Dark matter- why do we think it's there?
Just what dark matter is, is one of the most involving questions of modern physics, and it is one the fundamental questions that the CDMPP is addressing.
But let’s take a step back and ask “why”. Why do we need dark matter? What has led us to believe that there is indeed such a thing, and what concordances do we see between dark matter and observation in modern physics, astrophysics and cosmology?
The concept of dark matter- some unseen but yet observable component of our galaxy and, perhaps, the universe as a whole- has been around for some time. In a broad sense, it was suggested in the late 1800’s, with work in the early decades of the 20th Century on stellar velocities in our galaxy suggesting the presence of unseen mass in our galaxy.
In the 1930’s, the realm of investigation moved outward- quite a way outward, to around 300 million light-years, in fact- with the astronomer Fritz Zwicky’s study of the Coma galaxy cluster. The motions of the galaxy’s outer members suggested that the cluster contained much more mass than could be accounted for by the visible material alone. Whilst his estimate of the non-visible : visible mass ratio proved somewhat too large, the foundation was solid- the existence of dark matter was, in a sense, in plain, observable sight.
The next suggestion for the presence of dark matter came from observations of spiral galaxies. In essence a flat, thin disc with a bulge in the middle (well, that’s what they look like in visible light), spiral galaxies rotate around their nucleus. They’re not a single, solid object, so stars at different distances from the nucleus orbit the nucleus at different speeds.
What is observed is that the stars’ orbital speeds increase with distance out to a certain radius from the nucleus, and then the speeds level out, and stay approximately constant as the orbital radius increases.
But this is not what one would expect, based on the visible distribution of material in the galaxy (and this applies not only to the optically-visible gas, dust and stars, but also to the wider disc of neutral hydrogen that spiral galaxies also contain, as observed by radio telescopes). Considering only the visible matter and the neutral hydrogen disc, the stars’ orbital speeds should reach a peak at some distance from the nucleus, and then those speeds should decrease as orbital radius increases.
The best explanation for this discrepancy is that galaxies actually contain more mass- more material- than is observable. One can imagine such a galaxy (ours included) to be within a dark matter halo- invisible, and yet having a major effect on the dynamics of the galaxy.
Much of the work in this regard was done by Vera Rubin and her colleagues in the 1970’s, using optical telescopes to undertake spectroscopic observation of edge-on spiral galaxies, where the orbital motions of such a galaxy’s stars is most readily detected.
Continuing on in the galactic theme, more evidence comes in the form of observations of the internal motions of stars in elliptical galaxies. Whilst not rotating in the neat kind of way that spiral galaxies do, it is seen that the distribution of stellar velocities in most elliptical galaxies (i.e. the range of velocities) requires that more mass is present in such galaxies than is seen optically or by radio methods.
On a larger scale, looking at clusters of galaxies, estimates of the entire mass contained within a galaxy cluster may come from a number of methods. We can study
the motions of galaxies within the cluster (hearkening back to Zwicky’s original observations in the 1930’s);
the characteristics of the hot intergalactic gas (the intracluster gas) within the cluster, when observed with X-ray satellites;
gravitational lensing of background galaxies by the foreground cluster’s total mass.
These different methods result in a mass ratio of visible : non-visible mass that is consistent between the different methods.
And for a single, specific example, there is a galaxy cluster known as the Bullet cluster that provides strong evidence for dark matter. In this cluster, the location of the centre of the cluster, as determined from the distribution of visible (baryonic) matter, is different from the centre of mass of the cluster, as determined from other observational methods. The presence of a cluster-scale cloud of dark matter resolves this difference, in a way that other proposed non-DM methods (such as modified Newtonian dynamics) cannot.
Fear not, we’re getting there-just a couple more things to cover…
Let’s now expand to an even wider scale- to the very structure of the cosmic web itself, that diaphanous, web-like distribution of matter throughout the cosmos.
Computer simulations of the evolution of the universe, on a scale large enough to show clusters of galaxies like pearls on a string, show that dark matter is essential to the process of baryonic matter (i.e., the stuff that we do see, and that we’re made of) eventually being distributed in the web-like structures that we do observe.
Without dark matter, the influence of radiation on baryonic matter within the universe is such that the baryonic matter would not have the chance to coalesce, under gravity, to form that network. Since dark matter does not interact with electromagnetic radiation, it will do its own thing, so to speak, forming those patterns, and providing the gravitational “seeds” for the later collapse of baryonic matter onto- and into- the patterns that we do see.
Now- lastly- we’ll go further and, indeed, earlier- right back to the Cosmic Microwave Background (CMB).
The CMB is a relic of the Big Bang itself, a glow that covers the entire sky. It was discovered as a radio hiss that was the same everywhere, no matter where one observed in the sky. Over time, observations of the CMB have improved vastly; we can now measure minute differences in the temperature of the CMB that tell us about conditions in the very early universe.
When considering the distribution of these temperature variations over the entire sky, it is seen that there are variations on a number of angular scales, and these are related to various fundamental cosmological parameters. One of the peaks in these so-called “acoustic oscillations” relates to dark matter; and the observations in the CMB sky map agree extremely well with expectations from the Lambda-CDM cosmological model (“CDM” = “Cold Dark Matter”).
In all of the above, there is a strong agreement as to the relative amount of dark matter in the universe. We think that some 23% of the total mass-energy of the universe is in the form of dark matter, whereas only around 4.6% is in the form of the ordinary (baryonic) matter with which we are most familiar.
Thus, our desire to understand just what dark matter is, is a very fundamental one- there is more of it than all of the gas, dust, stars and galaxies put together…