zero energy universe hypothesis

What's the Total Energy In the Universe?

Considering the amount of energy packed in the nucleus of a single uranium atom, or the energy that has been continuously radiating from the sun for billions of years, or the fact that there are 10^80 particles in the observable universe, it seems that the total energy in the universe must be an inconceivably vast quantity. But it's not; it's probably zero.

Light, matter and antimatter are what physicists call "positive energy." And yes, there's a lot of it (though no one is sure quite how much). Most physicists think, however, that there is an equal amount of "negative energy" stored in the gravitational attraction that exists between all the positive-energy particles. The positive exactly balances the negative, so, ultimately, there is no energy in the universe at all.

Negative energy?

Stephen Hawking explains the concept of negative energy in his book The Theory of Everything (New Millennium 2002): "Two pieces of matter that are close to each other have less [positive] energy than the same two pieces a long way apart, because you have to expend energy to separate them against the gravitational force that is pulling them together," he wrote.

Since it takes positive energy to separate the two pieces of matter, gravity must be using negative energy to pull them together. Thus, "the gravitational field has negative energy. In the case of a universe that is approximately uniform in space, one can show that this negative gravitational energy exactly cancels the positive energy represented by the matter. So the total energy of the universe is zero."

Astrophysicists Alexei Filippenko at the University of California, Berkeley and Jay Pasachoff at Williams College explain gravity's negative energy by way of example in their essay, "A Universe From Nothing": "If you drop a ball from rest (defined to be a state of zero energy), it gains energy of motion (kinetic energy) as it falls. But this gain is exactly balanced by a larger negative gravitational energy as it comes closer to Earth’s center, so the sum of the two energies remains zero."

In other words, the ball's positive energy increases, but at the same time, negative energy is added to the Earth's gravitational field. What was a zero-energy ball at rest in space later becomes a zero-energy ball that is falling through space.

Sign up for the Live Science daily newsletter now

Get the world’s most fascinating discoveries delivered straight to your inbox.

The universe as a whole can be compared to this ball. Initially, before the big bang, the universe-ball was at rest. Now, after the big bang, it is falling: light and matter exist, and they are moving. And yet, because of the negative energy built into the gravity field created by these particles, the total energy of the universe remains zero.

Ultimate free lunch

The question, then, is why the ball started falling in the first place. How did something – composed of equal positive and negative parts, mind you – come from nothing?

Physicists aren't exactly sure, but their best guess is that the extreme positive and negative quantities of energy randomly fluctuated into existence. "Quantum theory, and specifically Heisenberg’s uncertainty principle, provide a natural explanation for how that energy may have come out of nothing," wrote Filippenko and Pasachoff.

They continued, "Throughout the universe, particles and antiparticles spontaneously form and quickly annihilate each other without violating the law of energy conservation. These spontaneous births and deaths of so-called 'virtual particle' pairs are known as 'quantum fluctuations.' Indeed, laboratory experiments have proven that quantum fluctuations occur everywhere, all the time."

Cosmologists have constructed a theory called inflation that accounts for the way in which a small volume of space occupied by a virtual particle pair could have ballooned to become the vast universe we see today. Alan Guth, one of the main brains behind inflationary cosmology, thus described the universe as "the ultimate free lunch."

In a lecture, Caltech cosmologist Sean Carroll put it this way: "You can create a compact, self-contained universe without needing any energy at all."

Follow Natalie Wolchover on Twitter @ nattyover . Follow Life's Little Mysteries on Twitter @ llmysteries , then join us on  Facebook .

Natalie Wolchover was a staff writer for Live Science from 2010 to 2012 and is currently a senior physics writer and editor for Quanta Magazine. She holds a bachelor's degree in physics from Tufts University and has studied physics at the University of California, Berkeley. Along with the staff of Quanta, Wolchover won the 2022 Pulitzer Prize for explanatory writing for her work on the building of the James Webb Space Telescope. Her work has also appeared in the The Best American Science and Nature Writing and The Best Writing on Mathematics, Nature, The New Yorker and Popular Science. She was the 2016 winner of the  Evert Clark/Seth Payne Award, an annual prize for young science journalists, as well as the winner of the 2017 Science Communication Award for the American Institute of Physics. 

One of the universe's biggest paradoxes could be even weirder than we thought, James Webb telescope study reveals

Early galaxies weren't mystifyingly massive after all, James Webb Space Telescope finds

Pollution harms men's fertility, but traffic noise affects women's

Most Popular

• 2 Asteroid 10 times bigger than the dinosaur-killing space rock smashed Jupiter's largest moon off its axis
• 3 Stone Age burial ground in France used for 800 years is nearly all male — and ancient DNA reveals they're largely related
• 4 The oldest evidence of Earth's atmosphere may be hiding in rocks on the moon
• 5 Sexually frustrated dolphin behind spate of attacks on humans off Japan

More From Forbes

Ask ethan: can we really get a universe from nothing.

• Share to Facebook
• Share to Twitter
• Share to Linkedin

Our entire cosmic history is theoretically well-understood in terms of the frameworks and rules that ... [+] govern it. It's only by observationally confirming and revealing various stages in our Universe's past that must have occurred, like when the first stars and galaxies formed, and how the Universe expanded over time, that we can truly come to understand what makes up our Universe and how it expands and gravitates in a quantitative fashion. The relic signatures imprinted on our Universe from an inflationary state before the hot Big Bang give us a unique way to test our cosmic history, subject to the same fundamental limitations that all frameworks possess.

The biggest question that we're even capable of asking, with our present knowledge and understanding of the Universe, is where did everything we can observe come from? If it came from some sort of pre-existing state, we'll want to know exactly what that state was like and how our Universe came from it. If it emerged out of nothingness, we'd want to know how we went from nothing to the entire Universe, and what  if anything  caused it. At least, that's what our Patreon supporter Charles Buchanan wants to know, asking:

One concept bothers me. Perhaps you can help. I see it in used many places, but never really explained. "A universe from Nothing" and the concept of negative gravity. As I learned my Newtonian physics, you could put the zero point of the gravitational potential anywhere, only differences mattered. However Newtonian physics never deals with situations where matter is created... Can you help solidify this for me, preferably on [a] conceptual level, maybe with a little calculation detail?

Gravitation might seem like a straightforward force, but an incredible number of aspects are anything but intuitive. Let's take a deeper look.

Countless scientific tests of Einstein's general theory of relativity have been performed, ... [+] subjecting the idea to some of the most stringent constraints ever obtained by humanity. Einstein's first solution was for the weak-field limit around a single mass, like the Sun; he applied these results to our Solar System with dramatic success. We can view this orbit as Earth (or any planet) being in free-fall around the Sun, traveling in a straight-line path in its own frame of reference. All masses and all sources of energy contribute to the curvature of spacetime.

