Step by Step
1
Hubble's 1929 discovery
Edwin Hubble found that distant galaxies show redshift proportional to their distance — the farther away a galaxy is, the faster it appears to be receding from us.
2
The formula: v = H₀ × d
Recession velocity (v) equals the Hubble constant (H₀) multiplied by distance (d). H₀ is approximately 70 km/s/Mpc, meaning for every additional megaparsec of distance, a galaxy's recession speed increases by about 70 km/s.
3
The key insight: space itself is expanding
Galaxies aren't actually moving through space in the traditional sense — space itself is expanding, carrying galaxies along with it. A helpful analogy: raisins in rising bread dough all move apart from each other as the dough expands, with no single raisin being the "center" of the expansion.
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No center of expansion, and the Hubble tension
Because space itself is expanding uniformly, there's no actual center point the universe is expanding away from — every location sees every other location receding. The "Hubble tension" refers to a puzzling discrepancy: measuring H₀ from the CMB gives a slightly different value than measuring it from the local distance ladder — an unresolved issue that may point to new physics.
Applied Walkthrough
1
In 1929, Edwin Hubble observed that distant galaxies consistently show redshift proportional to their distance — the further away a galaxy is, the faster it appears to be moving away from us.
2
This relationship is captured by the formula v = H₀ × d, with H₀ (the Hubble constant) at roughly 70 km/s/Mpc — meaning a galaxy twice as far away recedes at roughly twice the speed.
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Rather than galaxies actually moving through a fixed, unchanging space, it's space itself that's expanding — much like raisins baked into a loaf of bread all moving farther apart from each other as the dough rises, without any single raisin being the actual center of that expansion.
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Complicating this picture somewhat is the Hubble tension: measuring H₀ using the cosmic microwave background gives a noticeably different value than measuring it using the local cosmic distance ladder — an unresolved discrepancy that remains one of the more puzzling open questions in modern cosmology.
Exam Application
Exams test whether you can apply v = H₀ × d correctly, whether you understand that galaxies recede due to the expansion of space itself (not motion through space), and whether you know what the Hubble tension refers to.
⚠ Common Trap
The most common trap is describing galaxy recession as galaxies physically moving through space away from us — the correct interpretation is that space itself is expanding, carrying galaxies apart, which is also why there's no single "center" the universe is expanding from.
✓ Quick Self-Check
1. What is Hubble's Law, in formula form?
v = H₀ × d — recession velocity equals the Hubble constant times distance.
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2. What is the approximate value of the Hubble constant?
About 70 km/s/Mpc.
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3. Are galaxies moving through space, or is something else happening?
Space itself is expanding, carrying galaxies apart — they aren't moving through a fixed, unchanging space.
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4. Why is there no "center" of the universe's expansion?
Because the expansion of space is uniform everywhere — every location sees every other location receding, with no privileged center point.
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5. What is the Hubble tension?
A discrepancy between the Hubble constant measured from the CMB versus measured from the local distance ladder.
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