Some physical objects are simple, but have profound applications. Take lenses,
for example. The magnifying glass you use to read teensy-weensy texts is a lens. The glasses
your grandma wears to recognize a stranger is a lens. The microscope used by biologists
to study cellular bodies is a lens. If you just casually observe a lens,
you can say that in a way a lens distorts the view of things behind it.
In this video, we’re going to learn about a very special kind of lens. A lens that is helping
us to discover new galaxies and see what is otherwise UNSEEABLE! It is a lens borne out from
one of the fundamental forces of nature: GRAVITY. Although our understanding of gravity is limited,
we can certainly say that it’s a pretty amazing force. And not just for pulling poor Coyote
from Looney Tunes abruptly off a cliff, but also for keeping our own feet on the ground.
Interestingly, the supercool force of gravity also has an eccentric bag of tricks up its sleeve,
one of which is its ability to act as a lens.
Gravitational lensing occurs when a huge amount of matter, like a star, galaxy,
or cluster of galaxies, creates a powerful gravitational field around it, strong enough
to bend light coming from distant galaxies (behind them). Gravitational lensing tends
to magnify distant sources that are otherwise blocked by the galaxies in front of them.
Ironically, these obstacle galaxies themselves act as a lens, curving the light emanating from
distant galaxies and making them visible to viewers in the far-off galaxy, although
the view is often distorted. In a way,
gravitational lensing is akin to peering through a gigantic telescope.
The bending of light in the vicinity of a massive body may sound like a surreal idea, but at the
heart of it is Einstein’s theory of relativity. In 1915, Einstein put forth his new theory of gravity
called general relativity. Einstein suggested that matter distorts the fabric of space and
time around it. That implies that gravity bends light waves, that is, making their paths curve
around massive objects like stars, galaxies, or even black holes. This can be understood with the
analogy of how a fallen ball on a trampoline curves or distorts the space around it.
Well, the curvature of space-time plays tricks on our eyes. Our brain,
and thus our intuitive understanding of the world, is very much confined to Euclidean space.
Perhaps we are hardwired to see the world in three dimensions that are static with respect to time.
For us, it seems that light always travels in a straight line, and this helps us map the world
around us. We simply catch photons with our eyes and trace their path backward. This works alright
till the time path of light is indeed straight. But make its path curved by adding a glass of
water or a carnival mirror and our view of the world is suddenly distorted. We get an illusion
of things around us. Our eyes deceive us. It turns out that those massive celestial
bodies like stars, galaxies, and black holes are essentially carnival mirrors altering our view
of the universe. Fortunately, enlightened with the knowledge of general relativity
and optical physics, we can leverage this light- and mind-bending phenomenon to discover galaxies
and other celestial bodies that lie beyond the realm of discovery with present-day technology.
To use gravity as a lens we need three things: the source, the lens,
and the observer. The source could be a star, galaxy, black hole, or region of dark matter.
The lens could also be a star, galaxy, or cluster of galaxies. The observer would be a telescope,
as it’s nearly impossible to explore the universe with the naked eye.
Now, if the source, lens, and observer are perfectly aligned and the lens and source
are spherically symmetric, we will see a beautiful phenomenon called the Einstein ring. An Einstein
ring was first observed in 1998 through the Hubble telescope. The light in this observation
was coming from a dark dwarf satellite galaxy. The light was bent equally in all directions by
an ancient galaxy (acting as a lens), leading to the formation of a smeared-out ring, confirming
Einstein’s prediction from 80 years earlier! Gravitational lensing doesn’t always distort
or magnify. Sometimes it results in the visual occurrence of the same object multiple times. So,
if we perfectly align the source, lens, and observer and the lens is elliptical or spiral,
instead of being spherical, the chances are high that you can see something even more
interesting—the Einstein cross. The Einstein cross was first discovered by astrophysicist John Huchra
and his team in 1985. This Einstein cross was a manifestation of a luminous distant Quasar powered
by a supermassive black hole. Each of these spots is of the same Quasar, lensed by the intervening
galaxy (ZW 2237+030), which lies between us in the Milky Way and the Quasar in question.
Using gravitational lensing, we have confirmed and weighed many previously unseen galaxies,
but gravitational lensing is also helping us detect mysterious things in the universe that
we cannot see… like dark matter! Dark matter is a matter that has mass, but doesn’t emit light,
heat, or other forms of electromagnetic radiation, making it incredibly stealthy. Although it is
invisible to our eyes, since it has mass, it also bends the light that comes close to it,
just like ordinary matter. Interestingly, it is estimated that
in totality, dark matter is around five times more massive than the ordinary matter of the
universe put together! Ordinary matter includes all the matter we can see, like asteroids,
planets, stars, and other celestial bodies. Studies like the Dark Energy Survey (DES) are
ongoing to understand the distribution of dark matter throughout the Universe
using gravitational lensing. Perhaps, DES study would be able to come with a precise
map of matter in the universe, including the huge amount of mysterious dark matter.
As you can see, Einstein's ground-breaking work on general relativity has paved the
way for astronomers to continue studying the very structure of the Universe—both
the visible and the invisible.
And the simple, yet powerful tool of gravitational lensing is helping us in that cosmic effort!