# Can the diminishing energy of the CMBR be the source of dark energy?

I was just told the CMBR energy diminishes with time. Can it be that the lost energy is somehow transferred to spacetime, causing the expansion to accelerate?

No - the decreasing energy in the CMB is already well modeled in the Friedmann equations. The term in the density parameter that is proportional to $a^{-4}$ is the contribution of radiation energy density to the evolution of the universe, the term proportional to $a^{-3}$ is matter density (mostly dark, but includes ordinary matter), $a^{-2}$ is the contribution of the curvature of space-time itself, and the term without any factors of $a$ is the contribution of dark energy. The size of the radiation density, today, is already a small fraction of the matter density (about 0.03% of the matter density, 0.01% of the density of the universe overall - ordinary matter is about 5% overall).

The last time the energy density in the radiation fields was the same size as what's in the matter fields was around $z=3,300$.

I also disagree with @J.Chomel's answer - the energy stored in the radiation field is decreasing. Then energy density in the radiation field scales like $a^{-4}$, and the volume scales like $a^3$. Since the total energy is the energy density times the volume, the total energy scales like $a^{-1}$, just as you would expect with the number of photons being fixed, but the energy in each photon scaling as $a^{-1}$ as the wavelength increases.

## Did a dark energy discovery just prove Einstein wrong? Not quite.

The largest galaxy survey ever made suggests that our cosmos isn't as clumpy as it's supposed to be. That lack of clumpiness could mean there's a discrepancy with Einstein's theory of general relativity, which scientists use to understand how the structures in our universe have evolved over 13 billion years.

"If this disparity is true, then maybe Einstein was wrong," said Niall Jeffrey, one of the co-leaders of the Dark Energy Survey (DES) and a cosmologist at École Normale Supérieure, in Paris, told BBC News

The DES team compiled a catalog of hundreds of millions of galaxies, and used tiny distortions in the shapes of those galaxies to measure the vital statistics of the universe. Almost all of those measurements confirmed the prevailing Big Bang model of cosmology, in which all the universe's matter expanded from a mind-bogglingly hot, incredibly tiny point.

But one of those measurements &mdash the clumpiness of matter &mdash was a little off. If the universe is smoother than thought, that would mean that our understanding of how structures evolve in the universe, which is based on Einstein's general theory of relativity, would be wrong.

While some news headlines are already proclaiming that Einstein was wrong and physicists need to revise their models, the reality is much more nuanced. That's because the discrepancy isn't a statistical slam dunk yet.

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## Dark Energy vs Dark Matter

There is no global conserved energy in an expanding universe. See here:

Strictly speaking, since in our best current model our universe is spatially infinite, the "total energy" of our universe would be infinite as well. But even in a finite universe with dark energy (for example, a spatially closed FRW spacetime with a positive cosmological constant), the "total energy" (dark energy density integrated over spatial volume) would change with time, and that's to be expected.

Sure, the origin of dark energy is an open question. I think the prevailing opinion in mainstream is that it has to be explained as energy of vacuum. According to quantum field theory, even in vacuum there exist field fluctuations due to Heisenberg uncertainty principle. So naturally, one would think that energy of these fluctuations could be the source of dark energy observed in cosmology. However, there is enormous discrepancy between the observed value of dark energy (a.k.a. cosmological constant) and the theoretical value coming from QFT. The predicted value asi higher by 120 orders when compared to observed value.
https://en.m.wikipedia.org/wiki/Cosmological_constant_problem

So clearly we don't have satisfactory explanation yet. Maybe we won't know until we have a complete theory of quantum gravity

@PeterDonis Ok so you say scalar fields can do this, IIRC examples of known scalar fields would be also the B field (static), E field(static) , EM field , gravity , others?
Can the E, B, EM or gravity fields do anything similar to what is happening to the dark/vacuum energy ?

Let me ask one more thing with regards to the topic at hand, normally we associate a field with a source, like the E field and it's source charge or gravity and mass , so the way I imagine in ordinary conditions if I have a object that has charge and mass it is a source for both E field and gravity
What is the source for dark/vacuum energy?
Is the source known or hypothetical as of now?

