The expanding universe

Hubble's 1995 Deep Field covered a patch of sky about 1/13th the width of the full Moon and found roughly 3,000 galaxies in it.

1923: Edwin Hubble finds the Cepheid V1 in Andromeda; galaxies lie far beyond the Milky Way
1927: Georges Lemaître concludes from Hubble's and Slipher's data that the universe is expanding
1929: Hubble publishes the link between galaxy distance and redshift
1990: The Hubble Space Telescope launches
1995: Hubble Deep Field: about 3,000 galaxies in a tiny patch of sky after 10 days of exposures
2016: A census estimates about 2 trillion galaxies in the observable universe, 10 times earlier estimates

Galaxies moving apart

When light from a galaxy is spread into a spectrum, the familiar dark lines of hydrogen, calcium and other elements show up, but for almost every galaxy they are shifted towards the red end. This redshift means the galaxy is moving away from us. NASA explains that Hubble tied galaxy distances, measured with Cepheids, to these redshifts and found that the shift increased in proportion to distance: a galaxy twice as far away recedes twice as fast. Two years before Hubble published, the Belgian physicist Georges Lemaître had already analysed the observations of Hubble and Vesto Slipher and concluded that the universe is expanding. The relation is now called the Hubble-Lemaître law.

Written as a formula, speed = H₀ × distance. H₀, the Hubble constant, is measured in kilometres per second per megaparsec, and one megaparsec is 3.26 million light-years. With H₀ = 70, a galaxy 100 megaparsecs away recedes at about 7,000 km/s. This is not galaxies flying through space away from us in particular. Space itself is stretching, so every galaxy sees the others moving away, just as raisins in a rising loaf all move apart from each other. Nearby galaxies can buck the trend: Andromeda, bound to us by gravity, is approaching.

Recession speed against distance for two measured values of the Hubble constant: 67.4 (from the early universe, Planck) and about 73 (from Cepheids and supernovae). The lines spread apart more with distance.

The Hubble tension

Running the expansion backwards gives an age for the universe. One divided by H₀ has units of time; for H₀ = 70 it is about 14 billion years, close to the 13.8 billion years that NASA gives from careful modelling. Yet two families of measurements disagree. Fitting the pattern in the cosmic microwave background measured by ESA's Planck satellite gives 67.4 ± 0.5 km/s per megaparsec. Measuring distances directly with Cepheids and Type Ia supernovae, using Hubble, gives about 73. The gap is about 8%, far larger than either team's stated error, and it has not gone away as the data improved. Astronomers call it the Hubble tension, and it may point to physics we do not yet understand.

How many galaxies

BackgroundIn 1995 Robert Williams, director of the Space Telescope Science Institute, spent his own discretionary observing time pointing Hubble for 10 days at a patch of sky that looked empty, about 1/13th the width of the full Moon. Some colleagues doubted Hubble would find anything new there. The resulting Hubble Deep Field showed roughly 3,000 galaxies at many stages of growth, and the data were released to everyone at once. Astronomer Julianne Dalcanton, quoted by NASA, says this open sharing has since become the model for most large astronomical projects. Counting galaxies in deep fields and multiplying by the area of the whole sky led to an estimate of about 200 billion galaxies.

In 2016 a team led by Christopher Conselice used Hubble and other surveys, plus models of galaxies too faint to see, and concluded there are at least 10 times more, about 2 trillion galaxies that live or have lived in the observable universe. Most were small and merged long ago into bigger galaxies. A 2021 study using NASA's New Horizons spacecraft, far from the Sun's glare, measured how dark space really is. NASA later corrected its announcement to say these measurements constrain the total light from galaxies, not their number. So the honest summary is that the count lies somewhere in the hundreds of billions to about two trillion, depending on how faint galaxies are counted.

