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R. Barrett and P. P. Delsanto
is a missing mass in the universe, and that it must be more than five times the
visible mass.
Today, the term dark matter has been coined for this missing mass. If dark
matter is assumed, the anomalous motion can be explained within the framework of General Relativity. The actual nature of the dark matter, however,
remains a mystery. Many conjectures have been made about the components
of dark matter (e.g. neutrinos and other exotic particles), but none have been
satisfactorily substantiated.
Alternative explanations of the anomalous motion of stars and clusters
involve modifications of the theory of General Relativity. One such theory
is MOND, or Modified Newtonian Dynamics, created in 1983 by Israeli
physicist Mordehai Milgrom [4]. In this theory, Newton’s law departs from
the well-known inverse square law when the accelerations involved are very
small, such as in the outer reaches of galaxies. The modification to Newton’s
theory is too small to be observed on earth or within the solar system, where
the accelerations are much larger. MOND has been successful in explaining a
number of galactic phenomena, but fails to predict the behaviour of galactic
clusters. It has attracted a small number of adherents, whereas the majority
of cosmologists are committed to the dark matter solution of the observed
anomalies.
Recent evidence of dark matter has been found by Seth Epps and Michael
Hudson of the University of Waterloo [5]. Current theory predicts that
galaxies are immersed in a halo of dark matter, and that galaxies that are relatively close to each other are connected by filaments of dark matter. Light
passing in the vicinity of any matter is bent by the curvature induced in
space–time by the presence of the mass. The effect is known as “gravitational
lensing” (see Chap. 7). Epps and Hudson averaged the results of gravitational
lensing from 23,000 pairs of neighbouring galaxies, and compared these with
similar averages from pairs of galaxies that were in the same region of the sky,
but actually well separated in distance.
Results from their study are shown in Fig. 11.1. A red bridge associated
with dark matter filaments between the pairs of neighbouring galaxies (white
regions) is evident in the upper false-colour image, but not in the lower
image, which displays the results from well-separated galaxies.
Further investigations are needed to confirm the results of Epps and
Hudson: meanwhile the search for the elusive dark matter continues.
R. Barrett and P. P. Delsanto
is a missing mass in the universe, and that it must be more than five times the
visible mass.
Today, the term dark matter has been coined for this missing mass. If dark
matter is assumed, the anomalous motion can be explained within the framework of General Relativity. The actual nature of the dark matter, however,
remains a mystery. Many conjectures have been made about the components
of dark matter (e.g. neutrinos and other exotic particles), but none have been
satisfactorily substantiated.
Alternative explanations of the anomalous motion of stars and clusters
involve modifications of the theory of General Relativity. One such theory
is MOND, or Modified Newtonian Dynamics, created in 1983 by Israeli
physicist Mordehai Milgrom [4]. In this theory, Newton’s law departs from
the well-known inverse square law when the accelerations involved are very
small, such as in the outer reaches of galaxies. The modification to Newton’s
theory is too small to be observed on earth or within the solar system, where
the accelerations are much larger. MOND has been successful in explaining a
number of galactic phenomena, but fails to predict the behaviour of galactic
clusters. It has attracted a small number of adherents, whereas the majority
of cosmologists are committed to the dark matter solution of the observed
anomalies.
Recent evidence of dark matter has been found by Seth Epps and Michael
Hudson of the University of Waterloo [5]. Current theory predicts that
galaxies are immersed in a halo of dark matter, and that galaxies that are relatively close to each other are connected by filaments of dark matter. Light
passing in the vicinity of any matter is bent by the curvature induced in
space–time by the presence of the mass. The effect is known as “gravitational
lensing” (see Chap. 7). Epps and Hudson averaged the results of gravitational
lensing from 23,000 pairs of neighbouring galaxies, and compared these with
similar averages from pairs of galaxies that were in the same region of the sky,
but actually well separated in distance.
Results from their study are shown in Fig. 11.1. A red bridge associated
with dark matter filaments between the pairs of neighbouring galaxies (white
regions) is evident in the upper false-colour image, but not in the lower
image, which displays the results from well-separated galaxies.
Further investigations are needed to confirm the results of Epps and
Hudson: meanwhile the search for the elusive dark matter continues.