# Dark Matter

“Dark matter?”  You cannot see it.  But there is something there.  As for what it is, it’s anybody’s guess!  Dark matter does not interact with light.  At all.  Which makes it difficult to detect.  But if you cannot see it?  How do you know it is in fact there?”  Well, it does interact with gravity, and as it does so it bends the path of any light ray passing nearby...  “And did it really kill the dinosaurs…?”

Just like dark matter, you never see it coming…  And then suddenly… “BAM”!!  You’re cornered!  Without warning, it’s Cocktail Party Physics time…

Dark matter is mysterious, invisible “stuff” (technical word) that accounts for up to 85% of the matter in the Universe.  Dark matter neither emits nor absorbs light or any other kind of electromagnetic radiation.  It is completely non-luminous, which makes it difficult to observe.

“… !!”

That quirky scientific stuff never fails to make an impression at cocktail parties.  😉

But if you cannot see it?!!  How do you know it is in fact there?”

Well, actually…

# Inferring Dark Matter

Scientists study distant galaxy clusters using two space telescopes.  They observe them in visible light with the Hubble Space Telescope, as well as in X-rays with the Chandra Observatory.

There are three main components in galaxies:

• stars, seen in regular, visible-light images
• gas clouds, hot enough to glow with X-rays
• dark matter is more difficult to “see”, BUT not impossible to detect!

As the path of light is bent by dark matter, our view of any stellar object on the other side is warped in an effect called gravitational lensing.

## Gravitational Lensing

In General Relativity, mass bends spacetime.

Light follows the curvature of spacetime.

Hence when it passes in the region of a very massive object – like a large galaxy cluster, light is subsequently bent.

This means that the light from a distant object, like a galaxy, on the other side of it, will be bent towards an observer’s eye, just like an ordinary lens.

### Since light always moves at a constant speed, lensing changes the direction of the velocity of the light, but not the magnitude.

Light rays are the boundary between the future, the spacetime, and the past regions.

The gravitational attraction can be viewed as the motion of undisturbed objects in a background curved geometry, or alternatively as the response of objects to a force in a flat geometry.

Looking through dark matter is almost like observing an object moving behind a pane of frosted glass…  The objects you do see appear a little distorted and warped

Scientists have been investigating the gravitational effect of dark matter on light from distant galaxies.  Galaxies exhibit the gravitational dynamics of astronomical objects with much more mass than they actually appear to have.

## Rotational Velocity Curves

Evidence has been accumulated in support of dark matter, which, unlike normal matter, does not give off or absorb electromagnetic radiation.

The rotation curve of a galaxy is a plot of how its speed varies against distance from the galactic centre.  The greater-than-expected rotational speeds of stars in the outer-lying regions of galaxies suggest that those galaxies contain more mass than can be accounted for simply by adding up all of the light.

Some astronomers have become convinced that the total mass of dark matter in the Milky Way must be about ten times greater than the total mass of stars, and 100 times greater than the total mass of gas and dust.

“So what does it prove?”

While you cannot see it, you know it has to exist.  Right?  Although far from dark, light seems to be everywhere in dark matter – shining straight through it, coasting around it…

Dark matter must exist because astronomical objects like galaxies exhibit the gravitational dynamic behaviour of much more massive objects than they do actually appear to be.  So, some form of exotic non-luminous matter is the only way to explain such a discrepancy…

So, okay…  Scientists believe there are at least four times as much dark matter in the Universe as there is ordinary matter.  Despite its great abundance, dark matter is generally thought to interact only very weakly with conventional matter, causing it to form amorphous haloes around galaxies that contrast with the richly structured galactic disks themselves.

Despite knowing little about dark matter, scientists know that much of dark matter interacts weakly with other matter and with itself.

Among the leading candidates for dark matter, there are so-called weakly interacting massive particles (WIMPs) and axions, which rarely ever collide with one another.  The existence of these particles is also suggested from work in other areas of physics – WIMPs being predicted by some forms of supersymmetry, while axions might explain why strong interactions obey charge-parity symmetry.

# On the Large Scale

When astrophysicists study the large scale structure of the Universe, it becomes clear that the amount of visible matter cannot possibly generate enough gravity to hold together the structures they can see.  Galaxy clusters and even galaxies themselves ought to fly apart, given the amount of ordinary matter they contain.

At the initial epoch ($z = 30$), when the age of the Universe was less than 1% of its current age, the distribution of matter appears to be uniform, because the seed fluctuations are still fairly small.  As time goes on, the fluctuations grow resulting in a wealth of structures from the smallest bright clumps with sizes and masses comparable to that of galaxies and large filaments.  Over time, filaments becomes more pronounced.

Little change occurs between redshifts z=0.5 and z=0 (i.e. the last two panels), because the expansion of the Universe is in the stage of acceleration, as the “dark energy” becomes dominant at z < 1.  On large scales, gravity cannot compete with the dark energy-driven acceleration and the growth of structures ceases.  As the contraction of large-scale structures is halted, they expand with the Universe and appear “frozen” in a co-moving system of coordinates.

