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WHAT if a lighthouse looked as bright from 100 kilometres as it did from 100
metres? Strange as it may sound, an astrophysicist is claiming beacons like this
exist in the Universe in the form of X-ray-emitting jets emerging from
quasars.

Common sense says that galaxies and stars should appear fainter the farther
away they are, which is why astronomers build ever more powerful telescopes.

But Dan Schwartz of the Harvard-Smithsonian Center for Astrophysics believes
X-ray jets from quasars look just as bright no matter how far away they are.
This makes them unique cosmic probes which may allow astronomers to see right
back to the edge of the “cosmic dark age” approximately 14 billion years ago,
when the Universe lit up like a Christmas tree as stars switched on for the
first time.

Astronomers believe that the X-ray jets are created by high-energy electrons
streaming from the vicinity of supermassive black holes within distant quasars,
though nobody knows exactly how they do this. Now Schwartz is proposing that the
electrons boost the photons in the cosmic background radiation—the
“afterglow” of the big bang—giving them X-ray energies.

This process would have been more efficient in the past, when the Universe
was smaller and more cosmic background photons were squeezed into a given
volume. So older, more distant quasars appear just as bright as younger, closer
ones.

Schwartz says the effect is the result of an extraordinary combination of
factors. “The increase in efficiency exactly compensates for the dimming effect
of distance,” he says. “So, unlike all other astronomical objects, or anything
in the familiar world, X-ray jets are beacons which never dim.”

But not all astronomers agree. Some believe that the X-rays are created when
powerful magnetic fields decelerate the high-energy electrons, forcing them to
give up energy in the form of X-rays. “However, in many cases this assumption
leads to a total energy a thousand times less than is needed to create the
observed X-ray luminosity,” says Schwartz.

If Schwartz’s model is right, scientists may be able to use these X-ray
beacons to observe the first stars switching on at the end of the cosmic dark
age, and also to probe the cosmic background radiation itself at early times.
These are important tasks for astronomers attempting to understand how the
Universe evolved.

“This is an absolutely mind-blowing possibility,” says quasar expert David
Hough of Trinity University in San Antonio, Texas. “Who’d have thought we’d
discover a beacon that could illuminate the beginning of time?”