Dark Matter and Dark Energy have again been in the news. They can be confusing concepts as I pointed out in a previous post. The recent news is important, especially to cosmologists and astrophysicists, because it provides very strong evidence that previous studies and theory are in close agreement with this new evidence. It adds confidence in our understanding of the structure and behavior of the universe since the Big Bang.
The Dark Energy Survey team of over 400 scientists from 25 institutions in 7 countries reported results from the first year of a five year study of 26 million galaxies which cover 1/30th of the sky. Their map shows the distribution of the dark matter. Red shows regions of more dark matter while blue indicates less than average.
Dark vs Ordinary Matter
The ordinary matter we can see, describe, and understand makes up about 5% of the entire universe. About 25% of the matter in the universe is not visible or detectable by any direct methods. Hence it is called dark. It tends to be clumped or gathered near large scale visible structures like galaxies. Because it interacts gravitationally, there are some ways that dark matter affects the behavior of ordinary visible matter. Thus, we can infer the presence of the dark matter without actually seeing it. For example, rotation rates of outer stars of galaxies was observed to be faster than expected. Vera Rubin first pointed out that anomaly in the 1960s.
When light from a distant source passes by a region of high concentration of dark matter, the light path is bent slightly. That distorts the image of the distant source into an arc or even multiple images. This effect is called gravitational lensing. The recently reported DES study used gravitational lensing to produce their map of the sky seen above. For an excellent discussion of this effect and how DES uses the technique, watch this Fermilab video from 2015. It also contains some preliminary data from this most recent report of findings by DES.
The previous large scale study of the distribution of dark matter and energy was released by the European Space Agency ESA in 2013. The Planck Mission mapped the Cosmic Microwave Background, it shows the entire sky after a 4 year survey. It shows the small fluctuations in the microwave frequencies of the very young universe only about 380,000 yrs after the Big Bang before stars, galaxies, or common structures formed. It is a bit like looking at the seeds strewn about the ground before they started growing. These ‘cosmic seeds’ evolved into the array of stars, galaxies, and clusters of galaxies we see today.
The map and data released by the DES team studied 26 million galaxies covering a smaller part of the sky than the ESA study. The DES study looked at the large scale structure of the universe at a much later time. The universe is just under 14 billion years old. The map by DES sees the distribution of matter several billion years after the Big Bang in contrast to the 380,000 years by the ESA.
The model from the ESA study at the early age of the universe predicted how the matter would be distributed billions of years later at the time of the DES study. The DES data agreed within 7% of what was predicted. Such confirmation caused great excitement in the astrophysical community. It leads to confidence that the model being used may be correct.
What About Dark Energy?
Perhaps you noticed that dark matter and ordinary matter have been the only things discussed so far. You might have done some math and added the 5% and 25% and wondered about the remaining 70% of stuff in the universe. That was very observant of you. The remaining 70% is what is known as dark energy. As with dark matter, scientists don’t know how to directly measure it. They can’t see it. According to the current cosmological model, it seems responsible for the expansion of space.
Much of the stuff of the universe is matter that attracts like gravity. It tries to collapse to more dense regions. There is also this invisible energy that tries to expand space. We have evidence that the universe continues to expand and is even accelerating that expansion rate. Nobel Prizes were awarded for that discovery from 1998.
Of course, there are many theories about the ultimate fate of the universe. They go by names such as Big Freeze, Heat Death, Big Rip, Big Crunch, Big Bounce, False Vacuum, and Cosmic Uncertainty. I won’t be around to witness any of them. More immediate concerns occupy my thoughts.
The Dark Energy Survey home site.
BBC World Service
ESA European Space Agency