Galaxies and the expanding universe

Kinds of galaxies

Galaxies come in a few main shapes. Edwin Hubble arranged them in a "tuning fork" diagram in 1926, and his labels are still used.

• Elliptical galaxies (E0 to E7, from round to flattened) are smooth balls of mostly old, yellow-red stars with little gas, so few new stars form. The biggest galaxies known are giant ellipticals, often at the centres of galaxy clusters.
• Spiral galaxies (Sa to Sc) have a central bulge and a flat disc with spiral arms, where gas and dust form new, blue stars. Barred spirals (SBa to SBc), like the Milky Way, have a bar of stars across the middle. The "a" types have big bulges and tightly wound arms; the "c" types small bulges and loose arms.
• Lenticular galaxies (S0) have a disc but no arms and little gas.
• Irregular galaxies have no clear shape, often because of the gravity of a neighbour. The Magellanic Clouds are irregular galaxies.

There are far too many galaxies for astronomers to classify by eye. Since 2007, the Galaxy Zoo project has asked members of the public to classify galaxies online; hundreds of thousands of volunteers have made tens of millions of classifications, and each galaxy is checked by many people to reduce errors. Volunteers have even discovered new kinds of objects.

Galaxies beyond the Milky Way

The debate was settled using Henrietta Leavitt's Cepheid variables (6B). In 1923 Edwin Hubble found a Cepheid in the Andromeda "nebula" using the 2.5 m Hooker telescope in California. Timing its pulses gave its true brightness, and comparing that with how bright it looked showed that Andromeda was far outside the Milky Way. Hubble's figure was about 900 000 light years; the modern value is 2.5 million (the Cepheid scale was later corrected). Andromeda was a galaxy as big as our own, and the countless other spiral nebulae were galaxies too.

Redshift

When an ambulance races past, the pitch of its siren drops. As it approaches, its sound waves are squashed together to a shorter wavelength (higher pitch); as it moves away, they are stretched out to a longer wavelength (lower pitch). This is the Doppler effect, and it happens to light too. Light from an object moving away from us is shifted towards longer, redder wavelengths, a redshift. Light from an object approaching is shifted towards the blue, a blueshift.

A rainbow-like spectrum alone doesn't show a shift, but the dark absorption lines of elements do (Year 7 showed how they identify elements). Each element's lines are always at the same wavelengths in a laboratory, so if a galaxy's lines are all shifted by the same proportion, the galaxy is moving.

The redshift z is the fractional change in wavelength. For speeds much less than the speed of light, the speed v is c × z:

z = (observed wavelength − laboratory wavelength) ÷ laboratory wavelength     v = c × z

Hubble's law

From 1912 the American astronomer Vesto Slipher measured the spectra of spiral nebulae and found that most were redshifted, some moving away at over 1000 km/s, much faster than any star. In 1927 the Belgian physicist and priest Georges Lemaître combined Slipher's speeds with distances and argued that the universe is expanding. In 1929 Hubble, using his own distances, published the relationship clearly: the further away a galaxy is, the faster it is moving away. This is Hubble's law (since 2018 the International Astronomical Union has recommended calling it the Hubble–Lemaître law):

v = H₀ × d

v is the speed of recession (km/s), d is the distance (in megaparsecs, Mpc; 1 Mpc = 3.26 million ly), and H₀, the Hubble constant, is the gradient of the graph. Its value is about 70 km/s per Mpc: for every extra megaparsec of distance, galaxies are moving away about 70 km/s faster.

Hubble's own estimate in 1929 was about 500 km/s per Mpc, seven times too big, because the Cepheids he used were brighter than he thought, which made his distances too small. As distance measurements improved, the value came down. It is a good example of science correcting itself as evidence improves.

It's space that's expanding

Hubble's law doesn't mean we are at the centre of an explosion. A better picture is a loaf of raisin bread rising in the oven. Every raisin moves away from every other raisin, and raisins that start further apart separate faster, because there is more dough between them to stretch. A raisin can't tell whether it is in the middle of the loaf: every raisin sees the same pattern. In the same way, observers in any galaxy would see the others moving away according to Hubble's law. The universe has no centre.

The galaxies aren't really flying through space; space between them is stretching. As light crosses expanding space, its wavelength is stretched too: that is what the redshift of distant galaxies really is. Gravity holds smaller things together against the expansion. Galaxies, our Local Group, and of course the Solar System and you, are not expanding. Andromeda, held by gravity in the Local Group, is actually approaching us at about 110 km/s and shows a blueshift. It may collide with the Milky Way in a few billion years.

How old is the universe?

If galaxies are all moving apart, then in the past they were closer together. Run the expansion backwards and everything was together at one time. If the expansion had always been at the same rate, the time since then would be 1 ÷ H₀: