Dark Matter
Our Milky Way is a spiralling Galaxy containing 100-400 billion stars, with a stellar disk diameter of approximately 150,000-200,000 light-years and depth of 2000 light-years. There are primary 4 spirals coming out from the centre, with other minor arms. Our sun is around 25,000-28,000 light-years away from the Galactic centre in the inner edge of Orion-Cygnus arm. The Galactic centre is known as Sagittarius A*, a supermassive black-hole.
Our sun revolves about the Galactic centre taking around 240 million years to complete one revolution (a galactic year). Like our sun, all the stars and gases in our galaxy revolve around the galactic centre. Interestingly, however, contradicting to the intuition and Kepler’s laws, most objects revolve at approximately the same speed of 220 km/s irrespective of the distance from the galactic centre. Thus, it seems to follow a rigid-body rotation and not the differential rotation as is observed with-in our own solar system. On top, the rate at which it revolves, the galaxies should be torn apart due to the mere force, while accounting for gravitation, but as we know they don’t, requiring extra force to keep galaxies together.
Red line – expected velocities depending on the mass-structure etc. of Milky Way. Blueline is the observed velocity. source: https://commons.wikimedia.org/wiki/File:Rotation_curve_(Milky_Way).JPG
The difference in expected vs observed velocities led cosmologists to postulate the existence of dark invisible stuff spread across, named as the dark matter. The
mathematical calculations for the amount of dark matter required to explain
the anomaly suggests it to be around 5 times (27%) that of baryonic matter (4%).
Einstein
in 1915 when working to understand gravity looked at the problem of Mercury’s
orbit. It was observed that the orbit of Mercury wasn’t doing what was expected
but was shifting slightly after every revolution. Some astronomers postulated
then the presence of a dark planet
called Vulcan, which was believed to be providing the extra gravitational pull to
explain the anomaly. However, Einstein with a better understanding of gravity
using the General theory of relativity explained the solution to the orbit
problem without any dark planet or matter. Therefore, to explain the current
anomaly, it might very well be that we may need a better understanding of
gravity to explain such anomalies and not the dark matter.
In
nutshell, there are two major fields which offer a solution to the above-mentioned
problem and other motions in the universe, that is,
(i)
The
presence of dark matter
(ii)
Understanding
gravitational force even better
Firstly
discussing the theory of dark stuff. The question arises that what constitutes
this dark stuff. The possibilities could be classified as,
(i)
Aggregate
matter, that is,
a.
dead
stars
b.
large
planets
c.
black
holes.
(ii)
The
other is Particulate matter, that is,
a.
the
known particles
b. Other hypothetical particles
The first set of possibilities are called Massive Astrophysical Compact Halo Objects (MACHOs). It consists of the aggregate matter, like the black holes, neutron stars, white dwarfs, brown dwarfs, the dark stars that formed very early in the universe. These could have been normal matter early during in the universe and as big bang theory suggests were responsible for all the Helium in the universe, being involved in thermonuclear reactions. Eventually, however, these then became dark matter. These objects are theoretically allowed to be in the range of 10-8-103 solar masses. This makes microlensing a powerful way to detect these objects since this technique is sensitive to any objects within the allowed limits of mass. Below the lower limit of mass, the objects are expected to evaporate due to microwave background in less than the present age of the universe. Above the upper limit of mass, the objects would disrupt globular clusters.
With MACHO, Earth and the source star being in relative motion, it's not necessary to obtain an Einstein ring but it's possible to have significant amplification. The star, then, would appear to brighten, reach its peak brightness and dilute down to original brightness levels. When such a lensing event is detected, one fits the light curve to extract peak magnification, time of the peak and event duration. Using calculations one can obtain information about MACHO masses. Upon assuming that the events are due to lenses in the galactic halo, the model of the galactic halo and the observations of MACHO collaboration, the most likely mass of MACHOs are around 0.5 solar masses. Also, EROS and MACHO collaboration, based on the model of dark halo, have given constraints that the MACHOs in the mass range of 10-7-10-5 makes up less than 10% of the dark halo and so other candidates would lie in the higher mass range region.
Since the source stars should be far enough for sufficient mass to come and create lensing, researchers have been looking towards Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). Studies have allowed researchers to put constraints on how much such compact baryonic stuff could account for dark matter, which comes around 20%. There could be, however, very tiny black holes, the masses of asteroids, which may not create strong lensing but be available in quite some numbers, may very well account for the remaining amount of dark matter. By staring at the Andromeda, there has arisen slight evidence of the existence of such black holes, however, more data is required to put this claim strongly.
Now coming
to the second possibility, the elementary stuff. But couldn’t it be the stuff
we already know, like the electron, protons etc? This possibility is rejected
because the dark stuff must be electrically neutral or else it would interact
easily with other stuff in the universe. But we know that dark matter doesn’t
interact much. So, we require neutrality and stability in the properties of the
stuff for it to be a dark matter candidate. Neutron although being neutral,
outside the nucleus is unstable with a mean lifetime of approximately 15 minutes
and so can't be the dark stuff. Neutrinos are another interesting class of
particles we know in the standard model which are indeed stable, neutral and
very weakly interacting and potential candidates. It is indeed possible to
calculate the availability of dark stuff based on neutrino’s present but these
calculations show that they would form a tiny fraction. Therefore, none of the
standard particles could form the dark matter.
This leads to hypothesizing of new particles which could be candidates for the dark stuff. Weakly Interacting Massive Particles (WIMPS) are these hypothetical particles which are the believed to be strong candidate particles for the dark matter. WIMPS are weakly interacting, that is, out of the four primary forces, they interact only by weak nuclear force, gravity and any other forces which is not the part of Standard Model and even weaker than the weak nuclear force. Furthermore, because of being heavy, they are expected to be slow-moving and therefore "cold", forming cold dark matter.
