Weighing the Milky Way

Image above of Milky Way galaxy - copyright Khlongwangchao

An international research project lead by scientists at Edinburgh University produced interesting findings around the Milky Way and highlighted issues around Dark Matter 

There is a chocolate bar in the UK called Milky Way whose advertising asserted that it was lighter than other bars. Science journalists have had a bit of fun recently when a team lead by the University of Edinburgh discovered that the Milky Way galaxy is roughly half the mass as the neighbouring Andromeda system. Dr Jorge Peñarrubia of Edinburgh’s School of Physics and Astronomy lead a collaboration with the University of British Colombia, Carnegie Mellon University and NRC Herzberg Institute of Astrophysics. The team’s findings supported previous research using different data and methods conducted by Jonathan Diaz from the Institute of Astronomy at the University of Cambridge. 

They calculated the mass of Andromeda and the Milky Way by using the data on the known distances between galaxies (and data on their velocities). Not only were they able to discover the mass, the research revealed that ‘Dark Matter’ makes up 90 percent of the matter in both galaxies. Dr Peñarrubia explains that the conventional ways of measuring the mass of a galaxy (such as gravitational lensing) suggests that there must be more mass than what we can see. ‘We usually call this Dark Matter because first we don’t see it, we cannot detect any radiation, but we infer its presence through motion or the gravitational bending of light,’ says Dr Peñarrubia. The CERN website (the European Organization for Nuclear Research) whose Large Hadron Collider may offer some insight into Dark Matter notes that Dark Matter cannot absorb, reflect or emit light. 

One Quarter of the Universe

We assume the existence of Dark Matter says Dr Peñarrubia, ‘Basically we see the stars moving very fast, much faster than they should move if there was only the light or the mass in the stars. We count the stars, we know basically the mass of the star and that cannot account for the motion of these stars.”  Swiss Astronomer Fritz Zwicky first used the term Dark Matter in the 1930s. But even though we can infer Dark Matter, and it is believed that it comprises about 25 percent of the Universe compared to 5 percent of ordinary matter (the rest is believed to be something called Dark Energy) it poses problems for current models. If General Relativity is the correct way to describe gravity then, ‘Dark Matter cannot be made of particles that we can find on earth, but of exotic particles,’ explains Dr Peñarrubia, ‘and these are currently not predicted by the Standard Model of particle physics. It has attracted a lot of attention from particle physicists because they see Dark Matter as a possibility to extend their models beyond the current Standard Model paradigm.’

So why has there been a sudden upsurge of interest in the area of Dark Matter? This interest in Dark Matter has always been there since the 1930s, says Dr Peñarrubia, but ‘right now we are getting more and more data, and are more convinced that the problem is real.’ So people are more aware of the serious consequences of either having to drop General Relativity or drop the Standard Model of particle physics. 

‘It is becoming a multidisciplinary approach,’ he says, ‘you get particle physicists interpreting the Dark Matter problem, cosmologists interpreting the Dark Matter problem and trying to unify the forces as an opportunity, which is what Einstein was trying to do until he died.’

Peñarrubia explains that because accelerators are reaching their limit, particle physicists are looking at other alternative ways to test their theories. ‘Even with the Large Hadron Collider there is a finite amount of energy they can probe. The cosmos is much more energetic that anything we can produce on earth, so that’s why they can use astronomical data to test their theories. More and more people are interested in this area.’