The Milky Way in the Universe

The Milky Way is the galaxy that contains the Solar System. It is one of many billions of galaxies that make up the Universe.

The Milky Way is approximately 100,000 light-years in diameter. A light-year is the distance light travels in one year. Despite the word “year” in its name, a light-year is a unit of distance, not time. A diameter of 100,000 light-years therefore means that light would take approximately 100,000 years to travel across the Milky Way.

Redshift

When a source of electromagnetic radiation moves away from an observer, the radiation is observed at a longer wavelength than it was emitted. This increase in observed wavelength is called redshift.

As a source moves away, each successive wave is emitted from a position farther from the observer. The waves therefore arrive more spread out, so the observed wavelength is longer. This can occur with light from a receding star or galaxy.

To identify redshift in light from a distant galaxy, scientists compare features in its spectrum with the corresponding features produced under the same conditions on Earth. If the galaxy’s features are observed at longer wavelengths, the light is redshifted.

The name refers to a shift towards the longer-wavelength, red end of the visible spectrum. Redshifted radiation does not have to appear red: it may still be another visible colour or lie outside the visible spectrum.

Redshift Evidence for an Expanding Universe

Light from distant galaxies reaches us with its wavelengths stretched. This shift towards longer wavelengths is called redshift.

Scientists detect redshift by comparing distinctive patterns of lines in a galaxy’s light spectrum with the same patterns measured in a laboratory. If the galaxy’s lines appear at longer wavelengths, its light has been redshifted.

Redshift and expansion

Redshift indicates that distant galaxies are moving away from us. As space expands, the light travelling through it is stretched, increasing its wavelength. The redshift observed in light from distant galaxies is therefore evidence that the Universe is expanding.

Redshift and the Big Bang

If the Universe is expanding now, then tracing its expansion backwards leads to a time when it was smaller and denser. This evidence supports the Big Bang theory, which describes the Universe as having developed from an early hot, dense state as space expanded.

The Hubble Constant

Distant galaxies generally move away from Earth. Their recession speed increases with their distance: a galaxy twice as far away has approximately twice the recession speed. The ratio of recession speed to distance is called the Hubble constant, $$H_0$$.

Here, $$v$$ is the galaxy’s recession speed and $$d$$ is its distance from Earth.

Measuring recession speed and distance

The recession speed can be found from the redshift of the galaxy’s starlight. Spectral lines in the light are observed at longer wavelengths than those measured in the laboratory. Comparing the change in wavelength with the original wavelength allows the recession speed to be calculated.

A galaxy’s distance can be estimated using a supernova in that galaxy whose intrinsic brightness is known. Light spreads out as it travels, so the supernova appears dimmer when it is farther away. Comparing its observed brightness with its intrinsic brightness therefore indicates its distance.

Units of the Hubble constant

Speed is distance divided by time. If speed and distance are expressed using consistent distance units, those units cancel in $$v/d$$, leaving inverse time. For example, dividing a speed in metres per second by a distance in metres gives units of $$\mathrm{s}^{-1}$$.

The current estimate is $$H_0 = 2.2 \times 10^{-18}\ \mathrm{s}^{-1}$$.

Estimating the Age of the Universe

Galaxies are moving apart as the Universe expands. If a galaxy has always receded from us at its present speed, the time needed to reach its present distance is its distance divided by its recession speed.

For a galaxy at distance $$d$$ with recession speed $$v$$, Hubble’s law relates these quantities by $$v = H_0d$$, where $$H_0$$ is the Hubble constant, describing the present rate of expansion. The estimated time since the galaxy was at zero separation from us is therefore:

This gives an estimate of the age of the Universe. The ratio $$d/v$$ is a time: the distance travelled at a constant speed divided by that speed. The units of $$H_0$$ must be made consistent before calculating its reciprocal.

Tracing the expansion backwards makes separations between galaxies smaller. This supports the idea that all matter in the Universe was once concentrated at a single point.

The result is only an estimate because it assumes recession speeds remained constant. The expansion rate may have changed over time, so galaxies may not have taken exactly $$d/v$$ to reach their present separations.