What Is the Proof of Dark Matter?
Introduction to Dark Matter
The current standard model of cosmology indicates the mass-energy balance of our universe to be:
- 4.9% - 'normal' matter
- 26.8% - dark matter
- 68.3% - dark energy
Therefore, dark matter makes up almost 85% of the total matter in the universe. However, physicists currently don't understand what dark energy or dark matter is. We do know that dark matter interacts with objects gravitationally because we have detected it by seeing its gravitational effects on other celestial objects. Dark matter is invisible to direct observation because it doesn't emit radiation, hence the name 'dark'.
The main piece of evidence for dark matter comes from the observation of spiral galaxies using radio astronomy. Radio astronomy uses large collecting telescopes to collect radio frequency emissions from space. This data will then be analysed to show evidence for extra matter which can't be accounted for from observed luminous matter.
The most commonly used signal is the hydrogen 21-cm line. Neutral hydrogen (HI) emits a photon of wavelength equal to 21 cm when the atomic electron's spin flips from up to down. This difference in spin states is a small energy difference, and hence this process is rare. However, hydrogen is the most abundant element in the universe, and hence the line is easily observed from the gas within large objects, such as galaxies.
A telescope can only take an observation of a certain angular segment of the galaxy. By taking multiple observations that span the whole galaxy, the distribution of HI in the galaxy can be determined. This leads, after analysis, to the total HI mass in the galaxy and hence an estimate of the total radiating mass within the galaxy, i.e. the mass that can be observed from emitted radiation. This distribution can also be used to determine the velocity of the HI gas and hence the velocity of the galaxy throughout the observed region.
The velocity of the gas at the edge of the galaxy can be used to give a value for the dynamic mass, i.e. the amount of mass causing the rotation. By equating the centripetal force and gravitational force, we obtain a simple expression for the dynamic mass, M, causing a rotation velocity, v, at a distance, r.
When these calculations are performed the dynamic mass is found to be an order of magnitude larger than than the radiating mass. Typically, the radiating mass will only be about 10% or less of the dynamic mass. The large quantity of 'missing mass' that isn't observed through radiation emission is what physicists call dark matter.
Another common way of demonstrating this 'fingerprint' of dark matter is to plot the rotation curves of galaxies. A rotation curve is simply a plot of the orbital velocity of gas clouds against the distance from the galactic centre. With only 'normal' matter, we would expect a keplerian decline (rotation speed decreasing with distance). This is analogous to the speeds of planets orbiting our sun e.g. a year on Earth is longer than on Venus but shorter than on Mars.
However, the observed data doesn't show the keplerian decline that was expected. Instead of a decline, the curve stays relatively flat up to large distances. This means that the galaxy is rotating at a constant rate independent of the distance away from the galactic centre. To maintain this constant rotation speed the mass must be linearly increasing with radius. This is the opposite of observations that clearly show galaxies that have dense centres and less mass as distance increases. Hence, the same conclusion as earlier is reached, there is additional mass within the galaxy that is emitting no radiation and hence hasn't been directly detected.
The Search for Dark Matter
The problem of dark matter is an area of current research in cosmology and particle physics. Dark matter particles would have to be something outside of the current standard model of particle physics, with the leading candidate being WIMPs (weakly interacting massive particles). The search for dark matter particles is very tricky but potentially achievable through either direct or indirect detection. Direct detection involves looking for the effect of dark matter particles, passing through the Earth, on nuclei and indirect detection involves searching for potential decay products of a dark matter particle. The new particles may even be discovered in high energy collider searches, such as the LHC. However it is found, the discovery of what dark matter is made out of will be a huge step forward in our understanding of the universe.
© 2017 Sam Brind