The dark sector

ps200306

Maintaining a healthy skepticism about the role of dark matter in cosmology seems sensible, according to some new publications. Dark matter, we recall, has three main lines of evidence. The first is the anomalous rotation curves of galaxies. Stars nearer the edges of galaxies should orbit the centres more slowly than stars closer in. This is what we see for planets in our Solar System, and is why Neptune takes 165 years to complete an orbit compared to our single year even though it is only 30 times as distant from the Sun. (Kepler's 3rd Law tells us the square of the period is proportional to the cube of the radius: ).

We don't expect stars to orbit galactic centres exactly like that. After all, our Solar System's mass is almost entirely concentrated right at the centre in the Sun, so the inverse square law of gravity produces decreasing orbital velocity with distance:



To a first approximation, a spiral galaxy is organised as a spinning disk of stars. In a disc of constant density the mass internal to a given point goes up with the square of distance from the centre. This would compensate for gravity's inverse square behaviour so we should expect that orbital velocities would remain constant throughout the disc. But, in fact, the density of luminous material in a galaxy falls off with distance, and the galaxy isn't quite a disc either -- it has a central spherical bulge with a large concentration of mass. The mass of a sphere goes up with the cube of its radius, and so the rotation curve we expect for a spiral galaxy increases within a short distance of the centre and then falls away, but at a not-quite-inverse-square rate (the red line below). What we actually observe instead is a flat rotation curve of constant orbital velocity (alright, "curves" aren't flat but that's what they call it):



This suggests a constant density disk or, more likely, an oblate spheroid with diminishing density. That leads to the theory that each galaxy is embedded in an extended halo of matter that we can't see but which exerts its gravitational influence on stars -- dark matter!

We mentioned two other lines of evidence. The first is the behaviour of light as it skirts the edge of large clusters of galaxies. These structures are huge assemblages of gravitationally interacting galaxies, hundred or even thousands of them with combined masses of a thousand trillion suns. But even this enormous mass is not enough to explain the bending of light that allows us to see even more distant galaxies behind these ones, as if the light had passed through an optical lens. Like our anomalous rotation curves, the degree of gravitational lensing is explained by the presence of large amounts of unseen -- but gravitationally active -- matter.

Finally, dark matter is used to explain how galaxies got there in the first place. Computer models of the early universe have difficulty explaining how the galaxies got going from initial density fluctuations in the matter from the Big Bang. Star formation requires shedding a lot of frictional heat in collapsing gas clouds, and this is more difficult in early galaxies before they were enriched with metals and dust from previous generations of stars. The comparatively short timescales for galaxy formation are explained by having even larger fluctuations in dark matter density, which cause streams of cold gas to be drawn into nascent galaxies, triggering rapid star formation.

The problem with dark matter is its "just-so" properties. We need five times as much of it as the ordinary luminous matter in stars we can see. Yet this huge gravitating mass doesn't fall into the centres of galaxies along with all the ordinary matter. So it must be moving around rather quickly and not losing energy by friction with either itself or ordinary matter. That's why it is generally thought to consist of Weakly Interacting Massive Particles -- WIMPs. It's bad enough that five sixths of the matter in the universe is completely undetectable except by its gravity, but now it's made out of something radically different to everything we can see!

Nevertheless we have to go where the observations lead us. The problem is there are numerous observations that do not match the dark matter hypothesis. The latest of these has just been published and comes from spectroscopic analyses of early galaxies which show their rotation curves don't match those of supposed dark matter dominated ones. It's an interesting read:

http://physicsworld.com/cws/article/news/2017/mar/15/early-galaxies-shunned-dark-matter

tac foley

An acquaintance of mine, a former professional astronomer and now retired to become a full-time amateur, once advised me that 'dark matter' was the name that we gave towards explaining the 99.999999999999999999999999999999999999999999999999999% etc of the universe that we don't yet understand.

tac

Maximus Alexander

I love all your posts. Now that you're starting threads too, this forum just got 1000x better.

ps200306

Andrew76

Maximus Alexander wrote: »

I love all your posts. Now that you're starting threads too, this forum just got 1000x better.

+1

I'm not on this forum that much but when I see a post by ps I know it's worth a read, or two, or three, before I understand what's being said. :pac:

It's fascinating and frustrating in equal measure how little we know about something that's seemingly all around us. Then there's dark energy and the mystery of the accelerating expansion of the universe, which we know even less about! According to the article below - dark energy makes up 68% of the stuff in the universe, dark matter 27% and the remaining 5% is what we actually know about. 5%, that's a shockingly small amount.

https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy

I'd love if in our lifetime we get some answers. Maybe advances in technology will lead to more discoveries and some clever soul(s) will come up with some new theories/laws.

tac foley

Andrew76 wrote: »

According to the article below [edited] - dark energy makes up 68% of the stuff in the universe, dark matter 27% and the remaining 5% is what we actually know about. 5%, that's a shockingly small amount.

The more that you think that you know, the more you know that you don't know.

The only thing about our knowledge of the Cosmos that is increasing is our ignorance.

tac