# Gravitational Lensing

In this activity you will learn about gravitational lensing, how to identify it in images, and the science behind it.

Although we are used to thinking of space and time as separate things, Physics tells us that space and time should be thought of as part of the same fabric: the fabric of space-time. Astronomical objects warp this “space-time” depending on the strength of their gravitational field, which in turn depends on how massive they are. Black holes warp space-time to a more extreme degree than, say, a planet – or you.

Q1: Think about the light that travels from a lightbulb to your eye. How would you describe its path? Hint

Light travels via the shortest path. These paths are called geodesics. Usually we think of them as straight lines, however this is not always the case.

The path that light can take can be altered by the presence of a gravitational field due to the curvature of space-time. Without gravitational interference, the geodesic is usually straight. When a gravitational field is involved, the geodesic is curved. This effect is called gravitational lensing.

The fact that light doesn’t always travel along straight lines results in very specific distortions in astronomical images. In this page you will learn to identify and classify these distortions.

## Scavenger Hunt Part 1: Getting Comfortable with Navigate

You will see that gravitational lensing is rare. Before you are ready to identify distortions due to gravitational lensing in astronomical images, you first need to become familiar with how “normal” images look like. In this section you will be using the SDSS Navigate website to explore the universe that SDSS has observed so far. Click the button opposite to open Navigate.

How to use Navigate:

• RA and DEC are a coordinate system used in observations of the sky.
• The values on the left change as you move around, they correspond to the centre of the image.
• You can also click anywhere on the image. The yellow numbers that show next to your cursor are the RA and DEC that correspond to the position where you clicked on the image.
• If you click on an object, it might give you extra information. The side panel will tell you the RA, DEC and the type of object (STAR or GALAXY), this classification isn’t always correct. It will also show the spectrum of the object.
• This information is only available if the astronomical objects have spectroscopic observations.
• If you click the box “Objects with spectra”, a red box will appear over all objects that have spectra.
• If you want more information about the object, just click on the image of the spectrum on the bottom right and that will tell you the redshift and the classification of star/galaxy (CLASS = QSO for a Quasar)

Navigate Side Bars

Exercise:
In the table below there are six different types of objects to look for in Navigate. There are some example images for each object.
Don’t try to find the exact objects in the example as it would take far too long, just look for objects that fit into the same category.
Note down the RA and DEC of each object you find on the worksheet.
(25-30mins)

You should now be comfortable with using Navigate and have an awareness of what the universe looks like without gravitational lensing effects.

Below is an image of a galaxy cluster called Abell 370.

Exercise:
Look at the image below and think about what you can see.
Consider the different types of galaxies, the colours and shapes that this image contains.
(5-10 mins)

Abell 370 was one of the first observed galaxy clusters with gravitational lensing effects. The arcs and streaks in the image are a result of gravitational lensing.

## How Does Gravitational Lensing Work?

Gravitational lensing is an important consequence of general relativity. There are three main types of gravitational lensing: strong, weak, and micro lensing. Each of these stem from the same basic phenomena, the bending of a light path due to the curvature of spacetime. Here we will focus on strong lensing but there is a section at the end of the page if you are interested in weak or micro lensing.

The following video describes this phenomena using diagrams to help you visualise it.

### Strong Lensing

Strong lensing is classified as a lensing event that produces resolvable visual effects.

Gravitational lensing has four main effects on background sources:

• Change of Position:
• This effect is generally not observable as information about the original position of the source isn’t available. However, this phenomena can be observed in dynamical situations where the configuration of the system changes with time.
• Although this effect is unlikely to be observed, it was the first to be observed. In 1919, it was recorded during a solar eclipse where stars in the background were seen to have changed position.
• Distortion:
• The perceived shape or size of a background source is changed due to gravitational lensing.
• Magnification:
• The background source gets magnified, similar to the magnifying glass analogy in the above video.
• Multiple Images:
• Multiple images of the background source are produced.

In strong lensing, the distortion of background sources is large enough that it produces observable visual effects. These visual effects are split into three categories: multiple images, arcs, and Einstein rings. The production of each effect depends on the alignment of the background source, lens and observer.

#### Einstein Rings

Einstein rings are the rarest type of visual effect, they require almost perfect alignment.

Einstein rings occur when the alignment of source, lens and observer are sitting on a rotationally symmetric line of sight. This can be seen in the diagram on the right.

In a theoretically perfect universe, an Einstein ring would be a perfect circle but this is not what happens in our own universe. The Einstein rings we have observed so far are not perfect circles with many being ovals or missing sections from its arc.

#### Multiple Images

Multiple images occur when there is a small misalignment with the source, lens and observer.

The alignment causes the light from the background source to take different geodesic paths around the lensing object thus multiple apparent images of the source with time delays will be produced.

Gravitationally lensed quasars are commonly observed in multiple image systems.

#### Arcs

Arcs are the most common lensing effect, due to the rarity of near perfect alignments. They are produced when the alignment of the source, lens and observer breaks rotational symmetry. Essentially the Einstein ring is broken and only arc remain.

## Scavenger HuntPart 2: Looking at Gravitational Lensing Systems in Navigate

In this section you will be looking at gravitational lensing systems in Navigate. You will be looking at three confirmed gravitational lensing systems.

