Dark Matter
At the bottom of this entry, I give the Weekly Pondering 12 assignment, for those of you in ASTR 1/2. Any text that you need to read is on Blackboard or linked to here.
Side note: the narrator in the video states that the CMBR formed around 300,000 years after the Big Bang; actually, the time period is around 380,000 years.
12: Dark Matter
We are currently learning about cosmology, which is the study of the composition and evolution of the universe. Although we do have a basic picture of how the universe has evolved, we also know that there are deep cosmic mysteries that we do not understand. One of these is dark matter, which—if it exists—is by far the most common form of matter in the universe. What is dark matter, and what is the evidence for it?
Dark matter is thought to be a particle that, since it has mass, can therefore create substantial gravitational fields if it exists in large enough numbers. It must, however, not interact with light or visible matter very much and it must not form nuclei or atoms. These properties explain a number of puzzling observations. Why should we think that such a particle exists?
There are two main forms of evidence for dark matter. The oldest form comes from galactic rotation curves. These are plots of how quickly stars rotate around the centers of spiral galaxies. For example, to the right we have a photo of a spiral galaxy. The red arrow represents the distance from the center of the galaxy to some star, which is located at the tip of the arrow. This star is rotating around the center with some speed, which we can measure. We can repeat this experiment with many stars in this galaxy, and if we plot these speeds and distances we get a galactic rotation curve.
This is a depiction of a spiral galaxy with the distance from the center of the galaxy to an arbitrary star denoted with an arrow.
This is a cartoon depiction of galactic rotation curves. Figure in the public domain.
Using the mass distribution that we see, we can apply Newton’s model of gravity and laws of motion to predict what these rotation curves should look like. Our prediction is that the speeds of the stars near the edges of spiral galaxies should be much slower than the stars near the galactic bulges. Thus, we predict curves that look like curve A in the leftmost figure.
What we find, however, is that the stars near the edges are traveling at roughly the same speeds as the ones near the bulges! This corresponds to curve B, which we refer to as a flat rotation curve. This tells us that either we are unable to see most of the matter in galaxies, or that our laws of physics and gravity aren’t correct. The dark matter hypothesis proposes that the answer is the former. We will discuss the latter, that our laws of physics and gravity are wrong, later on this page.
The second form of evidence for dark matter comes from gravitational lensing. In the photo on the right, we depict the Bullet Cluster, which is a cluster of colliding galaxies. Notice that there are pink and blue regions. The cluster doesn’t actually have these colors; instead, this is a color scheme that marks the areas where we see matter (in pink) and the areas where the matter must be via measuring gravitational lensing (blue). By observing the bending of light in this image, we are able to work backwards to determine where the matter in this cluster must be—since, remember, gravitational lensing occurs because matter bends space. In order to explain the lensing in this image, most of the matter must be located in the blue areas. When we look for matter in this region using the electromagnetic spectrum (in other words, light), however, we see it in the pink areas.
This photo is from Chandra. Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.;
Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.
This is evidence that either the blue volumes contain huge amounts of matter that we cannot see, or our understanding of gravity is very wrong. Notice that the image is not symmetrical with respect to the blue and pink areas, and this makes it difficult to write down a model of gravity which can explain these results and also agree with all of the other observations of gravity. For example, we know that Newton’s gravitational model works very well in the Solar System; if it didn’t, we wouldn’t be able to use it to send rovers to Mars, for example.
Yet, how do we know that dark matter is a particle? Despite multiple experiments designed to detect this particle, no convincing evidence of detection has been observed. Since dark matter must interact only weakly, this is not too surprising—but, we cannot claim to understand this phenomenon without having clearly directly detected such a particle. We must keep an open mind and consider many other potential solutions. For example, several decades ago astronomers proposed that these gravitational effects might be coming from neutrinos. As we’ve learned, these are small particles that mostly pass straight through solid objects like the Earth; they also do not form atoms, since they do not interact by the strong nuclear force, and so they seem like a promising candidate. Further, they have been directly observed, and, although they do interact with light, they interact only weakly. Unfortunately, neutrinos are too light and not numerous enough to explain these observations. Astronomers have also proposed that this missing mass might be black holes, as described in the above-linked video.
Instead of hypothesizing missing matter, some astronomers propose that these effects are instead due to a breakdown of Newton’s laws. This is referred to as modified Newtonian dynamics (MOND). MOND can explain galactic rotation curves quite well, but has trouble fitting all of the other data. We know that Newton’s laws work well on cosmological and Solar System scales, and this places stringent limitations on MOND models; these models then end up adding in some dark matter. Nonetheless, this is an active area of research.
Please watch the linked video about the possibility that dark matter consists of black holes. In a paragraph or two, briefly list two questions you have about dark matter. We will discuss these during the WP session this week.
Submit WP 12 here:
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