Independent Techniques

As we have said, using the cosmological redshift as a measure of distance requires a cosmological model, including a set of parameters. Such a model is in ongoing discussion among cosmologists and new observations and theories may change the value of some parameters or even replace the whole model. This makes this type of distance indicator vulnerable to change. Illustrative is the ongoing discussion about the value of Hubble’s parameter H0 that we discussed above. Moreover, there is always an uncertainty about the validity of any of the model assumptions, and it is very desirable to find alternative methods of determining distance to the furthest objects for comparison. This may even help to improve the cosmological model.

Fortunately there are such independent methods and various new ideas are being pursued for physical indicators of distance and for independent verification of H0. We briefly describe three of these techniques, with links for further reading. We will not go into much detail as these techniques are really complicated and require an expert background to fully understand.

 

Gravitational Lens Time Delay

Click image for larger version

 

One promising technique is the use of the effect of gravitational lensing of images of quasars. When we see a quasar through two different paths because of a strong gravitational lens, the two paths differ in length and therefore also the travel time of the light.

 

When the quasar varies in intensity abruptly (as they do), we see that variation with a time delay in one path as compared to the other. This time delay is a measure for the distance to the lens, the “time-delay distance”. This time delay is also directly related to the size of the gravitational lens, which basically gives a measure of the expansion rate of the Universe, which is the Hubble parameter.

 

This method has already been proposed in the 1960’s but both availability of strongly lensed quasars and sufficiently accurate measurement of light curves, etc. only came about around the latest turn of the century. A complication is that the lensing effect depends on the mass distribution of the lensing system, which usually is a cluster of galaxies. In the modern era this method is an important indicator for large distances, because it is weakly dependent on any cosmological model and its parameters.

Some further reading here and here.

 

 

CMB from the Planck satellite.
Source

 

 

Sunyaev-Zel'dovich Effect

The Cosmic Microwave Background radiation (CMB) is an observable remnant radiation from the early Universe. Discovery of this microwave radiation has been a game changer for cosmology.

This CMB radiation is affected in certain parts of the sky by hot gas in galaxy clusters if it happens to pass through those clusters.

With this complicated technique, the observed angular diameter of the galaxy cluster could be a measure for its distance. Importantly this distance is independent from redshift.

More here

 

 

 

Gravitational Wave Astronomy

Since 2017 astronomers have been able to detect gravitational waves caused by merging neutron stars or black holes in a binary system. In some cases it has been possible to also measure the same event in the Electro-Magnetic spectrum as a gamma-ray burst.


Credit: ligo.caltech.edu




The amplitude of the gravitational waves enables scientists to determine distance to this event, for which the EM radiation provides a redshift value. These are very infrequent chance observations but importantly, this method gives a calibration between cosmological redshift and distance because it is independent from any other technique in the distance ladder or cosmological model.

More here.