Added by greenfieldc
13th Sep 2018

Exploring the dark universe with Einstein’s telescope. Report by Charlotte Daniels.

Exploring the dark universe with Einstein’s telescope.

Dark Matter and Einstein’s Theory of General Relativity are closely intertwined when discussing the effects of Gravitational Lensing. Gravitational lensing is predicted by Einstein’s theory, and is pivotal to proving the existence of Dark Matter. Dr. Nick Bate provided a dynamic and inspiring talk that made these complex themes accessible.
To those who didn’t attend, below is a summary that barely covers an iota of the detail Dr. Nick provided, however I hope it reminds each of us at NAS exactly why we are so passionate about our Universe and its never-ending set of mysteries!
As the talk covered, our Universe is made up of approximately 5% baryonic or ‘normal’ matter; the remaining 95% (ish) is comprised of Dark Matter and Dark Energy. Dark matter’s existence is supported by the effects of gravitational lensing. Clusters of galaxies are hypothesised to form in dense regions of dark matter, in fact Dr. Nick described dark matter as potentially ‘scaffolding’ for normal matter; paving the way for our early Universe to form.

The talk
NAS were lucky enough to receive Dr. Nick in late August, when he provided the perfect blend of presentation and discussion on gravitational lensing, also referred to as ‘Einstein’s Telescope’ due to the relationship between light and gravity demonstrated. General Relativity predicts that a mass, M, will deflect light from a source by an angle θ via the following equation:

Θ= 4GM



Where G is the gravitational constant, and b is the impact parameter of the light.

Gravitational lensing occurs when light/electromagnetic waves emitted by a background star are ‘bent’ and focussed by the gravitational influence of a foreground galaxy or star, causing an observer to see a magnified and distorted image of the background star. As we saw, this phenomenon has also been witnessed for entire galaxies! The extent/angle to which this light is deflected will be influenced by the mass of the foreground massive object, as Relativity predicts. We also learnt that lensing is affected by both normal and dark matter, which is key!

Gravitational lensing teaches much about the formation and behaviour of galaxies, clusters, stars… our Universe as a whole. It allows astronomers to view information, on a magnified scale, of distant galaxies that would otherwise not be visible, and as they would have appeared billions of years ago. By measuring the size and shapes of clusters and galaxies, mass distribution in the Universe can be inferred in addition to (over time) determining how the Universe has evolved.
Throughout Dr. Nick’s presentation, NAS members saw some incredible images that demonstrated the true extent of the distortions experienced via lensing. We also witnessed how the effects can vary wildly depending on the alignment between the telescope, and background and foreground objects (for instance, when perfectly aligned an Einstein Ring would be witnessed).

The image provided of Quasar 2237 (Einstein’s Cross), displayed a lensed image of a background quasar via a lensing galaxy and exemplifies the distortion and magnification potential of lensing. The image of this background quasar was lensed four times via the foreground-lensing galaxy!





Dr. Nick then showed members the image of Galaxy cluster Abell 2218. This is a well-known and powerful lensing galaxy that is situated in constellation Draco. Given its size and density of stars, Abell 2218 distorts and multiple images background galaxies into thin arcs (as shown). In denser regions of the cluster the distortion of the background galaxies is further increased, providing a wealth of information for astronomers on early galaxy formations and clusters in addition to suggesting that pockets of dark matter is also behind a significant part of the distortion.







The direct proof of Dark Matter?

Dr. Nick also presented an image of the Bullet Cluster, which comprised overlays of X-ray and infrared imaging. The image actually shows two clusters of galaxies that have collided with one another. The blue, infrared elements of the image show dense regions of clusters and dark matter that have passed straight through the collision (because dark matter doesn’t react with ordinary matter). Meanwhile the smaller portion of normal matter, has collided as expected and heated up emitting X-rays (pink image). The resulting overlay provides strong proof of dark matter’s existence!





Utilising the Gravitation Lens:

We subsequently learnt how lensing has become a tool for physicists today. An example is via planetary microlensing, which has been used to discover almost 80 planets in distant galaxies to-date; this number is rising all the time. Dr. Nick provided images of light curves to demonstrate the ‘spikes’ demonstrating temporary brightness when the lensing star moves in front of the source star; magnifying its image and allowing astronomers to infer the size and mass of the object. Naturally, the utilisation of planetary microlensing has significantly increased the potential to discover new (and Earth-like) planets!


What next?

