Astronomy 2e · The Big Bang
A Model of the Universe
On this page 9 sections
In 30 seconds
Cosmology The study of the structure, history, and fate of the universe as a whole Full entry → studies the universe as a whole — its structure, history, and ultimate fate. This topic assembles the standard model of the universe the rest of the chapter builds on, resting on three big ideas: the Cosmological principle The assumption that the universe is homogeneous and isotropic on large scales Full entry → (the universe looks the same everywhere and in every direction on the largest scales), the expansion of space (galaxies recede from one another because space itself stretches), and the competition between expansion and gravity (the universe's future depends on whether its density is high enough for gravity to halt the expansion). From just these ideas, cosmologists derive a small set of possible universes — closed, open, or flat — and the key question becomes: which one do we actually live in?
Why this matters
Every later topic depends on this model: the CMB is uniform only because the universe is Homogeneous The same density of matter at every location (on large scales) Full entry → and Isotropic The same in every direction Full entry → (Topic 4); dark energy was discovered when supernovae broke the simple three-model picture (Topic 5); inflation explains why the universe is so close to flat (Topic 6). The model also answers an old question: will the universe expand forever, or collapse? And seeing how a model is built from assumptions plus observations — then tested and revised — is a general science skill beyond astronomy.
The college version
Core Concepts
The cosmological principle
The cosmological principle states that, on the largest scales, the universe is homogeneous (the same density of matter everywhere) and isotropic (the same in every direction). No place is special: Earth is not the center, the Milky Way is not the center, and there is no preferred direction in space. This principle is an assumption, not a proven fact, but it is supported by galaxy surveys (matter is spread evenly on scales of hundreds of millions of light-years) and by the near-uniform cosmic microwave background. It fails on small scales — the solar system and Milky Way are lumpy — so it applies only to the universe viewed as a whole.
The expansion of space
In the 1920s, Edwin Hubble (building on Vesto Slipher's redshift measurements) found that distant galaxies recede from us, and the farther away a galaxy is, the faster it recedes. This is Hubble's law v = H₀d: recession speed grows with distance Full entry →:
v = H₀ × d
where v is the recession speed, d the distance, and H₀ (the Hubble constant) the current expansion rate — a commonly taught reference value of about 70 km/s per megaparsec (Mpc); different methods give roughly 67–73 km/s/Mpc, so verify against current sources. The key insight: space itself stretches — galaxies are not plowing through space like projectiles. Light from distant galaxies is stretched along with space, shifting it redward. Because every location sees the same pattern, there is no center: observers in any galaxy see all others receding.
The three models: closed, open, and flat
Whether expansion continues forever depends on average density. Gravity brakes the expansion; outward motion accelerates it. The balance point is the critical density (ρc), commonly taught as roughly 9.5 × 10⁻²⁷ kg/m³ — about 5–6 hydrogen atoms per cubic meter. Compare actual density ρ to ρc with the density parameter Ω = ρ/ρc:
- Closed universe (Ω > 1): Density exceeds critical. Space has positive curvature (like the surface of a sphere, but in one more dimension). The universe is finite but unbounded — you could travel around it forever without hitting an edge. Gravity eventually wins: expansion halts and reverses into a collapse known as the Big crunch The hypothetical collapse of a closed universe back into a dense state Full entry →.
- Open universe (Ω < 1): Density is below critical. Space has negative curvature (like the surface of a saddle). The universe is infinite and expands forever, slowing down but never quite stopping.
- Flat universe (Ω = 1): Density exactly equals critical. Space obeys familiar Euclidean geometry on the largest scales. The universe is infinite and expands forever, with the expansion speed approaching zero asymptotically.
