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The universe may have existed forever, according to a new model that
applies quantum correction terms to complement Einstein's theory of
general relativity. The model may also account for dark matter and dark
energy, resolving multiple problems at once.
The widely accepted age of the universe, as estimated by general
relativity, is 13.8 billion years. In the beginning, everything in
existence is thought to have occupied a single infinitely dense point,
or singularity. Only after this point began to expand in a "Big Bang"
did the universe officially begin.
Although the Big Bang singularity arises directly and unavoidably
from the mathematics of general relativity, some scientists see it as
problematic because the math can explain only what happened immediately
after—not at or before—the singularity.
"The Big Bang singularity is the most serious problem of general
relativity because the laws of physics appear to break down there,"
Ahmed Farag Ali at Benha University and the Zewail City of Science and
Technology, both in Egypt, told Phys.org.
Ali and coauthor Saurya Das at the University of Lethbridge in Alberta,
Canada, have shown in a paper published in Physics Letters B that the
Big Bang singularity can be resolved by their new model in which the
universe has no beginning and no end.
Old ideas revisited
The physicists emphasize that their quantum correction terms are
not applied ad hoc in an attempt to specifically eliminate the Big Bang
singularity. Their work is based on ideas by the theoretical physicist
David Bohm, who is also known for his contributions to the philosophy of
physics. Starting in the 1950s, Bohm explored replacing classical
geodesics (the shortest path between two points on a curved surface)
with quantum trajectories.
In their paper, Ali and Das applied these Bohmian trajectories to an
equation developed in the 1950s by physicist Amal Kumar Raychaudhuri at
Presidency University in Kolkata, India. Raychaudhuri was also Das's
teacher when he was an undergraduate student of that institution in the
'90s.
Using the quantum-corrected Raychaudhuri equation, Ali and Das derived
quantum-corrected Friedmann equations, which describe the expansion and
evolution of universe (including the Big Bang) within the context of
general relativity. Although it's not a true theory of quantum gravity,
the model does contain elements from both quantum theory and general
relativity.
Ali and Das also expect their results to hold even if and when a full theory of quantum gravity is formulated.
No singularities nor dark stuff
In addition to not predicting a Big Bang singularity, the new model does
not predict a "big crunch" singularity, either. In general relativity,
one possible fate of the universe is that it starts to shrink until it
collapses in on itself in a big crunch and becomes an infinitely dense
point once again.
Ali and Das explain in their paper that their model avoids singularities
because of a key difference between classical geodesics and Bohmian
trajectories. Classical geodesics eventually cross each other, and the
points at which they converge are singularities.
In contrast, Bohmian trajectories never cross each other, so singularities do not appear in the equations.
In cosmological terms, the scientists explain that the quantum
corrections can be thought of as a cosmological constant term (without
the need for dark energy) and a radiation term. These terms keep the
universe at a finite size, and therefore give it an infinite age. The
terms also make predictions that agree closely with current observations
of the cosmological constant and density of the universe.
New gravity particle
In physical terms, the model describes the universe as being
filled with a quantum fluid. The scientists propose that this fluid
might be composed of gravitons—hypothetical massless particles that
mediate the force of gravity. If they exist, gravitons are thought to
play a key role in a theory of quantum gravity.
In a related paper, Das and another collaborator, Rajat Bhaduri of
McMaster University, Canada, have lent further credence to this model.
They show that gravitons can form a Bose-Einstein condensate (named
after Einstein and another Indian physicist, Satyendranath Bose) at
temperatures that were present in the universe at all epochs.
Motivated by the model's potential to resolve the Big Bang singularity
and account for dark matter and dark energy, the physicists plan to
analyze their model more rigorously in the future. Their future work
includes redoing their study while taking into account small
inhomogeneous and anisotropic perturbations, but they do not expect
small perturbations to significantly affect the results.
"It is satisfying to note that such straightforward corrections can potentially resolve so many issues at once," Das said.
