Oct 10 2017

Half the Matter in the Universe Just Found

filamentsBy now most people are familiar with dark matter – that mysterious substance which has gravity but otherwise does not seem to interact with the normal matter with which we are most familiar. About 27% of the stuff (matter and energy) in the known universe is dark matter, 68% is dark energy, and only about 5% is made of known particles (baryons – protons, neutrons; leptons – electron; and more exotic particles).

We currently don’t know what dark matter is. We know it’s there because we can see its gravitational effect, first noticed because galaxies spin faster than they should. Based just on the gravity from stuff we can see, galaxies should be flying apart. They stick together because there is significantly more gravity than we can account for. There must be additional matter we can’t see, or dark matter.

It is perhaps less well-known that we also haven’t found about half of the normal matter that should exist in the universe. Even if we just consider that 5% that is made of standard particles, about half of it is missing. That is – until now, if recent reports are accurate.

This really wasn’t much of a mystery (not like dark matter) – astronomers suspected that the missing matter was present in the form of diffuse gas between galaxies. There is a lot of space out there, and even a wispy vapor could contain a lot of particles, as much as is contained in all the visible galaxies. The problem is, this thin gas is too wispy to see with conventional means.

Two groups of astronomers, however, have found a way to detect it. The two teams,  one at the Institute of Space Astrophysics (IAS) in Orsay, France, and the other from the University of Edinburgh, used data from the Planck satellite. They also both used a phenomenon known as the Sunyaev-Zel’dovich effect. When the cosmic background radiation passes through hot plasma it tends to brighten a little. Therefore, we can image the temperature of the CBR and use that to map out the hot plasma in the universe.

When the two teams did this they both found that there is hot plasma in the form of filaments that stretch between the visible galaxies. These filaments are 3-6 times denser than the background gas in the universe – which adds enough matter to the visible universe to account for the missing 50% of normal matter.

Astrophysicist Ralph Kraft is quoted as saying:

“This goes a long way toward showing that many of our ideas of how galaxies form and how structures form over the history of the universe are pretty much correct.”

Essentially astronomers pretty much knew this stuff was out there, but these are the first observations to actually demonstrate it. Sometimes new discoveries challenge what we think we know about the universe. And sometime discoveries confirm what we thought we knew. The former tend to get more media and public attention, but it is important to recognize how science progresses in all its facets.

I like to think of this in terms of a jigsaw puzzle analogy. Trying to figure out how the universe works, or any complex scientific question, is like putting together a jigsaw puzzle, without any reference picture, and without any edges, or even knowing how many pieces there are. Sometimes when we find a new piece it fits into the picture we are building. But sometimes the piece doesn’t fit – it expands the puzzle and shows us it is bigger and more complex than we previously thought.

This finding puts a piece right in the middle of our puzzle of the universe, and pretty much right where we thought it should go.

That still leaves 95% of the universe as a mystery. Dark energy and dark matter were definitely pieces that fit outside of the known edges of the picture. We suddenly realized the picture was 20 times bigger than we thought.

However, we should resist the temptation to overhype the significance of the mysterious nature of dark matter and dark energy. Often those who wish to cast doubt (either in general or on some specific area of science) will simplistically assume that because there is something we don’t know that automatically casts doubt one something else we think we do know. This is not necessarily correct, however.

It is possible to have confident scientific knowledge in one area, even if other areas remain unknown. Further, it’s even possible to be confident about one level of knowledge in an area even when deeper questions in that same area are unknown.

For example, we could be very confident that DNA is the primary molecule of inheritance before we understood how it worked. We can be confident that humans and other apes share a recent common ancestor, even before we fully flesh out all the complexity of our ancestry.

This is where the jigsaw puzzle analogy breaks down. As the picture emerges we aren’t just adding new crystal clear pieces. Some of the pieces and the resultant images they contain are blurry or low resolution. As science progresses the picture becomes more clear and more detailed. We are zooming into the picture, not just adding pieces. (So we have to invoke a digital jigsaw puzzle with individual pieces that can vary in terms of their resolution and focus.)

So, for example, when looking at a blurry picture of a tree, at some resolution you can be highly confident that it is, in fact, a tree and nothing else, even before you can see in enough detail to know what kind of tree it is. As the picture becomes clearer, perhaps at some point you can conclude it is a deciduous tree, and then with more detail that it is a maple tree. But there are still many species of maple and it may not be clear which one. Not knowing which species of maple the tree is, however, does not call into question whether or not it is a tree at all.

To bring this back to the current topic – the standard model of particle physics is wildly successful, has made many highly accurate predictions, and is a very useful construct to understand normal matter. The existence of dark matter adds a mystery to our understanding of the universe, but it does not invalidate the standard model.

The missing normal matter was an even smaller mystery. We basically knew it was there and where it was, we just needed to develop a technique for seeing it – and we did. A few more puzzle pieces snap into place with a satisfying click.

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