Hubble's Law and the Hubble Constant

In 1929, Edwin Hubble presented one of the most remarkable graphs in the history of science: distance plotted against recession velocity, forming a straight line. That line revealed a universe in motion — and its slope became one of the most important numbers in cosmology.

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Q1 Question 1 of 10

What does Hubble's Law state?

All galaxies in the universe have the same recession velocity, regardless of distance The recession velocity of a galaxy is proportional to its distance — more distant galaxies are receding faster Galaxies further than 100 Mpc are receding faster than light and cannot be observed The recession velocity of a galaxy is inversely proportional to its distance — nearby galaxies move fastest

Q2 Question 2 of 10

What are the units of the Hubble constant?

Light-years per billion years — the rate at which distance grows over time Kilometers per second per megaparsec (km/s/Mpc) — it gives the recession speed per unit of distance Solar masses per cubic megaparsec — the average density of matter driving expansion Meters per second per light-year — the Hubble constant in SI units

Q3 Question 3 of 10

If H0 = 70 km/s/Mpc, how fast is a galaxy 100 Mpc away receding?

70 km/s — the same as H0 itself, because distance does not matter 700 km/s, which is about 0.23% of the speed of light 7,000 km/s, which is about 2.3% of the speed of light 700,000 km/s, which exceeds the speed of light

Q4 Question 4 of 10

How does the Hubble constant give an approximate age of the universe?

H0 directly equals the age of the universe in billions of years H0 times c equals the radius of the observable universe, and dividing by c gives the age 1/H0 gives a rough timescale for the expansion, yielding about 14 billion years — close to the measured age of 13.8 billion years H0 squared gives the energy density, and dividing by the Big Bang energy gives the age

Q5 Question 5 of 10

What is the Hubble tension?

The difficulty of measuring H0 because Hubble himself never published his final value A significant discrepancy between the H0 value measured from the cosmic microwave background (~67 km/s/Mpc) and the value from local distance measurements (~73 km/s/Mpc) The fact that H0 changes over time, making a single value meaningless The international debate over whether to use km/s/Mpc or km/s/light-year as the unit

Q6 Question 6 of 10

What was wrong with Hubble's original 1929 value of H0 around 500 km/s/Mpc?

He forgot to correct for the motion of Earth around the Sun, introducing a systematic error His velocity measurements were wrong because spectroscopy technology was too primitive in 1929 His distance measurements were wrong by about a factor of 7 due to miscalibrated Cepheid distances — two types of Cepheids were confused He included the Milky Way itself as one data point, which skewed the linear fit

Q7 Question 7 of 10

What does it physically mean that the Hubble constant is around 70 km/s/Mpc?

The Milky Way moves through space at 70 km/s in the direction of the Virgo cluster For every 3.26 million light-years of distance, space is expanding at an additional 70 km/s relative to us The universe expands by exactly 70 km every second, regardless of scale Light travels 70 km/s slower in expanding space than in empty flat space

Q8 Question 8 of 10

How is the cosmic distance ladder used to measure the Hubble constant?

Astronomers use only Type Ia supernovae for all distance measurements — no other rungs are needed Nearby distances are measured by parallax, then Cepheids extend the reach, then Type Ia supernovae reach distant galaxies where recession velocities are measured The distance ladder measures H0 directly from the expansion of nearby galaxies visible to the naked eye Only the Hubble Space Telescope can measure H0 — no ground-based telescopes have sufficient precision

Q9 Question 9 of 10

What would it mean for cosmology if the Hubble tension is confirmed to be real and not a measurement error?

The Big Bang theory would be disproven and replaced with the Steady State model The standard cosmological model would be incomplete — something about the early or late universe is not captured by our current understanding Einstein's general relativity would need to be completely discarded The Hubble constant would be shown to have always been constant throughout cosmic history

Q10 Question 10 of 10

Why is the Hubble constant called a constant even though it changes over cosmic time?

It is called constant because it never changes — the expansion rate has been the same since the Big Bang It is called constant because it was defined by Hubble to be constant as a simplifying assumption It is the current expansion rate at this moment in cosmic history — the word constant means it is uniform across all directions of space today, not that it is unchanging over time It is called constant because it approaches a constant value in the far future due to dark energy

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