Redshift	Spectral lines shifted to longer wavelengths: moving away
Hubble-Lemaître law	Speed = H₀ × distance
Hubble tension	About 67.4 from Planck against about 73 from Cepheids and supernovae
Age of the universe (NASA)	About 13.8 billion years
Hubble Deep Field	1995, 10 days, about 3,000 galaxies

What does a redshift in a galaxy's spectrum show?
- The galaxy is moving away from us
- The galaxy is very hot
- The galaxy is approaching
- The galaxy is made of iron
With H₀ = 70 km/s per Mpc, how fast does a galaxy 50 Mpc away recede?
A galaxy recedes at 14,000 km/s. With H₀ = 70, how many Mpc away is it?
Who first concluded, in 1927, that the universe is expanding?
- Georges Lemaître
- Isaac Newton
- Henrietta Swan Leavitt
- Tycho Brahe
Why do all distant galaxies seem to move away from us?
- Space itself is stretching, so every galaxy sees the others recede
- Earth is at the centre of the universe
- Galaxies are afraid of the Milky Way
- Light slows down over distance
The Hubble tension compares 73 with 67.4. By what percentage is 73 higher? (one decimal)
Which galaxy is approaching us instead of receding?
- Andromeda
- Every galaxy in the Hubble Deep Field
- M87
- Markarian 231
About how many galaxies did the 1995 Hubble Deep Field show?
- About 3,000
- About 30
- About 3 million
- Only one
What did NASA's correction say about the New Horizons measurement?
- It limits the total light from galaxies, not their number
- It proved there are exactly 200 billion galaxies
- It found no galaxies at all
- It measured the Hubble constant
1 ÷ H₀ for H₀ = 70 km/s/Mpc is about how many billion years? (whole number)

Key points

Almost all galaxies are redshifted: they are moving away.
Hubble-Lemaître law: speed = H₀ × distance (Lemaître 1927, Hubble 1929).
Space itself expands; nearby Andromeda is the exception, approaching.
H₀ about 67.4 (Planck) or about 73 (Cepheids and supernovae): the Hubble tension.
Galaxy count: hundreds of billions to about 2 trillion.

Sources for this course (17)

NASA Galaxies (science.nasa.gov/universe/galaxies)
NASA Galaxy types (science.nasa.gov/universe/galaxies/types)
NASA Hubble's Galaxies (science.nasa.gov/mission/hubble/science/universe-uncovered/hubble-galaxies)
ESA/Hubble tuning fork (esahubble.org/images/heic9902o)
NASA Sun facts (science.nasa.gov/sun/facts)
NASA Stars (science.nasa.gov/universe/stars)
NASA/JPL Milky Way spiral arms (science.nasa.gov/resource/the-milky-way-galaxy)
JPL Large Magellanic Cloud (jpl.nasa.gov/images/pia07137-whats-old-is-new-in-the-large-magellanic-cloud)
NASA APOD Magellanic Clouds and Leavitt, NASA Hubble V1 asset (science.nasa.gov/asset/hubble/cepheid-variable-star-v1-in-andromeda-galaxy)
NASA Edwin Hubble (science.nasa.gov/people/edwin-hubble)
NASA Hubble 100 years (science.nasa.gov/missions/hubble/nasa-celebrates-edwin-hubbles-discovery-of-a-new-universe)
NASA and ESA Andromeda collision 2025 (science.nasa.gov/missions/hubble/apocalypse-when-hubble-casts-doubt-on-certainty-of-galactic-collision, esahubble.org/news/heic2508)
JPL Event Horizon Telescope Sagittarius A* (jpl.nasa.gov/edu/resources/teachable-moment/telescopes-get-extraordinary-view-of-milky-ways-black-hole)
NASA Hubble Deep Fields (science.nasa.gov/mission/hubble/science/universe-uncovered/hubble-deep-fields)
NASA 2016 galaxy census (science.nasa.gov/missions/hubble/hubble-reveals-observable-universe-contains-10-times-more-galaxies-than-previously-thought)
NASA New Horizons cosmic background with correction (science.nasa.gov/missions/hubble/new-horizons-spacecraft-answers-question-how-dark-is-space)
CERN Courier Hubble tension (cerncourier.com/a/the-hubble-tension).

Checked: 2026-10-17