If you have two point masses located some distance apart in your Universe, they'll experience an attractive force that compels them to gravitate towards one another. But this attractive force that you perceive, in the context of relativity, comes with two caveats.

The first caveat is simple and straightforward: these two masses will experience an acceleration towards one another, but whether they wind up moving closer to one another or not is entirely dependent on how the space between them evolves. Unlike in Newtonian gravity, where space is a fixed quantity and only the masses within that space can evolve, everything is changeable in General Relativity. Not only does matter and energy move and accelerate due to gravitation, but the very fabric of space itself can expand, contract, or otherwise flow. All masses still move through space, but space itself is no longer stationary.

The 'raisin bread' model of the expanding Universe, where relative distances increase as the space ... [+] (dough) expands. The farther away any two raisin are from one another, the greater the observed redshift will be by time the light is received. The redshift-distance relation predicted by the expanding Universe is borne out in observations, and has been consistent with what's been known going all the way back to the 1920s.

The second caveat is that the two masses you're considering, even if you're extremely careful about accounting for what's in your Universe, are most likely not the only forms of energy around. There are bound to be other masses in the form of normal matter, dark matter, and neutrinos. There's the presence of radiation, from both electromagnetic and gravitational waves. There's even dark energy: a type of energy inherent to the fabric of space itself.

Now, here's a scenario that might exemplify where your intuition leads you astray: what happens if these masses, for the volume they occupy, have less total energy than the average energy density of the surrounding space?

The gravitational attraction (blue) of overdense regions and the relative repulsion (red) of the ... [+] underdense regions, as they act on the Milky Way. Even though gravity is always attractive, there is an average amount of attraction throughout the Universe, and regions with lower energy densities than that will experience (and cause) an effective repulsion with respect to the average.

You can imagine three different scenarios:

• The first mass has a below-average energy density while the second has an above-average value.
• The first mass has an above-average energy density while the second has a below-average value.
• Both the first and second masses have a below-average energy density compared to the rest of space.

In the first two scenarios, the above-average mass will begin growing as it pulls on the matter/energy all around it, while the below-average mass will start shrinking, as it's less able to hold onto its own mass in the face of its surroundings. These two masses will effectively repel one another; even though gravitation is always attractive, the intervening matter is preferentially attracted to the heavier-than-average mass. This causes the lower-mass object to act like it's both repelling and being repelled by the heavier-mass object, the same way a balloon held underwater will still be attracted to Earth's center, but will be forced away from it owing to the (buoyant) effects of the water.

The Earth's crust is thinnest over the ocean and thickest over mountains and plateaus, as the ... [+] principle of buoyancy dictates and as gravitational experiments confirm. Just as a balloon submerged in water will accelerate away from the center of the Earth, a region with below-average energy density will accelerate away from an overdense region, as average-density regions will be more preferentially attracted to the overdense region than the underdense region will.

So what's going to happen if you have two regions of space with below-average densities, surrounded by regions of just average density? They'll both shrink, giving up their remaining matter to the denser regions around them. But as far as motions go, they'll accelerate towards one another, with exactly the same magnitude they'd accelerate at if they were both overdense regions that exceeded the average density by equivalent amounts.

You might be wondering why it's important to think about these concerns when talking about a Universe from nothing. After all, if your Universe is full of matter and energy, it's pretty hard to understand how that's relevant to making sense of the concept of something coming from nothing. But just as our intuition can lead us astray when thinking about matter and energy on the spacetime playing field of General Relativity, it's a comparable situation when we think about nothingness.

A representation of flat, empty space with no matter, energy or curvature of any type. With the ... [+] exception of small quantum fluctuations, space in an inflationary Universe becomes incredibly flat like this, except in a 3D grid rather than a 2D sheet. Space is stretched flat, and particles are rapidly driven away.

You very likely think about nothingness as a philosopher would: the complete absence of everything. Zero matter, zero energy, an absolutely zero value for all the quantum fields in the Universe, etc. You think of space that's completely flat, with nothing around to cause its curvature anywhere.

If you think this way, you're not alone: there are many different ways to conceive of "nothing." You might even be tempted to take away space, time, and the laws of physics themselves, too. The problem, if you start doing that, is that you lose your ability to predict anything at all. The type of nothingness you're thinking about, in this context, is what we call unphysical.

If we want to think about nothing in a physical sense, you have to keep certain things. You need spacetime and the laws of physics, for example; you cannot have a Universe without them.

A visualization of QCD illustrates how particle/antiparticle pairs pop out of the quantum vacuum for ... [+] very small amounts of time as a consequence of Heisenberg uncertainty. The quantum vacuum is interesting because it demands that empty space itself isn't so empty, but is filled with all the particles, antiparticles and fields in various states that are demanded by the quantum field theory that describes our Universe. Put this all together, and you find that empty space has a zero-point energy that's actually greater than zero.

But here's the kicker: if you have spacetime and the laws of physics, then by definition you have quantum fields permeating the Universe everywhere you go. You have a fundamental "jitter" to the energy inherent to space, due to the quantum nature of the Universe. (And the Heisenberg uncertainty principle, which is unavoidable.)

Put these ingredients together — because you can't have a physically sensible "nothing" without them — and you'll find that space itself doesn't have zero energy inherent to it, but energy with a finite, non-zero value. Just as there's a finite zero-point energy (that's greater than zero) for an electron bound to an atom, the same is true for space itself. Empty space, even with zero curvature, even devoid of particles and external fields, still has a finite energy density to it.

The four possible fates of the Universe with only matter, radiation, curvature and a cosmological ... [+] constant allowed. The top three possibilities are for a Universe whose fate is determined by the balance of matter/radiation with spatial curvature alone; the bottom one includes dark energy. Only the bottom "fate" aligns with the evidence.

From the perspective of quantum field theory, this is conceptualized as the zero-point energy of the quantum vacuum: the lowest-energy state of empty space. In the framework of General Relativity, however, it appears in a different sense: as the value of a cosmological constant, which itself is the energy of empty space, independent of curvature or any other form of energy density.

Although we do not know how to calculate the value of this energy density from first principles, we can calculate the effects it has on the expanding Universe. As your Universe expands, every form of energy that exists within it contributes to not only how your Universe expands, but how that expansion rate changes over time. From multiple independent lines of evidence — including the Universe's large-scale structure, the cosmic microwave background, and distant supernovae — we have been able to determine how much energy is inherent to space itself.

Constraints on dark energy 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’d need dark energy for certain, and also that there are uncertainties and degeneracies between the amount of dark matter and dark energy that we'd need to accurately describe our Universe.

