Out There | Einstein’s Telescope
A century after Albert Einstein proposed
that gravity could bend light, astronomers now rely on galaxies or even
clusters of galaxies to magnify distant stars.
Video by Jason Drakeford, Jonathan Corum and Dennis Overbye on
Publish Date March 5, 2015.
In the 1993 Bill Murray movie, a weatherman finds himself reliving the same day over and over again. Now astronomers using the Hubble Space Telescope
say they have been watching the same star blow itself to smithereens in
a supernova explosion over and over again, thanks to a trick of
Einsteinian optics.
The
star exploded more than nine billion years ago on the other side of the
universe, too far for even the Hubble to see without special help from
the cosmos. In this case, however, light rays from the star have been
bent and magnified by the gravity of an intervening cluster of galaxies
so that multiple images of it appear.
Four
of them are arranged in a tight formation known as an Einstein Cross
surrounding one of the galaxies in the cluster. Since each light ray
follows a different path from the star to here, each image in the cross
represents a slightly different moment in the supernova explosion.
This
is the first time astronomers have been able to see the same explosion
over and over again, and its unique properties may help them better
understand not only the nature of these spectacular phenomena but also
cosmological mysteries like dark matter and how fast the universe is expanding.
“I
was sort of astounded,” said Patrick Kelly of the University of
California, Berkeley, who discovered the supernova images in data
recorded by the space telescope in November. “I was not expecting
anything like that at all.”
Dr. Kelly is lead author of a report describing the supernova published on Thursday in the journal Science.
Robert
Kirshner, a supernova expert at the Harvard-Smithsonian Center for
Astrophysics who was not involved in the work, said: “We’ve seen
gravitational lenses before, and we’ve seen supernovae before. We’ve
even seen lensed supernovae before. But this multiple image is what we
have all been hoping to see.”
Supernovas
are among the most violent and rare events in the universe, occurring
perhaps once per century in a typical galaxy. They outshine entire
galaxies, spewing elemental particles like oxygen and gold out into
space to form the foundations of new worlds, and leaving behind crushed
remnants called neutron stars or black holes.
Because
of the galaxy cluster standing between this star and the Hubble,
“basically, we got to see the supernova four times,” Dr. Kelly said. And
the explosion is expected to appear again in another part of the sky in
the next 10 years. Timing the delays between its appearances, he
explained, will allow astronomers to refine measurements of how fast the
universe is expanding and to map the mysterious dark matter that
supplies the bulk of the mass and gravitational oomph of the universe.
The
heavens continue to light candles for Albert Einstein. On March 14 he
would have been 136, and this year marks a century since his greatest
achievement, the general theory of relativity that transformed our
understanding of space, time and gravity. Dr. Kelly’s paper appears in a
special issue of Science devoted to the anniversary of that theory.
Einstein
proposed that matter and energy warp the geometry of space the way a
heavy body sags a mattress, producing the effect we call gravity. One
consequence of this was that even light rays would be bent by gravity
and follow a curved path around massive objects like the sun, as
dramatically confirmed during a solar eclipse in 1919.
In effect, space itself could become a telescope.
How
this cosmic telescope works depends on how the stars are aligned. If a
star and its intervening lens are slightly out of line, the distant
light can appear as arcs. If they are exactly lined up, the more distant
star can appear as a halo known as an Einstein ring, or as evenly
separated images — the Einstein Cross.
Astronomers
have learned how to use entire galaxies and galaxy clusters as
telescopes to see fainter objects beyond them that would otherwise be
lost in the fog of time.
Hubble
scientists have recently been using this trick in a program known as
Glass, or Grism Lens-Amplified Survey from Space, to explore around
clusters of galaxies, the most massive and thus most powerful
gravitational lenses in the universe. This has enabled them to extend
Hubble’s already powerful vision deeper into the past, in one case to a
galaxy that existed when the universe was only half a billion years old.
Dr.
Kelly’s job was to inspect the images for distant supernovas. He was
not expecting to see four versions of the same explosion at once.
They
appeared in images recorded in November of a spiral galaxy roughly nine
billion light-years from here. The light from this spiral has been bent
and magnified both by the gravity of the intervening cluster, which is
five billion light-years distant, and by one very massive galaxy in the
cluster.
As
a result, ghost images of the spiral appear throughout the cluster and
in particular in an Einstein Cross around that one galaxy. Because the
lensing effect gathers light that would not otherwise be sent to our
eyes or a telescope, the image of the host galaxy is not split so much
as multiplied, explained Adi Zitrin, a team member from the California
Institute of Technology.
“We see simply see more appearances than we would if the lens were not present,” he said.
So
far the supernova, named after a Norwegian astrophysicist, Sjur
Refsdal, has been detected in only the four images in the Einstein
cross. Based on computer modeling of the cluster, Dr. Kelly and his
colleagues suspect that Supernova Refsdal has appeared before, around
1964 and 1995, in other lensed images of the spiral galaxy.
It
should appear again elsewhere in the same cluster within the next few
years, Dr. Kelly’s team predicts. The exact timing of Supernova
Refsdal’s reappearance depends on how the dark matter in the galaxy
cluster is distributed, which will tell astronomers much about a part of
the universe they cannot see any other way. The longer the path length
or the stronger the gravitational field the light ray goes through, the
longer the delay.
Or, as Dr. Kirshner said he liked to tell Dr. Kelly, who was his student, “The visible traces the invisible, Grasshopper.”
It
takes only about 100 seconds for a star to collapse in a typical
supernova, but the resultant outburst of light can last two or three
months before declining sharply. Dr. Kelly suspects that his team caught
the supernova within weeks of its initial outburst, and it might be a
long vigil waiting for the downturn.
Because
of the expansion of the universe, the star and its galaxy are receding
from us so fast that, according to relativity, clocks there appear to
run markedly more slowly than clocks here. As a result, two months from
the point of view of the supernova corresponds to nearly six months on
Earth.
From our point of view, Dr. Kelly said, “it’s going on in slow motion.”
A star might die only once, but with Einstein’s telescope, if you know where to look, you can watch it scream forever.
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