Cambridge, MA – The early evolution of the universe has confounded
astronomers for years. Observations seem to show that giant black
holes containing as much mass as three billion suns formed less than
a billion years after the Big Bang. Collecting so much material so
quickly was as unlikely as building a 20-room mansion in a day’s time.

Now, theoretical astrophysicists Stuart B. Wyithe and Abraham Loeb at
the Harvard-Smithsonian Center for Astrophysics (CfA) have explained
this paradox. They calculated that the light from a significant
fraction of the most-distant quasars is likely being magnified by
intervening matter, making the quasars’ central black holes seem 10
to 100 times larger than they actually are. It’s the equivalent of
learning that what you thought was a 20-room mansion actually was a
one-room shed, easily constructed in a day.

The scientists reported their findings in the June 27, 2002, issue of
the scientific journal Nature.

Astronomers use quasars to study the early universe because they are
bright enough to be seen from billions of light years away. Quasars
are powered by supermassive black holes in the centers of galaxies.
The black holes swallow large amounts of gas, creating powerful
beacons of light that shine across the universe like cosmic

“We can infer the minimum mass of the central black hole by measuring
the amount of light it is generating. The brighter the light, the
more massive the black hole ‘engine’ must be,” said Wyithe.

Within the past few years, astronomers have found quasars at very
high redshifts that existed when the universe was less than one
billion years old. Yet the light from these quasars indicates that
they are powered by black holes containing as much matter as three
billion suns.

“This is a challenge. How do you form such big black holes so early?”
said Loeb. “These supermassive black holes are found to exist in the
young universe when it was less than one-tenth of its current age,
yet they weigh as much as the most massive black holes in the
universe today.”

Wyithe and Loeb solved the mystery by showing that the light of
these distant quasars is likely to be gravitationally lensed. In
gravitational lensing, the light traveling from the quasar is bent
and magnified by the gravity of a galaxy between the quasar and the
earth. Thus the quasar seems brighter, and the black hole seems more
massive, than they actually are.

The scientists’ calculations show that as many as one-third of
quasars with redshifts larger than 6 may be lensed. In comparison,
less than one in a hundred of nearby quasars are lensed. The lensing
of a high-redshift quasar may magnify its brightness by a factor of
10 or more. “The exact fraction of lensed quasars depends on how many
fainter quasars existed at these early times,” said Wyithe.
“Inferring the lensed fraction from future observations will teach us
about the abundance of quasars of different luminosities in the early

The next step in this research is to verify the frequency of
gravitational lensing by direct observational data. Often, the
intervening, lensing galaxy is too faint to be seen by any currently
available telescope. However, the same lensing that magnifies the
light from a quasar also tends to create multiple images of the
quasar. These images are typically very close together in the sky,
with separations of less than one second of arc (the width of a dime
seen at a distance of about two miles). But large ground-based
telescopes or NASA’s Hubble Space Telescope can resolve the separate
images in cases where they exist.

Observational programs to take high-resolution images of
high-redshift quasars are currently being planned, and will provide a
direct measurement of the lensing rate. “Whether or not our
calculations are confirmed by these observations, the results will
tell us something useful about the early universe,” said Loeb.

Wyithe and Loeb’s paper is available online at

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian
Center for Astrophysics (CfA) is a joint collaboration between the
Smithsonian Astrophysical Observatory and the Harvard College
Observatory. CfA scientists organized into seven research divisions
study the origin, evolution, and ultimate fate of the universe.