It seems that unlike matter and the fields associated with matter the source for dark energy seems to be everywhere at once and fundamentally linked to space time.

## Dark Energy

In the late 1990s, astronomers found evidence that the expansion of the universe was not slowing down due to gravity as expected. Instead, the expansion speed was increasing. Something had to be powering this accelerating universe and, in part due to its unknown nature, this &ldquosomething&rdquo was called dark energy.

Hubble plays an important role in verifying, characterizing and constraining dark energy. Both Hubble and ground-based observations measures a special type of stellar explosion, a white dwarf supernova, to measure accurate distances to galaxies.

Within the Hubble Deep Field-North region, astronomers pinpointed a blaze of light from one of the farthest supernova ever seen. In a close-up view of that region (left) a white arrow points to a faint elliptical, the home of the exploding SN 1997ff. The supernova itself (right) is distinguished by the white dot in the center. Credit: NASA, Adam Riess (STScI) NEWS RELEASE: 2001-09 >

A galaxy located a billion light-years away provides a data point for the universe as it was a billion years ago. Meanwhile, as the universe expands, the light traveling to Earth from distant galaxies (and their supernovas) is stretched out to longer wavelengths &mdash a phenomenon called cosmological redshift. The cosmological redshifts of galaxies at different distances provides a history of the expansion of the universe over time.

However, only Hubble had the resolution to extend these observations to very distant galaxies. The discovery of supernova 1997ff, located about 10 billion light-years away, provided evidence for dark energy.

About halfway into the universe&rsquos history &mdash several billion years ago &mdash dark energy became dominant and the expansion accelerated. While ground-based studies had measured this accelerating period, Hubble&rsquos observation of 1997ff stretched back to the decelerating part of the expansion. This shift between two different eras of the universe &mdash a change from a decelerating universe to an accelerating universe &mdash showed that dark energy exists.

Hubble continued to explore the nature of dark energy with observations such as the Great Observatories Origins Deep Survey (GOODS), structured to help uncover distant supernovas.

The 42 supernovas found by Hubble not only solidified the conclusions about dark energy, but also began to constrain some of its possible explanations. Later Hubble results identified how early in the universe dark energy began to influence the expansion as well as constrained the current expansion rate.

The view that emerged was that dark energy was consistent with the slow, steady force of Einstein&rsquos cosmological constant, a concept that the physicist had initially introduced into his equations to prevent his theoretical universe from collapsing, then later retracted when the expansion of the universe was discovered. But instead of holding the universe in a steady state, dark energy is pushing outward to expand the universe faster and faster. The discovery of dark energy was recognized by the Nobel Prize in Physics in 2011.

Astronomers now know that there is much more to the universe than meets the eye. The luminous and non-luminous normal matter makes up about 4 percent of the total mass and energy density of the universe. Dark matter, which emits no light and cannot be directly observed, comprises another 24 percent of the total, while dark energy dominates with about 72 percent. Most of the universe is unknown and only indirectly detected. We can see its effects on galaxies and the expansion of the universe, but we have yet to identify the underlying source. That may seem unsettling, but to a scientist, it is exciting. There are more great mysteries to explore and solve!

## Answers and Replies

"Dark energy" is an attractive name for something that we have no evidence is an energy.
It may tend to confuse and mislead people to use the word "energy" in this context.

There is no evidence that space is a material substance. Basically we are talking about geometry and the equations that have been proposed governing its development and change.

It is important not to take some traditional Greek idea of geometry for granted. We've discovered it changes according to its own law. We have no right to assume that angles of a triangle always add up to 180. The reason they do in some circumstances and do not in other is explained by the accepted law of geometry.We have no right to assume that distances once they start increasing will not continue to increase (even if "dark energy" or the cosmological curvature constant were zero). The gradual changes in the rate of increase are according to the accepted law of geometry.

