Dark Matter, Neutrinos, and Our Solar System

Nirmala Prakash

• Publisher: WSPC
• Year: 2012
• ISBN:
  9789814449205
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

Dark Matter, Neutrinos, and Our Solar System is a unique enterprise that should be viewed as an important contribution to our understanding of dark matter, neutrinos and the solar system. It describes these issues in terms of links, between cosmology, particle and nuclear physics, as well as between cosmology, atmospheric and terrestrial physics. It studies the constituents of dark matter (classified as hot warm and cold) first in terms of their individual structures (baryonic and non-baryonic, massive and non-massive, interacting and non-interacting) and second, in terms of facilities available to detect these structures (large and small). Neutrinos (an important component of dark matter) are treated as a separate entity. A detailed study of these elusive (sub-atomic) particles is done, from the year 1913 when they were found as byproducts of beta decay — until the discovery in 2007 which confirmed that neutrino flavors were not more than three (as speculated by some).

The last chapter of the book details the real-time stories about the “regions” that were not explored thus far, for lack of advanced technology. Their untold fascinating stories (which span up to 2010) are illustrated here datewise in full.

The book concludes with the latest news that the Large Hadron Collider team at CERN has finally succeeded in producing 7 trillion electronic Volts of energy by creating head-on-collisions of protons and more protons (in search of God-particle). The energy produced was three times more than previous records.

Foreword
Foreword (48 KB)

Contents:

• The Advent of Dark Matter: Galaxies, Clusters, Planet Formation, and Comet Collision
• Stars of Poor Visibility and the Methods to Track Them
• Models in Cosmology, the Luminosity of a Star, White Dwarfs and Neutron Stars
• Black Holes: The Stars with no Shine
• Particles We Encounter (A Historical Overview)
• Dark Matter and Dark Energy (A Peep into the Deep): Some Questions and Answers
• Neutrino — The Puzzle and the Power
• Detection of Lightest Supersymmetric Particles (LSPs) in Dark Matter, and the Search for WIMPs
• The Years 2004–2010: A Boom for Planetary Scientists

Readership: Graduate students, researchers and all readers interested in cosmology and astrophysics.

Chapter 1

The Advent of Dark Matter: Galaxies,
Clusters, Planet Formation, and
Comet Collision

1.1 A Historical Perspective

Long before the words such as milky way, red shift, supernovae, clusters and so on—were coined in reference to bluish large dome over us, the sky-gazers were already engaged in unraveling the mysteries of non-luminous objects. As far as the written records go, the existence of these objects, collectively known as “dark matter” was predicted for the first time around the middle of the 18th century.

Astronomers then had begun to realise that the disturbed motion of a planet, or a changed position of a star, was due to the gravitational effects of a celestial body—a body that formed part of dark matter. Thus when planet Neptune was discovered in 1846, it was considered a constituent of dark matter (DM). However, it took some eighty years for them to make a serious suggestion that there was widely spread non-luminous matter in our galaxy, more precisely near the Sun. The extent of this (non-luminous) matter could be determined using the exertions of its gravitational force.

The only tools that 19th century astronomers had, were manual telescopes which they positioned at right angles to the plane of our galaxy to observe the motion of nearby stars. Using these observations, they obtained the total density of matter near the Sun. (See [34], [28] and [40] for early attempts.)

It was Oort ([53], [54]) who established for the first time that there was DM near the Sun. He used the observed vertical motions of stars for his study, and derived the value of total density of matter near the Sun as ~ 0.02M

pc⁻³.¹ This amount is about twice as large as the (present) observed density of matter near the Sun.

Ironically, the question regarding the amount of DM near the Sun is unsettled even to this day—due to lack of agreement on the choice of tracer populations of stars, and on the premises that need to be made before the equations governing the content of DM can be solved.

In the early 1990s, scientists were focused not only on the existence of DM, but also on its non-existence. The latter opinion is due to Kuijken and Gilmore [36], while the former is attributed to Bachall, Flynn and Gould [4]. In fact, the latter group concluded that “a model with no dark matter is inconsistent with the data at the 86% confidence level.”

Quite naturally, astronomers expanded their study to much larger arena, in the sense that they not only observed the positions of stars and galaxies, and their clusters, but examined their halos and X-ray emissions as well, to find the DM content. (See [21] for presence of dark matter in galactic halos; and [62] for X-ray emissions.)

As galaxies and clusters play a key role in study of ‘dark matter’, we devote next two sections to a brief description of galaxies and clusters. A few related terms are explained in App. 1.1.A.

1.2 Galaxies in the Universe and Our Own Galaxy

We recall that a large independent system (of stars), typically containing hundreds of millions of stars, is a galaxy. Galaxies are classified according to their shape and size. For instance, they are either spiral, barred spiral, elliptical² or elongated. They are sometimes very large or very small depending on their content and distribution of mass. These are respectively referred to as giant or dwarf galaxies. Since it is mainly the stars³ that make up a galaxy, the ‘luminosity’ is another factor that characterises a galaxy. Using this factor, one can further classify them as blue galaxies, red galaxies⁴ and cD galaxies. (We shall return to mathematical computations of luminosity function in Chap. 3.)

Galaxies (naturally) have a core, thus they are compared to each other by their core radii and by their brightness. We mention here only cD galaxies which are special in many ways—as we shall see in the next section.

cD Galaxy: These galaxies are defined as the ones that have a nucleus consisting of an extremely luminous elliptical galaxy—which in turn is embedded in an extended amorphous halo of low surface brightness. (See [42].)

cD galaxies are a class by themselves, since if one excludes the nuclear sources, e.g. the Seyfert galaxies, N galaxies and quasars, these would be the most luminous galaxies of the Universe. The core regions of these galaxies are much larger than those of giant elliptical galaxies. Our own galaxy is not a cD galaxy.

