[Everything else in the universe is either a particle or field.
Dark energy behaves as neither, and it may be a property inherent to
space itself.] [[link removed]]

DARK ENERGY MIGHT BE NEITHER PARTICLE NOR FIELD  
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Ethan Siegel
September 22, 2021
Big Think [[link removed]]

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_ Everything else in the universe is either a particle or field. Dark
energy behaves as neither, and it may be a property inherent to space
itself. _

There is a large suite of scientific evidence that supports the
picture of the expanding Universe and the Big Bang, complete with dark
energy. Dark energy, is required to explain what we observe., NASA /
GSFC

 

What is it, at a fundamental level, that makes up the universe? When
we ask this question, we typically think about starting with things
that we directly observe — things like stars, planets, humans, gas,
dust, plasma, and other forms of the matter we know — and dividing
them up until you reach something that is indivisible. Although we
originally thought that atoms would be these “uncuttable” things,
we soon discovered they could be further divided: into electrons and
atomic nuclei, which themselves are composed of quarks and gluons.

As we mastered the laws of physics and began to manipulate these
subatomic particles, we gained the ability to accelerate and collide
them, enabling the creation of a wide slew of particles and
antiparticles: everything described by the Standard Model of particle
physics. And yet, if we add up the sum total of all of these forms of
matter, including photons, neutrinos, and everything that does not
compose atoms, we fall far short of what is needed to describe our
universe. Two additional components are necessary: dark matter and
dark energy. Moreover, although we fully expect there to be a particle
responsible for dark matter, that is not the case at all for dark
energy. Here’s why.

The particles and antiparticles of the Standard Model obey all sorts
of conservation laws, with fundamental differences between fermionic
particles and antiparticles and bosonic ones. Each set of particles
possesses its own unique quantum numbers, but no particles here can
explain dark matter or dark energy. (Credit: E. Siegel / Beyond the
Galaxy)

Particles, at least as we know them, all have a few things in common.
They have a set of “quantum numbers,” or properties that are
inherent to them that determine their masses, spins, and various
charges. All particles of the same type — electrons, down quarks,
Z-bosons, etc. — have identical properties to one another, and you
could replace any one of them with any other identical particle and
everything would remain the same. The only things that differ between
them are either random, like their decay lifetime (if they are
unstable), or situational: things like their momentum, orbital angular
momentum, or energy levels within a bound system.

But there is another way to break these various particles up: into
massive and massless categories. Massive particles slow down as the
universe expands and cools during the hot Big Bang and eventually
gravitationally clump together, as every mass universally attracts
every other mass. Massless particles, however, simply travel through
curved space at the only permissible speed, the speed of light, until
they interact with another particle in their path. Massive (matter)
and massless (radiation) particles evolve in fundamentally different
ways with respect to the expanding universe.

While matter and radiation become less dense as the universe expands
owing to its increasing volume, dark energy is a form of energy
inherent to space itself. As new space gets created in the expanding
universe, the dark energy density remains constant. (Credit: E. Siegel
/ Beyond the Galaxy)

Astrophysically, when we survey the universe in all the different ways
we know of, it reveals a variety of aspects of the cosmic story. By
observing how abundant the lightest elements and their isotopes are,
we can determine how much normal matter, total, makes up our universe.
By measuring how galaxies clump and cluster together, as well as their
large-scale distribution and individual, internal properties, we can
determine how much total “stuff” there is that behaves as though
it is made of massive particles. And when we look at the details
embedded in the leftover glow from the Big Bang — the cosmic
microwave background — it tells us that the universe is spatially
flat, telling us how much total “stuff” is present in the
universe, overall.

From this information, we learn that all of the normal, Standard Model
material in our universe comes out to just 5 percent of the total.
Another ~27 percent is dark matter, which cannot behave like any of
the known particles. It clumps and gravitates like normal matter but
appears to have zero cross-section — i.e., doesn’t collide —
with normal matter, light, or other dark matter particles. Although we
can only detect dark matter’s presence through its gravitational
influence, it is immediately apparent that dark matter is distributed
far more diffusely than normal matter; it isn’t as clumpy,
particularly on small cosmic scales. Unfortunately, all attempts at
direct detection experiments have failed to yield a robust, positive
signal. Its true nature remains mysterious.

Light may be emitted at a particular wavelength, but the expansion of
the universe will stretch it as it travels. Light emitted in the
ultraviolet will be shifted all the way into the infrared when
considering a galaxy whose light arrives from 13.4 billion years ago.
The more the expansion of the universe accelerates, the greater the
light from distant objects will be redshifted and the fainter it will
appear. (Credit: Larry McNish / RASC Calgary)

Even with normal matter and dark matter combined, though, we haven’t
gotten close to finding everything. The remaining ~68 percent of the
universe is unaccounted for, and our big clue toward what that
“stuff” is first came in the 1990s, when observations of distant
supernovae appeared fainter than our models of the universe were
predicting. It was as though something else beyond what we expected
— various forms of matter and radiation — was contributing to the
universe. As the evidence poured in, bolstered by the cosmic microwave
background and large-scale clustering data, we realized that a wholly
novel form of energy, inconsistent with the properties of any form of
matter or radiation, must be present: what we call dark energy today.