Among the dark matter candidate particles, there's controversy about their formation, that is, if they were thermally created in the early universe or non-thermally through a phase transition. These different ways of formation lead to the particles having different properties like mass and couplings. The particles created by the former way are WIMPS and those formed by the latter way are axions.
Supersymmetry (SUSY) principle predicts such particles which postulate that very
early in the universe there was an equal number of all particles, partners of
each other. These particles will have a different spin and the heavier one’s
being unstable would transform but the relatively lighter one’s (around the
mass of Higgs particle) might be stable and form the dark matter. Therefore,
some of the SUSY particles have desired properties and gives correct abundance
rates theoretically. However, the super-symmetry theory hasn’t been verified yet
and so have the particles it predicts. What makes SUSY interesting is that not
only it postulates symmetry early in the universe but string theory also
predicts supersymmetry, which makes it very compelling.
Standard
model particles (left) and Super-symmetry particles (right). source: WSF https://www.youtube.com/watch?v=1VajnuxMJmU&t=2176sJPG
The abundance of stable SUSY particles is predicted using the theory assumption on the way they interact with the standard model particles. This then enables researchers to form models on these interactions, for example, modelling an equilibrium reaction which looks like this,
the dark matter particles coming in together, annihilating and transforming into ordinary matter particles, while ordinary matter interacting to form dark matter. Now, if we bring in the time component, the universe expanded with time, affecting the interaction rates. As the universe expanded, the dark matter particles won’t be able to find each other anymore and their numbers would remain there, accounting the presence of dark matter today. By accounting for dark matter interaction rates, expansion of universe etc, using this theory, the calculations of the presence of dark matter today matches very closely with the observed amount.
For WIMPS, researchers are approaching it from three sides
(i) Create it in a laboratory: Powerful colliders like
LHC, smashes standard particles and use its incoming energy and transform to
form newer particles (WIMPS) through E=mc2
(ii)
Look in the sky: Very rarely these particles may
interact and annihilate to produce radiation (gamma rays). So, the astronomers
are looking at galaxies which have abundant dark matter and looking at the
excess brightness or radiations which might be due to their interactions
(iii)
Looking at interactions on earth using experiments
and waiting for them to pop. Deep under the surface of the earth, far from noises,
still atoms of Xenon or other elements, researchers wait for the WIMP interaction
with the atom, which would produce slight jiggle in the atom and emit some
radiation as well upon.
Expanding on the approach (ii), satellites looked deeply into the
centre of the galaxy (Fermi bubble) to find gamma rays and did find an excess of
it, which after accounting for all sources known at the time could not explain the
abundance. This led some cosmologists to see it as evidence for the dark
matter. However, sceptics argued that these may be small rapidly rotating
pulsars, which was also seen by fermi satellite, could be abundant in the centre
and might be causing this excess. So, now astronomers are looking at dwarf
galaxies, which have relatively few starts and no such pulsating pulsars or
other stuff but a lot of dark matter. By staring at such galaxies they are trying
to get better evidence.
Having said that, it could very well be that dark matter are not WIMPS
and something different with different properties. The scales at which the
anomaly arises, dark matter proponents argue, that relativity equations are
trustworthy and force fields given are good enough. The anomalies raise the
possibility of a different kind of particle than WIMPS. This is because when the mass
distribution is seen by turning gravitational forces, the distribution looks
slightly different than anticipation which might be due to different property
which the dark matter has. There are different models and different things
which may happen apart from WIMPS within the dark matter regime, which is
possible.
Role of Dark Matter in the formation of
structures?
Using the known amount of abundance of dark matter, models have been
formed to simulate the structure formation and the predictions have been tried
to empirically checked with the observations. Firstly, the models predict the fluctuations
in CMB. These fluctuations then turn on gravity, which forms the structure
in the universe. Computer simulations upon comparison with the data from Sloan
Digital Sky Survey which mapped the positions in 3D of about 2 million
galaxies, look quite similar.
The models which use dark matters are able to make good predictions.
Such models allow us to know how much dark matter it is and where it is but
what it doesn’t tell is what it is. There are different possibilities (some of
which was discussed above) which can give the same distribution of gravity and
mass.
All this was the talk when we observe the universe at a large scale. At
slightly smaller scales, focused, observations gives more clues about dark
matter. WIMP, for example, is one example of the type of particle called the Cold
Dark Matter (CDM), which basically is cold, heavy and doesn’t move very
quickly. The other one is Warm Dark Matter (WDM), which moves relatively
quickly and removes the small structures in the universe. One version of WDM
would consist of dark matter formed from neutrino’s since they are light and
move very fast. Such models then predict very few small galaxies, however,
observations contrast them, supporting CDM. More than 50 dwarf galaxies
actually have been found, going around Milky way, some of which only have few
hundred stars but have million times more mass than the sun in form of dark
matter. Therefore, such observations suggest the presence of CDM, or almost cold
but not very warm.
Cold
Dark Matter (left) and Warm Dark Matter (right). source: WSF https://www.youtube.com/watch?v=1VajnuxMJmU&t=2176sJPG
Actually, the dark matter need not be a simple particle of matter. It might as well be a melange of both, like, black holes plus particles etc, but these things are not
known at present.
> Based on the World Science Festival's, Physics in the Dark and Shaking up the Dark Universe
References (short):
[2] - Earthsky
[6] - Caltech
Comments
Post a Comment