From light to dark – Exploring redshift and dark energy

Materials World magazine
,
1 Jan 2008

In 1666 in a darkened room, Newton placed a glass prism in a sunbeam emanating from a small hole in a shutter and generated a rainbow on the opposite wall. He correctly, though controversially, concluded that the colours he saw were components of white light.

Fraunhofer lines

What Newton did not detect was a sequence of dark lines – these were first noted by Wollaston in 1802 and have become known as Fraunhofer lines. They are due to the selective absorption of the sun's, or other star's, radiation at specific wavelengths by chemical elements in the atmosphere. Thus one could discover the chemical composition of stars. Indeed, helium was identified in the sun before it was found on Earth. Direct evidence for nucleosynthesis was obtained when the radioactive and relatively short-lived element technetium was discovered in some stars.

Identification of the presence of an element by its spectral 'bar code' in a star's spectrum is not the only information provided by their absorption lines – their position within the red to blue spectrum is also important. There are parallels with sound – the familiar change in tone as a train approaches and then recedes, known as the Doppler effect, has its counterpart with the passage of light-emitting objects such as a star.

Redshift

If the star was moving towards the earth, its light would be shifted towards the blue end of the spectrum, and contrariwise if receding it would be redshifted. The spectra from all galaxies outside the Milky Way are redshifted, and hence are moving away from us. In 1929, Edwin Hubble found that the recession velocity (or redshift) of a distant galaxy is directly proportional to the distance from the observer. These discoveries have been the basis of the now almost universally accepted view that the universe was created by an enormous explosion (the Big Bang) from a point source 10-20 billion years ago, and has been expanding ever since.

There has been much speculation over whether the expansion will continue forever or whether gravity will eventually slow the rate of expansion, and even reverse its direction. Two teams of scientists, the international High-z Supernova Search Team and the USA-based Supernova Cosmology Project, have been studying the spectra of distant supernovae, ie those with with extra-large redshifts, in the hope of discovering evidence for a reduction in the rate of expansion. Instead, their studies revealed that the expansion of the universe is accelerating and this has been described as possibly the most important astronomical discovery of recent times.

Dark energy

The magazine Physics World devoted much of its December issue to celebrating the 10th anniversary of this discovery, which, according to the Editor, 'points to the existence of a weird, gravitationally repulsive “dark energy” that is driving the acceleration and may account for some 75% of the entire mass-energy content of the universe'. Dark energy has been related to the 'cosmological constant' proposed by Einstein in 1917 as an extra term in his equation of general relativity and as a form of 'antigravity' that permeates all space.

New information on the possible ultimate fate of the universe and evidence for the existence of 'dark energy' may be of general interest to the public, but there have been few reports of these developments in the popular press. Interest in science seems to be decelerating at a rate comparable to the acceleration of the rate of the universe's expansion.

 

Further information:

High-Z Supernova Search Team
Supernova Cosmology Project