What is remarkable about the evidence for dark energy is how perfectly
uniform it is. There is no evidence that there’s more or less dark
energy in the space occupied by rich galaxy clusters than in the voids
of empty space. There is no evidence that dark energy correlates with
density, direction, location, or epoch of the universe. It appears to
be perfectly uniform, perfectly homogeneous, and perfectly constant:
unchanging throughout space and time. And yet, despite its simplicity,
it behaves fundamentally differently from all other known forms of
energy.

Various components of and contributors to the universe’s energy
density, and when they might dominate. Note that radiation is dominant
over matter for roughly the first 9,000 years, then matter dominates,
and finally, a cosmological constant emerges. (The others do not exist
in appreciable amounts.) However, dark energy may not be a
cosmological constant, exactly. (Credit: E. Siegel / Beyond the
Galaxy)

Every form of matter and radiation in the universe is linked to
quantum particles in some way. Normal matter is made up of subatomic
particles: particles of which there are a finite number. As the
universe expands, the number of particles stays the same while the
volume increases, hence matter gets less dense as time marches
forward. Similarly, radiation is quantized into particles as well
(even, theoretically, gravitational radiation, which should be
quantized into gravitons), but these particles are massless. As the
universe expands, not only does the number of particles remain the
same while the volume increases, but the energy of each individual
particle decreases as the universe expands.

Still, both of these descriptions fall apart for dark energy
[[link removed]]. As the
volume of the universe increases — as it expands — the energy
density does not change; it remains constant. It’s as though there
is something present through all of space that isn’t dependent on
anything else: matter density, radiation density, temperature, changes
in volume, etc. Although we can measure and quantify its effects on
the universe, we cannot say that we understand dark energy’s nature.
It could be a

* particle of some type,
* a field that permeates the universe,
* or even a property inherent to the fabric of space itself.

A universe with dark energy (red), a universe with large inhomogeneity
energy (blue), and a critical, dark energy-free universe (green). Note
that the blue line behaves differently from dark energy. New ideas
should make different, observably testable predictions from the other
leading ideas. And ideas which have failed those observational tests
should be abandoned once they reach the point of absurdity. (Credit:
Gabor Racz et al., 2017)

Of course, each of these scenarios leads to a vastly different
conception of the universe and what is present within it. If dark
energy is a particle, then either new particles must constantly be
created to keep the energy density constant, or the behavior of these
particles must evolve with time to keep their effects on the universe
constant. If dark energy is a field that permeates the universe, then
it is permitted to evolve in either space or time or both, and any
observed evidence (we have none) of such a variation would point in
this direction; models of quintessence
[[link removed](physics)] fall into this
category.

But if we follow the observations, there is no evidence that dark
energy is anything other than the most basic entity imaginable: a
property that is uniformly inherent to space everywhere and at all
times. This can come about in one of two different ways very easily:

* The universe can possess a positive, non-zero cosmological
constant, a term perfectly allowable in general relativity. It has to
be very, very small, but when you put it in everywhere over the whole
universe, it eventually comes to dominate.
* It could be a quantum property of space: the zero-point energy of
all the fields in the vacuum of space is not required to be zero but
could take on some positive, non-zero value. What we often interpret
as quantum fluctuations, or particle-antiparticle pairs popping in and
out of existence, could be the cause behind dark energy.

Visualization of a quantum field theory calculation showing virtual
particles in the quantum vacuum (specifically, for the strong
interactions). Even in empty space, this vacuum energy is non-zero.
(Credit: Derek Leinweber)

From a theoretical perspective, it is important to keep in mind that
until we understand the nature of dark energy, which is to say that we
acquire some sort of evidence that points toward one possibility over
another, we have to keep all of these options
[[link removed]] in mind. Dark energy could be
linked to the inflationary epoch that set up and gave rise to the Big
Bang; dark energy could have been important and impactful early on
[[link removed]] in the universe’s history before
decaying to its present, low-density state; dark energy could be
slowly evolving
[[link removed]] or
inhomogeneous, or could have a slightly higher or lower density
dependent on what else is around. Theoretically, all options remain on
the table.

But that is also why we don’t simply base our conclusions on theory
alone. The whole idea of science is based on the notion that the way
we find out information about the universe is by putting the universe
itself to the test: through measurement, experiment, and observation.
As we study:

* the cosmic microwave background down to smaller and smaller scales,
in more wavelength bands and with polarization included;
* the large-scale structure of the universe out to greater
distances, fainter objects, and larger areas on the sky;
* and individually luminous objects, out to greater precision and
greater distances,

we gain the ability to see whether there is any indication that dark
energy is anything other than a pure constant, and whether it shows
evidence for any evolution or inhomogeneities in time and/or space.