This form of energy is what we presently call dark energy, and it's responsible for the observed accelerated expansion of the Universe. Although it's been a part of our conceptions of reality for more than two decades now, we don't fully understand its true nature. All we can say is that when we measure the expansion rate of the Universe, our observations are consistent with dark energy being a cosmological constant with a specific magnitude, and not with any of the alternatives that evolve significantly over cosmic time.

Because dark energy causes distant galaxies to appear to recede from one another more and more quickly as time goes on — since the space between those galaxies is expanding — it's often called negative gravity. This is not only highly informal, but incorrect. Gravity is only positive, never negative. But even positive gravity, as we saw earlier, can have effects that look very much like negative repulsion.

How energy density changes over time in a Universe dominated by matter (top), radiation (middle), ... [+] and a cosmological constant (bottom). Note that dark energy doesn't change in density as the Universe expands, which is why it comes to dominate the Universe at late times.

If there were greater amounts of dark energy present within our spatially flat Universe, the expansion rate would be greater. But this is true for all forms of energy in a spatially flat Universe: dark energy is no exception. The only different between dark energy and the more commonly encountered forms of energy, like matter and radiation, is that as the Universe expands, the densities of matter and radiation decrease.

But because dark energy is a property of space itself, when the Universe expands, the dark energy density must remain constant. As time goes on, galaxies that are gravitationally bound will merge together into groups and clusters, while the unbound groups and clusters will accelerate away from one another. That's the ultimate fate of the Universe if dark energy is real.

The Laniakea supercluster, containing the Milky Way (red dot), on the outskirts of the Virgo Cluster ... [+] (large white collection near the Milky Way). Despite the deceptive looks of the image, this isn't a real structure, as dark energy will drive most of these clumps apart, fragmenting them as time goes on. Only the individually bound structures will remain together; everything else will accelerate away from whatever is unbound to it from its perspective.

So why do we say we have a Universe that came from nothing? Because the value of dark energy may have been much higher in the distant past: before the hot Big Bang . A Universe with a very large amount of dark energy in it will behave identically to a Universe undergoing cosmic inflation. In order for inflation to end, that energy has to get converted into matter and radiation. The evidence strongly points to that happening some 13.8 billion years ago.

When it did, though, a small amount of dark energy remained behind. Why? Because the zero-point energy of the quantum fields in our Universe isn't zero, but a finite, greater-than-zero value. Our intuition may not be reliable when we consider the physical concepts of nothing and negative/positive gravity, but that's why we have science. When we do it right, we wind up with physical theories that accurately describe the Universe we measure and observe.

• Editorial Standards
• Reprints & Permissions

• Classical Physics
• Quantum Physics Quantum Interpretations
• Special and General Relativity
• Atomic and Condensed Matter
• Nuclear and Particle Physics
• Beyond the Standard Model
• Astronomy and Astrophysics
• Other Physics Topics

Follow along with the video below to see how to install our site as a web app on your home screen.

Note: This feature may not be available in some browsers.

• Astronomy and Cosmology

Zero-Energy Universe Hypothesis: Questions & Answers

• Thread starter DiracPool
• Start date Aug 25, 2014
• Tags Energy Universe Zero
• Aug 25, 2014

A PF Organism

The zero-energy universe hypothesis states that the total amount of energy in the universe is exactly zero: its amount of positive energy in the form of matter is exactly canceled out by its negative energy in the form of gravity

• Massive merger: Study reveals evidence for origin of supermassive black hole at galaxy's center
• Solution to a cosmic mystery—the eccentric orbits of trans-Neptunian objects
• AI shines a new light on exoplanets

A PF Singularity

DiracPool said: What is the opinion of people on this site of the validity of this idea in general?

DiracPool said: What about electrical potential energy though, doesn't this also factor in?

DiracPool said: I read somewhere that observationally, the 3-d metric of space is found to be very close to flat

DiracPool said: which supports the zero-energy model

DiracPool said: the 4-d metric is supposed to be curved. Is this true?

DiracPool said: Does this refer to perhaps a negative curvature of Minkowski spacetime?

DiracPool said: How does that play into the zero-energy universe model

DiracPool said: how is dark energy supposed to fit in?

• Aug 26, 2014

PeterDonis said: Dark energy is basically treated as a kind of "matter" (in the generalized sense I gave above), which happens to have some peculiar properties not shared by other kinds of "matter". This is true in basically any cosmological model, whether it claims to be a "zero energy universe" one or not.

DiracPool said: does that mean that there is postulated corresponding negative potential gravity that exists to counteract the dark energy as well as "traditional" forms of positive matter-energy?

• Sep 2, 2014

I find the zero-energy universe hypothesis to be a fascinating concept. It is a theory that challenges our understanding of energy and the universe as a whole. While there is still much research and debate surrounding this idea, I can offer some insights to your questions based on current scientific knowledge. 1) The validity of the zero-energy universe hypothesis is still a topic of debate among scientists. There are some who support this idea and see evidence for it in observations, while others argue that it is still too early to make such a claim. More research and observations are needed to fully understand the energy balance of the universe. 2) In the zero-energy universe model, it is believed that all forms of energy, including electrical potential energy, are canceled out by the negative energy of gravity. This is based on the principle of conservation of energy, which states that energy cannot be created or destroyed, only transformed. Therefore, any excess energy in one form must be balanced out by an equal amount of negative energy in another form. However, there is ongoing research to better understand the role of electrical potential energy in the overall energy balance of the universe. 3) The 3-D metric of space refers to the spatial dimensions of length, width, and height, while the 4-D metric includes the dimension of time. The observed flatness of the 3-D metric supports the zero-energy universe model, as it suggests that the total energy of the universe is indeed balanced. However, the curvature of the 4-D metric does not necessarily contradict this idea, as it is believed to be caused by the presence of matter and energy. As for dark energy, it is still a mystery and its role in the zero-energy universe model is still being studied. In conclusion, the zero-energy universe hypothesis is a complex and intriguing concept that requires further research and observations to fully understand. It challenges our current understanding of energy and the universe, and scientists are actively working to unravel its mysteries.  

FAQ: Zero-Energy Universe Hypothesis: Questions & Answers

1. what is the zero-energy universe hypothesis.

The Zero-Energy Universe Hypothesis proposes that the total energy of the universe is exactly zero. This means that the positive energy of matter is balanced out by the negative energy of gravitational attraction and expansion.

2. How does this hypothesis explain the expansion of the universe?

The hypothesis suggests that the universe is expanding due to the repulsive force of dark energy, which counteracts the attractive force of gravity. This expansion is believed to be accelerating, leading to the eventual heat death of the universe.

