Spotlight #2: Measuring the Mass of Cosmic Giants

The universe we live in has a hierarchy of structure. We live on a planet which orbits an averaged sized star that we call the Sun. The Sun has neighbouring stars which all form into a large galactic city that we call the Milky Way. Each of these galaxies is gravitationally combined with all the other neighbouring galaxies into the Local Group. The Local Group is moving toward an even larger structure of galaxies called the  Laniakea supercluster. These galaxy clusters as the Cosmic Giants which are the largest structures in the Universe. These are nodes between the thread in our vast and maybe infinite cosmic web.

An image taken from the Millennium Simulation of Dark Matter particles it displays the Cosmic Web in the Universe.

In the early moments of the 20th century a young German physicist working in a patent office released a paper which described the way that light works dependent on observer. Albert Einstein’s Special Relativity described a new way of thinking about the world. One in which time and space were not fixed for everyone and even the order of events could be different for those who observe from far enough apart.

It was only 10 years from Special Relativity until Einstein went one step further and redesigned the way we think of the most fundamental building block of the Universe, gravity. From the time that the metaphorical “apple” fell on Issac Newton’s head,  scientists thought that gravity was an invisible force that just acted between objects with mass. Now Einstein was saying that space and time are one 4 dimensional sheet that mass warps.

The idea was met by criticism from many, however it was a British astronomer Arthur Eddington who designed an experiment to test Einstein’s theory. Indeed the story is one which just shows how scientific discovery can transcend war (If you are interested further please watch the movie  Einstein and Eddington <- linked here). He showed that by using the total solar eclipse you can see that the mass of the Sun bends space-time in such a way as to shift the background stars by a fraction.

Although when these preliminary results are reviewed they would seem dubious by today’s standards, over the last 100 years this theory has been challenged multiple times and has always come out on top.

It is this theory which allows us to measure our cosmic giants. Much like Eddington observing the curvature of space-time by the Sun we now must observe the same effect but of Galaxy Clusters. The principle is that light always travels in a straight line. But this only hold through space-time, by bending the fabric we appear to bend the path of the light. Strangely this can be seen!

When observing many clusters we see that the curving of space-time produces lensing of background galaxies around the centre of the cluster. The perfect example of gravitational lensing is if a background galaxy is exactly behind the cluster centre then a perfect Einstein ring is seen. Below is an example of a galaxy cluster with gravitational lensing around the centre.


Gravitational lensing seen around the centre of Abell 2218

The scale of the hard lensing seen above allows us to determine how much mass is located in the centre of the cluster.

More recently a secondary and more powerful technique has also been developed. This involves measuring the weak lensing of very faint background galaxies further from the centre. The small shearing of the background galaxies also allows scientists to map the distribution of the mass within the cluster. As most of this mass is concentrated in the mysterious Dark Matter it gives astrophysicists a unique opportunity to map out the DM clumping.

Looking forward to the future, by analysing a large number of galaxy clusters we will be able to determine the amount of Dark Energy in the Universe. Using simulations in which the contribution of accelerating Dark Energy is varied we see that the density of clusters varies proportionally. Over a large number of galaxy cluster we can measure this parameter and hence infer the amount of Dark Energy in the Universe.

In the next decade the introduction of the new cluster surveying telescopes such as EUCLID will move us from an era of analysing tens of clusters at a time to measuring thousands. This will be a golden age for cluster lensing analysis and will move towards answering important questions about the structure and dark side of our universe.


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