Astronomers came up with the idea of dark matter – mysterious, invisible and non-interacting stuff that fills the Universe, generating the gravity necessary to hold everything together.

Far from being a small discrepancy, the problem is huge!  No tiny amount of extra mass must be found.

#### An enormous amount of mass is “missing”.

According to the latest picture of the large-scale structure of the Universe from the Planck space mission, ordinary visible matter makes up a mere 5% of the total mass/energy of the Universe.

Dark matter makes up 27% of it – the remainder is thought to be the even more mysterious and exotic dark energy…

The new analysis by the Planck team of more than 400 galaxy clusters gives us a new look at their masses, which range between 100 to 1,000 times the mass of our Milky Way galaxy.  In one of the first-of-its-kind efforts, the Planck team obtained the cluster masses by observing how the clusters bend background microwave light.  A huge step in better understanding dark matter and dark energy.

One cosmic property that appears to have changed with this new batch of data is the length of time in which our Universe remained in darkness during its infant stages.  A preliminary analysis of the Planck data suggests that this epoch – the Dark Ages of Cosmology – that took place prior to the first stars and other celestial objects ignited, lasted more than 100 million years or so longer than previously thought.

Specifically, the Dark Ages ended 550 million years after the Big Bang that created our universe, later than previous estimates of 300 to 400 million years by other telescopes.

A phenomenological model for cold dark matter has proved hugely successful on cosmological scales where its gravitational influence dominates the growth and evolution of cosmic structure.  But there are several challenges on smaller scales…

# Dark Matter Haloes

Around 1011 stars make up our Galaxy – the Milky Way.  But the modern view of the Galaxy is that the largest and seemingly more massive of its components, is in effect a huge, and roughly spherical cloud that surrounds and pervades it.  That cloud consists of non-luminous matter.  Since this invisible matter has never been observed at any wavelength, it has become known as dark matter.

The huge cloud of dark matter that is the main structural component of the Milky Way and other galaxies, is often referred to a the dark-matter halo.  Its mass is so great that it is the gravity exerted by this elusive exotic matter that is primarily responsible for holding our Galaxy together, rather than the gravity due to the total mass of the stars.

As dark matter exerts a gravitational influence on luminous material, astrophysicists use this to infer the shape of each dark-matter halo.  From this tell-tale signature of dark matter, they were able to conclude that it forms an oblate spheroid – a three-dimensional figure formed by rotating an ellipse about one of its axes, and flattened at its poles with a shortest-to-longest axis ration of about 0.8.

# Does Dark Matter affect Motion in our Solar System?

If dark matter permeates space through the Solar System, and the Sun is surrounded by a sea of dark matter particles, we ought to be able to see its gravitational influence on the orbits of the outer planets, moons and asteroids. And if there is enough of it, dark matter ought to be detectable.

Adler (2009) reviewed constraints on solar system-bound dark matter, and discussed the possibility that dark matter could be gravitationally bound to the Earth and other planets and surveyed various empirical constraints on such planet-bound dark matter, and discuss effects it could produce if present, including anomalous planetary heating and flyby velocity changes.

Pitjev & Pitjeva (2013) compiled an impressive set of data consisting of some 677,000 measurements of planetary positions taken since 1910. These include optical measurements from observatories on Earth, ranging measurements from various spacecraft such as Cassini at Saturn and the Mars and Venus Express missions and Russian radar measurements of planetary positions.

The astrophysicists used these measurements to model the behaviour of the Solar System, taking into account the perturbations caused by the major planets, the Moon, the 301 largest asteroids, as well as the other asteroids modelled as a uniform ring, and the 21 largest trans-Neptunian objects, and so on.  Then they looked for anomalous gravitational effects that might be the result of dark matter.

“The conclusion?”

They found no evidence of the gravitational effect of dark matter in their analysis!  And they concluded that if it is present, its effect must be smaller than the errors in the data.  To satisfy this limit, the dark matter mass in the sphere within Saturn’s orbit must be tiny – 1.7 x 10-10 M.

## How much is the mass of the Dark Matter?

M = Sun’s mass = 2.004 x 1030kg.

So,

${1.7 \times 10^{-10}} \times {2.004 \times 10^{30}kg} = 3.4 \times 10^{20} \mathrm{kg}$.

1 ton ~ 1,000 kilograms.

This is about 3.4 x 1017 tons.

About the mass of a large asteroid.

At the heart of the argument, dark matter must hold our galaxy together in a vice-like gravitational grip.  On the other hand, dark matter is negligible when it comes to Solar System orbits, and it is not even close to being detectable.

“Mmmh…  Tricky!”  :-/

We may one day come to understand the Solar System well enough that such tiny differences will be detectable, but until then…

“Maybe we ought to think of them as the local dark-matter potholes of the Solar System?”

# Non-Gravitational Interactions of Dark Matter

Using the Chandra X-ray Observatory and Hubble Space Telescopes, Harvey et al. (2015) have observed 72 collisions between galactic clusters, including both major and minor mergers

Galaxy clusters span vast distances, and they contain huge amounts of dark matter, so when they are seen colliding over billions of years, observers get a unique opportunity to glimpse at its dynamics.