3. Is there any evidence to support the Zero-Energy Universe Hypothesis?

While there is no direct evidence, the hypothesis is consistent with current observations of the universe, such as the flatness of the universe and the observed acceleration of expansion. However, more research and evidence is needed to fully support this hypothesis.

4. How does this hypothesis relate to the Big Bang Theory?

The Zero-Energy Universe Hypothesis is a modification of the Big Bang Theory. It suggests that the universe began with a singularity and has been expanding ever since, but the total energy remains at zero. This differs from the traditional Big Bang Theory, which assumes a non-zero total energy of the universe.

5. What implications does this hypothesis have for the future of the universe?

If the Zero-Energy Universe Hypothesis is correct, it means that the universe will continue to expand and eventually reach a state of maximum entropy, known as the heat death of the universe. This would result in a universe with no energy or activity, effectively leading to the end of all life and processes as we know them.

Similar threads

• Jan 11, 2022
• Jul 7, 2018
• Jun 15, 2022
• Jan 5, 2024
• Dec 15, 2023
• Aug 8, 2015
• Oct 10, 2018
• Jan 17, 2022
• Jul 21, 2020
• Aug 26, 2022

Hot Threads

• A   Hubble tension -- any resolution?
• I   Questions about the accelerating Hubble expansion
• B   Dark energy might not be constant after all
• I   Is there an equivalent "redshift" for cosmic rays due to expansion?
• I   Has JWST solved the crisis in cosmology?

Recent Insights

• Insights   Brownian Motions and Quantifying Randomness in Physical Systems
• Insights   PBS Video Comment: “What If Physics IS NOT Describing Reality”
• Insights   Aspects Behind the Concept of Dimension in Various Fields
• Insights   Views On Complex Numbers
• Insights   Addition of Velocities (Velocity Composition) in Special Relativity
• Insights   Schrödinger’s Cat and the Qbit

May 24, 2023

The Universe Began with a Bang, Not a Bounce, New Studies Find

New research pokes holes in the idea that the cosmos expanded and then contracted before beginning again

By James Riordon

Science History Images/Alamy Stock Photo

How did the universe start? Did we begin with a big bang, or was there a bounce? Might the cosmos evolve in a cycle of expansion and collapse , over and over for all eternity? Now, in two papers, researchers have poked holes in different models of a so-called bouncing universe , suggesting the universe we see around us is probably a one-and-done proposition.

Bouncing universe proponents argue that our cosmos didn’t emerge on its own out of nothing. Instead, advocates claim, a prior universe shrunk in on itself and then regrew into the one we live in. This may have happened once or, according to some theories, an infinite number of times .

So which scenario is correct? The most widely accepted explanation for the history of the universe has it beginning with a big bang, followed by a period of rapid expansion known as cosmic inflation. According to that model, the glow left over from when the universe was hot and young, called the cosmic microwave background (CMB), should look pretty much the same no matter which direction you face. But data from the Planck space observatory, which mapped the CMB from 2009 to 2013, showed unexpected variations in the microwave radiation. They could be meaningless statistical fluctuations in the temperature of the universe, or they might be signs of something interesting going on.

On supporting science journalism

If you're enjoying this article, consider supporting our award-winning journalism by subscribing . By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.

One possibility is that the CMB anomalies imply that the universe didn’t emerge out of nothing. Instead it came about after a prior universe collapsed and bounced back to create the space and time we live in today.

Bouncing universe models can explain these CMB patterns as well as account for lingering quibbles about the standard description of the universe’s origin and evolution. In particular, the big bang model of the universe begins with a singularity—a point that appeared out of nothing and contained the precursors of everything in the universe in a region so small that it had essentially no size at all. The idea is that the universe grew from the singularity and, after inflation, settled into the more gradually expanding universe we see today. But singularities are problematic because physics, and math itself, doesn’t make sense when everything is packed into a point that’s infinitely small. Many physicists prefer to avoid singularities.

One bouncing model that averts singularities and makes the CMB anomalies a little less anomalous is known as loop quantum cosmology (LQC). It relies on a bridge between classical physics and quantum mechanics known as loop quantum gravity, which posits that the force of gravity peters out at very small distances rather than increasing to infinity. “Cosmological models inspired by loop quantum gravity can solve some problems,” says University of Geneva cosmologist Ruth Durrer, “especially the singularity problem.” Durrer co-authored one of the two new studies on bouncing universes. In it, she and her colleagues looked for astronomical signs of such models .

In an LQC model, a precursor to our universe might have contracted under the force of gravity until it became extremely compact. Eventually quantum mechanics would have taken over. Instead of collapsing to a singularity, the universe would have started to expand again and may even have gone through an inflationary phase, as many cosmologists believe ours did.

If that happened, says physicist Ivan Agullo of Louisiana State University, it should have left a mark on the universe. Agullo, who was not affiliated with either of the recent analyses, has proposed that the mark would turn up in a feature in the CMB data known as the “bispectrum,” a measure of how different portions of the universe would have interacted in a bouncing scenario. The bispectrum would not be apparent in an image of the CMB, but it would show up in analyses of the frequencies in the ancient CMB microwaves.

“If observed,” Agullo says, the bispectrum “would serve as a smoking gun for the existence of a bounce instead of a bang.” Agullo’s group previously calculated the bispectrum as it would have appeared shortly after a cosmic bounce. Durrer and her colleagues took the calculation further, but when they compared it with the present-day Planck CMB data, there was no significant sign of a bispectrum imprint.

Although lots of other bouncing cosmos models may still be viable, the failure to find a significant bispectrum means that models that rely on LQC to deal with the anomalies in the CMB can be ruled out. It’s a sad result for Agullo, who had high hopes of finding concrete evidence of a bouncing universe. He still considers many bouncing universe models viable, however. And Paola Delgado, a cosmology Ph.D. candidate at Jagiellonian University in Poland, who worked on the new analysis that was co-authored by Durrer, says there’s one potential upside. “I heard for a long time that [attempts to merge quantum physics and cosmology] cannot be tested,” Delgado says. “I think it was really nice to see that for some classes of models, you still have some contact with observations.”

Ruling out signs of an LQC-driven cosmic bounce in Planck data means the CMB anomalies remain unexplained. But an even larger cosmic issue lingers: Did the universe have a beginning at all? As far as advocates of the big bang are concerned, it did. But that leaves us with the inscrutable singularity that started everything off.

Alternatively, according to theories of so-called cyclic cosmologies, the universe is immortal and is going through endless bounces. Although a bouncing universe may experience one or more cycles, a truly cyclic universe has no beginning and no end. It consists of a series of bounces that go back for an infinite number of cycles and will continue for an infinite number more. And because such a universe doesn’t have a beginning, there’s no big bang and no singularity.