The fate of the universe
The models predict different far futures: a closed universe ends in a contraction to high density; an open universe expands and cools forever. For decades astronomers hunted for the slowdown of expansion — the "deceleration parameter." In 1998, two teams using distant Type Ia supernovae found the opposite: the expansion is accelerating. The simple competition between gravity and expansion is incomplete; something called dark energy (Topic 5) pushes the universe apart.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Galaxies moving through space | Space itself expanding | Galaxies are nearly at rest locally; distances grow because the space between them stretches. Within a cluster, gravity holds members together. |
| The universe has a center or edge | The observable horizon | Every location sees the same recession pattern, so there is no preferred center; the "edge" of the observable universe is a light-travel limit, not a real boundary. |
| "Closed" means inside a hollow sphere | Curved geometry of space itself | A closed 3-D universe is finite but boundaryless — like a sphere's surface, where walking straight eventually returns you to your start. The curvature is of spacetime, not a container. |
| "Flat" means pancake-shaped | Euclidean geometry on cosmic scales | Flat means parallel lines stay parallel and triangles have 180° on the largest scales; it says nothing about the universe's shape being thin. |
| Recession speeds exceeding the speed of light are impossible | Relativity's limit on motion through space | The limit applies to motion through space; stretching space itself can separate distant galaxies faster than light — a classic exam trap. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine raisins in rising bread dough. Every raisin sees every other raisin moving away, and the farther apart two raisins are, the faster they separate — no raisin is the center. Heavy dough stops rising and slumps back; light dough rises forever. Our universe is the dough; galaxies are the raisins.
Worked example
The balloon trick. Draw ten dots on an uninflated balloon — each dot is a galaxy. Now inflate it. Pick any dot and watch the others: every one moves away from your dot, and the dots that started farther apart separate fastest. No matter which dot you pick, the pattern is the same. That is the cosmological principle plus Hubble's law in one toy: there is no center, and recession speed is proportional to distance.
Now add numbers. With H₀ = 70 km/s/Mpc (a commonly taught reference value), a galaxy at 100 Mpc recedes at 7,000 km/s; one at 200 Mpc recedes at 14,000 km/s — exactly twice as fast. Finally, connect the balloon to fate: if you could weigh the balloon's material against the critical density, the "dough" would tell you whether it keeps rising forever or eventually slumps. Astronomers do that weighing with galaxy surveys and the CMB — and the answer (Ω very close to 1, plus an extra ingredient) is what Topics 4–6 explain.
Key takeaways
- The cosmological principle: the universe is homogeneous and isotropic on the largest scales — no center, no edge, no preferred direction.
- Expansion is space stretching, not galaxies moving through space; distant galaxies show redshifted light because wavelengths stretch with space.
- Hubble's law: v = H₀d — recession speed grows linearly with distance.
- Fate is set by density: Ω > 1 closed (recollapse/big crunch), Ω < 1 open (expand forever), Ω = 1 flat (expand forever, asymptotically slowing).
- Critical density is where expansion energy and gravity exactly cancel; commonly taught value ≈ 9.5 × 10⁻²⁷ kg/m³ (verify against current sources).
- Observational surprise: distant Type Ia supernovae (1998) revealed accelerating expansion, which the simple three-model picture could not explain — leading to dark energy.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
State the cosmological principle and explain why it is an assumption rather than a proven law.
Show answer
The universe is homogeneous (same density everywhere) and isotropic (same in every direction) on the largest scales, so no location or direction is special. It is an assumption because we cannot observe the whole universe; we adopt it and check it against observations (galaxy surveys, the CMB), which so far support it.
A galaxy is measured to recede at 14,000 km/s with H₀ = 70 km/s/Mpc. How far away is it?
Show answer
d = v / H₀ = 14,000 / 70 = 200 Mpc.
If the density parameter Ω = 1.5, which model applies and what is the universe's eventual fate?
Show answer
Ω > 1 means a closed universe with positive curvature; expansion will eventually halt and reverse into a big crunch.
Why is there no center to the expansion of the universe?
Show answer
Because the pattern is the same from every location: each observer sees all others receding with speed proportional to distance, so no point is singled out as the origin.
What physical quantities does the critical density balance against each other?
Show answer
The outward momentum of the expansion against the inward pull of gravity (the average density of matter and energy).
What observation in 1998 surprised cosmologists and forced a revision of the simple three-model picture?
Show answer
Distant Type Ia supernovae appeared dimmer (farther) than expected, showing the expansion is accelerating — not decelerating — which led to the idea of dark energy.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Cosmology
- The study of the structure, history, and fate of the universe as a whole
- Cosmological principle
- The assumption that the universe is homogeneous and isotropic on large scales
- Homogeneous
- The same density of matter at every location (on large scales)
- Isotropic
- The same in every direction
- Hubble's law
- v = H₀d: recession speed grows with distance
- Critical density (ρc)
- The density that exactly balances expansion against gravity
- Density parameter (Ω)
- The ratio of actual density to critical density (Ω = ρ/ρc)
- Closed / open / flat universe
- The three geometries of space allowed by the model (positive, negative, and zero curvature)
- Big crunch
- The hypothetical collapse of a closed universe back into a dense state
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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