Exercise:
Clicking on the buttons below will take you to a confirmed gravitational lensing systems.
You will need to fully zoom in to get the best resolution.
Check if any of the objects have spectra and if they do, note down the redshift and type of object.
(10-15 mins)

###### Click to view the RA and DEC values

Q2: Which visual effects are these three systems demonstrating? What types of objects are present in these systems? Hint

## Part 3: Comparing the SDSS to The Hubble Space Telescope

In the following section you will be comparing the SDSS to the Hubble Space Telescope.

The SDSS telescope is a ground-based optical telescope at the Apache Point Observatory in New Mexico, USA. The telescope began collecting photometric and spectroscopic data in 2000 until its final imaging release in 2011, when the telescope switched to solely spectroscopic observations. You can learn more about the SDSS telescope here.

The Hubble Space Telescope (HST) is a space telescope that orbits the Earth. It was launched in 1990 and it is still in operation. The HST has many advantages compared to the SDSS telescope. As it observes from space, it is not impacted by atmospheric effects, also called seeing, where turbulence in the atmosphere can reduce the angular resolution that can be achieved by a ground based telescope. This is the main reason as to why HST has been so important to study the distant Universe.

The following images were taken by the HST.
They correspond to the three systems that you have just looked at in Part 2.

###### Click to reveal the HST images

Q3: Compare these images to the ones you see on Navigate. What makes them different? And why?
Hint

Exercise:
In the worksheet, there is an exercise where you are asked to identify the lensing object from the apparent images. (5-10 mins)

When System 2 was discovered, scientists were surprised by the separation of its images. Previously, the largest separation of images in a gravitational lensed quasar system was 7 arcseconds. In System 2 the images are separated by 14.62 arcseconds, over double the separation of any other system known at the time. Systems like this are very rare which means they can only really be discovered in very large and widespread surveys like the SDSS. You can read more about this here.

## Identifying and Sorting Lensing Effects

This activity can be done in small groups or individually. All you need to have is a printed version of the headings and cards.

Exercise:
In this activity, you will be given a set of images. Each image contains at least one visual effect.
Your task is to place the images under its appropriate heading corresponding to the visual effects you can see.
(20-30 mins)

Whilst you are sorting the images, have a think about the following questions. Discuss with your peers if you are working in a group.

Q4: How common do you think gravitational lensing is? Think about different observing locations. Hint

Q6: How many gravitational lensing systems do you think we have discovered? Hint

Q5: Is it hard to decide if objects in the images are due to lensing? Why? Hint

Q7: How might scientists work out if images like these are due to gravitational lensing? Hint

## Additional Information

### Microlensing

Microlensing occurs when a low mass object passes in front of a background source. Due to the low mass of the lensing object, it is difficult to observe any change in the position of light or distortions. However, magnification of the background source can still occur allowing the changing in brightness to be observed. To confirm the presence of microlensing, the statistical data must be analysed.

Microlensing can be used to detect exoplanets. This occurs when a planet orbiting around its host star acts as a lens thus magnifying the star. From this, calculations for the planetary mass and orbital distance can be made.

### Weak Lensing

Weak lensing is similar to microlensing due to them both being statistical measurements.

Weak lensing occurs when the lensing mass is large but there aren’t any background sources aligned behind the lens leading to slight distortions in distant background galaxies. Since the distortion of each background source is small, it can’t be detected by observing the individual galaxies.

In order to confirm weak lensing, the observations must be averaged over a large number of background sources. In doing this, only then will the statistical anomaly be consistent with weak lensing.

If you are really stuck with the questions, here are some hints to get you on the right track.

#### Question Hints

###### Q1: Think about the light that travels from a lightbulb to your eye. How would you describe its path?

Visualise the situation. How does light normally propagate?
Back to question.

###### Q2: Which visual effects are these three systems demonstrating? What types of objects are present in these systems?

Do these systems look similar to the images below? System 3 has similarities to two of these images below, which two?

Think back to part 1 of the Scavenger Hunt. How did you find out more information about the objects you found? Have a look at spectra and the object classification.
Back to question.

###### Q3: Compare these images to the ones you see on Navigate. What makes them different? And why?

Firstly re-read the differences between the HST and SDSS telescopes.
Think about the visual differences like colour and resolution.
What’s the main difference between HST and SDSS? They are both telescopes but where are each of them?
Back to question.

###### Q4: How common do you think gravitational lensing is? Think about different observing locations.

Think about all the things that need to go perfectly for strong gravitational lensing to occur. It requires near perfect alignment of observer, lensing object and background source so how many times do you think that happens?

For the different observing locations, this refers more to extremely large differences such as if we were observing from another solar system or galaxy. Would we see the same gravitational lensing as we see on Earth?
Back to question.

###### Q5: Is it hard to decide if objects in the images are due to lensing? Why?

Which lensing effects are easier to identify?
Usually the Einstein rings and arcs are most distinguishable as an effect of lensing than a multiple images system. Can you think of situations where a multiple images system could be mistaken as something else?
Back to question.

###### Q6: How many gravitational lensing systems do you think we have discovered?

There have been over 1000 systems observed.
For comparison we have observed billions of galaxies thus strong lensing is extremely rare.
It is also quite difficult to detect strong lensing systems thus making them even more rare.
Back to question.

###### Q7: How might scientists work out if images like these are due to gravitational lensing?

The Einstein rings and Arcs are easier to classify but the multiple images systems are harder to confirm.
Think back to the Scavenger Hunt part 2 and the spectra of the multiple images system. What does spectra tell us? Did the spectra from the multiple images system look similar?
Back to question.