The study that Durrer and Delgado co-authored doesn’t rule out immortal cyclic cosmologies. Plenty of theories describe such a bouncing universe in ways that would be difficult or impossible to distinguish from the “big bang plus inflation” model by looking at Planck CMB data.

But a critical flaw lurks in the idea of an eternally cycling universe, according to physicist William Kinney of the University at Buffalo, who co-authored the second recent analysis. That flaw is entropy, which builds up as a universe bounces. Often thought of as the amount of disorder in a system, entropy is related to the system’s amount of useful energy: the higher the entropy, the less energy available. If the universe increases in entropy and disorder with each bounce, the amount of usable energy available decreases each time. In that case, the cosmos would have had larger amounts of useful energy in earlier epochs. If you extrapolate back far enough, that implies a big bang–like beginning with an infinitely small amount of entropy, even for a universe that subsequently goes through cyclic bounces. (If you’re wondering how this scenario doesn’t violate the law of conservation of energy, we’re talking about available energy. Although the total amount of energy in the cosmos remains static, the amount that can do useful work decreases with increasing entropy.)

New cyclic models get around the problem, Kinney says, by requiring that the universe expands by a lot with each cycle. The expansion allows the universe to smooth out, dissipating the entropy before collapsing again. Although this explanation solves the entropy problem, Kinney and his University at Buffalo co-author Nina Stein calculated in their recent paper that the solution itself ensures that the universe is not immortal . “I feel like we’ve demonstrated something fundamental about the universe,” Kinney says, “which is that it probably had a beginning.” That implies a big bang occurred at some point, even if that event happened many bouncing universes ago, which in turn suggests that it took a singularity to get everything going in the first place.

Kinney’s paper is the latest in the debate over cyclic universes, but proponents of a universe without beginning or end have yet to respond in the scientific literature. Two leading proponents of a cyclic universe, astrophysicists Paul Steinhardt of Princeton University and Anna Ijjas of New York University, declined to comment for this article. If the history of the debate is any indication , though, we may soon hear of a work-around to counter Kinney’s analysis.

Some physicists say the Planck data only rule out a bounce under a loop quantum cosmology model that can explain away the CMB anomalies through the bispectrum, not other LQC bounce models that address anomalies using different mechanisms. Cosmologist Nelson Pinto-Neto of the Brazilian Center for Physics Research, who has studied bouncing and other cyclic models, agrees that LQC bounces that account for the CMB anomalies are likely off the table now, but he’s more sanguine on the question of a cyclic universe. “Existence is a fact. We are all here and now. Nonexistence is an abstraction of the human mind,” Nelson says. “This is the reason I think that a [cyclic universe], which has always existed, is simpler than one that has been created. However, as a scientist, I must be open to both possibilities.”

Editor’s Note (6/29/23): This article was edited after posting to better clarify scientits’ views on loop quantum cosmology bouncing universe models that account for the cosmic microwave background. The text was previously amended on June 1 to better clarify Ivan Agullo’s position on bouncing universe models.

An editorially independent publication supported by the Simons Foundation.

Get the latest news delivered to your inbox.

Type search term(s) and press enter

• Comment Comments
• Save Article Read Later Read Later

Why the Tiny Weight of Empty Space Is Such a Huge Mystery

March 12, 2018

Lucy Reading-Ikkanda/Quanta Magazine

Introduction

The controversial idea that our universe is just a random bubble in an endless, frothing multiverse arises logically from nature’s most innocuous-seeming feature: empty space. Specifically, the seed of the multiverse hypothesis is the inexplicably tiny amount of energy infused in empty space — energy known as the vacuum energy, dark energy or the cosmological constant. Each cubic meter of empty space contains only enough of this energy to light a lightbulb for 11-trillionths of a second. “The bone in our throat,” as the Nobel laureate Steven Weinberg once put it , is that the vacuum ought to be at least a trillion trillion trillion trillion trillion times more energetic, because of all the matter and force fields coursing through it. Somehow the effects of all these fields on the vacuum almost equalize, producing placid stillness. Why is empty space so empty?

While we don’t know the answer to this question — the infamous “cosmological constant problem” — the extreme vacuity of our vacuum appears necessary for our existence. In a universe imbued with even slightly more of this gravitationally repulsive energy, space would expand too quickly for structures like galaxies, planets or people to form. This fine-tuned situation suggests that there might be a huge number of universes , all with different doses of vacuum energy, and that we happen to inhabit an extraordinarily low-energy universe because we couldn’t possibly find ourselves anywhere else.

Some scientists bristle at the tautology of “anthropic reasoning” and dislike the multiverse for being untestable. Even those open to the multiverse idea would love to have alternative solutions to the cosmological constant problem to explore. But so far it has proved nearly impossible to solve without a multiverse. “The problem of dark energy [is] so thorny, so difficult, that people have not got one or two solutions,” said Raman Sundrum, a theoretical physicist at the University of Maryland.

To understand why, consider what the vacuum energy actually is. Albert Einstein’s general theory of relativity says that matter and energy tell space-time how to curve, and space-time curvature tells matter and energy how to move. An automatic feature of the equations is that space-time can possess its own energy — the constant amount that remains when nothing else is there, which Einstein dubbed the cosmological constant. For decades, cosmologists assumed its value was exactly zero, given the universe’s reasonably steady rate of expansion, and they wondered why. But then, in 1998, astronomers discovered that the expansion of the cosmos is in fact gradually accelerating, implying the presence of a repulsive energy permeating space. Dubbed dark energy by the astronomers, it’s almost certainly equivalent to Einstein’s cosmological constant. Its presence causes the cosmos to expand ever more quickly, since, as it expands, new space forms, and the total amount of repulsive energy in the cosmos increases.

However, the inferred density of this vacuum energy contradicts what quantum field theory, the language of particle physics, has to say about empty space. A quantum field is empty when there are no particle excitations rippling through it. But because of the uncertainty principle in quantum physics, the state of a quantum field is never certain, so its energy can never be exactly zero. Think of a quantum field as consisting of little springs at each point in space. The springs are always wiggling, because they’re only ever within some uncertain range of their most relaxed length. They’re always a bit too compressed or stretched, and therefore always in motion, possessing energy. This is called the zero-point energy of the field. Force fields have positive zero-point energies while matter fields have negative ones, and these energies add to and subtract from the total energy of the vacuum.

The total vacuum energy should roughly equal the largest of these contributing factors. (Say you receive a gift of $10,000; even after spending $100, or finding $3 in the couch, you’ll still have about $10,000.) Yet the observed rate of cosmic expansion indicates that its value is between 60 and 120 orders of magnitude smaller than some of the zero-point energy contributions to it, as if all the different positive and negative terms have somehow canceled out. Coming up with a physical mechanism for this equalization is extremely difficult for two main reasons.

