Dark energy, the force believed to be responsible for the accelerating expansion of the universe, is under renewed scrutiny following recent research by University of Chicago astrophysicists. Traditionally, scientists have considered dark energy as a constant phenomenon, with its effects attributed to the energy present in empty space.
However, findings from last year’s Dark Energy Survey and Dark Energy Spectroscopic Instrument have prompted some cosmologists to reconsider this view. The data suggest that dark energy may not be static but could instead be evolving over time.
“This would be our first indication that dark energy is not the cosmological constant introduced by Einstein over 100 years ago but a new, dynamical phenomenon,” said Josh Frieman, University of Chicago Professor Emeritus of Astronomy and Astrophysics.
In a new study, Frieman and Anowar Shajib—a NASA Hubble Fellowship Program Einstein Fellow at UChicago—analyzed data from several astronomical probes. Their work indicates that models allowing for evolving dark energy provide a better fit to current observations than models assuming it remains unchanged. These findings raise the possibility that an undiscovered particle much lighter than an electron could exist.
Frieman explained the importance of understanding dark energy: “We now know precisely how much dark energy there is in the universe, but we have no physical understanding of what it is. The simplest hypothesis is that it is the energy of empty space itself, in which case it would be unchanging in time, a notion that goes back to Einstein, Lemaitre, de Sitter and others in the early part of the last century.”
He added: “It’s a bit embarrassing that we have little to no clue what 70% of the universe is. And whatever it is, it will determine the future evolution of the universe.”
Shajib described how interest in evolving dark energy has grown recently: “Although there has been interest in the dynamical nature of dark energy since its discovery in the 1990s to resolve some observational discrepancies, until recently, most of the major and robust datasets were consistent with a non-evolving dark energy model, which is accepted as the standard.”
He continued: “However, interest in evolving dark energy was vigorously rekindled last year from the combination of supernovae, baryon acoustic oscillation, and cosmic microwave background data from the Dark Energy Survey, Dark Energy Spectroscopic Instrument and Planck experiments. This combination of datasets indicated a strong discrepancy with the standard, non-evolving model of dark energy.”
Frieman elaborated on how these surveys inform our understanding: “The data from these surveys allow us to infer the history of cosmic expansion–-how fast the universe has been expanding at different epochs in the past. If dark energy evolves in time, that history will be different than if dark energy is constant.”
He noted: “The cosmic expansion history results suggest that over the last several billion years or so, the density of dark energy has decreased by about 10%—not much, and much less than the densities of other matter and energy, but still significant.”
Describing their study’s objectives and findings jointly with Frieman and Shajib: “The goal of this study is to compare the predictions of a physical model for evolving dark energy with the latest data sets and to infer the physical properties of dark energy from this comparison.”
They added: “In our paper, we directly compare physics-based models for evolving dark energy to the data and find that these models describe the current data better than the standard, non-evolving dark energy model.”
They also pointed out future prospects: “We also show that near-future surveys such as the Dark Energy Spectroscopic Instrument and the Vera Rubin Observatory Legacy Survey of Space and Time will be able to definitively tell us whether these models are correct or if instead dark energy really is constant.”
Frieman discussed why their models offer improved explanations: “These models are based on particle physics theories of hypothetical particles called axions. Axions were first predicted by physicists in the 1970s who sought to explain certain observed features of strong interactions. Today axions are considered plausible candidates for dark matter; experiments worldwide are actively searching for them including physicists at Fermi Lab and the University of Chicago.”
He clarified further: “The models in our paper are based on a different ultra-light version of axion that would act as dark energy not dark matter. In these models
dark
energy would,
in fact,
be constant for
the first several billion years
of cosmic history,
but
the axion would then start
to evolve—like
a ball on
a sloping field that's released from rest
and starts
to roll—and its density would slowly decrease,
which is what
the data appear
to prefer.”
“So
the data suggest
the existence
of a new particle
in nature that's about 38 orders
of magnitude lighter than
the electron.”
Shajib commented on implications for cosmic expansion:
“In these models,
the
dark
energy density decreases with time.
Dark
energy is
the reason for
the universe's accelerated expansion,
so if its density decreases,
the acceleration will also decrease with time.”
“If we consider
the very far future
of
the universe,
different characteristics
of
dark
energy can lead
to different outcomes.
Two extremes
of these outcomes are
a Big Rip,
where
the accelerated expansion itself accelerates
to
the point
that it rips everything apart,
even atoms,
and a Big Crunch,
where
the universe stops expanding at some point
and re-collapses,
which will look like
a reverse Big Bang.”
“Our models suggest
that
the universe will avoid both
of these extremes:
it will undergo accelerated expansion
for many billions
of years,
yielding
a cold,dark universe—a Big Freeze.”
Reflecting on his experience working on this problem,Frieman said:“When we began working on
Dark Energy Survey
in 2003 ,
our goal was
to constrain
properties
of
dark
energy
to determine whether
it was constant or changing .”
“For two decades ,data indicated
that
it was constant .
We almost gave up
on
that question because
data consistently supported
assumption .
However ,we now have first hint
in over 20 years
that
dark
energy might be changing ,and if
it is evolving ,
it must be something new ,
which would change our understanding
of fundamental physics .
That feeling
is reminiscent
of where we were at beginning .”
“It could still turn out
that
these hints are incorrect ,
but we may be on cusp
of answering that question ,and that's quite exciting .”
Shajib summarized their approach :“For this paper ,
we gathered all major datasets —from Dark Energy Survey ,Dark Energy Spectroscopic Instrument ,Sloan Digital Sky Survey ,Time-Delay COSMOgraphy ,Planck and Atacama Cosmology Telescope —and combined them to get most constraining measurement of dark energy to date .”
“All these measurements come from extensive experiments ,
so in way ,
they represent collective knowledge that cosmological community has gathered as whole .”
Resources supporting this research included computing facilities at University of Chicago Research Computing Center.
The full article appeared originally on Physical Sciences Division website.
Citation :“Scalar field dark energy models :Current and forecast constraints .”Anowar J.Shajib and Joshua A.Frieman ,Phys.Rev.D112 ,063508
Funding :NASA .