We don't know that the law is absolutely correct. It's merely the best (most precise, widely applicable, simple, reliable) law of geometry we have so far. It works.
It might be improved some day but it is accepted for the time being. It has a curvature constant in it. So far there is no convincing evidence that attributes this constant to an "energy". The observational evidence has been running the other way---that it is simply a naturally occurring constant, like other basic physical constants. We will have to wait and see.

The cosmological constant Lambda (which some people call "dark energy") has so far played a very minor role. Even if that constant were zero, geometry would still be expanding. The mental image you offer of pumping something into space to make it expand is not a very good mental image. Geometry expands because that is what a lot of solutions to the GR equation do, and we happen to be in one of the expanding solutions. The equation has in it a natural tendency for processes like this to continue (though the rate may gradually change) without any outside input.

You might take a look at an article called "Why all these prejudices against a constant?" by Rovelli.
Just google "prejudices rovelli" and you should get it. If that does not work, please let me know.

## Focal plane section.png

The DESI instrument was installed on the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory, a program of the National Science Foundation’s (NSF) NOIRLab, which has allowed the Department of Energy to operate the Mayall Telescope for the DESI survey. The instrument includes new optics that increase the field of view of the telescope and includes 5,000 robotically controlled optical fibers to gather spectroscopic data from an equal number of objects in the telescope’s field of view.

“We’re not using the biggest telescopes,” said Berkeley Lab’s David Schlegel, who is DESI project scientist. “It’s that the instruments are better and very highly multiplexed, meaning that we can capture the light from many different objects at once.”

In fact, the telescope “is literally pointing at 5,000 different galaxies simultaneously,” Schlegel said. On any given night, he explains, as the telescope is moved into a target position, the optical fibers align to collect light from galaxies as it is reflected off the telescope mirror. From there, the light is fed into a bank of spectrographs and CCD cameras for further processing and study.

“It’s really a factory that we have – a spectra factory,” said survey validation lead, Christophe Yeche, also a cosmologist at CEA. “We can collect 5,000 spectra every 20 minutes. In a good night, we collect spectra from some 150,000 objects.”

“But it’s not just the instrument hardware that got us to this point – it’s also the instrument software, DESI’s central nervous system,” said Klaus Honscheid, a professor of physics at Ohio State University who directed the design of the DESI instrument control and monitoring systems. He credits scores of people in his group and around the world who have built and tested thousands of DESI’s component parts, most of which are unique to the instrument.

Spectra collected by DESI are the components of light corresponding to the colors of the rainbow. Their characteristics, including wavelength, reveal information such as the chemical composition of objects being observed as well as information about their relative distance and velocity.

As the universe expands, galaxies move away from each other, and their light is shifted to longer, redder wavelengths. The more distant the galaxy, the greater its redshift. By measuring galaxy redshifts, DESI researchers will create a 3D map of the universe. The detailed distribution of galaxies in the map is expected to yield new insights on the influence and nature of dark energy.

“Dark energy is one of the key science drivers for DESI,” said project co-spokesperson Kyle Dawson, a professor of physics and astronomy at University of Utah. “The goal is not so much to find out how much there is – we know that about 70% of the energy in the universe today is dark energy – but to study its properties.”

The universe is expanding at a rate determined by its total energy contents, Dawson explains. As the DESI instrument looks out in space and time, he said, “we can literally take snapshots today, yesterday, 1 billion years ago, 2 billion years ago – as far back in time as possible. We can then figure out the energy content in these snapshots and see how it is evolving.”

Jim Siegrist, associate director for High Energy Physics at DOE, said, “We are excited to see the start of DESI, the first next-generation dark energy project to begin its science survey. Along with its primary mission of dark energy studies, the data set will be of use by the wider scientific community for a multitude of astrophysics studies.

DESI is supported by the DOE Office of Science and by the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. Additional support for DESI is provided by the U.S. National Science Foundation, the Science and Technologies Facilities Council of the United Kingdom, the Gordon and Betty Moore Foundation, the Heising-Simons Foundation, the French Alternative Energies and Atomic Energy Commission (CEA), the National Council of Science and Technology of Mexico, the Ministry of Economy of Spain, and by the DESI member institutions.

