Lost genius who merits a place on the podium
Who knows about Vesto Slipher, the astronomer who inspired the work of Edwin Hubble, asks CORMAC O'RAIFEARTAIGH
ONE OF the great surprises of 20th century science was the discovery that our universe is expanding. The finding caused a paradigm shift in cosmology and eventually led to today’s “Big Bang” model of the origin of the universe.
It is therefore quite puzzling that a scientist who played a key role in the discovery remains virtually unknown to scientists and the general public alike. Step forward Vesto Slipher, the American astronomer who first established that the most distant objects in the sky are moving away from one another at high speed.
Vesto Melvin Slipher was born in the US state of Indiana in 1875. He was educated at Indiana University and took up a position at the Lowell Observatory in Arizona after receiving his degree in 1901. There, he showed himself to be an astronomer of exceptional talent. In particular, he pioneered a method of measuring the motion of stars with the use of a spectrograph, an instrument that analyses the spectrum of light emitted by a body.
Slipher used his new technique to study the most enigmatic astronomical objects of the day, indistinct aggregations of gas and stars known as the nebulae. Exactly 100 years ago, he discovered that the spectrum of light emitted by the Andromeda nebula was significantly “Doppler-shifted”, indicating that it was moving at high speed.
In physics, the Doppler effect is a well-known phenomenon whereby the light measured by an observer is affected by the relative motion of source and observer: the light is measured as shifted in frequency towards the higher (blue) end of the spectrum if the source is moving towards the observer, and towards the lower (red) end if the source is moving away. By 1917, Slipher had established that the light from many of the distant nebulae was significantly red-shifted; they were moving away at speeds of up to 1,000km per second.
The discovery that the distant nebulae were receding at high speed was a great surprise to science and it was the first hint that the nebulae might constitute distinct galaxies of stars outside of the Milky Way. This was confirmed a few years later, when the astronomer Edwin Hubble used a new method of measuring astronomical distance to show that many of the nebulae lay far beyond our own galaxy. Combining his own measurements of distance with Slipher’s measurements of motion, Hubble made an even more astonishing discovery: the more distant a galaxy, the faster it was speeding away!
Over the next few years, Hubble and his assistant, Milton Humason, measured distances and redshifts for more than 40 galaxies. All obeyed the simple velocity/ distance relation above and, in time, it became known as Hubble’s law. However, it should really be called the Hubble-Slipher law, as Slipher was the first to detect the motion of the galaxies and, indeed, Hubble used Slipher’s measurements to derive the relation.
Today, one often reads that “Hubble discovered the expansion of the universe”, while Slipher’s contribution is largely forgotten. In fact, the statement is doubly inaccurate as Hubble’s law does not in itself imply an expanding universe. The latter phenomenon arises when one seeks a theoretical explanation for the outward motion of the galaxies – an explanation that is furnished by the theory of relativity.
According to general relativity, space and time are not fixed entities but affected by mass and motion. Applying relativity to the universe as a whole leads to the prediction of a universe in which space is either expanding or contracting. So the galaxies are not really moving; space itself is expanding and the galaxies ride the wave.
Einstein himself dismissed this prediction at first, famously modifying his equations in an attempt to force them to describe a static universe. However, other theorists such as Alexander Friedmann and Georges Lemaître realised that relativity predicts a universe that evolves over time. In particular, Lemaître made the connection with astronomy, pointing out that relativity gave a natural explanation for the redshifts of the galaxies. When Hubble’s graph became well known, this explanation was gradually accepted by the physics community.
So who discovered the expanding universe? As occurs so often in science, it isn’t really possible to attribute the discovery to one particular step or one particular person. Many scientists made key contributions. What is certain is that Slipher deserves a place on the podium alongside Hubble, Humason, Einstein, Friedmann and Lemaître.
Cormac Ó Raifeartaigh lectures in physics at Waterford Institute of Technology and writes the science blog Antimatter.
A conference recognising Slipher’s contribution to “Origins of the Expanding Universe” will take place on September 13th- 15th at Lowell Observatory, Flagstaff, Arizona