This story was updated at 2:40 p.m. on Aug. 18.
Mathematicians have proposed an alternative explanation for
the accelerating expansion of the universe that does not rely on the mystifying
idea of dark energy.
According to the new proposition, the universe is not
accelerating, as observations suggest. Instead, an expanding wave flowing
through space-time has caused distant galaxies to appear to be accelerating
away from us. This big wave, initiated after the
Big Bang that is thought to have sparked the universe, could explain why
objects today appear to be farther away from us than they should be according
to the Standard Model of cosmology.
"We're saying that maybe the resulting expanding wave
is actually causing the anomalous acceleration," said Blake Temple of the
University of California, Davis. "We're saying that dark
energy may not really be the correct explanation."
The researchers derived a set of equations describing
expanding waves that fit Einstein's theory of general relativity, and which
could also account for the apparent acceleration. Temple outlines the new idea
with Joel Smoller of the University of Michigan in the Aug. 17 issue of the
journal Proceedings of the National Academy of Sciences.
While more research will be needed to see if the idea holds
up, "the research could change the way astronomers view the composition of
our universe," according to a summary from the journal.
To convince other cosmologists, the new model will have to
pass muster with further inquiry.
"There are many observational tests of the standard
cosmological model that the proposed model must pass, aside from the late phase
of accelerated expansion," said Avi Loeb, director of the Institute for
Theory and Computation at the Harvard-Smithsonian Center for Astrophysics.
"These include big bang nucleosynthesis, the quantitative details of the
microwave background anisotropies, the Lyman-alpha forest, and galaxy surveys.
The authors do not discuss how their model compares to these tests, and whether
the number of free parameters they require in order to fit these observational
constraints is smaller than in the standard model. Until they do so, it is not
clear why this alternative model should be regarded as advantageous."
Johns Hopkins University astrophysicist Mario Livio agreed
that to be seriously considered, the model must be able to predict properties
of the universe that astronomers can measure.
He said the real test "is in whether they are able to
reproduce all the observed cosmological parameters (as determined, e.g. by a
combination of the Hubble Constant and the parameters determined by the CMB
observations). To only produce an apparent acceleration is in itself
interesting, but not particularly meaningful."
Inconvenient truths
Dark energy is itself a hasty fix to an inconvenient
truth discovered by astronomers in the late 1990s: that the universe is
expanding, and the rate of this expansion seems to be constantly picking up
speed.
To explain this startling finding, cosmologists invoked dark
energy, a hypothetical form of energy that is pulling the universe apart in all
directions (note that dark energy is wholly separate from the equally
mysterious concept of dark
matter - a hypothetical form of matter that populates the universe,
interacting gravitationally with normal matter, but which cannot be seen with
light). In this interpretation, the whole universe is blowing up like a
balloon, and from any given point within it, all distant objects appear to be
speeding away from you.
But not everyone is happy with the dark energy explanation.
"It just seems like an unnatural correction to the
equations - it's like a fudge factor," Temple told SPACE.com. "The
equations don't make quite as much physical sense when you put it in. You just
put it in to fit the data."
Temple and Smoller think the idea of an expanding wave makes
more sense.
"At this stage we think this a very plausible
theory," Temple said. "We're saying there isn't any acceleration. The
galaxies are displaced from where they're supposed to be because we're in the
aftermath of a wave that put those galaxies in a slightly different
position."
Ripples in a pond
Temple compared the wave to what happens when you throw a
rock into a pond. In this case, the rock would be the Big Bang, and the
concentric ripples that result are like a series of waves throughout the
universe. Later on, when the first galaxies start to form, they are forming
inside space-time that has already been displaced from where it would have been
without the wave. So when we observe these galaxies with telescopes, they don't
appear to be where we would expect if there had never been a big wave.
One potential issue with this idea is that it might require
a big coincidence.
For the universe to appear to be accelerating at the same
rate in all directions, we in the Milky Way would have to be near a local
center, at the spot where an expansion wave was initiated early in the Big Bang
when the universe was filled with radiation.
Temple concedes that this is a coincidence, but said it's
possible that we are merely in the center of a smaller wave that affects the
galaxies we can see from our vantage point - we need not be in the center of
the entire universe for the idea to work.