The DESI collaboration is honored to be permitted to conduct astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.

Editor’s note: This story is based on a press release from Lawrence Berkeley National Laboratory.

For questions or comments, contact the SLAC Office of Communications at [email protected]

SLAC is a vibrant multiprogram laboratory that explores how the universe works at the biggest, smallest and fastest scales and invents powerful tools used by scientists around the globe. With research spanning particle physics, astrophysics and cosmology, materials, chemistry, bio- and energy sciences and scientific computing, we help solve real-world problems and advance the interests of the nation.

## Dark Energy: Map Gives Clue About What It Is—but Deepens Dispute About the Cosmic Expansion Rate

Dark energy is one of the greatest mysteries in science today. We know very little about it, other than it is invisible, it fills the whole universe, and it pushes galaxies away from each other. This is making our cosmos expand at an accelerated rate. But what is it? One of the simplest explanations is that it is a “cosmological constant”—a result of the energy of empty space itself—an idea introduced by Albert Einstein.

Many physicists aren’t satisfied with this explanation, though. They want a more fundamental description of its nature. Is it some new type of energy field or exotic fluid? Or is it a sign that Einstein’s equations of gravity are somehow incomplete? What’s more, we don’t really understand the universe’s current rate of expansion.

Now our project, the extended Baryon Oscillation Spectroscopic Survey (eBOSS), has come up with some answers. Our work has been released as a series of 23 publications, some of which are still being peer reviewed, describing the largest three-dimensional cosmological map ever created.

Currently, the only way we can feel the presence of dark energy is with observations of the distant universe. The farther galaxies are, the younger they appear to us. That’s because the light they emit took millions or even billions of years to reach our telescopes. Thanks to this sort of time-machine, we can measure different distances in space at different cosmic times, helping us work out how quickly the universe is expanding.

Using the Sloan Digital Sky Survey telescope, we measured more than two million galaxies and quasars—extremely bright and distant objects that are powered by black holes—over the last two decades. This new map covers around 11 billion years of cosmic history that was essentially unexplored, teaching us about dark energy like never before.

SDSS telescope. Image credit: Sloan Digital Sky Survey/wikipedia, CC BY-SA

Our results show that about 69 percent of our universe’s energy is dark energy. They also demonstrate, once again, that Einstein’s simplest form of dark energy—the cosmological constant—agrees the most with our observations.

When combining the information from our map with other cosmological probes, such as the cosmic microwave background—the light left over from the big bang—they all seem to prefer the cosmological constant over more exotic explanations of dark energy.

### Cosmic Expansion in Dispute

The results also provide a better insight into some recent controversies about the expansion rate of the universe today and about the geometry of space.

Combining our observations with studies of the universe in its infancy reveals cracks in our description of its evolution. In particular, our measurement of the current rate of expansion of the universe is about 10 percent lower than the value found using direct methods of measuring distances to nearby galaxies. Both these methods claim their result is correct and very precise, so their difference cannot simply be a statistical fluke.

The precision of eBOSS enhances this crisis. There is no broadly accepted explanation for this discrepancy. It may be that someone made a subtle mistake in one of these studies. Or it may be a sign that we need new physics. One exciting possibility is that a previously unknown form of matter from the early universe might have left a trace on our history. This is known as “early dark energy,” thought to be present when the universe was young, which could have modified the cosmic expansion rate.

Recent studies of the cosmic microwave background suggested that the geometry of space may be curved instead of being simply flat, which is consistent with the most accepted theory of the big bang. But our study concluded that space is indeed flat.

Even after these important advances, cosmologists over the world will remain puzzled by the apparent simplicity of dark energy, the flatness of space and the controversial values of the expansion rate today. There is only one way forward in the quest for answers—making larger and more detailed maps of the universe. Several projects are aiming to measure at least ten times more galaxies than we did.