First, the vacuum energy’s only effect is gravitational, and so dialing it down would seem to require a gravitational mechanism. But in the universe’s first few moments, when such a mechanism might have operated, the universe was so physically small that its total vacuum energy was negligible compared to the amount of matter and radiation. The gravitational effect of the vacuum energy would have been completely dwarfed by the gravity of everything else. “This is one of the greatest difficulties in solving the cosmological constant problem,” the physicist Raphael Bousso wrote in 2007 . A gravitational feedback mechanism precisely adjusting the vacuum energy amid the conditions of the early universe, he said, “can be roughly compared to an airplane following a prescribed flight path to atomic precision, in a storm.”

Compounding the difficulty, quantum field theory calculations indicate that the vacuum energy would have shifted in value in response to phase changes in the cooling universe shortly after the Big Bang. This raises the question of whether the hypothetical mechanism that equalized the vacuum energy kicked in before or after these shifts took place. And how could the mechanism know how big their effects would be, to compensate for them?

So far, these obstacles have thwarted attempts to explain the tiny weight of empty space without resorting to a multiverse lottery. But recently, some researchers have been exploring one possible avenue: If the universe did not bang into existence, but bounced instead, following an earlier contraction phase, then the contracting universe in the distant past would have been huge and dominated by vacuum energy. Perhaps some gravitational mechanism could have acted on the plentiful vacuum energy then, diluting it in a natural way over time. This idea motivated the physicists Peter Graham, David Kaplan and Surjeet Rajendran to discover a new cosmic bounce model , though they’ve yet to show how the vacuum dilution in the contracting universe might have worked.

In an email, Bousso called their approach “a very worthy attempt” and “an informed and honest struggle with a significant problem.” But he added that huge gaps in the model remain, and “the technical obstacles to filling in these gaps and making it work are significant. The construction is already a Rube Goldberg machine, and it will at best get even more convoluted by the time these gaps are filled.” He and other multiverse adherents see their answer as simpler by comparison.

This article was reprinted on TheAtlantic.com .

Get highlights of the most important news delivered to your email inbox

Also in Abstractions blog

Scientists Find a Fast Way to Describe Quantum Systems

Mathematicians Marvel at ‘Crazy’ Cuts Through Four Dimensions

Insects and Other Animals Have Consciousness, Experts Declare

Comment on this article.

Quanta Magazine moderates comments to facilitate an informed, substantive, civil conversation. Abusive, profane, self-promotional, misleading, incoherent or off-topic comments will be rejected. Moderators are staffed during regular business hours (New York time) and can only accept comments written in English. 

Next article

Use your social network.

Forgot your password ?

We’ll email you instructions to reset your password

Enter your new password

Stack Exchange Network

Stack Exchange network consists of 183 Q&A communities including Stack Overflow , the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Q&A for work

Connect and share knowledge within a single location that is structured and easy to search.

Zero-energy universe - What is nothing?

I am a layman, so excuse me in advance for the stupidity of my questions, and I hope you can answer them in a way that I can understand.

I have read, here and there, that the Universe might have a total of zero energy and so that it might have appeared out of nothing.

First thing, I learned that we calculate the sum of the total energy of the Universe as zero because the scientific community reached a consensus (if that's really a consensus) that gravity was negative energy, all of this to satisfy the laws of conservation of energy.

Based on this, here are my questions:

1 - Can a quantum field, or anything really, have absolute zero energy in any local point?

2 - What is the "nothing" the Universe came from in the terminology of the advocates of the zero-energy universe theory?

3 - Why is it correct to attribute a negative value to the energy of gravity, would it not be possible to say that the laws of conservation of energy are wrong instead?

• quantum-field-theory
• energy-conservation

• $\begingroup$ Related: physics.stackexchange.com/q/2838/2451 , physics.stackexchange.com/q/39595/2451 and links therein. $\endgroup$ –  Qmechanic ♦ Commented Jan 16, 2019 at 16:57

It's well known that zero-point energy is not actually zero. The ground system of a quantised classical system has non-zero energy.

However it turns out with super-symmetry, we can actually obtain such a ground state. Unfortunately, so far the only evidence for super-symmetry has been theoretical, and not experimental...

Typically advocates of a universe that came into existence from nothing by a quantum fluctuation fall on this very point. It's a hugely speculative idea, and for that reason, should be taken with the degree of regard reserved for hugely speculative ideas, ie not very much.

Its actually quite difficult to come up with a generally covariant form of the conservation of energy; and more-over, it hasn't been universally held; for example, Fred Hoyles and Hermann Bondis Steady State Theory, which was the leading cosmological contender before the Big Bang held that there was a cosmological creation of energy. Caroll writes :

The point is pretty simple: back when you thought energy was conserved, there was a reason why you thought that, namely time-translation invariance... [however], in general relativity that’s simply no longer true. Einstein tells us that space and time are dynamical, and in particular that they can evolve with time. When the space through which particles move is changing, the total energy of those particles is not conserved...and one that has been experimentally verified! The success of Big Bang Nucleosynthesis depends on the fact that we understand how fast the universe was expanding in the first three minutes, which in turn depends on how fast the energy density is changing. And that energy density is almost all radiation, so the fact that energy is not conserved in an expanding universe is absolutely central to getting the predictions of primordial nucleosynthesis correct.

Your Answer

Sign up or log in, post as a guest.

Required, but never shown

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy .

Not the answer you're looking for? Browse other questions tagged quantum-field-theory gravity cosmology energy-conservation universe or ask your own question .

• Featured on Meta
• Announcing a change to the data-dump process
• Bringing clarity to status tag usage on meta sites

Hot Network Questions

• What was the first "Star Trek" style teleporter in SF?
• What was the typical amount of disk storage for a mainframe installation in the 1980s?
• Confusion about time dilation
• Why would autopilot be prohibited below 1000 AGL?
• When can the cat and mouse meet?
• "It never works" vs "It better work"
• Are all citizens of Saudi Arabia "considered Muslims by the state"?
• The question about the existence of an infinite non-trivial controversy
• Navola: Series or Standalone?
• How does this MOSFET-based Schmitt trigger work?
• Stained passport am I screwed
• how did the Apollo 11 know its precise gyroscopic position?
• How would you format to be cleaner and explain the steps better? Do I have any no no's in my latex?
• I'm not quite sure I understand this daily puzzle on Lichess (9/6/24)
• Did Babylon 4 actually do anything in the first shadow war?
• How can I play MechWarrior 2?
• Etymology of 制度
• do-release-upgrade from 22.04 LTS to 24.04 LTS still no update available
• Book about a wormhole found inside the Moon
• Is it a good idea to perform I2C Communication in the ISR?
• Is there a problem known to have no fastest algorithm, up to polynomials?
• Star Trek: The Next Generation episode that talks about life and death
• Does the average income in the US drop by $9,500 if you exclude the ten richest Americans?
• How do I learn more about rocketry?

