Much of the group's work is connected with the CLASS lens search. Work on the CLASS database continues
at the present time, although the data collection, selection and calibration has been completed. Members of the group use CLASS in areas such as:
- Estimating values of the Cosmological parameters from gravitational lensing statistics
- Establishing the redshift distribution of the lensed quasar population
- Analysing the CLASS radio source population (see, for example, the CERES pages)
The successor lens search to CLASS will take advantage of upgraded instruments, such as EVLA and
e-MERLIN. A survey using these super-sensitive interferometers could find many more lenses than the 60+
known today. A larger sample of lens systems will reduce uncertainties in estimates of cosmological parameters derived from lensing statistics.
Lensing statistics can be used to estimate the values of the matter density and the cosmological constant. These cosmological
parameters underpin both the large-scale geometry and the expansion history of the universe. Since the number of lenses we expect to see
depends on the large-scale geometry, we can use the number of lenses found in a survey to constrain the geometry of the universe.
In practice, there are still too few lenses known to provide reliable constraints on the cosmological parameters, which is a major reason
for doing more lens surveys.
Mass Modelling of Strong Lenses
The image positions, flux ratios and other information derived from observations of a lens system can be used to constrain the mass distribution
of the system's lensing galaxy. Gravitational lensing is an excellent probe of galaxy mass distributions at cosmological distances, where methods
useful for nearby galaxies are ineffective. Further, lensing is sensitive to the total mass distribution (dark matter + luminous matter), unlike
other techniques which are sensitive only to luminous matter.
If a suitable mass model of a lensing galaxy is obtained, and the lensed source varies in brightness with time, the lens system can be used to estimate
Hubble's constant. Hubble's constant is extremely important in cosmology, since it appears in expressions for many of the most important
cosmological problems - knowing Hubble's constant accurately allows us to determine the age of the universe.
Some lens systems are unsuitable for use in the
determination of Hubble's constant, because the lensed source is not variable or because the lensing of the source is due to a complex arrangement of
galaxies that make the system difficult to model. Larger surveys that find more lenses are the obvious solution to this problem, since we then have a
larger selection of systems that could be used to determine Hubble's constant.
Galaxy formation is one of the most active research areas in astrophysics. In standard hierarchical models dissipationless
dark matter aggregates into larger and larger clumps as gravitational instability amplifies the weak density perturbations
produced at early times. Gas associated with such dark haloes cools and condenses within them, eventually forming
the luminous galaxies we see today.
Image Copyright © 1999 by Princeton University Press
There are still many unanswered questions in galaxy formation, including even the most basic questions such as the origin of the Hubble
sequence of galaxies. Active research is being conducted at JBO on the formation of
disk galaxies, the hot (X-ray) haloes of galaxies and the feedback effects during early phases of galaxy assembly.
Microlensing is gravitational lensing in motion. Unlike strong lensing
where the images are usually resolvable, microlensing images are of
milli-arcsecond separation for microlensing in the local group and hence
currently unresolvable. However, since the system is in motion, what is
observed is a temporary brightening, and the study of the resulting light
curves is proving to be an exciting and stimulating area of research.
Here at Jodrell bank we are mainly interested in microlensing within
our own Galaxy (although microlensing itself is not restricted to
Galactic distances, e.g., observations are carried out using the LMC,
the Andromeda galaxy M31, and even distant quasars). To date more than
1000 microlensing events have been seen toward the Galactic bulge, and
this number is increasing daily. The most important advancement in
recent years is the advent of real-time microlensing detections, where
events can be studied as they are happening (see, for example, the
A light curve for a microlensing event. The source gradually gets brighter and
then dies away back to its usual constant baseline. The two fits (solid and dashed
lines) are theoretical models, one of which incorporates the Earth's motion around the
Sun (the so-called parallax effect). It is currently believed that
this event may have been caused by a stellar mass black hole acting as the lens.
The analysis of these microlensing light curves has many uses, since it
is one of the only ways to probe objects (in the form of the
intervening lenses) which do not necessarily have to be
luminous. Obviously this is a distinct advantage, and may soon result
in direct detections of isolated stellar mass black holes in our
Galaxy. Indeed, some may already have been identified (see the above
figure for one such example). Galactic Microlensing can also be used
to study the kinematics of the disk and bulge, since the analysis of
microlensing statistics becomes more important as the number of detected
events rises. In fact, one of the most exciting problems in microlensing
is the explanation as to why there seem to be many more detected events
toward the Galactic bulge than the standard models of our Galaxy predict.