If the maps from eBOSS were the first to explore a previously missing gap of 11 billion years of our history, the new generation of telescopes will make a high-resolution version of the same period of time. It is exciting to think about the fact that future surveys may be able to resolve the remaining mysteries about the universe’s expansion in the next decade or so. But it would be equally exciting if they revealed more surprises.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

## Dark energy is driving universe apart: NASA's Galaxy Evolution Explorer finds dark energy repulsive

A five-year survey of 200,000 galaxies, stretching back seven billion years in cosmic time, has led to one of the best independent confirmations that dark energy is driving our universe apart at accelerating speeds.

The survey used data from NASA's space-based Galaxy Evolution Explorer and the Anglo-Australian Telescope on Siding Spring Mountain in Australia.

The findings offer new support for the favored theory of how dark energy works -- as a constant force, uniformly affecting the universe and propelling its runaway expansion. They contradict an alternate theory, where gravity, not dark energy, is the force pushing space apart. According to this alternate theory, with which the new survey results are not consistent, Albert Einstein's concept of gravity is wrong, and gravity becomes repulsive instead of attractive when acting at great distances.

"The action of dark energy is as if you threw a ball up in the air, and it kept speeding upward into the sky faster and faster," said Chris Blake of the Swinburne University of Technology in Melbourne, Australia. Blake is lead author of two papers describing the results that appeared in recent issues of the Monthly Notices of the Royal Astronomical Society. "The results tell us that dark energy is a cosmological constant, as Einstein proposed. If gravity were the culprit, then we wouldn't be seeing these constant effects of dark energy throughout time."

Dark energy is thought to dominate our universe, making up about 74 percent of it. Dark matter, a slightly less mysterious substance, accounts for 22 percent. So-called normal matter, anything with atoms, or the stuff that makes up living creatures, planets and stars, is only approximately four percent of the cosmos.

The idea of dark energy was proposed during the previous decade, based on studies of distant exploding stars called supernovae. Supernovae emit constant, measurable light, making them so-called "standard candles," which allows calculation of their distance from Earth. Observations revealed dark energy was flinging the objects out at accelerating speeds.

The new survey provides two separate methods for independently checking these results. This is the first time astronomers performed these checks across the whole cosmic timespan dominated by dark energy. Astronomers began by assembling the largest three-dimensional map of galaxies in the distant universe, spotted by the Galaxy Evolution Explorer.

"The Galaxy Evolution Explorer helped identify bright, young galaxies, which are ideal for this type of study," said Christopher Martin, principal investigator for the mission at the California Institute of Technology in Pasadena. "It provided the scaffolding for this enormous 3-D map."

The team acquired detailed information about the light for each galaxy using the Anglo-Australian Telescope and studied the pattern of distance between them. Sound waves from the very early universe left imprints in the patterns of galaxies, causing pairs of galaxies to be separated by approximately 500 million light-years.

Blake and his colleagues used this "standard ruler" to determine the distance from the galaxy pairs to Earth. As with the supernovae studies, this distance data was combined with information about the speeds the pairs are moving away from us, revealing, yet again, the fabric of space is stretching apart faster and faster.

The team also used the galaxy map to study how clusters of galaxies grow over time like cities, eventually containing many thousands of galaxies. The clusters attract new galaxies through gravity, but dark energy tugs the clusters apart. It slows down the process, allowing scientists to measure dark energy's repulsive force.

"Observations by astronomers over the last 15 years have produced one of the most startling discoveries in physical science the expansion of the universe, triggered by the big bang, is speeding up," said Jon Morse, astrophysics division director at NASA Headquarters in Washington. "Using entirely independent methods, data from the Galaxy Evolution Explorer have helped increase our confidence in the existence of dark energy."

## What Can Swiss Cheese Teach Us About Dark Energy?

About 10 years ago, scientists reached the astonishing conclusion that our universe is accelerating apart at ever-increasing speeds, stretching space and time itself like melted cheese. The force that's pushing the universe apart is still a mystery, which is precisely why it was dubbed "dark energy."