The Bullet Cluster provides the best current evidence for the nature of dark matter and “evidence against some of the more popular versions of Modified Newtonian Dynamics (MOND)” as applied to large galactic clusters.  At a statistical significance of 8σ, it was found that the spatial offset of the centre of the total mass from the centre of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law alone.

A 2010 study concluded that the velocities of the collision, as currently measured, are “incompatible with the prediction of a Lambda Cold Dark Matter (ΛCDM) model”.

With multiple views showing the collisions, the research tracked the movement of stars, clouds of gas and dark matter.  It provides a unique look at how the stuff behaves.  Using gravitational lensing, the scientists from the University of Edinburgh, University College London (UCL) and Switzerland’s École Polytechnique Fédérale de Lausanne (EPFL) mapped the clusters’ dark matter as they collided.

They measured three fundamental quantities: the position of stars, the position of gas, and the position of mass.  If there was no dark matter present, all the mass not accounted by stars would have to be associated with the gas.  They did find an offset in their data, which indicated the existence of dark matter in the clusters to a probability of greater than 99.99999999999%.

Unlike the gas clouds, which appear to grind to a turbulent halt, unlike the stars, which glide past each other, dark matter appears to pass through everything in a ghostly fashion, then re-emerges undisturbed without interacting with anything at all.

The Planck data support the idea that the mysterious force known as dark energy is acting against gravity to push our Universe apart at ever-increasing speeds.  Some scientists have proposed that dark energy does not exist.  Instead, what we do know about gravity from Albert Einstein’s General Theory of Relativity, needs refining.  In those theories, gravity becomes repulsive across great distances, eliminating the need for dark energy.

Independent scientific lines of evidence seem to suggest that most of the matter in the Universe is in a form outside the Standard Model of Particle Physics.

Many theories can now be ruled out, including the idea that dark matter is a type of ordinary matter made of “dark” atoms.

“Really?”

Really.  It’s more outlandish than that…

# Extinction of Dinosaurs?

Right…  I gather that one story has been doing the rounds…

Several astrophysical mechanisms have been proposed to explain cyclic impacts and extinctions on the Earth.  In 1984, one proposal involved the Sun’s oscillation through the Galactic mid-plane.

The Sun takes around 250 million years to orbit around the centre of the Milky Way.  But it does not revolve around it in a flat line.  Instead it bobs up and down – a bit like a cork stopper bobbing up and down on a river current.

The Solar System has a wave pattern motion as it crosses through the dense spiral plane of the Galaxy.

In 2013, Harvard scientists suggested that a significant amount of dark matter might be sandwiched in the dense middle layer of the Galaxy.  And two physicists even put forward an idea that this might explain the seemingly regular pattern of extinction events on Earth.

According to scientists, every 30 – 42 million years, the Solar System could pass through that dark matter sandwich, altering the orbits of comets in the Öpik–Oort cloud, and thus creating a rise in impact events at the surface of the Earth, and the other planets… and cause mass extinctions.

In 2014, Michael Rampino – a biologist at NYU – went one step further and proposed that dark matter could accumulate inside the Earth’s core as it passes through the Galactic disk.  Eventually those dark matter could interact and annihilate each other by creating a lot of energy that would in turn heat up the inner core.  He theorised that it could explain the sudden increases in geological activity – like massive volcanic eruptions that would alter the Earth’s environment enough for some species to become extinct and others to survive:

A cycle in the range of 26 – 30 Myr has been reported in mass extinctions, and terrestrial impact cratering may exhibit a similar cycle of 31 ± 5 Myr.  These cycles have been attributed to the Sun’s vertical oscillations through the Galactic disk, estimated to take from ∼30 to 42 Myr between Galactic plane crossings.  Near the Galactic mid-plane, the Solar system’s Oort Cloud comets could be perturbed by Galactic tidal forces, and possibly a thin dark matter (DM) disc, which might produce periodic comet showers and extinctions on the Earth.  Passage of the Earth through especially dense clumps of DM, composed of Weakly Interacting Massive Particles (WIMPs) in the Galactic plane, could also lead to heating in the core of the planet through capture and subsequent annihilation of DM particles.  This new source of periodic heating in the Earth’s interior might explain a similar ∼30 Myr periodicity observed in terrestrial geologic activity, which may also be involved in extinctions.  These results suggest that cycles of geological turmoil and biological evolution on the Earth may be partly controlled by the rhythms of Galactic dynamics.

“… !!”

No, really!  It’s peer-reviewed and everything…

“… !!”

It’s been published in the Monthly Notices of the Royal Astronomical Society!

As daring as this theory might sound for the present, it could serve to explain why extinction events that were not caused by comet impact events also fit the timeline.

“… !!”

“Oh, well… That’d better no’ happen again…!”

Bah…!!  I’m running home.  🙁

Enough cocktail parties for a while!

“… !!”

I’m a WIMP…