A supernova explosion that disappeared will reappear again in 2037!
A team of astronomers published the first days of September a study in the journal Nature,
in which it is predicted that in 2037 will return to be visible a supernova exploded 10
billion years ago on the other side of the Universe, and whose light had already been
captured by the Hubble Telescope in 2016. Put like that, the news might seem really
curious... How, in fact, can the same star explode twice? We are not talking about a Recurrent Nova,
which "turns on" at intervals of years or decades, but a real powerful Supernova. And Supernovae
are stars that destroy themselves completely, without any possibility of a second life...
And what's more, how can astronomers claim to know even the year in which the supernova - not
by chance called with the unsympathetic name of REQUIEM - will return to shine?
What do you say, let's try together to figure it out?
The sky is full of mirages: stars that appear where they should not be,
images that split several times, galaxies that twist and thin like threads.
As in a great kaleidoscope or hall of mirrors, the play of images depends on how we are aligned with
distant sources. But the illusions, in this case, are not created by mirrors or prisms,
but by the gravitational pull of stars and galaxies. Because gravity also acts on light
rays, deflecting them as a lens does. You will say: but to feel gravity you
must have mass, and light has no mass, so how can it feel gravity?
This is one of the many ideas of relativity that unfortunately are not underlined enough:
gravity is not - as Newton said - the effect of a force, but it is the effect of the curvature
of the space-time "sheet". And in the universe, EVERYTHING is forced to follow the ripples of
space-time. Everything, even light. It is simply not possible not to follow the folds of the sheet.
The deflection of light by a large mass resembles the behavior of a
lens that deforms the image of an object. These phenomena, called "gravitational lensing",
until thirty years ago were considered theoretical curiosities. Today, however,
thanks to instruments such as the Hubble Space Telescope hundreds of them have been localized.
And they are very useful to study the distribution of matter and gravity in the universe.
The German astronomer Johann Georg von Solder realized in 1801 that gravity could deflect light.
But what before him was a hypothesis, for Einstein was a certainty:
matter exerts a gravitational attraction that also acts on light, curving its path.
Einstein's calculation foresaw an effect two times more intense than what was foreseen
by Newton's law of gravitation. And it turned out to be correct,
as it was verified for the first time in 1919, in a historic experiment led by Arthur S. Eddington.
The position of stars next to the Sun, Eddington observed during an eclipse, is
slightly altered, because the gravity of our star deflects the light rays that pass near it.
In practice, it is a mirage like those that occur in the desert,
although there the light rays bend because they pass through layers of
air with different temperature and density. This is why the image of an oasis (which exists,
but is elsewhere) can appear where there is, for example, only sand.
The gravity of the Sun, however, is not sufficient to create spectacular optical effects.
The first true gravitational lens was observed and recognized only in 1979 by astronomer Dennis Walsh
of the Jodrell Bank Observatory, in Great Britain. Walsh saw two quasars (which are very distant,
primordial and very bright galaxies), angularly very close and apparently equal, one of which
stood out just on the edge of a galaxy much closer to us. After a moment of perplexity,
Walsh understood that, in reality, they were two images of the same quasar, which reached him
through two different optical paths: one direct, and one deviated by the gravity of the galaxy,
which was at an intermediate distance. It was the first one, and it made a lot
of sensation, but today we know hundreds of gravitational lenses, sometimes much more
spectacular, and able to double, quadruple, sixfold the image of distant sources.
But are we really sure to have well understood the mechanism that underlies
this fascinating cosmic phenomenon? Before going into the explanation of
the Supernova Requiem it will be good to further deepen the theoretical aspect.
To understand the nature of these events, it is necessary to start from some considerations of
cosmological nature. If we look from a distance a large region of the universe, it appears
"homogeneous and isotropic". This property of the universe has been assumed to the rank of
"cosmological principle" and is the basis of the model with which cosmologists describe space-time,
that is the space in four dimensions consisting of time and the three spatial coordinates.
With this model it is possible, for example, to calculate the distance traveled by a photon from
the instant of its emission from a light source to the instant in which an observer receives it.
Moreover, it is possible to predict the trajectory of the photon in space-time.
Let's consider instead what really happens in the proximity of a body with mass. As we know,
around it a gravitational field develops, whose intensity decreases as the distance increases.
This perturbation, therefore, results in a change of the path of light with respect to its
original trajectory, so that rays are focused on the observer that, in the absence of mass,
it would not have been possible to receive. The effect produced by the presence of a
celestial body is therefore comparable to the refraction by a common optical lens:
just as light propagates through the latter with a different speed than in
vacuum, in the same way in the presence of a gravitational field light propagates "slower",
as if photons were attracted by the mass that produces the gravitational field.
This similarity also explains the name given to the phenomenon: gravitational lensing.
Only in 1916, thanks to the completion of the Theory of General Relativity, Einstein
was the first to obtain the exact estimate of the deflection angle of a light ray passing
at a certain distance from a body with mass. In the specific case of the Sun, he predicted a
deflection angle of 1.74 arc seconds. Subsequent experimental verifications started in 1919,
have fully confirmed the result predicted by Einstein. The deflection of light due to
gravity is therefore one of the most important proofs of the Theory of General Relativity.
