Space Warps Talk

My Easy Explanation of Strong Gravitational Lenses

  • Budgieye by Budgieye moderator

    What is a gravitational lens?

    It is a galaxy that bends light that is traveling past it.

    No, that is wrong. Light doesn't bend, light travels in a straight line.

    Try again:

    A gravitational lens is an area of space around a massive galaxy
    and that galaxy is so massive that it bends space,
    and the path of light coming from a distant galaxy which goes through that warped space is also bent.
    The light is focused and combined so we get distorted but brighter images of the distant galaxy.

    We often say "the galaxy bends the light" because it is easier to say, but we must remember that is not what is happening.

    Here is the Google definition
    A region of space containing a massive object whose gravitational field distorts electromagnetic radiation passing through it in a similar way to a lens, sometimes producing a multiple image of a remote object

    All objects with mass will bend space..
    ..black holes, Dark Matter, the Sun, the Earth, an apple, a proton and a Higgs-Boson particle.

    But it takes a massive object to bend space enough for us to measure easily.

    Why are gravitational lenses important?

    1. They make distant galaxies brighter, so we can see them better, and learn more about the history of the Universe.

    2. We can measure the amount of Dark Matter in a galaxy. About 85% of matter in the Universe is made up from something that nobody knows much about. We can't see Dark Matter, but we know that it is there because scientists have found that galaxies weigh much more than the number of stars, gas, and black holes in them. One way of weighing galaxies is by measuring how much light paths warp space around them, and for that we need........ gravitational lenses.

    The galaxies that do the lensing are far away, and the galaxies whose light is lensed are even further, usually twice as far away.

    The galaxies that do the lensing appear to be small, but they are actually large elliptical galaxies, much bigger than our Milky Way galaxy.

    How far way?

    13,700,000,000 ly: the edge of our observable Universe, the "Big Bang"

    8,000,000,000 ly: the distance of distant lensed galaxies, at about z=1.0

    5,000,000,000 ly: the distance of the massive lensing galaxies, at about z=0.5

         2.000,000 ly to Andromed galacy
    
                       4 ly to the nearest star Alpha Centauri
    

    The orange galaxies that we are looking at are a third of the way across the Universe.

    The blue arcs that we are looking for are half way across the Universe.

    The lensed quasars may be most of the the way across the Universe.

    one light-year (ly) = 10 trillion kilometers, or 6 trillion miles

    Posted

  • Budgieye by Budgieye moderator

    enter image description here

    not to scale

    Here is a distant blue galaxy emitting light.
    But the white spiral galaxy is stopping the light, so on the Earth (green disc), we can't see the distant galaxy.

    Spiral galaxies are smaller than large elliptical galaxies. They don't have enough mass to be an effective gravitational lens.

    (Exception: maybe it will possible if the spiral galaxy is viewed edge-on.)

    For a galaxy to bend light, we need a "heavier" galaxy ie. a more massive galaxy.

    Posted

  • Budgieye by Budgieye moderator

    enter image description here

    Here is the same view, but with a "heavier" galaxy in the way.

    We can't see the blue galaxy, because our view is being blocked by a massive elliptical galaxy.

    But this galaxy is so massive that it warps the space around it.

    Some light from the distant galaxy travels though the warped space, and the light takes changes direction.

    Light that was initially traveling in another directions is now traveling towards the Earth.

    More light is arriving at the Earth, and the distant galaxy is brighter than we might expect it to be.

    Posted

  • Budgieye by Budgieye moderator

    enter image description here

    Here is a 2D representation of the gravity field around the massive elliptical galaxy.
    .

    The light from the distant galaxy changes direction as it travels through the gravity "hole.

    It is similar to putting a heavy weight on a bed mattress. It is difficult to roll a small ball, because the ball tends to roll towards the low area.

    Of course, lenses exist in 3D space. To show this correctly in three dimensions, a 3D diagram would be required.

    Posted

  • Budgieye by Budgieye moderator

    enter image description here

    When the light reaches the Earth, we can see several images of the distant galaxy (the lensed galaxy), which have arrived by different routes. The images are distorted, since some parts of the images have gone through different parts of spacewarp and are not bent evenly.

    Posted

  • Budgieye by Budgieye moderator

    enter image description here

    So far, we have been looking at the Earth, the lensing galaxy and the distant lensed galaxy from a perspective off to the side.

    Let us move closer to the Earth.

    Posted

  • Budgieye by Budgieye moderator

    enter image description here

    Here we are on Earth, looking at the massive elliptical galaxy.
    We don't have a direct view of the distant lensed galaxy behind the elliptical, but we can see the distorted images of the lensed galaxy around the lensing elliptical.

    You can make a similar image with a wineglass. Hold an empty wineglass on its side, and look at a bright light through the bowl and stem. You can make the bright light form distorted circles and arcs, much like a gravitational lens.

    Posted