Scientists find Milky Way weighs less than expected

A team of scientists from Edinburgh University has found that the Milky Way galaxy is a lot lighter than we thought. It has about half the mass of a neighbouring galaxy, known as Andromeda. VoR's Brendan Cole spoke to Dr Jorge Penarrubia who headed the study.
Dr Jorge Penarrubia  told VoR: “There has been a long-standing controversy over the calculations of the Milky Way mass and the Andromedae 51 masses. It has been uncertain for a number of years now.
“What we did was to try to measure both of them simultaneously using the same data set. That’s the novelty of our approach, really.
“We measured the total mass of the Milky Way, plus the Andromeda masses. We got both masses, now, accurately determined, we think.
“Our results indicate that the Milky Way mass is about half the mass of Andromeda. Dark matter is the problem. What we can do is to measure the position and velocities of objects we can see, for example, stars and gas. But the problem is that those objects seem to be located mostly in the inner regions of galaxies and when we go and measure those velocities, what we see is that the further away we go from the centre of a galaxy, the more mass there seems to be. And that’s in contradiction to what we see.
“The number of visible objects is falling off very quickly. This is commonly interpreted as evidence of dark matter. But it poses a big problem, because since we don’t have any more visible objects out there, then we cannot really tell, historically, we have not been able to tell what was the total mass of a galaxy.
“Nobody understands what dark matter is. The person who understands what dark matter is will get a Nobel prize. Dark matter [as an issue] has been around for almost a hundred years. It’s a big challenge. What have to understand is that what we can measure is distances and velocities. And from there what we do is to assume how gravity works. From using that gravity model, we translate velocities and positions into masses."

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