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.
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