But is dark energy really real? Is our universe really accelerating? These questions hang around in the mind of Ali Vanderveld, a post-doctoral cosmologist at JPL. Vanderveld and her colleagues recently published a paper in the journal Physical Review looking at how giant holes in our "Swiss-cheese-like" universe might make space look as if it's accelerating when it's really not. They concluded these holes, or voids, are not sufficient to explain away dark energy nevertheless, Vanderveld says it's important to continue to question fundamental traits of the very space we live in.

"Sometimes we take dark energy for granted," said Vanderveld. "But there are other theories that could explain why the universe appears to be moving apart at faster and faster speeds."

Why do scientists think the universe is accelerating? A large part of the evidence comes from observations taken over the last decade or so of very distant, colossal star explosions called supernovae. JPL's Wide-Field and Planetary Camera 2 on NASA's Hubble Space Telescope contributed to this groundbreaking research. Astronomers had already figured out that space, since its inception about 13.7 billion years ago in a tremendous "Big Bang" explosion, is expanding. But they didn't know if this expansion was happening at a constant rate, and even speculated that it could be slowing down. By examining distant supernovae billions of light-years away, scientists could get a look at how the expansion of space behaves over time.

The results were baffling. The more distant supernovae were dimmer than predicted, which would suggest they are farther away than previously believed. If they are farther away, then this means the space between us and the supernovae is expanding at ever-increasing speeds. Additional research has since pointed to an accelerating universe.

A group of researchers from Fermi National Accelerator Laboratory in Batavia, Ill., recently invoked what's called the Swiss-cheese model of the universe to explain why these supernovae might appear to be moving faster away from us than they really are. The universe is made up of lumps of matter interspersed with giant holes, or voids, somewhat like Swiss cheese. In fact, last year, astronomers at the University of Minnesota, Twin Cities, reported finding the king of all known voids, spanning one billion light-years. In other words, it would take light -- which holds the title for fastest stuff in the universe -- one billion years to go from one side of the void to the other!

The researchers at Fermi said these voids might lie between us and the supernovae being observed, acting like concave lenses to make the objects appear dimmer and farther than they really are. If so, then the supernova might not be accelerating away from us after all. Their theory claimed to provide a way in which dark energy might go poof.

Vanderveld and her colleagues at Cornell University, Ithaca, N.Y., looked more closely at this theory and found a few "holes." The group at Fermi had assumed a bunch of voids would line up between us and the supernovae, but Vanderveld's group said, in reality, the voids would be distributed more randomly -- again like Swiss cheese. With this random distribution, the voids are not enough to explain away dark energy.

"The lumpiness of the universe could still be tricking us into thinking it's accelerating," said Vanderveld. "But we did not find this to be the case with our best, current models of the universe."

There is, however, one other freakish possibility that could mean a void is creating the illusion of an accelerating universe. If our solar system just happened to sit in the middle of a void, then that void would distort our observations. Said Vanderveld, "It's really hard to tell if we're in a void, but for the most part this possibility has been ruled out."

## Putting theory to the test

Excitingly, Maeder’s theory is testable. For example, we can observationally determine the rotation speed of galaxies and compare the data to the predictions made by his model of empty space. We can even examine the motion of galaxies inside clusters of galaxies to test if there is agreement with the model proposed.

Scientists only proposed dark matter to explain how galaxies and galaxy clusters move due to a gravitational pull. But what if space itself could make them move in this way? Thus far, the tests that Maeder describes are in agreement with the observations made.

However, there are many more tests that need to be run, Maeder has only investigated two galaxy clusters. And let’s not forget the huge body of work suggesting that dark matter and dark energy do exist. Yet it is tantalising that, if the hypotheses that Maeder has put forward are correct, then it points to a large revision of our ideas about cosmology.

While we are not there yet, ultimately, the pie chart of mass and energy density of the entire universe may need to be revisited to scrub out the two biggest parts! It’s an exciting time to be a cosmologist.