PX436 images

Here are some images and figures relevant to the course.

Gravitational lensing

There are some beautiful examples of gravitational lensing in clusters of galaxies. Here is a section of an HST image of a cluster known as SDSSJ1004+4112. The objects I have marked 1, 2, 3, 4 and 5 are actually 5 images of the same quasar that is seen behind the cluster. For more information, go to the original press release. The full-sized image is well worth a look.

Gravitational lensing in a galaxy cluster

Gravitational micro-lensing

If the object bending the light has a low mass (e.g. a star), then the separation between the images can be too small to resolve. However, it can cause an amplification of the flux of the background object. This is called gravitational microlensing. If multiple objects bend the light then the resulting lightcurves (flux versus time) can be remarkably complex owing to caustics (lines of infinite amplification). These are now being used to find planets, and this is a technique that is sensitive to very low mass planets. In the little animation below (taken from a nice website on planetary microlensing), a background star (blue dot) is lensed by a star and a planet. Purple blobs are the images produced, with a size representing brightness. The dashed circle is called the Einstein radius and is about 0.5 milliarcseconds. Remembering typical blurring from Earth's atmosphere of 1 arcsecond, none of the images are resolvable from each other, but their light curve can be measured, as shown along the bottom og the animation. The strange red cross-shaped figure near the lensing star is the pattern of caustics produced by the combined star/planet gravity. Note the sharp spikes as the background star moves across these caustics. In many ways, the physics of gravitational microlensing is more optics than GR, but it is GR that underlies the predictions of light deflection.

 

Cartoon of gravitational microlensing by a star and planet

Here are some real data obtained on the first multiple-planet system found with micro-lensing (the figures are from the OGLE project's website, worth a visit to see many other examples of micro-lensing). The example here is thought to be a case of two planets, similar to Jupiter/Saturn. It is the subject of an article in Science.

 

Multiple planet lensing, OGLE data Multiple planet lensing, all data
OGLE data only All data plus model

Isotropy of the Universe

Here is a picture of the distribution of radio sources on the sky (equal area projection). These are primarily distant radio galaxies and nicely show how isotropic our Universe is on large scales.

Radio source positions
Figure from Gregory and Condon (1991) and also shown on page 43 of Principles of Physical Cosmology by Peebles (first edition, Princeton).

Accelerating Universe

The luminosity distance is relevant for the observations of supernovae that indicate that the expansion of the Universe is accelerating, driven by a cosmological constant or dark energy component. Here is diagram showing the status of the supernova work (figure taken from Leibundgut (2008), based on the data of Davis et al (2007).

Type Ia supernova Hubble diagram

The red line is for an empty universe (no matter, no Lambda, also known as the Milne model of the Universe). The blue line is an Einstein-de Sitter Universe (matter at the critical density). This was for many years the favoured model, but is now clearly not favoured by the data. The green line is for 70% cosmological constant, 30% matter. Note that this diagram does not distinguish baryonic from dark matter as they both have the same effect upon the expansion history of the Universe. The data are the grey points, while the black points are binned versions. The deviation from the Einstein-de Sitter prediction is around 0.3 to 0.4 magnitudes, or 30 to 40% in flux, with the supernovae appearing dimmer than expected.

 

Gravitational waves

The slides shown in the lecture on gravitational waves are here. Computer simulations are required to understand GR in complex situations, and are even needed to detect waves in the first place. Considerable progress has been made in modelling of the GR in the strong field case. This movie taken from this NASA site is an example of such simulations. Gravitational wave chirp (sound file, very short!).