Scientists Visualize Waves in Space Caused by Black-Hole Mergers

Merging black holes will rock the fabric of space and time with
gravitational waves that start quiet, grow to a thunderous roar at
the moment of impact, and then resonate from the final gong,
according to international team of scientists who have created a
novel computer model of such a merger based on Einstein’s equations.
Scientists present these results this week at the Fourth
International LISA Symposium on gravitational radiation at Penn State
University in University Park.

Gravitational waves constitute a form of radiation predicted by
Einstein but which has yet to be directly detected. “The collision
of two black holes is the ultimate manifestation of Einstein’s theory
of general relativity,” said Lee Samuel Finn, Director of the Center
for Gravitational Wave Physics at Penn State and chair of the LISA
Symposium Scientific Organizing Committee. “Anything we can do to
understand that process better is a step toward the success of the
LISA mission.”

“We can only observe black holes plunging into each other through
gravitational waves,” said John Baker of NASA Goddard Space Flight
Center in Greenbelt, Maryland. “This model is an important first
step toward understanding what such waves will look — or sound —
like.”

Baker and his colleagues, Manuela Campanelli and Carlos Lousto of the
University of Texas at Brownsville, and Ryoji Takahashi of the
Theoretical Astrophysics Center in Copenhagen, collectively known as
the Lazarus Team, have recently published a journal article about
this modeling in the journal Physical Review D. Baker presents his
novel computer model during the LISA meeting this week.

Gravitational waves ripple through space like waves upon an ocean.
These exotic waves offer an entirely new window on the Universe and
may carry direct information about black holes and stellar
explosions, or information about the Big Bang itself. Gravitational
waves are produced by massive objects in motion. The waves travel at
light speed with a wide range of frequencies, carrying energy away
from the source.

Unlike light waves (electromagnetic radiation), gravitational waves
do not interact strongly with matter. Passing gravitational waves
alter the distance between objects, gently shifting them so they bob
like buoys rising and falling on the sea surface with each passing
wave. Even for objects as far apart as the Earth and the Moon
though, gravitational waves might alter their separation only by a
length a thousand times smaller than an atom.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) in
Washington and Louisiana — funded by the National Science Foundation
and now in the commissioning phase — hopes to detect distances
between test objects altered by gravitational waves. A proposed NASA
– European Space Agency mission called the Laser Interferometer Space
Antenna (LISA) would do the same from space. But these observatories
need models in order to interpret the data they hope to collect.

“These calculations are the first to give us some insight into
certain kinds of signals that we’ll be receiving with LISA,” said
Robin Stebbins, the NASA Project Scientist for LISA. “Since
gravitational waves produce such tiny effects, even in the best
receivers we know how to build, any knowledge of this complex
phenomenon is very valuable. Now that the Lazarus team is able to
model the expected signals, we can better optimize the detectors.”

Events that produce detectable gravitational waves include black-hole
and galaxy mergers, neutron-star mergers, and massive-star
explosions. The Lazarus team focused on binary black holes in their
final orbit, just as they are about to merge. This model also
includes galaxy mergers — the coalescence of supermassive black
holes in galaxy cores.

This latest computer model uses a combination of treatments, each
specialized to a different stage in the process of binary-black-hole
coalescence. Most importantly, the model employs Einstein’s
nonlinear equations to describe the critical moment when the black
holes plunge together, requiring a supercomputer with at least 100
gigabytes of RAM. Simple Newtonian theory does not account for the
warping of spacetime by gravity, a prediction of General Relativity,
and is thus inadequate for describing the physics of strong gravity
near a black hole.

The model supports previous predictions that the gravitational waves
from coalescing black holes will be relatively weak until just
moments before the merger. Then, the wave would grow louder,
culminating in a thunderous impact. After this, the newly formed
single black hole would resonate with that final gong from the
merger. Stellar-size black holes would produce waves with a
frequency of about 10 hertz, in the range of the ground-based LIGO
detector. Supermassive black holes would produce waves with a
frequency of a thousandth of a hertz, in the range of the space-based
LISA detector.

John Baker works at Goddard’s newly formed gravitational wave Group
in the Laboratory for High-Energy Astrophysics and is funded through
a grant with the National Research Council. A copy of the journal
article, “Modeling Gravitational Radiation from Coalescing Binary
Black Holes”, is available at http://arxiv.org/abs/astro-ph/0202469.

This work began while all the collaborators were working at the Max
Planck Institute for Gravitational Physics in Germany. The
institute’s director, Bernard Schutz, who is a European member of the
LISA International Science Team, commented that this study “… is a
perfect illustration of the international nature of work in
gravitational waves and the interpretation of Einstein’s theory.
From detector development to the prediction and interpretation of
observations, we need to involve the best scientific minds from
around the world if we are to do justice to Einstein’s great vision.”

For more information, refer to the Lazarus web site at
http://www.phys.utb.edu/lazarus.