In a paper published in Nature today, physicists detail a new method of looking at faraway galaxies that may help shed light on dark energy.
In order to learn more about dark energy – the mysterious force that is believed to be responsible for the ever increasing rate of expansion of the Universe – astronomers need to be able to peer further back in time. Luckily, due to the finite speed of light, all we need to do to see further back in time is to look at objects further away. For example, if we look at a galaxy that is ten light years away, we see it as it was ten years ago. Simple.
What is not so simple is working out how we can observe far away objects in enough detail to make the observations useful. When observing galaxies, astronomers look at their emission spectra, which are made up of spectral lines that each correspond to a certain wavelength of light that has been emitted by a particular element. From these lines astronomers can determine which elements are present, but spectral lines can have other uses too.
The emission line with a wavelength of 21cm is caused by neutral hydrogen, and is potentially very useful when it comes to probing dark energy. So far, astronomers have been able to detect the 21cm line in galaxies with a redshift up to 0.24. Cosmological redshift is a measure of how fast an object is moving away from the Earth and is a consequence of the expanding Universe. Objects with higher redshift are moving away from the Earth faster than those with lower redshifts, and are further away too. Galaxies more distant than those with a redshift of 0.24 are too faint for the 21cm line to be detected using traditional methods. It is believed that observations of the 21cm line from galaxies with redshifts between 0.5 and 2.5 would allow us to gain a better understanding of dark energy, so physicists have been trying to find a way to make these observations.
Chang et al have come up with a new technique called intensity mapping that may help to solve this problem. Rather than trying to look at the 21cm emission line from individual galaxies that are too far away, they have measured the total emission from a “cosmic web” containing thousands of galaxies. Combining this with data from the DEEP2 optical galaxy redshift survey they have produced a 3D intensity map for redshifts between z=0.53 and z=1.12, giving an insight into the 21cm emission line in the region needed to probe dark energy. In doing this they have avoided having to look at every individual galaxy and instead jumped straight to determining the large scale structure of the Universe.
The large scale structure, which describes the distribution of galaxies in the Universe on scales of millions of light years, can give us an insight into the effects of dark energy on the Universe. In developing a new method that allows us to look at the 21cm emission line for even further away galaxies, and therefore find the large scale structure at times closer to the big bang, Chang et al have given us a first taste of what we may be able to do with further study of the large scale structure of our Universe.
Chang, T., Pen, U., Bandura, K., & Peterson, J. (2010). An intensity map of hydrogen 21-cm emission at redshift z ≈ 0.8 Nature, 466 (7305), 463-465 DOI: 10.1038/nature09187