Article: Modified Gravity v. Dark Energy

Modified gravity v. dark energy

For many years now, scientists have wondered why the universe is expanding
faster than it should be. Through conventional knowledge of physics, the
universe should be expanding at a slower pace that observations show that it
is. "There are two main theories for why the universe is expanding so fast,"
Martin Kunz tells PhysOrg.com. "One is modified gravity and the other is the
presence of dark energy. We want to figure out how to distinguish between
the two."

Detecting the difference between dark energy and modified gravity would
provide physicists with a better understanding of how the universe works.
And detecting modified gravity would add a further benefit by contributing
to the understanding of one the fundamental forces in the universe.

Kunz, a scientist at the University of Geneva, and his colleague Domenico
Sapone have published a Letter in Physical Review Letters addressing the
difficulties of distinguishing between modified gravity and dark energy. The
Letter is titled "Dark Energy versus Modified Gravity."

"There are theoretical problems with dark energy," explains Kunz, "and this
had led people to modified Einstein's general relativity in order to get
modified gravity, which some think would explain the expansion of the
universe." The problem, he says, comes in when one tries to observe one of
these phenomena. "We cannot observe either dark energy or modified gravity
directly. We can only observe how galaxies behave."

Kunz points out that in many models, the universe is shown as smooth,
assuming that the energy is evenly distributed and homogenous. "This is not
completely the case," Kunz says. "There are small fluctuations. But many
measurements only probe the smooth universe." He continues: "In this simple
model, you can make everything look like a component with negative pressure,
there is no way to decide whether it is due to dark energy or a modification
of gravity."

Some scientists have looked at the growth of structure in the universe as
way to distinguish between the effects of modified gravity and the effects
of dark energy. It is these measurements that Kunz and Sapone find lacking
in terms of ability to detect differences between the two. Through some
modeling and equations of their own, the two have found that dark energy
perturbations can affect the distribution of galaxies in a way that matches
the effects of modified gravity. "At a certain level, dark energy and
modified gravity look the same," Kunz explains.

"People hoped that you could prove general relativity wrong by studying how
structure forms in the universe," Kunz says. "We showed that you could rule
out certain models of dark energy, but not general relativity itself."

So, while Kunz and Sapone did not manage to show how to distinguish dark
energy from modified gravity, they did discover new avenues that need to be
explored in the debate of modified gravity versus dark energy. And they
discovered something else: "One thing we saw that was really essential was
anisotropic stress," says Kunz. "The presence or absence of anisotropic
stress might help to distinguish between the two. If we measure the presence
of anisotropic stress in the universe, it is more likely to be modified
gravity."

Kunz also sees hope for the future of settling this debate. "Over the next
few years, precise measurements of weak gravitational lensing will become
possible, which also measures anisotropic stress. Combined with the next
round of distance measurements with supernovae, we will be able to get good
constraints."

Copyright 2007 PhysOrg.com.
http://www.physorg.com/news95593687.html

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Posted by
Robert Karl Stonjek

Posted by
Robert Karl Stonjek