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What is Dark Energy?

Cosmology is the study of the origin and fate of the Universe. At the end of the last millennium, it became clear that the expansion of the Universe was actually accelerating. This discovery, which was an immense surprise, was awarded the Nobel Prize for Physics


SKA Science - Dark Energy 300dpi

What is dark energy?

Prior to this, astronomers had thought that the expansion of the Universe should be slowing under the mutual gravitational attraction of galaxies. Instead, it seems that the Universe contains some additional component, which astronomers have termed “dark energy”

This mysterious force appears to counteract and even surpass the mutual gravitational attraction causing acceleration in the expansion. It could however indicate that our understanding of gravity, as described by Einstein’s General Theory of Relativity, is incomplete.


Image showing the effects of redshift: Credit SKA Organisation

What is redshift?

Astronomers typically measure distances to very distant galaxies using the redshift effect. Consider observing the hydrogen gas in a distant galaxy. To an astronomer in that galaxy, the hydrogen gas emits radio emission at its characteristic 21 cm wavelength.

However, because the galaxy is distant, it takes some time for the radio emission to travel to astronomers here on Earth. During that time, the Universe expands, which has the effect of increasing the wavelength at which we detect the radio emission from the hydrogen gas.

The difference between the observed wavelength and expected wavelength is then a measure of how distant the galaxy is as well as being a measure of how much time has elapsed since the light was emitted.

For instance, with a large sample of galaxies, one can track how galaxies form into clusters.

How quickly these clusters form is partially a balancing act between gravity, which causes galaxies to fall together into a cluster, and dark energy, which acts to separate the galaxies. How big, and how rapidly, clusters form is then a measure of the strength of dark energy, which in turn can help astronomers understand better what it is.

There are two broad approaches that the SKA will pursue in studying cosmology:

  • The first approach involves a large survey of galaxies, searching for their (redshifted) 21 cm hydrogen emission. An extremely large survey of galaxies is required in order to sample a large enough of volume in the Universe that one can detect relatively subtle effects.
  • Another approach by which the SKA can study cosmology and dark energy is to observe the gravitational effects of galaxies and clusters of galaxies on the path of radio waves through the Universe.

Einstein’s Theory of Relativity relates mass to energy and shows that both mass and energy contribute to gravitation. Concentrations of mass have the effect of disturbing the path that radio waves take in their path to the Earth. In effect, concentrations of mass, such as galaxies and clusters of galaxies, can act as giant lenses in space.

Thus, if there is a galaxy behind another galaxy, or behind a cluster of galaxies, the shape of the background galaxy will appear distorted because the path(s) that the radio waves have taken has been distorted by the foreground galaxy or cluster.

By measuring the amount of distortion of background galaxies, astronomers can infer how much mass (both regular matter and dark matter) is between the background galaxies and us and a measure of how this mass is distributed. In turn, how the mass is distributed can be affected by the properties of dark energy, and aspects of cosmology. Thus, measurements of the shapes of large numbers of galaxies can be used to constrain models of the cosmology of the Universe.

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