For about fifty years the study of gravitational lensing proceeded slowly,
mainly because of the impossibility to acquire other observational data. However,
already in the mid-’20s, it was hypothesized that one of the consequences of the deflection
of light by massive celestial bodies could be the production of multiple images of the same source
or even the formation of a ring, when particular conditions were verified,
such as the circular symmetry of the lens-object and its alignment with the source.
The decisive push to the study of lensing came in 1979 with the discovery of the double quasar,
which we have already discussed, and since then the attention of astronomers
to this phenomenon has gradually grown. In 1985, then, was observed an arc structure
(the largest ever discovered with 30" long) at the center of the cluster of galaxies called Abell
370, located approximately 4 billion light-years away from the Earth in the constellation Cetus.
And this was the discovery of the so-called "strong lensing". No one had previously
thought that clusters could have a core so dense as to produce this kind of effect.
Finally, at the turn of the 1990s, it was understood that distortion by galaxy clusters on
images of galaxies behind them is not is manifested not only by the formation of
large arcs or rings but also by the appearance of small arcs (the so-called “weak lensing”).
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To simplify what has been said so far… The elements involved in an event of
gravitational lensing are three: the astronomical source that emits light, the so-called lens,
that is the celestial body that produces the gravitational field with which the light from the
source goes to interact, and finally the observer, that is the astronomer who with his instruments
detects the effect of this interaction. the effect of this interaction. All together,
these three elements constitute a real optical system.
Many factors influence how a lensing event occurs.
First, much depends on how the matter that makes up the lens is distributed. In general,
for example, the lens becomes very efficient when it reaches very high mass densities at the center.
But a lot also depends on how the source is made. If it is a star or a quasar,
i.e. a light source with angular dimensions so small that it can be considered punctiform,
the lensing effect is not detectable as a large distortion of the images (which will
tend to remain punctiform), but as a result of the appearance of multiple images of the same source.
Multiple images are formed when the lens converges on the observer light rays that
the source has emitted in different directions. If then, as in the case we are dealing with,
the light rays emitted by a transient phenomenon such as the explosion of a supernova
meet different densities of matter, then you can verify the appearance of NOT simultaneous images!
OK, now that we know a little more about lensing we can deal with the real news,
that is the discovery of the Requiem supernova, the prediction
of its reappearance, and the cosmological consequences of all this strange story.
It all starts in 2019 when the Danish astronomer Gabriel Branner is comparing an image captured
by the Hubble Space Telescope during the REQUIEM (REsolved QUIEscent Magnified Galaxies) program,
with another of the same field, taken, however, in 2016. (Incidentally,
here is explained why the nickname, although then, as we shall see, another astronomer will
give on the thing a more romantic version...) The object portrayed in both photos is the galaxy
cluster MACS J0138, located in the constellation Cetus at a distance of 4 billion light years.
It's just a routine comparison, but to his surprise, Branner notices that three
points of light appeared in the 2016 photo, but were completely absent in the most recent one!
Branner quickly discarded the most far-fetched hypotheses and realized
that he was observing the "before" and "after" of a supernova that exploded in a galaxy much
farther away from the cluster, but on the same line of sight as the Earth's observer.
And that the gravitational lensing that distorts the galaxy into reddish arcs is
also responsible for the triple image of the supernova in the 2016 photo.
The light that Hubble captured from the MACS J0138 cluster, took about four billion years to reach
Earth. The light from Supernova Requiem needed an estimated 10 billion years for its journey,
based on the distance of its host galaxy. The supernova then obviously faded until
it disappeared altogether in the 2019 photo. This fact alone would be fascinating enough,
but there's more. Branner and Steve Rodney, an American astronomer, taking a closer look at it
all managed to make a startling prediction: A fourth image of the same supernova will ignite
in 2037, 21 years after the other three! And the two astronomers also pinpointed
the precise location where the supernova should reappear!
Are you wondering how they did it? Well… The team's prediction of the
supernova's return appearance is based on computer models of the gravitational field of the cluster,
which describe the various paths the supernova light take through the galactic grouping.
Each lensed image takes a different route through the cluster and arrives at Earth at a different
time, due, in part, to differences in the length of the pathways the supernova light followed.
We can compare the different paths of the supernova light to trains that leave the
same station at the same time, travel at the same speed, and head to the same destination. However,
they travel on tracks of different lengths and therefore will never arrive at the same time.
But that's not all. The supernova image predicted in 2037 is further behind other images of the
same supernova because its light travels directly through the center of the cluster,
where the densest matter resides. After the one in 2037, astronomers say,
there will be more supernova resurrections... starting with the one in 2042, but that one
will be too faint to be detected by current instrumentation. Perhaps future instruments such
as NASA's James Webb Space Telescope will succeed. Ah, I forgot... According to Steve Rodney, the
alternative meaning of the nick "Requiem" is this: "A requiem for a dying star and a sort of elegy
for the Hubble Space Telescope, which in 2037 will no longer be with us to receive the star's light".
Very sad, but also very appropriate. Isn't it?