Dr. Nick mentioned within the talk that there is approximately a one-in-a-million chance of spotting a star lensing another star given the resources used to-date. The focus is now on developing 3-Dimensional models, such as the one pictured by the Cosmic Evolution Survey (“COSMOS”), in order to compare observations directly with the simulations of dark matter developed to-date.

This ‘cosmic web’ as Dr. Nick described shows the distribution of dark matter growing ‘clumpier’ (and therefore eventually forming galaxies in the densest regions).






I caught up with Dr. Nick after the talk, where he explained the need for more data to capture more lensing events and establish changes over time.

Could the use of gravitational lensing as a ‘tool’ for planetary/cosmological lensing be further refined? How so?

Dr. Nick: More data helps us find more gravitational lenses. The more of the sky that you can continuously monitor, the more of these events you will see. And likewise, searching for gravitationally lensed galaxies and (especially) quasars is like searching for a needle in a haystack. If you've covered more of the sky, down to fainter and fainter magnitudes, then you can find more lensed galaxies and quasars. And when you do find them, you ideally want to monitor them for long periods of time, so you can watch them change.

The good news is we're about to have a deluge of new data. Surveys like LSST (the Large Synoptic Survey Telescope) will image the entire sky once every few nights, which will give us a huge (and constantly-updating) dataset to search for new lenses. Next-generation telescopes like ALMA (the Atacama Large Millimetre Array) and 30-metre class telescopes (such as the Extremely Large Telescope, under construction in Chile) will give us extremely high resolution images of very faint objects, which is wonderful for studying detailed structure in lensed galaxies or quasars.

Do you anticipate any further applications for gravitational lensing in the near future?

Dr. Nick: First, I think we'll discover more gravitationally-lensed supernovae, and that's exciting both because we'll be able to study what supernovae were like when the Universe was young, and because it will help us to build more precise models of the mass in the foreground galaxies and galaxy clusters that are acting as lenses.

Scientists are hoping to use gravitational lensing to map out the shape of dark matter halos in individual galaxies (rather than in statistical samples of galaxies). By doing that, we'll be able to learn about whether star formation worked differently in the early Universe, compared with now. A lot of our measurements are based on the assumption that star formation has always been the same, but that may not be true. And if it's not, then there'll be a lot of consequences! It's a tricky thing, though, measuring dark matter distributions in individual galaxies.
The real hope, of course, is that our measurements will get precise enough that we'll be able to map out how dark matter is distributed in three dimensions.

You mentioned in your talk that the focus is now on trying a develop a direct, rather than statistical look at Dark Matter. What more needs to be done in order to accomplish this?

Dr. Nick: To do this, we need two things. The first is still more (and better) data. In the case of dark matter distributions in individual galaxies, we need long-term monitoring. That lets us build up gravitational lensing light curves, which encode detailed information about the mass distribution of the lens galaxies (how much of the mass is in compact objects like stars, how much is in smooth dark matter, what masses are the stars, etc). Currently that is a serious effort, but in the near future, (with surveys like LSST) we'll get that data for free -- light curves observed once every few nights over a period of 10 years.

We're then going to have to develop new tools for analysing those light curves. They contain a lot of information, but they're tricky to decode. Current methods require huge computer simulations which are only feasible for a handful of gravitational lensing systems. In the future we'll have hundreds or thousands of these systems, and so we'll need a new way to analyse that data. We haven't figured that one out yet, but machine learning might be useful. You can train an algorithm on large sets of simulated data, and then give it a real observation, and hopefully it'll be able to tell you what set of parameters (dark matter fraction, mass distribution of stars...) led to that real observation.

I guess the short answer (as always for astronomers!) is more data, and better simulations!

The name "dark energy" is really just code -- we know something is causing the expansion of the Universe to accelerate, but we have no idea what it is, so we call it "dark energy" Dr. Nick Bate.

Last, any final words on Dark Energy?

Dr. Nick: I would say that unlike dark energy, dark matter is at least generous enough to experience one of the four fundamental forces. Ordinary matter experiences electromagnetism, the strong force, and the weak force, as well as gravity. Dark matter (at least as far as we can tell) experiences only gravity, but at least that's something familiar. Dark energy is something else entirely. It is causing the expansion of the universe to accelerate, but how it is doing that is a complete mystery. There are almost as many ideas for what dark energy might be as there are physicists who study it! 

Many thanks to Charlotte for writing this up for Cygnus and our web site!


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