This week: the arXiv Accessibility Forum

Help | Advanced Search

General Relativity and Quantum Cosmology

Title: eternally oscillating zero energy universe.

Abstract: The question of whether the universe is eternal or if it had a singular moment of creation is deeply intriguing. Although different versions of steady state and oscillatory models of eternal universe have been envisaged, empirical evidence suggests a singular moment of creation at the big bang. Here we analyze the oscillatory solutions for the universe in a modified theory of gravity THED (Torsion Hides Extra-Dimension) and evaluate them by fitting Type 1a supernovae redshift data. THED-gravity exactly mimics General Relativity at the kinematical level, while the modifications in its dynamical equations allow the universe to bounce between a minimum size and a maximum size with a zero average energy within each oscillation. The optimally fit oscillatory solutions correspond to a universe with (i) a small matter density requiring little to no dark matter, (ii) a significantly negative spatial curvature, (iii) a tiny negative dark energy. Alternatively, there exists non-oscillating solutions that appear as an ever-expanding universe from a single bounce preceded by a collapse from the infinite past. These ever-expanding solutions provide marginally better fits to the supernova redshift data, but require larger matter densities and positive dark energy along with a positive spatial curvature. A qualitative analysis of CMB power spectrum in the modified theory suggests a significant negative spatial curvature, which is in stark contrast to a near-zero curvature in the standard big bang theory. An independent constraint on the spatial curvature can further shed light on discriminating the ever expanding and oscillatory universe scenarios.

Submission history

Access paper:.

• Other Formats

References & Citations

• INSPIRE HEP
• Google Scholar
• Semantic Scholar

BibTeX formatted citation

Bibliographic and Citation Tools

Code, data and media associated with this article, recommenders and search tools.

• Institution

arXivLabs: experimental projects with community collaborators

arXivLabs is a framework that allows collaborators to develop and share new arXiv features directly on our website.

Both individuals and organizations that work with arXivLabs have embraced and accepted our values of openness, community, excellence, and user data privacy. arXiv is committed to these values and only works with partners that adhere to them.

Have an idea for a project that will add value for arXiv's community? Learn more about arXivLabs .

11 min read

What is Dark Energy? Inside our accelerating, expanding Universe

Some 13.8 billion years ago, the universe began with a rapid expansion we call the big bang. After this initial expansion, which lasted a fraction of a second, gravity started to slow the universe down. But the cosmos wouldn’t stay this way. Nine billion years after the universe began, its expansion started to speed up, driven by an unknown force that scientists have named dark energy .

But what exactly is dark energy?

The short answer is: We don't know. But we do know that it exists, it’s making the universe expand at an accelerating rate, and approximately 68.3 to 70% of the universe is dark energy.

A Brief History

It all started with cepheids.

Dark energy wasn't discovered until the late 1990s. But its origin in scientific study stretches all the way back to 1912 when American astronomer Henrietta Swan Leavitt made an important discovery using Cepheid variables, a class of stars whose brightness fluctuates with a regularity that depends on the star's brightness.

All Cepheid stars with a certain period (a Cepheid’s period is the time it takes to go from bright, to dim, and bright again) have the same absolute magnitude, or luminosity – the amount of light they put out. Leavitt measured these stars and proved that there is a relationship between their regular period of brightness and luminosity. Leavitt’s findings made it possible for astronomers to use a star’s period and luminosity to measure the distances between us and Cepheid stars in far-off galaxies (and our own Milky Way).

Around this same time in history, astronomer Vesto Slipher observed spiral galaxies using his telescope’s spectrograph, a device that splits light into the colors that make it up, much like the way a prism splits light into a rainbow. He used the spectrograph, a relatively recent invention at the time, to see the different wavelengths of light coming from the galaxies in different spectral lines. With his observations, Silpher was the first astronomer to observe how quickly the galaxy was moving away from us, called redshift, in distant galaxies. These observations would prove to be critical for many future scientific breakthroughs, including the discovery of dark energy.

Redshift is a term used when astronomical objects are moving away from us and the light coming from those objects stretches out. Light behaves like a wave, and red light has the longest wavelength. So, the light coming from objects moving away from us has a longer wavelength, stretching to the “red end” of the electromagnetic.

Discovering an Expanding Universe

The discovery of galactic redshift, the period-luminosity relation of Cepheid variables, and a newfound ability to gauge a star or galaxy’s distance eventually played a role in astronomers observing that galaxies were getting farther away from us over time, which showed how the universe was expanding. In the years that followed, different scientists around the world started to put the pieces of an expanding universe together.

In 1922, Russian scientist and mathematician Alexander Friedmann published a paper detailing multiple possibilities for the history of the universe. The paper, which was based on Albert Einstein’s theory of general relativity published in 1917, included the possibility that the universe is expanding.

In 1927, Belgian astronomer Georges Lemaître, who is said to have been unaware of Friedmann’s work, published a paper also factoring in Einstein’s theory of general relativity. And, while Einstein stated in his theory that the universe was static, Lemaître showed how the equations in Einstein’s theory actually support the idea that the universe is not static but, in fact, is actually expanding.

Astronomer Edwin Hubble confirmed that the universe was expanding in 1929 using observations made by his associate, astronomer Milton Humason. Humason measured the redshift of spiral galaxies. Hubble and Humason then studied Cepheid stars in those galaxies, using the stars to determine the distance of their galaxies (or nebulae, as they called them). They compared the distances of these galaxies to their redshift and tracked how the farther away an object is, the bigger its redshift and the faster it is moving away from us. The pair found that objects like galaxies are moving away from Earth faster the farther away they are, at upwards of hundreds of thousands of miles per second – an observation now known as Hubble’s Law, or the Hubble-Lemaître law. The universe, they confirmed, is really expanding.

Expansion is Speeding Up, Supernovae Show

Scientists previously thought that the universe's expansion would likely be slowed down by gravity over time, an expectation backed by Einstein's theory of general relativity. But in 1998, everything changed when two different teams of astronomers observing far-off supernovae noticed that (at a certain redshift) the stellar explosions were dimmer than expected. These groups were led by astronomers Adam Riess, Saul Perlmutter, and Brian Schmidt. This trio won the 2011 Nobel Prize in Physics for this work.

While dim supernovae might not seem like a major find, these astronomers were looking at Type 1a supernovae , which are known to have a certain level of luminosity. So they knew that there must be another factor making these objects appear dimmer. Scientists can determine distance (and speed) using an objects' brightness, and dimmer objects are typically farther away (though surrounding dust and other factors can cause an object to dim).