Milky Way:⁵ The galaxy that we live in—is a spiral galaxy which contains our Sun. It is seen from the Earth as a broad, faintly luminous band of stars and interstellar gas arching across the night sky, with the constellation⁶ Sagittarius⁷ marking the direction to the center.

The important role that galaxies play in the formation of clusters will be the subject of the next section. Here we simply state that it is the ‘radiation’ from galaxies—that is of great interest. By radiation we mean here, the X-ray emissions and the radio emissions. (See App. 1.A for explanations on emissions.)

Galaxies with strong radio sources are known as ‘radio galaxies’. We shall also examine in the next section the relationship between the two emissions of galaxies, as well as those of clusters.

At the end of the chapter we have given a table (Table 1.1) of various kinds of electromagnetic radiations as a help to understand the problem of radio and X-ray emissions.

1.3 Clusters—Their Formation and Classification

Clusters in plain words stand for groups of similar objects. In astrophysics however, they stand for something very important—the clusters here are very large groups of galaxies (see App. 1.D for clusters of stars). These clusters (as we know now) are the single most organised structures in the Universe—in the sense that they are the largest dynamically bound entities available for ‘dark matter probes’ in the Universe.

How does one use them as cosmological laboratories though, is the subject matter of the next two chapters. Here we give their brief history and shed some light on their formation and classification.

According to Lundmark [41], William Herschel in the late 1700s had visually explored the ‘blue dome’ and had recognised many distinctive concentrations of nebulae.⁸ These nebulae were mostly galaxies that appeared to be single or in pairs in some cases, but often times they were in clusters or metagalactic clouds.⁹ It was soon recognised that clusters originated in an early epoch as small perturbations to a given uniform surface density. These perturbations continued to grow, until they reached the stage of collapse owing to the phenomenon of expanding Hubble flow.

After the cluster has undergone these violent relaxations, it settles down to a state of dynamical equilibrium. And it is at this point, that it is categorised with respect to its content, shape and size. It is worth noting here that the galaxies which eventually made up the cluster were formed either before or after the collapse phase.

We define below some of the important classes of these clusters.

1.3.1 Regular Clusters

These clusters are highly symmetric in shape and have a core with a high degree of concentration of galaxies towards the center. In other words, the mass density in the vicinity of the center is much higher as compared to overall density. There is almost no subclustering in regular clusters. These clusters are often referred to as ‘dynamically relaxed systems.’

1.3.2 Irregular Clusters

These clusters have much less symmetry (compared to regular ones) or central concentration. They are dynamically less-evolved and thus their distribution of formation is almost preserved. They also show significant subclustering.

1.3.3 Rich Clusters¹⁰

The clusters that have abundantly many galaxies are naturally designated as rich. Their density profile is very different from the non-rich (poor) clusters, as we shall see in the next chapter. A catalogue of 2, 712 rich clusters identified by visual inspection of the original POSS (Palomar Observatory Sky Survey) was prepared by Abell [1].

In subsection 1.3.6 we describe two rich clusters which have been studied extensively for the past many decades. With improved observational technology even the clusters formed by low-mass dwarfs (e.g. brown dwarfs) have become a subject of great interest. These two clusters, which are close to our galaxy are the Hyades and the Pleiades. Both these clusters have been searched from various angles to obtain the definitive answers on their stellar and substellar population. The goal of these searches is to find their mass function with maximum accuracy, and to study the properties of young brown dwarfs of different ages. (See [57] for enlightening articles on the topic.)

1.3.4 Classification Based on Galactic Content

The clusters whose galaxies are mostly spiral are known as (Sps) clusters; the ones with disk galaxies without spiral structure are (SOs); and with elliptical galaxies are (Es). (See [47].) This classification was further refined into three categories by Oemler [52]. The clusters with maximum number of spiral galaxies, i.e. (Sps) are known as spiral-rich; on the other hand, clusters where spiral galaxies are less common and (SOs) are most common are spiral-poor; and clusters which are dominated by a central cD galaxy are called cD clusters. The central cD galaxies are supergiant galaxies (See [46]).

Apart from these, there are clusters where the brighter galaxies tend to concentrate near the cluster center and the fainter ones tend to spread out—these are called C clusters.

1.3.5 Clusters’ Contributions to Cosmology

There are three important contributions that a cluster makes to cosmology: its individual morphology and substructure preserves the remnants of initial condition before collapse, its local statistics carries the information on sites and dynamics of the collapse process, and its distribution in redshift reflects the clustering in action.

We shall see the influence of these contributions when we examine the content of dark matter in the Universe (see Chap. 6, Sec. 4).

Our next subsection describes in brief two of our neighboring clusters.

1.3.6 Virgo and Coma Clusters

Both these clusters are rich clusters, in the sense that they are clusters with abundantly many galaxies.

Virgo Cluster—whic...

Table of contents

• Cover Page
• Half title
• Title
• Copyright
• Dedication
• Foreword
• Preface
• Contents
• 1. The Advent of Dark Matter: Galaxies, Clusters, Planet Formation, and Comet Collision
• 2. Stars of Poor Visibility and the Methods to Track Them
• 3. Models in Cosmology, the Luminosity of a Star, White Dwarfs and Neutron Stars
• 4. Black Holes: the Stars with no Shine
• 5. Particles We Encounter (A Historical Overview)
• 6. Dark Matter and Dark Energy (A Peep into the Deep) Some Questions and Answers
• 7. Neutrino—the Puzzle and the Power
• 8. Detection of Lightest Supersymmetric Particles (LSPs) in Dark Matter, and the Search for WIMPs
• 9. The Years 2004-2010: A Boom for Planetary Scientists
• Epilogue
• Index