This snippet from a structure-formation simulation, with the expansion
of the universe scaled out, represents billions of years of
gravitational growth in a dark matter-rich universe. Even though the
universe is expanding, the individual, bound objects within it no
longer expand. Their sizes, however, may be impacted by the expansion;
we do not know for certain. (Credit: Ralf Kahler and Tom Abel (KIPAC)
/ Oliver Hahn)

Of course, it hasn’t. Fifteen years ago, we were able to constrain
that dark energy was a constant to a precision of ±30 percent or so.
Today, that has improved to a precision of ±7 percent or so, with the
next generation of space-based and ground-based observatories —
particularly ESA’s Euclid, NSF’s Vera Rubin observatory, and
NASA’s Nancy Roman telescope — poised to take us to a precision of
just ±1 percent. If there are any imperfections, inhomogeneities, or
evolutionary effects that occur in the dark energy sector, these
upcoming surveys will be our best bet at uncovering them.

However, there are other methods that could reveal some more exotic
interpretations. Recently, the XENON experiment claimed to see an
excess of events over the anticipated background, beyond what
conventional sources could explain. There are three main
interpretations on the table, at present:

* the result is an experimental fluke that will go away with better
statistics, which is within the realm of possibility;
* that this is our first evidence of an unexpected type of dark
matter, whose explanations would require additional contortions over
what was theorized previously; or
* a new source of background that hasn’t been included in the
analysis (such as tritium in the water) is causing it.

Of these explanations, most physicists favor the last one. But, as we
said earlier, all possibilities, no matter how exotic or strange, have
to be kept in mind.

That is where, as was recently shown by a small team of
scientists, the idea
[[link removed]] of
chameleon dark energy comes into play. If dark energy is actually a
very specific, exotic type of particle that has its clumping and
density restricted in the most matter-rich regions of space, it could
have potentially created the signal seen by the XENON experiment
[[link removed]].
With some additional theoretical contortions, the team was able to
conclude
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the ~95 percent confidence level, that this interpretation is favored
over the null hypothesis: that the result is a mere fluke.

Of course, what most people do not realize is that this is precisely
what most ideas in theoretical physics look like: you add in one or
two new free parameters to explain one or two new phenomena. Most
ideas like this are not new, but rather are variations on an old idea,
and most ideas in this vein are colossally bad: they are ill-motivated
and are considered only because experiments are nearing the precision
necessary to rule them out, either wholly or at least in part. In
physics, as well, a signal at ~95 percent confidence is barely worth a
second look; this idea of chameleon dark energy particles likely will
never rear its head again in any experimental setting.

Constraints on the total matter content (normal + dark, x-axis) and
dark energy density (y-axis) from three independent sources:
supernovae, the CMB (cosmic microwave background), and BAO (which is a
wiggly feature seen in the correlations of large-scale structure).
Note that even without supernovae, we would need dark energy for
certain, and also that there are uncertainties and degeneracies
between the amount of dark matter and dark energy that we would need
to accurately describe our universe. (Credit: Supernova Cosmology
Project, Amanullah et al., ApJ, 2010.)

When you take away wishful thinking and look only at the evidence that
we have, the story the universe tells is very simple, albeit
counterintuitive. The stuff that we thoroughly understand — the
matter and radiation composed of all the known particles of the
Standard Model plus gravitational waves — makes up only 5 percent of
the total of what’s out there. There is another form of mass, dark
matter, that makes up an additional ~27 percent or so. But the
majority of what is present, the ~68 percent of the universe that is
dark energy, doesn’t appear to either be a particle or change with
time. It behaves neither as a particle nor as a field, but rather as a
property that is inherent to space itself.

Although it is a fun exercise to consider what might happen under a
variety of exotic conditions, particularly when experiments or
observations are reaching the sensitivities necessary to probe them,
it is vital to treat them as the fringe hypotheses that they are. The
default, working hypothesis of what dark energy is doesn’t include
extra couplings, clumpiness, time-or-space evolution, or anything else
beyond a simple constant in space. It is time to take seriously the
idea that dark energy might simply be a property inherent to the very
fabric of space. Until we learn how to calculate the zero-point energy
of empty space itself, or gain some bizarre, surprising, and
unanticipated evidence, this will remain one of the biggest
existential questions in all the universe.

_ETHAN SIEGEL is a Ph.D. astrophysicist and author of "Starts with a
Bang!" He is a science communicator, who professes physics and
astronomy at various colleges. He has won numerous awards for science
writing since 2008 for his blog, including the award for best science
blog by the Institute of Physics. His two books "Treknology: The
Science of Star Trek from Tricorders to Warp Drive"
[[link removed]] and "Beyond
the Galaxy: How humanity looked beyond our Milky Way and discovered
the entire Universe"
[[link removed]] are
available for purchase at Amazon. Follow him on
Twitter @startswithabang [[link removed]]._

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