This led the scientists to conclude that these supernovae were just much farther away than they expected by looking at their redshifts.

Using the objects’ brightness, the researchers determined the distance of these supernovae. And using the spectrum, they were able to figure out the objects’ redshift and, therefore, how fast they were moving away from us. They found that the supernovae were not as close as expected, meaning they had traveled farther away from us faster than ancitipated. These observations led scientists to ultimately conclude that the universe itself must be expanding faster over time.

While other possible explanations for these observations have been explored, astronomers studying even more distant supernovae or other cosmic phenomena in more recent years continued to gather evidence and build support for the idea that the universe is expanding faster over time, a phenomenon now called cosmic acceleration. 

But, as scientists built up a case for cosmic acceleration, they also asked: Why? What could be driving the universe to stretch out faster over time?

Enter dark energy.

What Exactly is Dark Energy?

Right now, dark energy is just the name that astronomers gave to the mysterious "something" that is causing the universe to expand at an accelerated rate.

Dark energy has been described by some as having the effect of a negative pressure that is pushing space outward. However, we don't know if dark energy has the effect of any type of force at all. There are many ideas floating around about what dark energy could possibly be. Here are four leading explanations for dark energy. Keep in mind that it's possible it's something else entirely.

Vacuum Energy:

Some scientists think that dark energy is a fundamental, ever-present background energy in space known as vacuum energy, which could be equal to the cosmological constant, a mathematical term in the equations of Einstein's theory of general relativity. Originally, the constant existed to counterbalance gravity, resulting in a static universe. But when Hubble confirmed that the universe was actually expanding, Einstein removed the constant, calling it “my biggest blunder,” according to physicist George Gamow.

But when it was later discovered that the universe’s expansion was actually accelerating, some scientists suggested that there might actually be a non-zero value to the previously-discredited cosmological constant. They suggested that this additional force would be necessary to accelerate the expansion of the universe. This theorized that this mystery component could be attributed to something called “vacuum energy,” which is a theoretical background energy permeating all of space.

Space is never exactly empty. According to quantum field theory, there are virtual particles, or pairs of particles and antiparticles. It's thought that these virtual particles cancel each other out almost as soon as they crop up in the universe, and that this act of popping in and out of existence could be made possible by “vacuum energy” that fills the cosmos and pushes space outward.

While this theory has been a popular topic of discussion, scientists investigating this option have calculated how much vacuum energy there should theoretically be in space. They showed that there should either be so much vacuum energy that, at the very beginning, the universe would have expanded outwards so quickly and with so much force that no stars or galaxies could have formed, or… there should be absolutely none. This means that the amount of vacuum energy in the cosmos must be much smaller than it is in these predictions. However, this discrepancy has yet to be solved and has even earned the moniker "the cosmological constant problem."

Quintessence:

Some scientists think that dark energy could be a type of energy fluid or field that fills space, behaves in an opposite way to normal matter, and can vary in its amount and distribution throughout both time and space. This hypothesized version of dark energy has been nicknamed quintessence after the theoretical fifth element discussed by ancient Greek philosophers.

It's even been suggested by some scientists that quintessence could be some combination of dark energy and dark matter, though the two are currently considered completely separate from one another. While the two are both major mysteries to scientists, dark matter is thought to make up about 85% of all matter in the universe.

Space Wrinkles:

Some scientists think that dark energy could be a sort of defect in the fabric of the universe itself; defects like cosmic strings, which are hypothetical one-dimensional "wrinkles" thought to have formed in the early universe. 

A Flaw in General Relativity:

Some scientists think that dark energy isn't something physical that we can discover. Rather, they think there could be an issue with general relativity and Einstein's theory of gravity and how it works on the scale of the observable universe. Within this explanation, scientists think that it's possible to modify our understanding of gravity in a way that explains observations of the universe made without the need for dark energy. Einstein actually proposed such an idea in 1919 called unimodular gravity, a modified version of general relativity that scientists today think wouldn't require dark energy to make sense of the universe.

Dark energy is one of the great mysteries of the universe. For decades, scientists have theorized about our expanding universe. Now, for the first time ever, we have tools powerful enough to put these theories to the test and really investigate the big question: “what is dark energy?”

NASA plays a critical role in the ESA (European Space Agency) mission Euclid (launched in 2023), which will make a 3D map of the universe to see how matter has been pulled apart by dark energy over time. This map will include observations of billions of galaxies found up to 10 billion light-years from Earth.

NASA's Nancy Grace Roman Space Telescope , set to launch by May 2027, is designed to investigate dark energy, among many other science topics, and will also create a 3D dark matter map. Roman's resolution will be as sharp as NASA’s Hubble Space Telescope's, but with a field of view 100 times larger, allowing it to capture more expansive images of the universe. This will allow scientists to map how matter is structured and spread across the universe and explore how dark energy behaves and has changed over time. Roman will also conduct an additional survey to detect Type Ia supernovae

In addition to NASA’s missions and efforts, the Vera C. Rubin Observatory, supported by a large collaboration that includes the U.S. National Science Foundation, which is currently under construction in Chile, is also poised to support our growing understanding of dark energy. The ground-based observatory is expected to be operational in 2025.

The combined efforts of Euclid, Roman, and Rubin will usher in a new “golden age” of cosmology, in which scientists will collect more detailed information than ever about the great mysteries of dark energy.

Additionally, NASA's James Webb Space Telescope (launched in 2021), the world’s most powerful and largest space telescope, aims to make contributions to several areas of research, and will contribute to studies of dark energy.

NASA's SPHEREx (the Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer) mission, scheduled to launch no later than April 2025, aims to investigate the origins of the universe. Scientists expect that the data collected with SPHEREx, which will survey the entire sky in near-infrared light, including over 450 million galaxies, could help to further our understanding of dark energy.

NASA also supports a citizen science project called Dark Energy Explorers , which enables anyone in the world, even those who have no scientific training, to help in the search for dark energy answers.

*A brief note*

Lastly, to clarify, dark energy is not the same as dark matter. Their main similarity is that we don't yet know what they are!

By Chelsea Gohd NASA's Jet Propulsion Laboratory

Related Terms

• Dark Energy
• Dark Matter
• James Webb Space Telescope (JWST)
• Nancy Grace Roman Space Telescope
• SPHEREx (Spectro-Photometer for the History of the Universe and Ices Explorer)
• Stellar Evolution
• The Big Bang
• The Universe

Explore More

Hubble Examines a Busy Galactic Center

NASA’s Webb Reveals Distorted Galaxy Forming Cosmic Question Mark

NASA’s Mini BurstCube Mission Detects Mega Blast