.
  • •Deviations from homogeneity on smaller scales, such as galaxies, clusters, and voids, are consistent with the principle because they represent local density fluctuations within an overall uniform distribution.

About this Study Pack

Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.

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