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.
Fermi National Accelerator Laboratory | Click to embiggen
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.
European Space Agency | Planck Collaboration | Click to embiggen
Zoomed in to a smaller portion of the sky | ESA
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.
References
The Dark Energy Survey home site.
http://www.darkenergysurvey.org/
Fermilab
http://vms.fnal.gov/asset/detail?recid=1949856
BBC World Service
http://www.bbc.co.uk/programmes/p05bgyn1
ESA European Space Agency
http://sci.esa.int/planck/53103-planck-cosmology/
http://www.esa.int/Our_Activities/Space_Science/Planck/Planck_and_the_cosmic_microwave_background
How I See It 
I enjoyed this post very much, Jim. Cosmology is fascinating. While it may seem very abstract to many people, it is all too real when it’s explained as you do here. I’m led to think about the coming total eclipse of the sun as meaningful evidence that astrophysics is altogether real and meaningful.
I can relate to dark matter, but not to dark energy which blows my mind. Matter is stuff. I get stuff, you can feel it and if it’s close, you can see it. Matter and gravity relate because that is actual experience. I went to the Wikipedia page on dark energy in search of more understanding:
My conclusion: Dark energy is an intangible thing that has consistent properties that help explain observations but which don’t relate in any way to common experience. I’m reminded of a quote by the great Arthur C. Clark:
I’m glad you found the post meaningful. The concepts are not easy to grasp and harder yet to explain, especially to an audience that might not have followed previous news, etc. Your comments were helpful.
I agree about being able to relate to dark matter. We have some experience with matter and how it affects us and other matter. The dark energy thing is very mysterious. I’ve read and listened to talks about vacuum energy. Virtual particles can appear and disappear in the vacuum field. Some say the dark energy behaves in a similar way, creating particles and space. That is where they lose me.
I can relate to things like electric and magnetic fields. They can store large amounts of energy. They can cause expansion as like charges or poles are held near each other and let go. But, with dark energy there is no known charge or pole. Maybe some kind of particle we have yet to discover.
I do love the magic of it all. Thanks for your visit. Have a good weekend. Will you go see the total eclipse somewhere? We are driving down toward Jefferson City.
No, we are staying home during the eclipse. I have seen one somewhere, sometime, but I can’t remember when or where. But I saw enough to make it no longer an abstract thing. I wish you safe travel to Jeff City!
All this is fascinating but I’ll confess I don’t understand much of it. I’m no specialist, so there’s no reason why I should. At the same time, I wonder to what extent physicists are on the right track. Might dark energy be a modern counterpart of the epicycles that were put forward as a way of justifying the observed movements of the planets?
It is through testing theories, challenging the data and assumptions, and openly discussing results that I think the physicists are on the right track. We move forward with baby steps most of the time. Once in a while, we get to take a big step. This seems to be one of them.
I don’t understand many of the details mentioned in papers and talks and am quickly in over my head. I’ve tried to keep pace with the progress for a long time. It is fun to see the excitement of the science community in stories like this.
What I find most fascinating about dark matter is that we really don’t know what it is. It is a true frontier of knowledge.
Someday I think we will know. I’m hopeful the frontier will reveal some secrets in my lifetime.
It is good that there are frontiers left to explore.
[…] placed into the framework of the Standard Model. Also missing are Dark Matter and Dark Energy. Much research is ongoing in these […]